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M. Rutigliano a, D. Santoro a , and M. Balat-Pichelin b
a CNR–IMIP, Istituto di Metodologie Inorganiche e dei Plasmi, Via G. Amendola 122/D, 70126 Bari, Italy
b Laboratoire Procédés, Matériaux et Energie Solaire, PROMES-CNRS, 7 rue du four solaire, 66120 Font-Romeu Odeillo, France
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
Outline
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
Introduction
Experiment Setup Evaluation method for recombination coefficient
Results
Simulations
Molecular Dynamics (MD)calculations
Results Comparison experimental vs theoretical for recombination coefficient
Summary
Introduction
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
The realization of nuclear fusion reactors like ITER has revealed the need of new data on hydrogen recombination on several kinds of materials at high temperature level.
In the literature, one can find only data at low temperature (around 300°K) for silica, stainless steel, several pure metals and carbon. For a same material, data are widely scattered depending on the gas temperature and gas pressure and on the surface temperature of the material.
According to the literature review, it seems necessary to perform new measurement of the recombination coefficient of atomic hydrogen at higher temperature levels.
Experimental Setup
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
1 - concentrator 2 - shutter 3 - mass flowmeter 4, 8 and 9- mirrors 5 - quartz reactor 6 - waveguide 7 - optical pyrometer 10 - magnetron 11 - support bracket 12 - monochromator 13 - CCD 14 - computer 15 - vacuum pump 16 - control valve 17 - pressure gauge
MESOX set-up (Moyen d’Essai Solaire d’OXydation) at the focus of the 6 kW Odeillo solar furnace
Experimental evaluation of recombination coefficient γH
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
Actinometry, Optical Emission Spectroscopy :
• the intensity ratio profile IH/IH2 of the two lines, Hβ at 486.1 nm and H2 at 492.7 nm was used for the determination of γH as these two lines can be acquired simultaneously.
Resolution of the diffusion equation at steady state : • mean free path of H atoms <<< reactor radius
• some hypotheses: convective transfer negligible, radial gradient negligible, the stability of the ratio IH/IH2 in the reactor (without sample) allows neglecting the recombination in volume and on the reactor wall
Finally :
γH =4.DH,H2
V.L
IH
IH2 x= L
IH
IH2 x=0
Ts
TL−1
γH=Hrec/Htot
Tungsten sample
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
AFTER
BEFORE
Heat treatment in H2 plasma at 2143°K
XRD pattern
The chemical composition of the W sample is: W 99.98%, Mo 0.004%, Fe 0.002%, P 0.002% and other components (0.012%)
Experimental Results for the recombination of hydrogen atoms
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
Process activation energy 14 kJ/mol
Recombination of atoms on surfaces
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
H H
H2
H
H
H
H
H2
H2
LANGMUIR-HINSHELWOOD
ELEY-RIDEAL
HOT ATOM
Semiclassical Molecular Dynamics calculations
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
The dynamical simulation of surface processes is worked out through three main steps:
building up of a 3D lattice crystal and phonon dynamics determination
determination via DFT calculations of Potential Energy Surface(PES)where the reaction takes place
Trajectory Propagation
G. D. Billing 2000 Dynamics of Molecule Surface Interactions John Wiley & Sons, New-York
Semiclassical collisional method
Recombination Probability: site and surface temperature effect
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
10-1.0 100.06 7 2 3 4 5 6 7 2 3 4 5 6 7
Kinetic Energy (eV)
0.0e0
2.0e-3
4.0e-3
6.0e-3
Rec
ombi
natio
n Pr
obab
ility
10-1.0 100.06 7 2 3 4 5 6 7 2 3 4 5 6 7
Kinetic Energy (eV)
0.00
0.01
0.02
0.04
Rec
ombi
natio
n Pr
obab
ility
3F SITE T SITE
TS=1000K TS=700K
3F T
Factors Influencing the surface temperature effect
The effect of surface temperature on the reaction dynamics is not wholly predictable due to the influence of different chemico-physical factors at both microscopic and macroscopic levels. The main factors include:
energy exchange mechanism between the chemisorbed species and the substrate
mobility of adsorbed species
surface coverage
surface structural modifications
reaction mechanism
Probability for other surface processes
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
TS=1000K
3F SITE T SITE
Had*W(1110) + Hgas ”products”
10-1.0 100.02 3 4 5 6 7 2 3 4 5 6 78
Kinetic Energy (eV)
0.00
0.20
0.40
0.60
0.80
1.00
Prob
abili
ty
Had + Hgas
[H2]ad
Had + Had
10-1.0 100.06 7 2 3 4 5 6 7 2 3 4 5 6 7
Kinetic Energy (eV)
0.00
0.20
0.40
0.60
0.80
1.00
Prob
abili
ty
Had + Hgas
[H2]ad
Had + Had
Recombination coefficient: theoretical versus experimental values
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
(γH)th
3F T
TS=Tgas=1000K 0.19 0.35
TS=Tgas=700K 0.17 0.32
TS=1000K Tgas= 800K 0.15 0.30
TS= 700K Tgas=800K 0.18 0.34
TS (K)
(γH)exp
700
0.169
0.170
0.181
0.185
1045 0.396
1075 0.416
1325 0.507
1350
0.581
0.588
In the experiment the gas temperature was assumed to be around 700 K for similar surface temperature and 800 K for surface temperatures up to 1350 K.
