DEUTERIUM RETENTION IN POLYCRYSTALLINE TUNGSTEN

By

Zhe Tian

A thesis submitted in conformity with the requirements for the degree of

Master of Applied Science

Graduate Department of Aerospace Studies

University of Toronto

© Copyright by Zhe Tian (2009)

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Deuterium Retention in Polycrystalline Tungsten Zhe Tian

Master of Applied Science Graduate Department of Aerospace Studies

University of Toronto 2009

Abstract

Deuterium retention in two types of polycrystalline tungsten was studied as a function

of ion fluence, irradiation temperature and ion energy. Fluence dependence: D retention at

300 K tends to saturate in both Rembar and Plansee PCW. At 500 K, D retention in the

Plansee PCW increases with increasing ion fluence, similar to previous results for Rembar

tungsten. Even at a fluence of 8×1025 D+/m2, no sign of saturation was observed.

Temperature dependence: D retention in Plansee PCW decreases with increasing irradiation

temperature (300 - 500 K). Energy dependence: varying the D+ energy from 100 to 500

eV/D+ plays a minor role in D retention in W, suggesting that D retention depends more on

the W structure, irradiation temperature and fluence, rather than on the ion energy when the

energy is below the displacement threshold.

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Acknowledgements

I would like to gratefully thank my supervisors Prof. A.A. Haasz and Dr. J.W. Davis for

giving me this opportunity to participate in a fresh research project. Prof. Haasz’s generosity,

patience, and excellent guidance are very much appreciated. Dr. Davis’s encouragement and

technical support for each particular problem during the research not only keeps me on the

right track but inspires me in new ways of thinking. Special thanks to Dr. Makoto Oyaidzu

for his valuable instructions in the beginning of my research, and to Mr. Charles Perez for his

high quality and prompt work on preparing specimens and fabricating components for my

experiments. The financial support provided by the Natural Science and Engineering

Research Council (NSERC) of Canada is gratefully acknowledged.

In my first two years’ life in Canada, I am pleased to have studied in such a

multicultural environment and friendly atmosphere at UTIAS. So, big thanks to Fusion lab

mates, Bernie Fitzpatrick, John Roszell, Cedric Tsui and Andre LeBelle, for their help,

support and communications during the work time. Also thanks to ASA members, UTIAS

soccer players, and everybody at the Institute to make the school life much more enjoyable.

Special thanks go out to Dr. Alan Yu and Dr. Joseph Chen for their wisdom and substantial

support in helping me accommodate to life in Toronto.

Finally, I would express my gratitude to my family for their endless encouragement

and love.

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Table of Contents Abstract……………………………………………………………………… ii

Acknowledgements………………………………………………………….. iii

List of figures………………………………………………………………... vi

List of tables…………………………………………………………………. viii

1. Introduction……………………………………………............... 1

1.1 Fusion energy…………………………………………………............. 1

1.2 Plasma-facing materials……………………………………………… 2

1.3 Tungsten……………………………………………………………….. 3

1.4 Thesis objective……………………………………………………….... 4

2. Background: deuterium retention in tungsten…..................... 4

2.1 Irradiation effects on structure evolution of tungsten materials…… 5

2.2 Fluence dependence of deuterium retention in tungsten………….… 7

2.3 Temperature dependence of D retention in tungsten……………….. 10

2.4 Flux dependence of deuterium retention in tungsten.......................... 10

3. Experimental apparatus................................................................ 11

3.1 Polycrystalline tungsten specimens…………………………………….. 11

3.2 Ion beam implantation system………………………………………… 12

3.2.1 Single-beam ion accelerator……………………………..………... 12

3.2.2 Specimen holder………………………………………..………….. 13

3.3 Thermal desorption system……………………………………………. 14

3.4 Scanning electron microscopy (SEM)……………………………........ 15

4. Experimental procedure………………………………………. 16

4.1 Specimen preparation and anneal……………………………………. 16

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4.2 Deuterium ion implantation…………………………………………... 17

4.3 Thermal desorption spectroscopy (TDS) …..…….. …………….…... 18

4.4 TDS analysis………………………………………………………...…. 19

5. Results and discussion…………………………………….…… 21

5.1 Fluence dependence of D retention in PCW…………………………. 21

5.1.1 D retention in Rembar PCW at 300 K……………………..……. 21

5.1.2 D retention in Plansee PCW at 300 K……………..…………….. 23

5.1.3 D retention in Plansee PCW at 500 K…………..……………….. 24

5.1.4 Discussion of fluence dependence………………………………... 25

5.2 Temperature dependence of D retention in PCW……....………….... 27

5.3 Ion energy dependence of D retention in PCW…………..…..…....… 28

6. Conclusions…………………………………….………………. 29

6.1 Fluence dependence………………………………….…………...…… 29

6.2 Temperature dependence……………………….…………………...... 30

6.3 Ion energy dependence………………………….………………......… 30

References………………………………………….…...……..… 32

Figures……………………………………………………...…... 36

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List of figures: Figure 2-1: Depth profiles of D trapped as D atoms (a) and D2 molecules (b) in single crystal

and hot-rolled W implanted with 6 keV D ions at 300 K determined by the SIMS/RGA method.

Figure 2-2: Retained vs. cumulative-fluence for 1 keV/D+ implantations at 500 K. Data are shown for specimens W2 (1023 D/m2 probe-fluence only), W1 (9×1023 D/m2 damage-fluence and probe-fluence), and W3 (1025 D/m2 damage-fluence and probe-fluence).

Figure 2-3: Retained vs. cumulative-fluence for 500 eV/D+ implantations at 500 K. Data are shown for specimens W5 (1023 D/m2 probe-fluence only), W4 (9×1023 D/m2 damage-fluence and probe-fluence), W6 (3×1024 D/m2 damage-fluence and probe-fluence), W7 (1025 D/m2 damage-fluence and probe-fluence), and W9 (3×1025 D/m2 damage-fluence and probe-fluence).

Figure 2-4: Fluence dependence of D retention in PCW at 300 K under various D ion energies

Figure 2-5: NRA measurements of the near-surface D depth profiles. (a) 1 keV and 500 eV D+ (1024 D+/m2 incident fluence) implanted into W at 300 K. Implantation profiles for 1 keV and 500 eV D+ as calculated by TRVMC are shown for comparison (normalized to the peak height of the measured profiles). (b) 500 eV D+ implanted at 500 K into W (1024 D+/m2) and W-1% La2O3 (3.3×1024 D+/m2).

Figure 2-6: Fluence dependence of D retention in W at elevated temperatures using ion beams.

Figure 2-7: Fluence dependence of D retention in W at elevated temperatures using plasma devices and tokamaks.

Figure 2-8: Temperature dependence of D retention in W and W-1%La2O3. (a) 1 keV/D+ at fluences of 1023 and 1024 D+/m2, (b) 500 eV D+ at fluence of 1023 D+/m2.

Figure 2-9: Temperature dependence of D retention in M-SCW with an incident fluence of 1024 D+/m2.

Figure 2-10 (a): Deuterium retention in single-crystal and polycrystalline fine-grain tungsten exposed to low-energy (200 eV/D+) and high flux (about 1×1021 D/m2s) D plasmas as a function of exposure temperature. For comparison, the temperature dependence of the D retention in polycrystalline coarse-grained W irradiated with 200 eV/D+ ions and flux of 4×1019 D m−2 s−1 to a fluence of 1×1024 Dm−2 is also shown. Note that the deuterium retention was calculated from deuterium depth profiles measured up to a depth of 7μm.

Figure 2-10 (b): Deuterium retention in polycrystalline tungsten exposed to low-energy (98–100 eV/DT) and high flux ((8.7–10)×1021 D(T)m−2 s−1) D or (D+ T) plasmas as a function of exposure temperature.

Figure 2-11: Deuterium retention as a function of incident D+ flux at three fluences (1021, 1022, and 1023 D+/m2) at room temperature.

Figure 3-1: Schematic of single-beam ion accelerator. Figure 3-2: Implantation specimen holder. Figure 3-3: Schematic of the TDS system.

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Figure 4-1: SEM images of irradiated Plansee PCW (after TDS test). (a) On-spot area; (b) Off-spot area.

Figure 4-2: SEM images of irradiated Rembar PCW (after TDS test). (a) On-spot area; (b) Off-spot area.

Figure 4-3: Cross-sectional SEM photograph of Plansee PCW specimen. Figure 4-4: Signals of H2, HD, and D2 in two thermal desorption runs. (a) 200 eV/D+, 500 K,

Plansee PCW; (b) 200 eV/D+, 500 K, Plansee PCW. Figure 4-5: Example calculation of deuterium retention in Plansee PCW (200 eV/D+, 500 K).

The vertical bar indicates the estimated HD contribution (i.e., integration of HD signals over the two different time spans).

Figure 5-1: 500 eV/D+ ion implantation on PCW at 300 K. Figure 5-2: 200 eV/D+ ion implantation on PCW at 300 K. Figure 5-3: Deuterium retention in different types of PCW at 300 K. Figure 5-4: Fluence dependence of D retention in PCW at elevated temperatures. Figure 5-5: Collection of fluence dependence data on deuterium retention in tungsten at

room temperature. Figure 5-6: Fluence dependence of D retention in Plansee PCW for different energy and

temperature combinations. Figure 5-7: Collection of fluence dependence data on D retention in tungsten at elevated

temperatures. Figure 5-8: Irradiation temperature effects on D retention in PCW. Figure 5-9: Incident ion energy dependence of D retention in Plansee PCW. Figure 5-10: Energy dependence of D retention in Rembar PCW.

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List of tables: Table 1: Impurity content of polycrystalline tungsten specimens…………………………12

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1. Introduction

1.1 Fusion Energy

The global energy demand is growing rapidly due to the increase in world population

and the increasing energy use per capita. The present global energy system relies heavily on

fossil fuels, which supply almost 80% of the world’s energy demand [1]. However, the

current use of fossil fuels is facing many challenges and will be restricted due to concerns

about atmospheric pollution and resource depletion [2]. In order to address our present and

future energy demand, there is an urgent need to develop a sustainable energy mix. Nuclear

fusion is considered to be one of the alternative energy options because of several

advantages, such as limitless fuel supply, no greenhouse gas emission, suitability for

large-scale electricity production and low levels of radioactive waste [3].

