Plasma neutralizers - Pure · types of plasma creetion, the electron temperature mey be much smaller then 732 Plasma N eutralizers the 5 ev quoted in the exemple, leeding to smeller

Plasma neutralizers

Citation for published version (APA):Vallinga, P. M., Schram, D. C., & Hopman, H. J. (1990). Plasma neutralizers. In Production and Neutralization ofNegative Ions and Beams : fifth international symposium / Ed. A. Hershcovitch (pp. 729-740). (AIP ConferenceProceedings; Vol. 210). New York: American Institute of Physics.

Document status and date:Published: 01/01/1990

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PLAS_MA NEUTRALIZERS

PJ'1. Vallinga and D.C. Schram Enehoven Lhiversi1y of TechrologJ I P. 0. BG< 5131

N.. -5600MB Eirdhcwenl The 1\etherlerds

HJ. Hopman fiEr te6'111 cl o Mex-Plend<-lrstittt ter Plesmer:tlysik,Boltzmenrstr. 21

D-0046 Gerchirg bei M Cncten, FRG I frld FOM hstittt.efa- Atomie erd Mola:ula-- Pl'Y::Jsics.

Amsterdanl Tte Nettena1ds

ABSTRACT

Presented are results on the modelllng of a cascaded are. Under suitable condltions, the plasma effusing out of the are has a high degree of ionization. lt is proposed to use this are as the plasma souree of a plasma neutralizer in neutral beam injectors for fusion research. The neutrallzation of a o- beam results in the generation of an electron beam. The power in this e-beam is toa small to sustain the plasma in the neutralizer for the 1.3 MeV fusion beams.

INTRODUCTION

The neutral beams that at present are being discussed for heating of the plasma and partiel drive of the toroidal current in next step fusion devices like NET and ITER, have partiele energies between 1 and 1.3 MeV [ 1 ,21. These beams are obtained by the neutralization of negative ion beams, bath H- and o-. The powers envisaged range from z 50 (NET) to ~ 75 MW (ITER). Therefore, it is important to generate these beams with the highest possible efficiency. Areas where efficiency gains are likely to be possible are the negattve ion souree and the neutraHzer. Negative ion sourees do get attention in the neutral beam community [see this conference], but this is nat sa with neutralfzers. In the available designs of neutral beam injectors, the werking hypothesis is to use a gas neutralizer. However, the use of a plasma neutralizer [3] would give a considerable increase of overall beam line efficiency, possibly from z 42% to z 58%.

Neutralizers for negative ion beams have two particularities that d!stinguish them from these for positive ion beems. Firstly, one is dealtng with three charge fractions, which, 1n the case of e deuterium beam, are o-, 0°, end o+. This property necessitates a modification of energy recovery techniques when they ere eerried over from positive to negative ion beams [41.

© 1990 American Institute of Physics 729

730 Plasma N eutralizers

Secondly, during the stripping of the negetive ion, en electron is releesed thet is trevelljng et epproximetely the seme speed as the perent ion. At e o- energy of 1.3 MeV the neutrelizetion results in the generetien of 350 ev electrons. In the case of positive ion beems, there is no stripping end the ibnizetion of neutrelizer etoms or molecules results in electrans oflow energy only.

Fest electron generetien occurs in ges es well es in plesme neutrelizers. The question then erises if these electrans een be used to simplify the neutrelizer. In the cese of e neutrelizer to which only ges is edmitted, they mey ionize the ges end farm e plesme. To creete e sufficient number of ion peirs the fest electrens must be confined, for instanee with e multl-pole megnetic field. Then, the situetion is enalogous to a negetive ion souree in which'the hot-cethode discharge has been repleced by e negetive ion beam. The camperisen ellows us to predict some espects of this neutrelizer. A prellminery estimete indicetes thet the beem current is toa smell to ionize the ges to the necessery high degree of ionizetion [3], a 150%.

