Fusion Engineering and Design 58–59 (2001) 307–311

On an Electric Pulsed Hydride Injector for gas puffing intotokamaks

Yu.A. Kareev a, U. Tamm b,*, I.S. Glushkov a, Yu.G. Gendel a, G. Mueller c,R.-D. Penzhorn c, V.P. Novikov a

a Troitsk Institute for Inno�ation and Fusion Research (TRINITI), 142190 Troitsk, Moscow Region, Russiab Forschungszentrum Karlsruhe GmbH, Institut fur Hochleistungsimpuls- und Mikrowellentechnik, P.O. Box 3640,

76021 Karlsruhe, Germanyc Forschungszentrum Karlsruhe GmbH, Tritium Labor Karlsruhe, P.O. Box 3640, 76021 Karlsruhe, Germany

Abstract

The potential application of an Electric Pulsed Hydride Injector as a fuelling device for tokamaks was examined.The developed injector, which does not make use of moving parts, is equipped with 20 molybdenum elements coatedwith titanium for the interim storage of deuterium/tritium in a chemically bound way. Gas generation is accomplishedby electrically heating these elements. The pressure of the generated gas flow is comparable to the gas pressure intokamaks. To feed tokamaks of ITER-FEAT size with deuterium/tritium the individual elements must be replaced bypackages of 16 elements. 20 packages constitute a module with a volume of about 3×10−3 m3. The module candeliver a deuterium flow of 3.7×10−6 kg/s during 100 s or 2×10−6 kg/s during 400 s. The electrical powerconsumption is estimated to be approx. 2 kW. To maintain the plasma density in a tokamak constant a combinationof a single element with packages of elements is proposed. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Electric Pulsed Hydride Injector; Gas puffing; Tokamaks

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1. Introduction

Piezo valves used for feeding tokamaks withgas fuel have essential drawbacks. The diameterdp of the aperture in the wall between the gasstorage vessel and the tokamak chamber is �1mm. At larger hole sizes the gas flow rate into thetokamak becomes unacceptably high. To achievethe required flow rate the gas pressure in the gas

storage vessel provided with the piezo valve mustbe maintained within 0.1–1 MPa. The pressure ofthe flowing gas, which differs by about four tofive orders of magnitude from that in a tokamakchamber, can have a negative influence on theplasma confinement. The high tritium inventoriesat risk at elevated pressures constitute a contami-nation hazard. Other disadvantages of piezovalves are the poor control of the gas flow rateand the potential failure of active parts.

The Electric Pulsed Hydride Injector (EPHI)has none of the above mentioned drawbacks.Besides having no moving parts, it provides in-

* Corresponding author. Tel.: +49-7247-823053; fax: +49-7247-822289.

E-mail address: ulrich.tamm@ihm.fzk.de (U. Tamm).

0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

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terim storage of the hydrogen isotopes in a metalgetter using specially designed elements, whichconsist of molybdenum foils whose surface hasbeen coated with titanium. Gas is generated byheating these elements with an electric current.The kinetics of deuterium liberation from theseelements has been reported in previous papers[1,2]. In the case of EPHI diameter di the apertureto the torus vacuum vessel is not as critical aswith the piezo valve. Therefore, to obtain withEPHI the same gas flow as with piezo valves a gaspressure lower by a factor of �= (dp/di)2 is ac-ceptable. With a diameter di=20 mm we obtain�=2.5×10−3. Thus, a pressure inside the EPHIof 2.5×103 Pa will already be sufficient. Themuch lower tritium inventory at risk undoubtedlyconstitutes one of the favourable properties ofEPHI.

For tokamaks such as ITER-FEAT a deu-terium fuel supply of 0.3×10−3 kg/s is needed,and for T-11M at TRINITI [3], whose vacuumchamber has a volume of 0.8 m3, the deuteriumflow is about 100 times smaller. The parametriccharacteristics that EPHI has to comply with tomeet the corresponding flow requirements will bediscussed below. The characteristics of the EPHI-1C [1] will also be addressed briefly.

The EPHI-1C contains 20 elements, each ofwhich can be connected to a separate powersource. The elements installed in the injector eachhave a titanium content of (0.07–0.15)×10−3 kg.The total titanium mass in the injector amounts toabout 2–3×10−3 kg. The injector has a totalvolume of 2.7×10−3 m3.