Nuclear fusion involves the reaction between two light nuclei which combine to form

a heavier nucleus, accompanied by a release of energy. Hydrogen isotopes, namely

deuterium and tritium, have been regarded as the most likely elements suitable for a

terrestrial fusion reactor [4]. Among the possible fusion reactions, the D-T reaction is

currently under investigation because of the large reaction cross-section at relatively low

energy [5]. In order for fusion reactions to occur, one must provide enough kinetic energy

to overcome the Coulomb repulsion force between the two positively charged hydrogen

nuclei. Heating the fuel particles to temperatures on the order of 100 million ºC will speed

up the hydrogen ions to sufficiently high velocities to undergo fusion; this method is

thermonuclear fusion. However, atoms with high thermal velocities will easily escape even

a large reaction volume in a short period of time, so it is necessary to find some means of

confining the hot fusion fuel, or plasma. During the past 50 years, two main confinement

techniques have been developed: inertial and magnetic confinement. For inertial

confinement, powerful lasers are employed to heat and compress sub-millimeter sized

capsules of fusion fuel to high densities and temperatures to reach the necessary conditions

for fusion reactions to occur [3]. In magnetic confinement, magnetic fields are used to

confine and stabilize the plasma.

The most advanced type of magnetic confinement concept is known as the tokamak,

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derived from Russian words meaning “toroidal chamber magnetic”. A tokamak employs

both toroidal and poloidal magnetic fields to confine the plasma [5]. Large experimental

tokamak devices have been studied, such as the Tokamak Fusion Test Reactor (TFTR) with

11 MW peak fusion power [6], and the Joint European Torus (JET) with fusion power of 16

MW [7]. However, such reactors are still far from the requirement of a commercial fusion

power station. The most recent major fusion device, now under construction in France, is

the International Thermonuclear Experimental Reactor (ITER) [8]. The goal of ITER is to

demonstrate fusion ignition conditions, and to maintain a burning plasma without external

heating sources.

1.2 Plasma-facing materials

In all magnetic confinement fusion devices, including tokamaks, some plasma

particles can escape from the magnetic confinement region and impact the surrounding

wall. Such plasma-wall interactions critically affect tokamak operation in many ways [9].

Plasma erosion determines the lifetime of plasma-facing components and creates a source

of impurities, which can play a role in cooling and diluting the plasma. Deposition of

eroded impurities onto plasma-facing materials alters their surface composition and via

co-deposition may lead to long-term accumulation of large in-vessel tritium inventories.

Retention and recycling of hydrogen affect fuelling efficiency, plasma density control and

the density of neutral hydrogen in the plasma boundary, which in turn, impacts particle and

energy transport.

The first wall will be responsible for withstanding the intense heat load and particle

flux from the core plasma, over months or years of operation, with little or no maintenance.

Thus, the choice of plasma-facing materials is a critical issue for the development of ITER.

The ideal candidate would have [9]: low erosion, thereby extending the lifetime of

components; high thermal conductivity to dissipate high heat loads; thermal stress

resistance to prevent fracture due to temperature changes; resistance to the detrimental

effects of neutron irradiation; and low tritium retention for safety and fuelling issues.

Although it is impossible to find a single material which meets all of the above

requirements, the use of the divertor magnetic geometry may provide the opportunity to

employ different materials for different regions of the reactor wall. Currently, a

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combination of beryllium, carbon and tungsten has been selected as plasma-facing

materials for ITER [10]. Beryllium, with a low atomic number, excellent oxygen gettering

capabilities but low melting temperature and high sputtering yield [11], has been chosen as

the armor material for ~80% of the total surface exposed to the plasma (primary wall, upper

baffle and as the primary option for the port limiters) [11]. Another low Z material, carbon,

which has the advantages of good power handling and thermal shock resistance, has been

selected for the primary high heat flux plasma-contact surfaces in the divertor. A particular

advantage of carbon is that it does not melt and thus can preserve its shape under high

temperature circumstances. But, carbon also has shortcomings, such as a high chemical and

physical erosion rate as well as high deuterium retention, primarily in co-deposited layers

in the divertor region [9]. Tungsten is favorable as another plasma-facing material

candidate due to its high melting temperature, low physical sputtering yield, high threshold

for physical sputtering, and no chemical sputtering [9]. However, such a high Z material

could cause high radiative losses if sputtered tungsten impurities reach the core plasma.

Therefore, the use of tungsten must be avoided in regions where exposure to significant

fluxes of energetic particles might occur.

1.3 Tungsten

This study focuses on deuterium retention in tungsten. Therefore, a brief description

about tungsten, including tungsten’s physical and chemical properties as well as their

connections to fusion use, is given in this section.

Known for its high temperature characteristics, such as the highest melting

temperature of 3683 K, lowest vapor pressure of 6.5×10-7 Pa at 2300 K, and highest tensile

strength at elevated temperatures among all metals, together with good thermal

conductivity (178 W/m·K) [12] and low sputtering yield [13] for minimizing impurity

generation, tungsten is regarded as a favored armor material for selected wall components

of future tokamaks.

The use of tungsten was initially suggested in the 1970s when it was briefly

considered an alternative to stainless steel as plasma-facing material [14]. To prevent high

radiative losses by tungsten impurities leading to unacceptable plasma cooling, it was

suggested that tungsten only be used in low ion energy regions (< 40 eV) to avoid erosion.

4

However, the limitation of tokamak technology at that time made it quite difficult to

produce sufficiently low energy plasmas. Therefore, for several decades, the research focus

shifted to low Z materials, such as carbon and beryllium.

The interest of using tungsten as a plasma-facing material has been renewed due to

advances in tokamak operation over the past 20 years, which resulted in the production of

edge plasmas with energies of 3-100 eV. As a result, recent studies have concentrated on

updating the database on hydrogen interaction with tungsten. A study on the assessment of

tungsten for use in the ITER plasma facing components indicated that tungsten armor

structures were capable of surviving as many as 200 cycles at 16 MW/m2 without any

damage [15]. In the meantime, research on low Z plasma-facing materials, such as carbon

and beryllium, suggested the high erosion rates may require more frequent reactor

maintenance and downtime to replace and repair the first wall [16].

1.4 Thesis objective

The objective of this thesis is to extend the database to improve our understanding on

D retention in polycrystalline tungsten by examining the effects of material structure and,

very importantly, extending the irradiation fluence with our ion beam accelerator to the

high fluences typically achieved in plasma devices (≥ 1026 D+/m2). Discrepancies in the

existing retention results in the literature, especially between measurements by different

research groups, have led this study to investigate deuterium retention in polycrystalline

tungsten as a function of incident fluence, irradiation temperature, ion energy, and surface

structure.

2. Background: deuterium retention in tungsten

Plasma-wall interactions are among the critical issues in fusion devices. In particular,

hydrogen retention in plasma-facing components can affect fuelling efficiency, plasma

density control, and the density of neutral hydrogen in the plasma boundary [9]. This in

turn affects particle and energy confinement. Tritium inventory and permeation through the

wall or into coolant channels are also concerns for the safety of reactors. Therefore, the

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underlying mechanism of hydrogen trapping in plasma-facing materials has been widely

studied, but with inconsistent results. Generally, two types of tungsten materials have been

studied, namely single-crystal tungsten (SCW) and polycrystalline tungsten (PCW). It has

been suggested that the concentration and nature of trapping sites in the bulk material

would largely determine the transport and retention of hydrogen [17]. Since PCW is

planned for use in ITER [10], deuterium retention in polycrystalline tungsten has been

selected as the primary topic to be investigated in this thesis.

Previous investigations on hydrogen retention in tungsten have been performed using

various experimental techniques and under different irradiation conditions. Generally, ion

implantation has been used to simulate the ions and neutrals incident on plasma-facing

materials in tokamaks. Common ion sources include accelerators, ion guns and linear

plasma devices (e.g. PISCES [18]). The hydrogen remaining in a specimen following ion

implantation can be determined by thermal desorption spectroscopy (TDS) or nuclear

reaction analysis (NRA). The depth profile of hydrogen in tungsten could be obtained from

secondary ion mass spectroscopy (SIMS) or nuclear reaction analysis (NRA). The

microstructure of the materials could be observed by scanning electron microscopy (SEM).

In addition, several simulation codes, such as TMAP, have been developed to interpret

hydrogen trapping mechanism in tungsten [19]. The TAMP-7 code was used by Poon et al.

to derive D trapping energies in SCW [19a].

2.1 Irradiation effects on structure evolution of tungsten materials

The minimum energy of hydrogen and deuterium ions for displacement damage

production is ~2050 eV for H+ [20] and ~960 eV D+ [21]. Irradiation with H or D ions

above the threshold leads to elastic collisions resulting in nucleation and growth of

dislocation loops. When the incident D+ ion energy is below the displacement threshold in

W, ion-induced damage due to D+ does not occur, but it is still possible to produce damage

by knock-on collisions between D+ ions and surface impurities [22]. At essentially all

incident energies, hydrogen accumulation, extending to depths much greater than the ion

range, may cause lattice distortion, even leading to the formation of vacancy clusters or

nano-bubbles, micro-voids and in some cases blistering [21].

Sakamoto et al. [20] studied hydrogen ion bombardment effects on tungsten

6

microstructure. For the energy range of 0.5-8 keV H+ and implantation temperatures of

300-1073 K, TEM observations showed that for H+ < 2 keV, an incident fluence of

1.5×1022 H+/m2 did not lead to any change in the single-crystal tungsten’s inner structure at

room temperature. However, when using 3 keV H+, which is above the threshold energy for

displacement damage, hydrogen clusters and dislocation loops were created. For 8 keV H+

implanted into single-crystal tungsten, a temperature dependence study showed dislocation

loops coalesced to form dislocation networks at 373 K and below; between 473 K and 773

K, dislocation loops were formed but without networks; at 873 K or above, fine hydrogen

bubbles were observed [20]. This study also showed that defect accumulation was more

prominent in polycrystalline tungsten than in single-crystal tungsten, suggesting higher D

retention levels would be found in polycrystalline tungsten.

Alimov et al. [23] compared deuterium retention and lattice damage in two types of

W, namely SCW and hot-rolled PCW at two ion energies (6 keV/D+ and 10 keV/D+) and

two temperatures (300 K and 650 K). They found at least two types of ion-induced defects

causing trapping of deuterium: (1) micro-voids filled with D2 located in the implantation

zone; (2) dislocations distributed from the surface to depths far beyond 1 μm. SIMS/RGA

depth profiles (Figure 2-1) revealed that 6 keV/D+ irradiation of W (both SCW and PCW)

at 300 K led to the accumulation of D atoms both in the ion stopping zone and at depths of

up to several microns. The existence of D trapped as D2 molecules in micro-voids was

inferred from the release of D2 during sputter-SIMS measurements. After D+ implantation

at 650 K, D2 molecules were not observed and deuterium was accumulated solely in the

form of D atoms at depths of up to 500 nm. In the stopping zone of SCW irradiated with 6

and 10 keV/D+ at 300 K, the maximum concentration of D accumulated in both states

reaches a value of 0.10 D/W while the maximum value is 0.015 D/W for 6 keV/D+

irradiation at 650 K [23]. RBS/C analysis indicated that micro-voids and dislocation-type

defects were two existing types of hydrogen trapping, the former within the ion

implantation zone and the latter extending to depths far beyond 1 micron. Heating up to

900 K during implantation, the D2-filled micro-voids completely disappear while the

dislocation density decreases at depths beyond the implantation zone and increases in the

ion stopping zone [23].