Jt fellows thet the plesme in the neutrelizer needs to be gener:eted by extemal meens. We propose the use of cesceded ercs [5, 6] to inject plasma into e box lined with e multi cusp megnetic field. Presented will be the most relevent result of a rnadelling of this ere. lt is found thet the celculeted degree of i anizetion in the ere is neerly 100%. The ere power required for e full scele neutrelizer is estimeted et 30 kW for e 8.3 MW neutrel beem module.

A cesceded ere is e well stebilized ere, consisting of e cethode, e steek of electricelly insuleted caseede pletes, end en enode. The ere chennel is formed in the centrel bare of the caseede pletes, which ere 5 mm thick, weter cooled, capper pletes, thet ere sepereted by 1 mm geps meinteined by PVC specing rings. Ges is edmitted to the ere chennel et the cethode side end ple.sme flowsoutof the chennel through e hole (e nozzle) in the enode. The ere plesme is cherecterized by high electron densities, high degrees of ionizetion, moderate temperetures, ende lew power consumption.

PLASMA NEUTRALIZER REQUIREMENTS

Culhem hes proposed the use of e close coupled neutrelizer in their design of e neutral beem injector for NET [7]. Ase result of the short distence between souree end neutrelizer, end beceuse of the smell divergence of a high energy negetive ion beem, the individuel beemlets do nat merge. Therefore, neutrelizers een heve neerl~ closed front end beek side, contrery to the open structures used on present fu~~ion machines. A neutrellzer een be enviseged es e box on en sides covered by permanent megnets, with::::: 30 mm wide slotsin the front end bet:k wen to ellow the beemlets to pess through. Slots ere needed beceuse of the demend to sweep the beamlets in the vertical plene

V allinga et al. 731

(beem profile control). The number of slotsis equel to the number of aperture columns in the source. Usinga Culham type souree in the cese of e 83 MW, 1.3 MeV module (ITER NB injector concept [8J) , the neutrelizer cross sectien would be approximetely 1.5 by 1.5 m2 end 1t would heve ebout 10 slots [91.

In the following we give a rough estimete of the ionization rete needed to maintain the plasme in a neutre11zer box of length 1, and width end height d. The totel ion flow to the wens (Bohm sheethenten on) is equel to,

<p z 0.6 ni es Ac, (1)

where, nt is the plasma density, es=~ k(T e+Ti)/m is the ecoustic speed with m the ion mess, end the totelloss area Ac is given by the product of the length end the width ö of the cusp lines between the magnet rows. We assume Jineer 11ne cusps perpendiculer to the velocity of the negetive ion beem et a pitch D. This wey the magnatie f1eld is meinly parellel to the beem and the perturbetion on the beem is minimaL In the front end beek wen, the magnet

configuretion is determined by the s1ots. Fina11y, we have ö = 4 ~ [ 101, which is four times the hybrid Larmor redius. From these consideretions we obtein,

Ac z ö d (41 + d) ID. (2)

A further given fect is the optimum plasma target density TI, given by [3],

n = n1 1 z 2 x 1 o 19 m-2. (3)

With the eid of Eqs. ( 1, 2, 3) we een wnte,

.... me d (41 + d) _r-:. 1-4 ~

cp = 2.4 TI -\f m D 1 ~'I PePl oo B -'J ~ (4)

The sealing lew demonstratas thet it is beneficiel to increese the B fleld, en approach being studied by Culhem [9], or to reduce T 9. The ion mess hes less influence, but switching from deuterium to ergon ions, one geins a factor 2.1. As a numericel exemple, we take Te = 5 eV, Ti (D+) = 0.4 eV, (temperetures

.. meesured in buckets [ 111, in which the plasma is creeted by energetic

electrons), B 5= 0.15T, d = 1.5 m, 1 = 1 m, end D = 5 cm. we obtein cp= 2 x 1022

s-1; foren Ar+ plasma the seme quentlty is 9 x 1 o21 s-1. In the ebove derivetion, wen lossas were essumed dominant. Withether

types of plasma creetion, the electron temperature mey be much smaller then

732 Plasma N eutralizers

the 5 ev quoted in the exemple, leeding to smeller ditfusion losses. However, volume losses llke three-body end redietlve recombinetion of D+ i ons become more importent, es well es loss chennels invalving (vibretionelly excited) molecule rections. This suggests thet there is en optimum tempareture tor which the flux <p is minimel. This tempareture mey be quite different for etamie end moleculer geses, beceuse of the more complex chemistry of the letter.