The chemically bonded deuterium in the tita-nium deuteride can be obtained from theexpression

MD=�MTiAD/ATi (1)

where � is the mean number of deuterium atomsretained by a titanium atom and MTi is the tita-nium weight. AD=2 and ATi=47.9 denote theatomic masses of deuterium and titanium atoms,respectively. The gas flow W is given by

W= −dMD/dt=MTi� (2)

where

�= − (AD/ATi)(d�/dt) (3)

the derivative of � with respect to time gives thedeuterium generation rate, which is a function ofthe titanium temperature T, i.e. �= f(�, T). Then

d�/dt= (�f/��)T(d�/dt)+ (�f/�T)�(dT/dt) (4)

where (�f/��)T�0 and (�f/�T)��0. At constantgas flow the left term in Eq. (4) equals 0. FromEq. (3) one can obtain d�/dt= −�0/�0, where �0

and �0 are the initial values of the parameter �

and the duration of the flow, respectively. Atconstant gas flow Eq. (4) becomes

dT/dt= − [(�f/��)T/(�f/�T)� ](d�/dt) (5)

As expected from Eq. (5) the release rate ofdeuterium increases with the temperature of thetitanium.

Inserting the numerical values of the corre-sponding parameters into Eq. (3) gives �=4.18×10−2�0/�0. For �0=1 and �0=100 s a value of�=4.18×10−4 s−1 is calculated. Experimentallya value of �=1.84×10−4 s−1 was obtainedwhen � was reduced from 0.6 to 0.2 and thetemperature was increased from 500 to 600 °C [1].

2. Characteristics of the element packages

For the gas supply to a tokamak with EPHI atitanium weight is required that by far exceeds theone used for EPHI-1C. For the tokamak T-11Mwith its pulse duration of approx. 1 s a deuteriumflow of 3×10−6 kg/s is needed. When �0=100 sthe amount of titanium is estimated to be MTi=3×10−6/4.18×10−4=7.2×10−3 kg. In com-parison, for ITER-FEAT an amount of titaniumof MTi=3.23 kg would be required (W=0.3×10−3 kg/s) for a pulse duration of �0=450 s.

In order to accommodate a large number ofelements into EPHI, they need to be arranged incompact packages comprising a large number ofelements of identical dimensions. These elementscan be stacked one on top of the other. Ends ofthe same electrical sign are combined in a com-mon terminal, which is used to supply the current.

To assess the properties of the package a nu-merical calculation was performed using a code

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capable of modelling the physical processes takingplace in EPHI. In Fig. 1 the dependence of theresistance of the package from the current isshown. As seen the voltage drop in a single ele-ment exceeds that of a package of 16 elements bya factor of 8. Consequently, 8 packages in series,each consisting of 16 elements (total elements128), may be connected to the power source. Fora single element, a package of six elements, and apackage of 16 elements, currents of 64, 116, and178 A, respectively, are required to achieve amaximum element temperature of 900 °C understationary conditions (�=0). As obvious from thedata in Fig. 1, the energy consumption of a singleelement heated to a temperature of 900 °Camounts to 286 W, while the energy consumed bya package of 16 elements is only 95 W whenheated to the same temperature.

3. Deuterium gas supply for tokamaks

In the case of tokamaks the flow behaviour as afunction of time will depend upon the desiredplasma density ne. The plasma density is calcu-lated from

dne/dt=W/V−ne/�p (6)

where V is the plasma volume in the tokamakchamber and �p denotes the plasma confinement

time. The energy confinement time with ‘‘alcator’’scaling is �E=0.07aR2q (10−20 ne), where a and Rare the small and large radius in meters of theplasma, respectively, q is a safety factor and ne isthe electron density in units of m−3. Substituting�p�3�E into Eq. (6) and taking V=2�2a2R, weobtain the equation

dne/dt= (W−W0)/V (7)

in which W0=94×1020a/(qR) gives the particleflow of the plasma. For the T-11M tokamak(a=0.2 m, R=0.7 m, q=3) W0=8.95×1020

s−1 or 2.97×10−6 kg/s deuterium. Since theplasma density is constantly measured in the toka-mak the parameter dne/dt is known in real time.The stationary state (dne/dt=0) is realised auto-matically by decreasing the current through theelement package from the maximum to the mini-mum for dn/dt�0 and vice versa for dn/dt�0.