Haasz et al. [24] used both 3 keV D3+ ions (1 keV/D+: above displacement threshold)

7

and 1.5 keV D3+ ions (500 eV/D+: below threshold) to investigate the effect of ion-induced

damage on W. In this study, specimens were subjected to different damage histories by

varying the incident D+ fluence prior to exposing specimens to a “probe” fluence (TDS was

performed after each “damage” fluence). Figure 2-2 shows the effect of such an

implantation history on D trapping. For specimen W1, the retention values for

damage-fluence implantations of 1 keV/D+ and 9×1023 D/m2 showed a small increase with

cumulative fluence, but the values appeared to level off after ~8×1024 D/m2, approximately

a factor of 2 above the virgin damage implantation value. The damage and probe fluences

used for the 500 eV/D+ ion implantations were identical to those used for the 1 keV/D+ case,

with the addition of 3×1024 D/m2 and 3×1025 D/m2 damage-fluence runs. The results from

these specimens are shown in Figure 2-3. Retention values for a particular set of

damage-fluence runs increased as a function of cumulative fluence, but the dependence

appeared to be weak. It is noted that the retention values corresponding to the 3×1024 D/m2

damage-fluence runs (W6) were about twice the values obtained for the 9×1023 D/m2 runs

(W4), but increasing the damage fluence from 3×1024 D/m2 (W6) to 1025 D/m2 (W7) or

3×1025 D/m2 (W9) did not significantly increase the retention levels [24]. This suggests that

for 500 eV D+ implantation at 500 K, the amount of ion induced damage saturates for

incident-fluences above 3×1024 D/m2 [24].

The results for 500 eV/D+ ions for the “probe” fluence showed increased D retention

in the specimens, which was suggested to be due to swelling-induced stresses and/or

precipitation of W hydrides leading to dislocation creation and grain cracking [24].

Evidence of surface blisters at high fluences was also seen [24]. Such effects appear to be

dependent on both ion energy and incident fluence. Furthermore, the damage was not

removed during TDS to 2100 K so that more trapping sites are present in subsequent

implantations. Identical experiments using 1 keV/D+ ions did not show any significant

change in “probe” fluence retention due to prior implantations [24].

2.2 Fluence dependence of deuterium retention in tungsten

Since polycrystalline tungsten has been selected as one of the candidates for

plasma-facing components in ITER, deuterium retention in tungsten as a function of the

incident ion fluence has been widely investigated. It is anticipated that the typical

8

irradiation fluence in the ITER divertor is ~1028- 1029 DT/m2 [9]. Linear plasma devices,

such as PISCES [18], may be able to reach such high-fluence irradiation due to their

capability to produce high flux plasmas (~1022 D+/m2s). By comparison, accelerator beams

are limited to 1017-1020 ions/m2s and hence, incident fluences of 1017-1026 ions/m2.

Therefore, estimation of hydrogen inventories in ITER wall materials from ion beam data

requires extrapolating the lower fluence results. While the fluxes of ion beams are lower

than those of plasma device, ion beam accelerators do provide better control over ion

species, ion energy and ion flux. A well-characterized beam could help understand the

deuterium trapping mechanism in tungsten. Since this study is only focusing on deuterium

retention issues in polycrystalline tungsten, experimental results reviewed here are mainly

on PCW.

Several studies on PCW irradiated at 300 K by various deuterium ion energies are

shown in Figure 2-4. Haasz et al. [25] employed 500 eV and 1 keV D ions to irradiate

specimens cut from a 25 μm thick and 99.95 wt% pure polycrystalline tungsten foil made

by Rembar Corporation. They found that for both energies the retained amount of

deuterium saturated for fluence above 1023 D+/m2, with similar retention levels (~6×1020

D/m2). The similar deuterium inventory was verified by NRA measurements in the same

study [25], which showed D was trapped to depths of 500 nm, far beyond the ion range for

both energies (see Figure 2-5(a)). In another study, Ogorodnikova et al. [26] observed a

linear increase of deuterium retention in PCW at 300 K with no sign of saturation for

incident fluences up to 2×1024 D+/m2. It should be noted that the PCW specimens used by

Ogorodnikova were cut from a reduced-rolled, 0.5 mm thick, 99.96 wt% pure tungsten foil

made by Plansee Corporation. The disagreement was possibly due to the different surface

structure of the two types of specimens, which could be caused by different specimen

preparation techniques. The resolution of this disagreement is also one of the objectives of

this thesis project.

Further studies of D retention by 200 eV/D+ ions at 300 K by Ogorodnikova et al. [27]

(measured by TDS) and Alimov et al. [28] (measured by NRA) also yielded conflicting

results; see Figure 2-4. Alimov’s results [28] illustrated a trend of saturation for incident D

ion fluence above 5×1023 D+/m2, while Ogorodnikova’s [27] showed an increasing trend

with increasing fluence, without saturation. In the case of single-crystal tungsten, Poon et al.

9

[29] found the fluence dependence of D retention in SCW for 500 eV D+ irradiations at 300

K to be similar to that of PCW [25]. The retention initially increased with increasing D+

fluence and then tended to saturate at 6×1020 D/m2 for fluences above 1023 D+/m2.

Fluence dependence of deuterium retention in Rembar PCW has also been studied at

elevated temperatures. For 500 K implantation with both 500 eV/D+ and 1 keV/D+ ions,

Haasz et al. [25] observed no sign of saturation (see Figure 2-6), which was interpreted as

indication of a diffusion-limited trapping mechanism [25]. The NRA analysis of the front

and back surfaces of the tungsten specimen both showed ~0.05 at.% concentrations,

suggesting that the trapped D is uniformly distributed thorough the bulk of the material (see

Figure 2-5(b)). For 200 eV/D+ irradiation of Plansee PCW at 380-470 K, Ogorodnikova et

al. [27] also observed the retained D to increase with increasing incident fluence; however,

in her case the magnitude and slope of the curve differed from the results of Haasz et al.

[25]. Single-crystal tungsten irradiated by 500 eV/D+ ions (produced by accelerator) was

investigated by Poon et al. [29] at 500 K case. The fluence dependence observed in [29]

also indicated an increasing trend with increasing fluence without reaching saturation even

at 1025 D+/m2; see Figure 2-6. However, PCW and SCW differed in the slope of the curves

and the magnitude of the retained amount of deuterium, with the SCW showing less

retention [25,29], which suggests that there are significantly fewer trapping sites in

single-crystal tungsten [29].

Deuterium retention as a function of incident fluence has also been investigated in

linear plasma devices [30-36] and tokamaks (e.g. [37]) at elevated temperatures; see Figure

2-7. Doerner et al. [30] reported that 100 eV D+ irradiation on ITER grade PCW at 575 K

in PISCES showed an increasing trend for deuterium retention with incident fluence

ranging from 1025 D+/m2 to 1026 D+/m2. Kolasinski et al. [32] used 70 eV D plasmas to

irradiate Plansee PCW at 623 K and determined a saturation level for an incident fluence of

1026 D+/m2. Alimov et al. [36] implanted 38 eV D plasmas into W at 530 K and found

similar retention values for fluences between 1026 and 1027 D+/m2, suggesting the

occurrence of saturation for incident fluences above 1026 D+/m2. Differences in materials

and implantation conditions must still account for the large variation in trends and absolute

retention values observed at high fluences. From the collection of data in Figure 2-7, it is

not evident whether the retention levels off or continues to increase for fluence > 1026

10

D+/m2.

2.3 Temperature dependence of D retention in tungsten

Various studies on different types of tungsten (PCW or SCW) irradiated by different

deuterium sources (D plasmas or D+ ions) at elevated temperatures have shown that D

retention in tungsten is dependent on the exposure temperature. Haasz et al. [25] and Poon

et al. [29] used deuterium ion beams with different energies to investigate temperature

effects on D retention in both PCW and SCW. It was found, for PCW [25], that diffusion of

D into the bulk increased in importance with increasing irradiation temperatures. In Figure

2-8, Haasz et al. [25] found an enhancement showing a maximum of the retained amount of

D at ~500 K compared with the 300 K case for both 500 eV/D+ and 1 keV/D+ irradiation.

In Figure 2-9, Poon et al. [29] observed that deuterium retention in SCW decreased with

increasing temperature, with no sign of a peak or plateau in the temperature dependence, as

seen with polycrystalline tungsten (Figure 2-8). No retention was observed in specimens

implanted at temperatures above 700 K. Also, it was suggested that the large scatter in the

data in Figures 2-8 and 2-9 might be a result of varying levels of background impurities

during implantation [29].

Plasma devices have also been employed to study the temperature dependence of D

retention in W; see Figure 2-10. In the study by Alimov et al. [38], D retention in both

SCW and PCW was 1×1020 D/m2 at 300 K for a deuterium plasma with energy of 200

eV/D+, a flux of 1×1021 D+/m2s and incident fluence of 2×1024 D+/m2. As the temperature

increased, the retained D rose to the maximum value of (4~6)×1020 D/m2 at 460 to 490 K,

then decreased by an order of magnitude to 2×1019 D/m2 at 530 K for SCW and 640 K for

PCW; see Figure 2-10 (a). Results of other investigations [39-42] on the temperature

dependence of D retention in PCW by D plasmas are plotted in Figure 2-10 (b). A

maximum in the retention was observed, but it appears to be shifted to higher temperatures,

compared to the studies by Alimov et al. [38] or Haasz et al. [25].

2.4 Flux dependence of deuterium retention in tungsten

High flux (1×1020 D+/m2s) and high fluence (above 1×1024 D+/m2) ion irradiation was

found to be able to create μm size surface blisters on polycrystalline tungsten [24]. It is

11

evident that hydrogen bubble formation and growth in metals depends on the local

hydrogen concentration and vacancy concentration. Thus, a study was performed by Poon

et al. [43] to change the local hydrogen concentration in single-crystal tungsten by varying

incident ion fluxes in order to observe possible flux effects on D retention.

D retention in SCW implanted with 500 eV/D+ ions at 300 K was found to vary

significantly at low flux (<1018 D+/m2s) and low fluence (<1021 D+/m2) (see Figure 2-11).

The amount of retained D was sharply decreased for fluxes below 1018 D+/m2s, indicating a

possible flux threshold for D retention. But, for higher fluences of 1022 and 1023 D+/m2, D

retention did not show significant dependence on incident flux. Poon et al. [43] suggested

that a steady state was established between the incident ions and the D diffusing out of the

implantation zone, which implies that the local mobile D concentration was dependent on

the incident flux. Hydrogen trapping occurred only when the local D concentration

exceeded a threshold, and high enough to cause lattice distortion [43]. This explains why

there was no trapping for very low fluxes. Although flux higher than the threshold led to

high concentration of mobile D concentration and induced D trapping, the trapping rate at

the high fluxes did not depend on the fluxes [43].