PLASMA CREATION

As mentioned in the introduction, the neutrelizetlon of e negetlve ion beem is eccompenied by the formetion of e 350 eV electron beem. In the cese of e module deliVering e 8.3 MW, 1.3 MeV (DO) beem, the electron current emounts to = 6.5 A, end represents e power of = 2.24 kW. Beceuse the electrens ere releesed inside the neutrelizer, we essume thet they ere confined by the multi cusp megnetic field end loose their energy by i anizetion of the ges. Teking en expenditure of 64 eV per ion peir creeted [ 121. the electron beem could ionize 2.2 x 1 o20 etoms per second. lt is cl eerthet in the cese of beems for fusion the electron beem power is too smell to esteblish the required plesme terget. The situetion becomes more fevoureble with beems of higher energy end higher current density.

We conclude thet the plesme must be creeted by extemel power. We een choose between two epproeches. ( 1 ): One een inject e plesme [5, 13]. Then the neutrelizer must heve en open structure to ellow efficient pumping of the i ons thet neutrelize on the wells. The ges volume to be pumped is found from <p. In the cese of deuterium, it is= 280 Torrl Is. (2): One een inject power end ges seperetely into the neütrelizer end creete the plesme in situ [9, 121. In this cese, the neutrelizer must be closed for ges trensport to reduce the pumping requirements. Teking egein 64 eV for the creetion of e deuterium ion peir, the required power is found to be 200 kW.

To leem ebout consequences of injecting plflsme from en extemel souree we studied cesceded ercs, beceuse of their high power efficiency.

ARC MODEL

To describe the evolution of the ere plesme es tunetion of the exiel position elong the chennel between cethode end enode e self consistent ene­dimensienel model hes been set up. A two-dimensionel model, which tekes into eccount the rediel profiles, hes been formuleted [D. Milojevic, D.C. Schrem, end P.M. Vellinge,-to be published]. -In this peper, we present the 1-D model results obteined by numericel integretion of the conservetion lews for mess, momenturn end energy. Celculeted es tunetion of the coordinete x ere the

V allinga et al. 733

densities end the temperatures of heevy perticles end of electrons, the pressure, end the directed flow velocity. A further result is the ere voltage.

lt is essumed thet the hëavy partiele components 02, oo, end o+, with densities n2, n 1, end n1, ere closely coupled end have the seme tempareture Th end drift velocity u. The electron component hese density ne end tempareture Te. Then, the degree of ionizetion is defined by a= n9/(n2 + n 1 + n;), the degree of dissocietion by f3=nt/(nt + n2>. end the reduced mess velocity by M = u/c,

with c =~5kTh/3m. The energy input is to the electrans end is due to Ohmic dissipetion,

Oohm, of theerecurrent Ia in the ere chennel of diemeter D. The electrans loose energy by dissocietion of the molecules, Od, by ionizetion of the heevy perticles, Oi, by elestic energy transfer in electron heavy partiele collisions, Oeh, by werk performed on the plasma end leeding to the plasma expension, Ou, end through rediative processas like the escape of line redietion emitted by excited heavy partiel es, theescape of continuurn redietion due to free-free transit i ons end recombinatton to exclted levels.

Changes in the densities of species ere brought ebout by direct or indirect electron impeet ionizetion end dissociation, three partiele recombinetion, end redietive recombinetion. Momenturn transfer is by meens of alestic collisions between electrans end heavy perticles. In eddition, also friction between the plasma end the wellis teken into account.