M. R., D. Santoro, M. Balat-Pichelin, Surface Science 628 (2014) 66
Comments: theoretical versus experimental
❶ In MD simulations, we considered a perfect (110) crystal while in the sample used in the experiment, there are other crystalline planes, the considered surface plane can have some defects and the sample contains, some impurities.
❷ The adsorption density considered in the calculations is different from that of the real surfaces in which different sites can be occupied by hydrogen atoms at the same time altering the reaction dynamics. This latter circumstance could lead the activation of Langmuir–Hinshelwood mechanism. This issue requires further investigation.
❸ In the experiment it was observed that recombination acts for approximately 3 mm above the surface. This distance is too large to ascribe the decrease in the flux intensities just to the surface processes. In fact, surface processes act for distances of few Å from the surface, while for larger distances gas-phase atomic recombination can be effective.
Recombination Reaction Energetics
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
Ekin(eV) ∆Eph Evib(%) Erot(%) Etr(%)
4.0 <10-3 38 37 24
5.0 <10-3 42 44 13
6.0 <10-3 40 53 6
3F SITE
0.1 10.00
0.10
0.20
0.30
0.40
0.50
0.60
∆Eph
Etr
Evib
Tota
l Ene
rgy
Frac
tion
Kinetic Energy (eV)
ErotT SITE
Vibrational Distribution of H2 formed molecules
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
T SITE
100.06 7 8 2 3 4 5 6 7 8Kinetic Energy (eV)
0.0
0.2
0.4
0.6
P(v)
v=0v=1v=2v=3v=4v=5v=6
100.06 7 8 2 3 4 5 6
Kinetic Energy(eV)
0.0
0.1
0.2
0.3
P(v)
v=7v=8v=9v=10v=11v=12
3F SITE
0 1 2 3 4 5 6 7 8 9 10 11 12Vibrational Number
0.0
0.1
0.2
0.3
P(v)
Ekin=5 eV
0 1 2 3 4 5 6 7 8 9 10 11 12Vibrational Number
0.00
0.05
0.10
0.15
P(v)
Ekin=6 eV
Summary
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
The recombination coefficient of hydrogen atoms on W(110) has been evaluated by using a method based on the actinometry technique.
The experimental results obtained for the recombination coefficient of H atoms on W were presented in the high temperature range and have shown that this material is very catalytic with values going from 0.17–0.18 at 700 K up to 0.59 at 1350 K.
MD simulations on the two most active sites on W(110) surface for the surface temperatures of 700 and 1000 K and a gas temperature of 700, 800 and 1000 K have been presented.
A quite good agreement has been found between the experimental and the theoretical γH.
The energy transferred to the surface during recombination reaction is negligible.
H2 molecule are formed in excited vibrational levels.
Acknoldegments
ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014
This research was supported by :
CNRS, France (ref. EDC 25071) and
CNR, Italy (Bilateral Project n. 9 2012-2013)
through the funding project: “HYDROGEN ATOMS RECOMBINATION ON TUNGSTEN FOR ITER - EXPERIMENTS AND MOLECULAR DYNAMICS SIMULATION”