3. Experimental apparatus

3.1 Polycrystalline tungsten specimens

Two types of polycrystalline tungsten have been investigated in this study. One was

produced by Rembar Corporation and hot-rolled to a thickness of 25 μm. Specimens

measuring 5×10 mm2 were cut from this 99.95 wt% pure foil. The other type of PCW

tungsten (99.97 wt%) was made by Plansee Corporation without the reduced-rolled process.

Specimens were cut for us by Plansee in dimensions of 5×5×1 mm3. The impurity content

as specified by the manufacturers can be found in table 1.

12

Table 1: Impurity content of polycrystalline tungsten specimens

Impurity Rembar (99.95 wt% pure W) Plansee (99.97 wt% pure W)

Hydrogen (H) < 5 PPM < 5 PPM

Carbon (C) < 30 PPM < 30 PPM

Nitrogen (N) < 10 PPM < 5 PPM

Oxygen (O) < 30 PPM < 20 PPM

Molybdenum (Mo) < 100 PPM < 100 PPM

Silver (Ag) < 5 PPM < 10 PPM

Iron (Fe) < 30 PPM < 30 PPM

Aluminum (Al) < 15 PPM < 15 PPM

3.2 Ion beam implantation system

3.2.1 Single-beam ion accelerator

Deuterium ion implantation in PCW specimens was performed by employing the

single-beam ion accelerator facility in the University of Toronto Institute for Aerospace

Studies. A schematic layout is shown in Figure 3-1. For further details on the accelerator

facility, see Ref. [44]. Since the ion implantation needs to be conducted under high vacuum

conditions, three Leybold-Heraeus turbo-molecular pumps were used to provide differential

pumping for the system. The three vacuum sections comprise the first stage including the

first lens and bending magnet chamber, the second stage for the second lens chamber, and

the third stage for the target chamber.

The single-beam ion accelerator is capable of delivering a mass analyzed light ion

beam with energies from 0.1 to 10 keV. High purity deuterium was fed through a high

pressure regulator to a sapphire-seal variable leak valve and delivered into the

duoplasmatron ion source with a controlled flow rate. Nylon gas lines were connected to

the variable leak to provide electrical isolation. The pressure in the nylon line was kept

above atmospheric pressure to prevent possible contamination from atmosphere.

A platinum mesh filament was positioned in the center of the duoplasmatron ion

source chamber. The filament is coated with an electron emitting barium carbonate

compound and acts as the cathode for the plasma discharge. The plasma so generated is

13

confined by an axial magnetic field in the source chamber. Then a 10 kV extraction voltage

extracts the ions from the duoplasmatron source through a 0.25 mm diameter pinhole

aperture. With the acceleration caused by the extraction voltage, the ions pass through a

second orifice and arrive at the first Einzel lens. Focused by the first lens, the ions’ vertical

position is corrected when passing through a set of steering plates. A 30° bending magnet is

employed to select the desired species by mass selection; the bending magnet is capable of

selecting masses up to 16 amu. The selected ions are then decelerated by passing through a

double gridded deceleration gap to reach the final energy. Finally, a single-grid, second

Einzel lens is used for final focusing of the beam through a 3 mm diameter collimation

aperture onto the target specimen.

3.2.2 Specimen holder

During ion implantation, the target tungsten specimen was positioned in a specimen

holder (Figure 3-2). Following the procedure of Poon [45], the requirements for a specimen

holder include: allowing measurement of the beam current on both the mask and the

specimen, heating and measuring the temperature of the specimen, and specimen alignment

with the beam. To meet all the requirements, a series of layers were designed and

constructed on a stainless steel (SS) base. The base part has a 5 mm diameter aperture in

the center. The first layer on the base was a 10×15 mm2 piece of 25 μm, 99.96 wt% pure

polycrystalline tungsten foil (produced by Rembar Corporation), which was spot-welded

onto the stainless steel plate. A 1.5-mm diameter aperture was drilled through this foil in

order to define the ion exposure area of the specimen, so that it could serve to mask the

incident ion beam, allowing the central high-flux part of the beam to bombard the specimen.

On top of the tungsten foil, a 50 μm thick mica sheet with a slightly bigger aperture than

the one on the foil was used as the second layer to electrically isolate the specimen from the

tungsten foil mask, in order to make it possible to measure the beam current on the

specimen directly. With such a multi-layer set-up, the total beam current can be determined

by summing the current on the mask and the specimen.

Because tungsten irradiation was also performed at elevated temperatures, a

nickel-chromium/nickel-aluminum (chromel-alumel, type K) thermocouple was inserted

between the mica layer and the specimen to measure the temperature. A ceramic heater was

14

placed on top of the specimen and fixed by bolting through the holes on the stainless steel

plate. A graphite washer was placed between the heater and stainless steel washers to

provide a good electrical contact during heating. A thin strip of stainless steel was inserted

between the center of the heater and the back side of the specimen for measuring the beam

current on the specimen. All layers were held together by the bolts mounting the ceramic

heater to the stainless steel base plate. Three electrically isolated sections were used in the

specimen holder assembly [45], namely, (1) the specimen, thermocouple and current

measurement strip; (2) the stainless steel plate and tungsten foil mask; and (3) the bolts and

ceramic heater. In the target chamber, the specimen holder was placed vertically to face the

beam from the accelerator and was isolated from the rest of the vacuum vessel.

3.3 Thermal desorption system

Thermal desorption spectrometry (TDS) was performed in a separate (i.e., other than

the implantation chamber) vacuum system. For TDS, the specimen was placed and heated

on a 25 μm thick tungsten foil cradle made of Rembar PCW. The cradle was resistively

heated and the power to the cradle was controlled by programmed computer software.

During TDS, the temperature of the specimen was also measured by a chromel-alumel

thermocouple. In this case the thermocouple was spot-welded to the specimen; fine

platinum wires were used between the thermocouple and the specimen in order to

strengthen the bond. The released species during heating were monitored by a Hiden

quadruple mass spectrometer (QMS). The QMS was calibrated prior to each TDS run using

calibrated leak bottles of H2 and D2. The heating process was usually conducted under high

vacuum conditions, with a background pressure of ~5×10-8 Torr in the chamber. The

schematic of the thermal desorption system is shown in Figure 3-3. This system was also

used for high temperature annealing of specimens prior to ion implantation.

A Sorenson DCR 20-80B power supply, capable of providing 0-80 A current and 0-20

V voltage, was used to heat the cradle on which the specimens were placed. The power

supply was controlled by running a National Instruments LabWindows software program to

produce a smoothly linear temperature increase. This heating control program was based on

iterations and had to be adjusted for different type of specimens.

15

3.4 Scanning electron microscopy (SEM)

Previous studies [25,29] have indicated that surface preparation would influence the

surface structure of tungsten specimens, thus affecting the deuterium retention in tungsten.

Also, the surface structure of the specimen will be modified when exposed to ion beam or

plasma irradiation. As a result, the surface morphology is a good indication to interpret the

mechanism of plasma surface interactions. A Hitachi S-5200 scanning electron microscopy

(SEM) at the University of Toronto Center for Nanostructure Imaging was employed in this

study to provide high resolution images for the surface structure of tungsten specimens.

The SEM is a type of electron microscope that images the specimen surface by scanning it

with a high-energy electron beam. The electrons then interact with the atoms at or near the

surface on the specimen, producing signals that contain information about surface

topography, composition and other properties. The types of signals produced by SEM

include secondary electrons, back scattered electrons (BSE), and characteristic X-rays.

The basic operation of SEM is as follows [46]. An electron beam is emitted from an

electron gun fitted with a filament cathode. The beam typically has an energy ranging from

a few hundred eV to 40 keV. Being focused by condenser lenses to a spot about 0.4 nm to 5

nm in diameter, the beam passes through pairs of scanning coils or pairs of deflector plates

in the electron column, typically in the final lens, which deflect the beam in the x and y

axes so that it scans over a rectangular area of the sample surface. When the primary

electron beam interacts with the sample, the electrons lose energy by repeated random

scattering and absorption within a teardrop-shaped volume of the specimen known as the

interaction volume, which extends from less than 100 nm to around 5 µm into the surface.

The energy exchange between the electron beam and the sample results in the reflection of

high-energy electrons by elastic scattering, emission of secondary electrons by inelastic

scattering and the emission of electromagnetic radiation, each of which can be detected by

specialized detectors. The beam current absorbed by the specimen can be detected and used

to create images of the distribution of specimen current.

16

4. Experimental procedure

This study was undertaken to investigate deuterium retention in polycrystalline

tungsten materials, focusing on the fluence, temperature and energy dependencies, as well

as the effect of structure on D retention. A procedure consisting of four steps has been

followed in the experiment. The first step was specimen preparation, namely, high

temperature annealing. In the second step, the specimen was mounted in the specimen

holder and placed in the accelerator target chamber for irradiation by energetic deuterium

ions; the ion energy, ion flux, ion fluence and implantation temperature, were controlled so

that one could gain an understanding of the effects of these parameters on deuterium

retention. The third step was to conduct thermal desorption of the implanted specimens,

and the final step was the analysis of the thermal desorption profiles to derive D retention

levels for various irradiation conditions.

4.1 Specimen preparation and anneal

Prior to D+ implantation, the tungsten specimens were first annealed at 850-950 K for

30 minutes in the TDS chamber at a pressure of ~10-7 Torr, followed by cooling down at a

slow rate to avoid quenching. Up to 5 specimens could be annealed at once, and removed

from the heating cradle for implantation when the temperature returned to room

temperature. (We note that the maximum temperatures reached in this study (~950 K) are

much lower than those obtained in previous D retention experiments performed by the

UITAS Fusion group, which were typically ~1500 K. This happened inadvertently by using

the chromel-alumel thermocouple instead of the high-temperature tungsten-5% rhenium /

tungsten-26% rhenium.)

Referring to previous studies regarding thermal annealing effects, van Veen [47]

determined vacancy annihilation occurred at ~800 K but depended on temperature ramping

rates. Anderl et al. [48] indicated that PCW annealed to 1673 K led to a reduction of

dislocation density by a factor of 7. The study also suggested that annealing to 1500 K

increased the grain size of PCW by a factor of 3 to 5, which suggested a decrease of grain

boundaries [48]. Also, high temperature treatment played an important role in desorbing

hydrogen and other impurity gases trapped in the specimens during manufacturing.

17

However, one should note that annealing at around 900 K may not be able to sufficiently

remove the effects of irradiation history. Haasz et al. [24] found that sequential

implantations on the same location of the same specimen led to a significant increase of

deuterium retention in PCW. Therefore, in the present study, virgin specimens were used

for each implantation to avoid any cumulative effect of repeated irradiations on the same

spot.