When performing a calculation, the input paremeters ere the ere current Ja, the pressure et the chennel entrance p0, the chennel diem. D, the chennel length I, end the ges flow <j>. Of these, one paremeter is a dependent one beceuse of the boundery condition thet a sonic condit ion, M = 1, is reeched et the end of the chennel in the anode.

SOME RESULTS

The model described ebove hes been eppHed to en ere buming on ergon ges. Celculeted velues ofTe, ne, end pressure p, egreed within 5% with velues meesured et some ten different points elong the ere chennel, providing· the chennel was given a diemeter of 38 mm, insteed of the experimentel velue of 40 mm [61. This difference is releted to the existence of a 0.1 mm thick well layer. These results give confidence in the applicebility of the model to a deuterium ere, for which no experimentel date are aveileble.

The simuletions in generel show en increese of the degree of i anizetion a wïth are current end with chennellength, ende decreese with ges flow. Fig. 1 mustretes the letter point with results for three cases in deuterium ges:

734 E.lasma Neutralizers

ges flow (scc/s) ere length (mm)

case A 100 118

ceseC 150 55

case D 200

31

with further, D = 3.8 mm, Ie= 95 A, end Po= 0.35 bar. As mentioned before, not all paremeters ere free. rn this series of celculetions, the ere length was edjusted to obtein in the anode the sonic condition M = 1. Sterting velues et x = 0 ere a, (3 = 0.01 end T = 1000 K. The figure shows thet a increeses epproximetely lineerly with x end reaches e maximum velue > 0.8 for the smallest flow. Also the degree of dissocietion i3 is shown. in case A, nearly full disseciet ion is obteined in the first 40% of the ere length.

In case A, the gas is heeted to Th> 1 o4 Kin the first 1 0% of the chennel, in which thermel equilibrium is reached. From there on Te end Th increese neerly linearly from 13,000 K to 16,000 K et the anode side. The highest tempareture is found for the smallest flow. The electron density is plotted in Fig. 2. The highest density is reeched with the smallest flow. The initiel i"ncreese in ne is due to i anization of the ges; the decreese in the secend half of the channel is releted to the increese in plasma flow velocity. The combined result is a monotonic decrease of the pressure with position x. The major energy terms essociated with these processes ere plotted in Fig. 3 for case A. All terms ere normalized to the Ohmic energy input. lt is seen thet the electron energy used for dissociation, 0d, diseppears at the position where (3 tends to one; ionizetion losses Oi remein high up to the point where the electron density reeches its maximum. Beyend this position Q; decreases, even though a still increases. Seturetien of a only occurs neer the end of the channel. Further is indicated the elestic energy transfer from electrens to heavy pertïcles, Oeh. This quentity becomes the dominating one at the end of the chennel, where the plasma speed u increases repidly end the sonic condition M = 1 is reached. Oe is the sum of the three terms just discussed. In the energy belence, the redietion losses ere unimportant. Tagether they contribute less than e few % end are not presented. Figure 4 presents the energy contributions releted to the gredient terms. The energy needed for

plasma ecceleretion is Ou=~ kTe ne (V u), end the energy due to veriations in

electron density end tempareture ere On = ~ kT e u (V ne) end Ot = ~ ne u (V

kTe), respectively. Agein, the terms ere normallzed with respect to the Ohmic input. At the end of the chennel, the dominent term is the plasma acceleretion. lt reaches e value of 0.6 et the anode end makes üp for the deficlt in Fig. 3. Due to the expansion the tempareture decreeses at the anode (Qt < 0).