One of the scenarios tested in the T-11M toka-mak involved the disruptive breakdown in thedeuterium gas continuously fed into the tokamakchamber via a piezo valve (the gas flow was0.2×1020 s−1, which is much smaller than W0) inwhich a plasma density of n=0.1×1020 m−3 wasattained. After this, the EPHI will supply gas forincreasing the plasma density by a factor of fiveover a period of 10–40 ms. Then the plasmadensity has to be maintained at this level for aperiod of 0.7 s. This is achieved by the simulta-neous operation of one element and two packagesof six elements connected in series. Power wassupplied to these packages by two independentsources. Firstly the elements are heated up to atemperature of approximately 400 °C for about 4s. Following this, two capacitor batteries (421 and1670 A) are discharged through the packages fora period of approx. 10 ms. Finally, the elementsare again connected to the power sources supply-ing currents of 60 and 168 A, respectively. Tocontrol the gas flow the current through the ele-ment circuit is adjusted as described above. Fig. 2illustrates the calculated plasma densities and gasflows and Fig. 3 shows the variations in current ofa single element applied to generate the desiredplasma density.

Another method used to control the gas flow ofthe EPHI is based on the fact that its value is a

Fig. 1. Resistance of the package as a function of packagecurrent under stationary conditions. The curves 1, 2, 3, and 4correspond to 1, 2, 3, and 16 elements in the package, respec-tively. The curves 5, 6 and 7 are isotherms corresponding to500, 700 and 900 °C. The marks on the isotherms from left toright correspond to 1, 2, …, 10, 12, 14, and 16 elements in thepackage, respectively.

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Fig. 2. Gas flow and plasma density as a function of time. (1)plasma density, (2) injector gas flow, (3) gas flow for twoelement packages, and (4) gas flow of a single element.

Fig. 4. Gas flow of the module as a function of time atconstant pressures in the injector of 20 Pa (1) and 16 Pa (2),respectively.

known function of the gas pressure Pd inside theinjector volume. By comparing the pressure Pi

measured in the injector with Pd, an automaticsystem controls the package current in a waysimilar to that applied for the control of plasmadensity. For an implementation of EPHI at ITERscale, it is estimated that this flow can be achievedwith 20 packages of 16 elements connected inseries. These packages were considered to consti-tute the EPHI module having a volume of about3×10−3 m3 and containing 41.6×10−3 kg oftitanium and 1.18×10−3 kg of deuterium. Theproposed module current is 178 A (see above). Toreduce the time required for the gas flow toincrease, a current of 800 A was passed throughthe module for the first 1.2 s. Fig. 4 shows theflows calculated for Pd values of 20 and 16 Pa (thepressure in the tokamak chamber is 10 Pa and thepipe connecting the injector with the chamber hasa diameter of 20 mm).

Gas generation from titanium is characterisedby the fact that an increased gas flow of 3.7×10−6 kg/s at a pressure in the injector of 20 Pacan only be guaranteed for about 125 s. Underthese conditions the amount of gas released corre-sponds to 40% of the total stored gas. At apressure of 16 Pa, on the other hand, a gas flow of2×10−6 kg/s can be maintained for about 400 s.In the latter case the amount of gas releasedcorresponds to 67% of the gas stored. The electricpower required for the gas generation in the mod-ule is of the order of 1.4 kW at the beginning and1.9 kW towards the end of the process.

4. Conclusions

� Methods have been developed to control theplasma density in a tokamak by adjusting thegas flow of an EPHI. The overall accuracy ofthe injector is not affected by the scatter in thecharacteristics of the constituting individualpackage elements. This is important, as it isvery difficult to produce totally identicalelements.

� The energy losses of a package of N elementsare below the energy losses of a single elementat a given maximum element temperature.

� With the implementation of element packagesthe EPHI becomes highly versatile and applica-ble for the fuel supply to both small as well aslarge tokamaks.Fig. 3. Current of a single element as a function of time.

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References

[1] I. Glushkov, Yu. Kareev, Yu. Petrov, A. Savotkin, V.Frunze, E. Hutter, G. Mueller, R.-D. Penzhorn, U. Tamm,Generation of hydrogen isotopes with an electric pulsehydride injector, Int. J. Hydrogen Energy 24 (1999) 105–109.

[2] Yu. Kareev, U. Tamm, I. Glushkov, E. Hutter, V. Novikov,R.-D. Penzhorn, V. Frunze, Investigation of the deuteriumrelease rate in the electric pulsed hydride injector (EPHI), in:Proceedings of the 20th Symposium on Fusion Technology,Marseille, France, Sept. 7–11, 1998, pp. 929–932.

[3] V.S. Vlasenko, et al., in: Proceedings of the 6th InternationalConference on Plasma Physics and Contr. Nucl. FusionResearch, Berchtesgaden, Germany, 1976, p. 85.