In order to show the annealing effects and how incident deuterium ions interact with

the tungsten specimens, scanning electron microscopy (SEM) was employed to provide

images of the surface structure of both Rembar and Plansee polycrystalline tungsten

specimens after TDS tests. For each type of tungsten, SEM pictures were taken on both the

irradiated (on-spot) and the surrounding (off-spot) areas. Such characterization provided

information on the micro-structure of these two different types of tungsten, as well as the

evolution of the surface structure due to irradiation by deuterium ions.

SEM images for the Plansee PCW are shown in Figure 4-1, where image (a) indicates

the irradiated area (on-spot) and image (b) represents the surrounding area (off-spot). When

comparing Figure 4-1 (a) and (b), it is hard to find obvious differences in surface structure

of the Plansee PCW on-spot and off-spot areas. SEM photographs for the Rembar PCW are

shown in Figure 4-2; again, the image (a) represents the irradiated area (on-spot) and (b)

the surrounding area (off-spot). In image (b), the grain boundary and some unknown

impurity dots on the surface of the off-spot area are observed. However, the irradiated area

in (a) differs significantly from the surrounding off-spot area in (b). This implies that the

Rembar tungsten specimen surface has been modified by D+ ion bombardment. Overall, the

Plansee specimen appears to have greater surface roughness than the Rembar material.

Cross-sectional SEM photograph of Plansee PCW specimen shows the surface roughness is

about 5-10 μm; see Figure 4-3.

4.2 Deuterium ion implantation

After annealing, prepared specimens were placed into the specimen holder and

installed in the implantation target chamber. Following about 12 hours of pumping, the

pressure of the target chamber reached ~10-8 Torr, while the 1st accelerator stage achieved a

vacuum of ~5×10-7 Torr. Before introducing deuterium into the duoplasmatron, the filament

18

in the duoplasmatron ion source was heated to outgas the chamber to remove impurity

gases. Then, by gradually opening a leak valve, high-purity D2 gas was fed into the system

until the pressure of the first stage reached 1.5×10-5 Torr, which was found to be the

optimum for producing the highest beam current. A 150 V arc voltage (associated with a

2.5 μA arc current) was applied between the filament and plasma cavity to initiate the

plasma discharge. The 30° bending magnet was used to choose the desired species by mass

selection. A current of 1.55 A was applied to the bending magnet to generate D3+ ions. The

selected ions were then decelerated by passing through a double gridded deceleration gap

to reach the final energy.

The accelerator was operated such that the beam energy at the exit aperture was 3 keV.

Thus, to generate a 1.5 keV D3+ ion beam, a 1500 V bias voltage was applied on the

specimen. During implantation, both the total beam current and the current on the specimen

were monitored and adjusted periodically so that the variation of the currents was kept

within ±20% of the mean value. A total beam current of 25-40 μA associated with 4-7 μA

(4-7×1019 D/m2s) on the specimen was obtained during normal operations. To achieve an

ion fluence > 3×1024 D/m2, it was necessary to operate the accelerator for more than ~12 h.

For these cases, the implantation was broken up over a period of two or more days. The

longest implantation, 8×1025 D/m2, required 30 days of normal accelerator operation (8-10

hours per day).

4.3 Thermal desorption spectroscopy (TDS)

After implantation, the irradiated tungsten specimen was removed from the ion

accelerator, spot-welded to nickel-chromium/nickel-aluminum (chromel-alumel, type K)

thermocouple wires, and then installed in the cradle in the TDS system. Typically,

irradiated tungsten specimens were exposed to atmosphere for less than 30 minutes to

minimize the effect of time delay after implantation. Quastel et al. [22] found that thermal

desorption performed after 8 weeks after deuterium ion irradiation led to about a factor of 2

reduction in the measured total D retention compared with the <1 h time delay cases,

indicating that during D+ irradiation some mobile D diffuses deep into the bulk, requiring

relatively long post-irradiation times at room temperature to diffuse to the surface and be

released.

19

With the specimen loaded onto the heating cradle and inserted into the TDS system,

the TDS chamber was pumped down to ~1×10-7 Torr. Before heating the specimen by TDS,

mild baking of the test chamber to about 330-350 K was performed for 2 hours. In the same

study by Quastel et al. [22], it was suggested that mild baking of the test chamber to ~360

K resulted in the escape of about 40% of the trapped D. The loss of this weakly-trapped

deuterium corresponded to the near elimination of the 400 K desorption peak as well as a

reduction in the total amount of D retained. This suggested the existence of additional

lower energy traps, containing about 40% of the trapped D. For the present study, the

shallow traps were emptied by the 330-350 K baking, and only the more deeply trapped D

was measured; see Figure 4-4. The mild baking was also able to improve the vacuum in the

TDS chamber to below 5×10-8 Torr.

After the TDS chamber cooled down to room temperature from mild baking and a

good vacuum (< 5×10-8 Torr) was reached, TDS was ready to start. A complete thermal

desorption cycle is described as follows. First, various QMS signals were calibrated by the

corresponding leak bottles. QMS steady-state signals were measured with the leak bottle on

and off. Then the leak bottle flow rate was divided by the difference of the signals, yielding

the absolute numbers of molecules/s for a given QMS signal. In this study, D2 and H2 leak

bottles were used for quantifying these two species. The calibration factor for HD was

assumed to be the average of these two. Next step was to initiate the computerized heating

program run under LabWindows/CVI, which is a manually modified voltage profile to

control the ramping rate for a linear increase of specimen temperature. With a typical ramp

rate of 1.5-2 K/s, all the specimens were linearly heated up to 900-1000 K and dwelled

there for 2-3 minutes. During the whole process of heating and dwelling, the QMS

monitored the signals of masses 2 (H2), 3 (HD), and 4 (D2). The last step was to cool down

the system with a relatively slow cooling rate of 2 K/s in order to avoid possible quenching

of the specimen.

4.4 TDS analysis

Integration of the QMS signals produced during TDS yielded the total trapped

deuterium in a specimen. Details of the calculation for each result are shown below. The

release of D2 and HD from the specimen was considered to contribute to the deuterium

20

retention in tungsten; this is slightly different from Poon’s method [45] where only D2 was

considered. For the D2 release, the QMS mass 4 signal was used. These signals were

converted to the absolute number of D2 molecules by multiplying them with the calibration

factor. Then, the area confined by the background levels and desorption peaks was

integrated and divided by the beam spot area to yield the D2 release rate, in the unit of

m-2s-1. Finally, the retained amount of D2 was obtained by integrating the release rate by the

desorption time under the peak. This number was multiplied by 2 to yield the number of D

atom released in the form of D2 molecules; it is the number of D atoms, which is plotted in

the retention figures.

Interpretation of the HD signals was more difficult, as background contribution could

be significant. Thus, not the whole area below the HD peaks and above the background

level was integrated. Instead, only those traces that clearly tracked the D2 counterparts were

considered to contribute to D retention from the HD signal. In order to assess the

uncertainty from selecting HD signals, an ‘upper limit’ was also obtained by including a

larger time span for the HD signals. Figure 4-3 shows typical traces of H2, D2, HD QMS

signals in two thermal desorption runs, namely, 200 eV/D+ irradiation on Plansee PCW at

both 500 K (Figure 4-4 (a)) and 300 K (Figure 4-4 (b)). In Figure 4-4 (a), HD signals from

300 s to 500 s were regarded as HD’s main contribution part for D retention and extracted

to calculate the ‘best-guess’ value. To obtain the ‘upper limit’, HD signals from 200 s to

700 s were considered to cover all possible HD signals that contributed to D retention.

Similar calculation was used in Figure 4-4 (b). In the 300 K case shown in Figure 4-4 (b),

no noticeable desorption peak was observed at 400 K, implying that mild baking of the test

chamber to about 330-350 K before TDS has released the majority of the weakly-trapped

deuterium from low-energy trapping sites. Normally only less than 20% of the calculated

total D retention was contributed by HD signals. To give a clear illustration of this

calculation, results of deuterium retention in Plansee tungsten under 200 eV/D+ irradiation

at 500 K are plotted in Figure 4-5. Comparison between trend lines of “D from D2 only”

and “upper limit” values shows that the QMS signal (mass 4) D2 is the major part (almost

80%) of the retained deuterium. Also, these two trend lines tend to be parallel, indicating a

consistent ratio of D2 and HD for the different TDS runs.

21

5. Results and discussion

This chapter describes the experimental results on deuterium retention in

polycrystalline tungsten measured in this study. Three different controlling parameters were

investigated, namely incident ion fluence, specimen temperature, and ion energy. Among

these, fluence dependence is the main focus of this thesis, leaving temperature and ion

energy dependence as brief investigations. For each controlling parameter, the objective,

experimental conditions and the results are given below, followed by discussions of the

experimental results.

5.1 Fluence dependence of D retention in PCW

Two types of PCW materials with different surface structures and preparation, namely

Rembar and Plansee, were tested in this study. The deuterium retention in these two

different types of tungsten made it possible to assess the effects of surface condition on D

retention in PCW. The irradiations were performed at 300 K and 500 K. D+ energies of 200

and 500 eV/D+ were selected; these energies are well below the 940 eV displacement

threshold to avoid deuterium-induced damage in tungsten. A significant objective of this

work was to increase the incident fluence an order of magnitude higher than fluences

achieved in previous ion-beam studies. Our fluence range was 1×1021 to 8×1025 D+/m2,

with the upper end being close to fluences achieved in plasma devices, allowing for

comparisons between the current beam results and those from linear plasma devices and

tokamaks [30-37].

5.1.1 D retention in Rembar PCW at 300 K

At room temperature (~300 K), D retention measurements were made with both the

Rembar and Plansee PCW specimens. For the Rembar PCW, a systematic study of 500

eV/D+ and 1 keV/D+ irradiations has already been performed by Haasz et al. [25]. Here we

only use energies below the threshold. This study started with 500 eV/D+ ion implantation

in Rembar PCW for repeating the experiments shown in [25]. Then new irradiations with

200 eV/D+ were performed to explore the fluence dependence in PCW at this energy.

22

Deuterium retention in Rembar PCW by 500 eV/D+ at 300 K as a function of incident

fluence is plotted in Figure 5-1. The incident fluence for this study ranges from 1×1022 to

1×1025 D+/m2. The ion flux was controlled at 5-7 ×1019 D+/m2s for these runs. Published D

retention in Rembar PCW [25] and Plansee PCW [26] under the same irradiation

conditions (500 eV D+, 300 K) are included in Figure 5-1 for comparison. The results of

Haasz et al. [25] for D retention in Rembar PCW agree well with the present Rembar PCW

results. For low fluences (<1×1023 D+/m2), D retention increases with increasing incident

D+ fluence. When the incident fluence reaches ~1×1023 D+/m2, the retention tends to

saturation at a level of 4-5×1020 D/m2, which is similar to the saturation level obtained by

Haasz et al. [25]. However, both these sets of results differ significantly from the results of

Ogorodnikova et al. [26], where D retention levels are higher, with no sign of saturation

(albeit she only has two data points). The speculation was that such a disagreement could

be caused by the different types of polycrystalline tungsten specimens used for these

studies, namely Rembar for Haasz et al. [25] and the present study, while Plansee for

Ogorodnikova et al. [26]. In order to further explore the effect of different materials, this

thesis has also investigated D retention measurement in Plansee PCW; see sections 5.1.2

and 5.1.3.