The calculáted cumulative plasma resistance, between the cathode and the anode amounts to 1.8 Ohm. With the chosen are current of 95 A, we anive

V allinga et al. 735

et en ere voltage of 170 V ecross the column of case A. So, this ere dissipetes 16 kW:

.. DISCUSSION

With the eim to fill e given volume with plasma, the relevent question is the maximum flow of ion pairs provided by the ere. lt is epproximetely etteined for the condit i ons of case A, end emounts to ~ = 3.8 x 1 o21 s-1. However, reducing the ges flow results in e higher degree of ionizetion end in relaxed pumping requirements. At q, = 50 scc/s, celculetions give a z 100% end

q>a = 2.4 x 1021 s-1. Teking this letter case, we find <p'cpe z 8. So, some 8 cesceded ercs would be needed for e full sized plasma neutralizer. This number is en upper bound, because we compered two situetions with different temperatures end, therefore, different ion life times. We estimeted <p using Te = 5 eV, whereas in the plasma jet squirting out of the cesceded are Te s. 1 eV. The sealing lew in Eq. (4) suggests thet the number of ercs might be es lew as 3, implying a gas flow of 150 scc/s or 115 Torr 1 I s.

The edventege of e cescaded are is the smell power consumption. From the calculeted plasma flow and are power, one obtains 26 eV per ion pair, e factor2.5 more fevourable than the low pressure discharge result [12]. Using this number, the required power is estimated at 30 kW, for three ercs of 50 scc/s gas flow.

The ebove estimetes of gas flow and power indicete thet plasma neutrallzers farm a realistic option besides ges neutralizers. Moreover, the sealing law suggests thet many improvements are possible. Because of the important benefits possible with plasma neutralizers, such as a much smaller eree occupied by the neutral beam system in next step fusion devices experimentel efforts in this area deserve a streng support.

ACKNOWLEDGEMENTS. This workis part of the research program of the associatfon agreement EURATOM-FOM, wlth financial support from NWO end EURATOM.

736 Plasma N eutralizers

REFERENCES

[ 1] R. Toschi, M. Chezelon, F. Engelmenn, J. Nihoul, J. Reeder, endE. Selpietro, Fusion Technology 14 ( 1988) 19.

[2] ITER Concept Definition, ITER Document Series no. 3, IAEA, Vienne (1989).

[3] K.H. Berkner, R.V. Pyle, S.E. Saves, end K.R. Stel der, Proc. 2-nd Int. Symp. on Production end Neutralization of Negative !ons end Beams, Broekhaven Netiona1 Laborator:J Report 51304, Upton NV USA ( 1980), p. 291.

[4] HJ. Hopman, Nucleer Fusion 29 ( 1988) 685. [5] G.M.W. Kroesen, end D.C. Schram, in Pred. end Appl. of Light Negetive I ons,

Proc. 111-rd Eur. Workshop, Euroase Congress Center, Amersfoort, The Netherlends, 17- 19 February 1988, p. 209.

[6] G.M.W. Kroesen, J.C.M. de Hees, end D.C. Schram, submitted to Plasma Chem. Plasma Proc.

[7] J. Couplend, I. Gray, A. Hol mes, M. lnmen, L. Lee, K. Martel, end R. Perker, Culhem Leberetory Report 3 15/88 - 7 /FU UK NET ( 1989).

[8] ITER Conceptuel Design Interim Report, to be publishad in ITER Document Series, IAEA, Vienne; ITER Report TN-HD-9-1 (October 1989).

[9] AJ.T. Hol mes, private communicetion. [ 1 O] K.N. Leung, N. Hershkowitz, end K.R. Meckenzie, Phys. Fluids 19 ( 1976)

1045. [ 11] M. Becel, M. Cepitelli, C. Gèrse, D.A. Skinner, end J. Bretagne, in Pred. end

Appl. of Light Negetive lens, Proc. 111-rd Eur. Workshop, Euroase Congress Center, Amersfoort, The Netherlends, 17- 19 Februery 1988, p. 112.

[ 12] K.G. Moses, end J.R. Trow, in Prod. end Appl. of Light Negetive I ons, Proc. 111-rd Eur. Workshop, Euroase Congress Center, Amersfoort, The Netherlends, 17- 19 Februery 1988, p. 203; ibid. Production end Neutrelizetion of Negetive I ons end Beems, 4th Int. Symp., Broekhaven Natienel Leboretory, AlP Conf. Proc. 158, NV ( 1987), p. 651.