Now, we address new experiments with 200 eV/D+ irradiation of Rembar PCW at 300

K. D retention results are plotted in Figure 5-2 for the fluence range 1×1022 to 2×1025

D+/m2. Comparison is made with results obtained by Ogorodnikova et al. [27] and Alimov

et al. [28] under similar irradiation conditions. For the 200 eV/D+ case in the present study,

D retention at 300 K still shows a trend to saturate at a level of 5-6×1020 D/m2 for incident

fluences above 2×1024 D+/m2. For this 200 eV case, saturation occurs at an incident fluence

10 times higher than that for the 500 eV case; the retention value is also higher than the 500

eV case. When comparing the present 200 eV results to the results of Ogorodnikova et al.

[27] and Alimov et al. [28], we observe the following: Ogorodnikova et al. [27] observed

an increasing D retention with increasing incident D+ fluence without a clear sign of

saturation, although, with scatter in the data above ~1024 D+/m2, the start of saturation

cannot be ruled out. The incident fluence of Ogorodnivoka’s study was in the range 3×1021

to 6×1024 D+/m2, and for the highest fluence (6×1024 D+/m2), the retention level reached as

high as 3×1021 D/m2, almost 5 times higher than the saturation level observed in this thesis.

23

While the results of Alimov et al. [28] show a clear trend to saturation (Figure 5-2) for

incident fluences above 2×1024 D+/m2, which is similar to our current study, the saturation

level is ~2×1020 D/m2, about 3 times lower than the one observed in this thesis. A possible

explanation might be the use of different techniques for trapped D measurement – TDS

here and NRA by Alimov el al. [28].

5.1.2 D retention in Plansee PCW at 300 K

The motivation for performing experiments with Plansee PCW arose because of the

different fluence-dependence results of Haasz et al. [25] and Ogorodnikova et al. [26,27].

The fluence dependence of D retention in Rembar PCW irradiated by 500 eV/D+ ions at

room temperature by Haasz et al. [25] showed a clear trend to saturation. However, the

fluence dependence of D retention in Plansee PCW irradiated by 200 and 500 eV/D+ at 300

K by Ogorodnikova et al. [26,27] showed no sign of saturation. Also, different amounts of

trapped deuterium were reported by Alimov el al. [28] and Haasz et al. [25], although, the

two studies both observed a saturation trend; see Figure 5-2. With an attempt to resolve the

above discrepancies, a new set of D+ irradiation experiments with 200 and 500 eV/D+ ions

at 300 K were performed on Plansee PCW. The specific focus was on whether D retention

continues to increase with increasing fluence, as seen by Ogorodnikova et al. [26,27] or

does it saturate as seen by Haasz et al. [25]. In cases where saturation does occur, what

controls the “onset” incident fluence and the amount of retained D at saturation, what are

the reasons for the different levels seen by Haasz et al. [25] and Alimov et al. [28]?

The present D retention measurements in Plansee PCW irradiated by 200 and 500

eV/D+ ions at room temperature are shown in Figure 5-3, as a function of fluence from

3×1022 to 3×1024 D+/m2. The ion flux for these runs was well controlled at 3-4 ×1019

D+/m2s. In order to observe the effects (if any) of the different types of polycrystalline

tungsten, D retention data for Rembar PCW (shown in Figure 5-1 and 5-2) are also plotted

in Figure 5-3. Plansee PCW irradiated by 500 eV/D+ ions tends to saturate at about

3.7×1020 D/m2, which is slightly lower than the saturation level (~5×1020 D/m2) of Rembar

PCW irradiated at the same conditions. However, a significant difference in D retention is

seen between Plansee and Rembar PCW particularly at lower incident fluences (< 1×1023

D+/m2), where retention values are a factor of 4 to 5 lower in Plansee than in Rembar PCW.

24

High incident fluences of 200 eV/D+ ions implanted into Plansee PCW at 300 K lead

to a saturation level of 4×1020 D/m2, which is close to the 3.7×1020 D/m2 for the 500 eV

case (shown in Figure 5-3). The behavior of fluence dependence for 200 eV/D+ depicted in

Figure 5-3 tracks the 500 eV curve very well, showing that every 200 eV data point is close

to the corresponding 500 eV point at the same incident fluence. From Figure 5-3, we note

that the retention values for the Plansee PCW differ from those of the Rembar PCW,

although both of them show clear evidence of a saturation trend. It is possible that the

different retention levels for the Rembar and Plansee PCW are caused by the different

surface structures and preparation conditions for these two types of specimens. The SEM

photographs presented in section 4.1 do show different surface structures (Figures 4-1 and

4-2). For the Plansee PCW, ion irradiation appears to cause some changes on the specimen

surface, while for the Rembar PCW, the effects of ion bombardment are clearly observed –

the surface has become much smoother due to ion irradiation. Consequently, the difference

on the surface structure could play a role in determining different retention levels in these

two types of polycrystalline tungsten specimens.

5.1.3 D retention in Plansee PCW at 500 K

In contrast to the 300 K cases, where D retention in tungsten tends to saturate,

independent of the incident ion energy and type of tungsten materials, deuterium trapping

in tungsten at 500 K is reported to show an increasing trend with incident fluence

[25,27,29,30]. As for the 300 K studies in sections 5.1.1 and 5.1.2, a systematic study of the

fluence dependence of D retention in Plansee PCW at 500 K was also performed in this

thesis. Most of the experiments were performed with 200 eV/D+, however, to investigate

the dependence of D retention on incident ion energy, some runs were also done with 100

and 500 eV/D+; see section 5.3. A plot of D retention in Plansee PCW at 500 K is shown in

Figure 5-4. The 200 eV/D+ irradiation results at 380-470 K for Plansee PCW by

Ogorodnikova et al. [27] are also plotted for comparison. In order to discern possible

different deuterium trapping behavior in different types of polycrystalline tungsten, D

retention data in Rembar PCW at 500 K by Haasz et al. [25] are also included in Figure 5-4.

Further, since the incident fluence in this thesis has been extended to approach ~1×1026

D+/m2 to provide overlap with results from linear plasma devices, some results from

25

PISCES [30] are also shown.

For the present 500 K study, the incident 200 eV/D+ ion fluence spreads 5 orders of

magnitude, ranging from 1×1021 to 8×1025 D+/m2, and the ion flux was controlled at 5-7

×1019 D+/m2s. The D retention in Plansee PCW, as a function of incident fluence in Figure

5-4 illustrates that D retention increases with increasing incident fluence with a slope of

0.35-0.4, with no indication of saturation. Irradiation studies, also on Plansee PCW, under

the similar irradiation conditions (380-470 K and 200 eV/D+) by Ogorodnikova et al. [27]

also showed a constantly increasing trend in D retention, but with a slope above 0.5,

indicating a higher increasing rate in D retention. The highest incident D+ fluence reported

by Ogorodnikova et al. [27] was ~3×1024 D+/m2, more than an order of magnitude lower

than the highest fluence reached in the present study. However, for 3×1024 D+/m2, the

retention in [27] has already reached ~6×1020 D/m2, which is higher than the retained

amount for 8×1025 D+/m2 obtained in the present study. Although both [27] and the present

study used the same type of W material and similar irradiation conditions, the retention

data still differ in both the retained amount of D and the slope of the increasing trend line.

The differences must now be explained by different specimen preparation techniques or

different experimental procedures.

D retention in Rembar PCW at 500 K studied by Haasz et al. [25] is also shown in

Figure 5-4. The comparison between Plansee and Rembar PCW shows that the retained

deuterium for 500 eV irradiation in the Plansee PCW at 500 K at a fluence of ~1023 D+/m2

is about 3-4 times lower than in the Rembar PCW, which is possibly due to the different

structure and preparation procedures for the two types of tungsten specimens. As discussed

in 5.1.1, the difference in the surface structure could play a role in determining different

retention levels in these two types of polycrystalline tungsten specimens. In comparing the

current results with those from PISCES [30], we note that magnitudes of the amounts

retained are very similar, but there is a steeper fluence dependence slope in the PISCES

data.

5.1.4 Discussion of fluence dependence

A plot consisting of the existing published data and the present results for the fluence

dependence of deuterium retention in tungsten at room temperature (near 300 K) is shown

26

in Figure 5-5. Both polycrystalline (Plansee PCW and Rembar PCW) and single-crystal

tungsten (JM-SCW from Johnson-Matthey and M-SCW from State Institute of Rare Metals,

Moscow [29]) are included in this plot. It is noted that the fluence dependence data

obtained at the University of Toronto at 300 K consistently show a saturation trend at high

fluences, independent of the type of tungsten (Plansee or Rembar PCW, JM or M SCW)

and ion energy (200, 300 or 500 eV/D+). Similar saturation trend was also seen by Alimov

el al. [28]. In contrast, no saturation trend was found at room temperature by Ogorodnikova

et al. [26,27] at both 200 and 500 eV/D+, nor by Golubeva et al. at 323 K [49].

In assessing the D retention results from the various studies regarding the existence of

a saturation trend, the highest incident ion fluence achieved in the different experiments is

an important factor. For example, in the case of 200 eV/D+ irradiation of the Rembar PCW

performed in the present study, the saturation trend was not fully confirmed until an

incident fluence of 2×1025 D+/m2. However, for the experiments of Ogorodnikova et al

[26,27] and Golubeva et al. [49], the maximum fluences reported were 5×1024 D+/m2 and

7×1023 D+/m2, respectively. Thus, the absence of an observed saturation trend may simply

be due to the fact that a high enough fluence has not yet been reached.

A possible explanation for the saturation trend at 300 K was suggested by Haasz et al.

[25] based on NRA depth profiles measured on both the front and back surfaces of a

Rembar PCW specimen. The depth profile of the trapped D on the front surface of the

irradiated specimen showed that the trapped D extended well beyond the implantation zone

(~ 40 nm) to ~500 nm, while on the back surface no D was detected, implying that

diffusion may be too slow at 300 K to allow trapping sites deep in the bulk to be reached.

The D retention results at 500 K irradiations on Plansee PCW obtained in the present

study show that the amount of trapped D linearly increases with increasing incident D ion

fluence with a slope of 0.35-0.4, showing no sign of saturation; see Figure 5-4. A plot of D

retention in Plansee PCW as a function of incident fluence at different energy/temperature

combinations is shown in Figure 5-6. It is observed that at fluences below ~1024 D+/m2, the

retained amount of D at 500 K and 200 eV/D+ is less than that at room temperature, but at

the highest fluence measured (8×1025 D+/m2), the D retention value at 500 K is about 2

times larger than the saturated level for the 300 K cases. A similar trend was also observed

in the study of D retention in Rembar PCW at 500 K by Haasz et al. [25], which suggested

27

diffusion taking greater importance at elevated temperatures and the results were consistent

with a diffusion-limited trapping mechanism (slope = 0.5).

A composite plot containing most of the existing published and present data for the

fluence dependence of D retention in W at ~500 K is shown in Figure 5-7. For the ion beam

studies [25,27], D retention was seen to increase with increasing incident ion fluence, with

no sign of saturation. However, for irradiations performed in plasma devices or tokamaks,

no consistent trends were observed. For instance, the results of Kolasinski et al. [32] with a

70 eV D plasma used to irradiate Plansee PCW at 623 K show a saturation level for

incident fluences of ~1026 D+/m2. Alimov et al. [36] using a 38 eV D plasma at 530 K

found similar retention amounts for 1026 and 1027 D+/m2 irradiations, suggesting the

occurrence of saturation for an incident fluence above 1026 D+/m2. Differences in materials

and implantation conditions must still account for the large variation in trends and absolute

retention values observed at high fluences.

To explain the non-saturation trend of fluence dependence at 500 K, the study by

Haasz et al. [25] provides a good reference. The NRA profiles of a specimen implanted at

500 K by 500 eV D+ (1×1024 D+/m2) showed that a nearly uniform concentration of D was

found through the specimen ~0.05 at.% on both the front and back surfaces, indicating that

almost all of the traps in the bulk are accessible to the diffusing D [25]. According to such

results, it is suggested that one must go to higher fluences to reach bulk saturation, or there

is a source of traps produced by the irradiation process [25]. With the specimens of the

current study being 40 times thicker (1 mm compared to 25 μm) than those in [25], we

would not expect to see signs of saturation until considerably higher fluences than what we

were able to reach (i.e. 8×1025 D+/m2).

5.2 Temperature dependence of D retention in PCW

Here we have briefly examined the effect of specimen temperature during

implantation on the deuterium retention in Plansee PCW. As indicated in section 2.3, large

discrepancies exist in the current data base on the temperature dependence, motivating

further study.

At the University of Toronto, several previous studies have investigated the

temperature effects on deuterium retention in both single-crystal and polycrystalline

28

tungsten materials. For PCW, investigations using 500 eV/D+ on Rembar PCW [25] found

a localized peak in retention at an implantation temperature of 450 K. Lower amounts of

retained D are observed at both lower and higher temperatures. D retention at or above 700

K was below detectable levels. For single-crystal tungsten, Poon et al. [27] found that the

highest D retention was observed for 300 K irradiations, and the retention decreased with

increasing irradiation temperature, with no local maxima observed over the studied

temperature range. Alimov et al. [38] also observed a decreasing trend of D retention with

increasing irradiation temperature from 350 to 570 K. The purpose of the present study was

to determine the behavior of D trapping in another type of polycrystalline tungsten, namely

Plansee PCW.

An ion energy of 200 eV/D+ was selected to perform the implantations at fluxes of

4-6×1019 D+/m2s and a fluence of 1×1023 D+/m2. The implantation temperature was varied

between 300 K and 500 K. The measured D retention is shown as a function of irradiation

temperature in Figure 5-8. Results for 500 eV/D+ ion irradiation on Rembar PCW by Haasz

et al. [25], 500 eV/D+ ion irradiation on SCW by Poon et al. [29], and 200 eV/D+ ion

irradiation on Plansee PCW by Alimov et al. [38] are also included here for comparison.

For the new results obtained in this study, the highest retained deuterium amount was found

at room temperature (300 K) with a value of 1.7×1020 D/m2, and then D retention decreases

with increasing irradiation temperature, reaching 5×1019 D/m2 at 500 K. Clearly, the

observed D retention behavior for the Plansee PCW is significantly different from that of

the Rembar PCW. Interestingly, the temperature dependence of the Plansee PCW D

retention does show a similar trend to that observed for single-crystal tungsten [29].

We note that the different fluence dependencies observed at different temperatures –

e.g., saturation at 300 K, no saturation of 500 K – mean that any temperature dependence

profile will depend on incident fluence.

5.3 Ion energy dependence of D retention in PCW

Three different D+ energies were used for irradiation, namely 100, 200 and 500 eV/D+.

Although energy transfer from 500 eV D+ to a W atom (21 eV) is insufficient to create a

displacement [20], the energy transfer from 500 eV D+ to oxygen and carbon are 200 eV O+

and 250 eV C+, assuming two-body collisions. Due to their larger mass, the energy transfer

29

from O+ or C+ to W is more efficient than from D+. Thus, it is possible for 500 eV D+ to

create vacancies in W through intermediate recoil collisions with O and C impurities on the

surface of W [22]. But 200 eV and 100 eV D+ cannot induce such knock-on effect because

the energy transfer from D+ to O or C is insufficient to create displacement in W.

Irradiations were performed at 100, 200 and 500 eV/D+ energies and 300 K and 500

K temperature; see Figure 5-9. Three different fluence/temperature combinations are shown.

In the first case, 300 K and incident fluence of 1×1023 D+/m2, the retained D shows only a

slight increase as the energy is increased from 100 to 500 eV/D+. The second case, for an

incident fluence of 1×1024 D+/m2 at 300 K, similarly shows only a slight increase with

increasing energy. However, as expected from the fluence dependence curves at 300 K in

Figure 5-3, the D retention levels at a fluence of ~1024 D+/m2 are about a factor of 2 higher

than at ~1023 D+/m2; this difference prevails over the 100-500 eV/D+ energy range. For the

last case, a fluence of ~1023 D+/m2 was used and the irradiation temperature was raised to

500 K; the retention levels are noticeably lower than for the previous two cases, but a factor

of 3 increase in retention is noted as the energy increases from 200 to 500 eV.

Given the scatter in the experimental results, it is only possible to conclude that for

D+ energies below 500 eV/D+, D retention in the Plansee PCW at 300 K depends only

weakly on the incident ion energy. The dependence is stronger at 500 K and it is possible

that this is associated with recoil displacements due to O and C impurities. A similar

conclusion might be drawn from the results of [25], shown in Figure 5-10. Even though the

maximum energy, 1 keV/D+, is already above the displacement threshold of 940 eV/D+, it

is still difficult to observe any noticeable effects of ion energy on D retention [25]. The

spread of the retention measurements at 500 K and 1 keV in [25] has been attributed by the

authors to accumulated damage progressively created by successive implantations on the

same spot [24].

6. Conclusions

6.1 Fluence dependence

30

Irradiation at 300 K: deuterium retention in both Rembar and Plansee PCW foils at

room temperature, as a function of incident D+ ion fluence, indicates a trend to saturation.

For Rembar PCW, the saturation level for 500 eV/D+ irradiation was ~6×1020 D/m2 for

incident fluences above 1×1023 D+/m2, similar to the results of Haasz et al. [25]. Irradiation

performed by 200 eV/D+ ions caused the D retention to level off at ~7.5×1020 D/m2 (only

slightly higher than the 500 eV/D+ case) for incident fluence above 1×1024 D+/m2. For the

Plansee PCW, irradiations by 500 and 200 eV/D+ ions demonstrated similar trapping

behavior, with the retention reaching saturation levels of ~4×1020 D/m2 for incident

fluences above 5×1023 D+/m2.

Irradiation at 500 K: only Plansee PCW was studied here. D retention results for 200

eV/D+ irradiations suggest that D retention in the Plansee PCW increases linearly with

increasing incident D+ fluence without any indication of saturation. Even when the incident

fluence was increased to 8×1025 D+/m2, which is in the range of plasma devices, there was

still no sign of saturation. The Retention value corresponding to the highest incident

fluence (8×1025 D+/m2) was found to be 5.2×1020 D/m2 – only slightly higher than the

saturation levels in Plansee PCW irradiated with 500 and 200 eV/D+ at 300 K. However,

we note again that at 500 K, the retained D is seen to increase at least over the incident

fluence range 1021 to 1026 D+/m2. The retained amount of D obtained at the highest fluence

reached in our ion-beam experiment (~8×1025 D+/m2) is similar to some retention results

obtained in plasma devices. The remaining question is whether the increasing trend

continues above ~1026 D+/m2 into the ITER fluence regime.

6.2 Temperature dependence

Our temperature dependence study of D retention in the Plansee PCW irradiated with

200 eV/D+ ions showed that D retention decreases with increasing irradiation temperature

in the 300 to 500 K range. The highest retention level was found to be ~1.7×1020 D+/m2 at

300 K, followed by a linear decreasing trend, reaching ~5×1019 D+/m2 at 500 K.

6.3 Ion energy dependence

D retention in the Plansee PCW irradiated at 300 K to a fluence of ~1×1023 D+/m2

was seen to increase only slightly with ion energy from 100 eV/D+ to 500 eV/D+; the

31

retention values were all within the range 1-2×1020 D/m2. Similar results were found for the

Plansee PCW irradiated at 300 K over the same energy range to an incident fluence of

1×1024 D+/m2; the retained levels were slightly higher in the range 3-3.5×1020 D/m2. For

irradiations at 500 K, the D retention increased noticeably (a factor of ~3) as the energy

increased from 100 to 500 eV/D+. Based on these limited results, one can draw the

conclusion that the energy of incident D ions plays a minor role in affecting D trapping in

polycrystalline tungsten.

32

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308-312. [16] R. Behrisch, G. Federici, A. Kukushkin, D. Reiter, “Material erosion at the vessel walls of future fusion devices”, J. Nucl. Mater. 313–316 (2003) 388–392. [17] A.A. Haasz, M. Poon, R.G. Macaulay-Newcombe, J.W. Davis, “Deuterium retention in single crystal tungsten”, J. Nucl. Mat. 290-293 (2001) 85-88. [18] R. Doerner, R.W. Conn, D. Phelps, L.M. Waganer, “The PISCES-upgrade facility for fusion plasma-materials interactions research”, Proceedings of the 18th Symposium on Fusion Technology, p 775-8 vol.1, 1995. [19] G.R. Longhurst, D.F. Holland, J.L. Jones, B.J. Merrill, “TMAP4: Tritium Migration Analysis Program, Description and User’s Manual”, INEL report, EGG-FSP-10315, EG & Idaho Inc. (1992). [19a] M. Poon, A.A. Haasz, J.W. Davis, “Modelling deuterium release during thermal desorption of D+-irradiated tungsten”, J. Nucl. Mater. 374 (2008) 390-402. [20] R. Sakamoto, T. Muroga, N. Yoshida, “Microstructural evolution induced by low energy hydrogen ion irradiation in tungsten”, J. Nucl. Mater. 220–222 (1995) 819. [21] A.D. Quastel, J.W. Davis, A.A. Haasz, R.G. Macaulay-Newcombe, “Effect of post-D+-irradiation time delay and pre-TDS heating on D retention in single crystal tungsten”, J. Nucl. Mater. 359 (2006) 8–16. [22] M. Poon, R.G. Macaulay-Newcombe, J.W. Davis, A.A. Haasz, “Effects of background gas impurities during D+ irradiation on D trapping in single crystal tungsten”, J. Nucl. Mater. 337-339 (2005) 629-633. [23] V.Kh. Alimov, K. Ertl, J. Roth, “Deuterium Retention and Lattice Damage in Tungsten Irradiated with D ions”, Physica Scripta. T94 (2001) 34-42. [24] A.A. Haasz, M. Poon, J.W. Davis, “The effect of ion damage on deuterium trapping in tungsten”, J. Nucl. Mater. 266-269 (1999) 520-525. [25] A.A. Haasz, J.W. Davis, M. Poon, R.G. Macaulay-Newcombe, “Deuterium retention in tungsten for fusion use”, J. Nucl. Mater. 258-263 (1998) 889-895. [26] O.V. Ogorodnikova, J. Roth, M. Mayer, “Deuterium retention in tungsten in dependence of the surface conditions”, J. Nucl. Mater. 313-316 (2003) 469-477. [27] O.V. Ogorodnikova, J. Roth, M. Mayer, “Pre-implantation and pre-annealing effects on deuterium retention in tungsten”, J. Nucl. Mater. 373 (2008) 254-258. [28] V. Kh. Alimov, private communication, Institute of Physical Chemistry of the Russian

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Academy of Sciences, Moscow, Russia, 2008. [29] M. Poon, A.A. Haasz, J.W. Davis, R.G. Macaulay-Newcombe, “Impurity effects and temperature dependence of D retention in single crystal tungsten”, J. Nucl. Mater. 313-316 (2003) 199.

[30] R.P. Doerner, private commucation, University of California at San Diego, La Jolla, CA, USA, 2008. [31] W.R. Wampler, R. Doerner, L.-N. Luo, “The effect of displacement damage on deuterium retention in plasma exposed tungsten”, the 9th International Workshop on Hydrogen Isotopes in Fusion Reactor Materials, Salamanca, Spain, 2008. [32] R.D. Kolasinski, private communication, Hydrogen & Metall. Sci. Dept., Sandia Nat. Labs., Livermore, CA, USA, 2009. [33] G. Maddaluno, C. Alessandrini, G. Giacomi, L. Verdini, “Thermal desorption measurement of deuterium retention in fusion relavant materials”, Associazione EURATOM-ENEA sulla Fusione, Centro Ricerche Frascati, Frascati, Rome, Italy, 2008. [34] G. Wright, private communication, University of Wisconsin at Madison, 2009. [35] R. Causey, K. Wilson, T. Venhaus, W. R. Wampler, “Tritium retention in tungsten exposed to intense fluxes of 100 eV tritons”, J. Nucl. Mater. 266-269 (1999) 467. [36] V.Kh. Alimov, “Surface modification and deuterium retention in tungsten and molybdenum exposed to low-energy, high-flux deuterium plasmas”, in the 1st International Conference on New Materials for Extreme Environments, San Sebastián, SPAIN, 2008. [37] V. Philipps, private communication, the ITPA SOL/DIV Avila, Spain, 2008. [38] V.Kh. Alimov and J. Roth, “Hydrogen isotope retention in plasma-facing materials: review of recent experimental results”, Phys. Scr. T128 (2007) 6–13. [39] G.-N. Luo, W.M. Shu and M. Nishi, “Influence of blistering on deuterium retention in tungsten irradiated by high flux deuterium 10-100eV plasmas”, Fusion Eng. Des. 81 (2006) 957-962. [40] R. Causey, K. Wilson, T. Venhaus, W.R. Wampler, “Tritium retention in tungsten exposed to intense fluxes of 100 eV tritons”, J. Nucl. Mater. 266–269 (1999) 467. [41] T. Venhaus, R. Causey, R. Doerner, T. Abeln, “Behavior of tungsten exposed to high fluences of low energy hydrogen isotopes”, J. Nucl. Mater. 290–293 (2001) 505. [42] K. Tokunaga, M.J. Baldwin, R.P. Doerner, N. Noda et al., “Blister formation and deuterium retention on tungsten exposed to low energy and high flux deuterium plasma”, J. Nucl. Mater. 337–339 (2005) 887.

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[43] M. Poon, R.G. Macaulay-Newcombe, J.W. Davis, A.A. Haasz, “Flux dependence of deuterium retention in single crystal tungsten”, J. Nucl. Mater. 307–311 (2002) 723–728. [44] A. Quastel, “Effects of impurities on deuterium retention in single crystal tungsten”, M.A.Sc. thesis, University of Toronto (2005). [45] M. Poon, “Deuterium trapping in tunsten”, Ph.D. thesis, University of Toronto (2004). [46] L. Reimer, “Scanning electron microscopy: physics of image formation and microanalysis”, Berlin; New York: Springer, c1998. 2nd completely rev. and updated ed. [47] A. van Veen, “Vacancies and interactions in metals and alloys”, C. Abromeit, H. Wollenberger (Eds.) (Berlin 1986), 3. [48] R.A. Anderl, R.J. Pawelko, S.T. Schuetz, “Deuterium retention in W, W1%La, C-coated W and W2C”, J. Nucl. Mater. 290-293 (2001) 38-41. [49] A.V. Golubeva, M. Mayer, J. Roth, V.A. Kurnaev, O.V. Ogorodnikova, “Deuterium retention in rhenium-doped tungsten”, J. Nucl. Mater. 363–365 (2007) 893–897.

36

Figures

Figure 2-1: Depth profiles of D trapped as D atoms (a) and D2 molecules (b) in single-crystal and hot-rolled W implanted with 6 keV D ions at 300 K determined by the SIMS/RGA method (Figures from Ref. [23]).

37

Figure 2-2: Retained vs. cumulative-fluence for 1 keV/D+ implantations at 500 K. Data are shown for specimens W2 (1023 D/m2 probe-fluence only), W1 (9×1023 D/m2 damage-fluence and probe-fluence), and W3 (1025 D/m2 damage-fluence and probe-fluence). (Figure from Ref. [24].)

Figure 2-3: Retained vs. cumulative-fluence for 500 eV/D+ implantations at 500 K. Data are shown for specimens W5 (1023 D/m2 probe-fluence only), W4 (9×1023 D/m2 damage-fluence and probe-fluence), W6 (3×1024 D/m2 damage-fluence and probe-fluence), W7 (1025 D/m2 damage-fluence and probe-fluence), and W9 (3×1025 D/m2 damage-fluence and probe-fluence). (Figure from Ref. [24].)

38

Figure 2-4: Fluence dependence of D retention in PCW at 300 K under various D ion energies. [25-28]

39

Figure 2-5: NRA measurements of the near-surface D depth profiles. (a) 1 keV and 500 eV D+ (1024 D+/m2 incident fluence) implanted into W at 300 K. Implantation profiles for 1 keV D+ and 500 eV D+ as calculated by TRVMC are shown for comparison (normalized to the peak height of the measured profiles). (b) 500 eV D+ implanted at 500 K into W (1024 D+/m2) and W-1% La2O3 (3.3×1024 D+/m2). (Figures from Ref. [25].)

40

Figure 2-6: Fluence dependence of D retention in W at elevated temperatures using ion beams. [25,27,29]

41

Figure 2-7: Fluence dependence of D retention in W at elevated temperatures using plasma devices and tokamaks. [30-37]

42

Figure 2-8: Temperature dependence of D retention in W and W-1%La2O3. (a) 1 keV/D+ at fluences of 1023 and 1024 D+/m2, (b) 500 eV/D+ at fluence of 1023 D+/m2. (Figure from Ref. [29].)

43

Figure 2-9: Temperature dependence of D retention in M-SCW with an incident fluence of 1024 D+/m2. (Figure from Ref. [29].)

44

Figure 2-10: (a) Deuterium retention in single-crystal and polycrystalline fine-grain tungsten exposed to low-energy (200 eV/D+) and high flux (about 1×1021 D/m2s) D plasmas as a function of exposure temperature. For comparison, the temperature dependence of the D retention in polycrystalline coarse-grained W irradiated with 200 eV D ions and flux of 4×1019 D m−2 s−1 to a fluence of 1×1024 Dm−2 is also shown. Note that the deuterium retention was calculated from deuterium depth profiles measured up to a depth of 7μm. (b) Deuterium retention in polycrystalline tungsten exposed to low-energy (98–100 eV/DT) and high flux ((8.7–10)×1021 D(T)m−2 s−1) D or (D+ T) plasmas as a function of the exposure temperatures. [38-42]

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Figure 2-11: Deuterium retention as a function of incident D+ flux at three fluences (1021, 1022, and 1023 D+/m2) at room temperature. (Figure from Ref. [43].)

46

Figure 3-1: Schematic of single-beam ion accelerator. (Figure from Ref. [44].)

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Figure 3-2: Implantation specimen holder.

Figure 3-3: Schematic of the TDS system.

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Figure 4-1: SEM images of irradiated Plansee PCW (after TDS test). (a) On-spot area; (b) Off-spot area.

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Figure 4-2: SEM images of irradiated Rembar PCW (after TDS test). (a) On-spot area; (b) Off-spot area.

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Figure 4-3: Cross-sectional SEM photograph of Plansee PCW specimen.

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Figure 4-4: Signals of H2, HD, and D2 in two thermal desorption runs. (a) 200 eV/D+, 500 K, Plansee PCW; (b) 200 eV/D+, 300 K, Plansee PCW.

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Figure 4-5: Example calculation of deuterium retention in Plansee PCW (200 eV/D+, 500 K). The vertical bar indicates the estimated HD contribution (i.e., integration of HD signals over the two different time spans).

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Figure 5-1: 500 eV/D+ ion implantation on PCW at 300 K. [25,26]

Figure 5-2: 200 eV/D+ ion implantation on PCW at 300 K. [27,28]

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Figure 5-3: Deuterium retention in different types of PCW at 300 K. [25]

Figure 5-4: Fluence dependence of D retention in PCW at elevated temperatures [25,27,30-32, 36]. (The red circle indicates the highest D+ fluence achieved in the present study.)

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Figure 5-5: Collection of fluence dependence data on D retention in tungsten at room temperature. [17,25-29,49]

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Figure 5-6: Fluence dependence of D retention in Plansee PCW for different energy and temperature combinations.

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Figure 5-7: Collection of fluence dependence data on D retention in tungsten at elevated temperatures [25,27,31-37]. (The red circle indicates the highest D+ fluence achieved in the present study.)

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Figure 5-8: Irradiation temperature effects on D retention in PCW. [25,29,38] (The specimen of the present data point (200 eV/D+, 450 K, 1×1023 D+/m2, 8×1020 D/m2) was annealed at 1500 K.)

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Figure 5-9: Incident ion energy dependence of D retention in Plansee PCW.

Figure 5-10: Energy dependence of D retention in Rembar PCW. [25]