[ 13] A. Hershcovitch, V. Koverik, end K. Prelec, in Prod. end Appl. of Light Negetive lens, Proc. 111-rd Eur. Workshop, Euroase Congress Center, Amersfoort, The Netherlends, 17 - 19 Februery 1988, p. 217.

,;;-'e -N 0

Ql

c

0.8

0.6

0.4

I I

/ /

/A

-

V allinga et al. 737

---

A

c ------------------· 0.2 ------

0 0.2 0.1 0.0 o.n

axinl posit.ion x/1

FJ91.re 1. lhe calctJat.ed lna-eese In thedegree of lonizaial (solldlines)ond dissociatlon (dosred mes) V'lith oxià pa;iti at in o coscaded ere c:homel, ncrmalized to tre totà en:: len~l. Theperametersere the ere wrrent, Ie= 95 A; treinlet t:ressure, Po= 0.35 ber da.rteriungos; g:~sfloward en:: Iengthere fa- case A) 100 s;c/s md 118 mm, fa-coseC) lSO s;c/s md 95 mm, md forcaseD) 200 scc/send31 mm.

:JO

20

10

axinl position x/1

F191.re 2. Axiel density profiles, cokul8f.ed for o cascadad ere h deuta-ium gas. Parameta-s ere the sarre as h Fl g 1 .

~

738 Plasma Neutralizers

-(IJ

a '"' ~ ~

~ u

'"' ::3 0 111 -a .c 0 0' ...... 0'

-1/l e '"' ~ ~

a 0 .... 111 ·a til Pc ~ ~ -e .a 0 0' ...... 0'

o.n

0.6

O..t

0.2

Qe

0 0.2 0.·1 ll.ft o.n

axial posilion x/I

FJJ~ 3. 'A:Iriws eectron Erlergy diss1leticn tams, norl'Tlflli~d to the 01mic energy Tput, es ft.ncticn of the p;>sifun in the cascooed are cha'lnet Parameters a--e those of case A in Fig 1 . Oe is the sum of the tlree tams presEJ~ted Od, the disscx:iaticn loss, Oi, tll:l icnizaUon loss, end Om, the EnErQI transferred to ionsin elastlc camsiena ·

11 0.2 0.4 !Uo ILO

... axial posilion x/1

FiglTe 4. Termsin the electronenergJ eQJ6lion, proportlonel to plasrrtl gmdiErlts, lke the-density gradiEnt ~, the tanpenstu~ gracient Qt, and the gradiEnt in the directed velocity, Ou.

V allinga et al. 739

DISCUSSION

Hershcovitch: I'm net asking this question te promate our past program. Why don't you consider hollew cathode discharges for which Daan Schram had an excellent pro­gram there. Hollew cathodes are much more efficient in terms of power and gas.

Hopman: In gas efficiencies matters, if the calcu­lations give you a high degree of ionization better than 95%, it is equal te the hollew cathode. The advantage is, even though there is no hard number on it, that the lifetime of the elements in the cascaded are is much langer than in the case of the hollew cathode. There is no need te have a tip at 3000 K like in the case of the hollew cathode. This is the main advantage that I see.

Herhcovitch: There have been experiments and studies that showed that hollew cathodes operatïng with LaB6 cathodes have a very long lifetime providing there are no contaminating elements. I would suspect a neutral­izer would be a clean environment where the only ele­ments would be the plasma.

Hopman: This is something that has te be discussed.

Moses: I certai~ly agree with your conclusion, I think we have te get te plasma neutralizers. The question of electron temperature, what was the electron temperature in your source?

Hopman: I forgot te bring the viewgraph, I think it's 12,000 degrees, for the plasma flame that would be 1 eV?! It must be much colder than 1 ev.

Moses: We have plasma temperatures on the order of 6 te 8 eV se that is relatively cold. The reasen why we have been advocating heavier gases in our werk is since the ionization potential goes down, se you save that way. In a 21 liter volume, we have been able te produce den­sities on the order over 1014, and in hydragen in about lo13. We have already used in that system the high field magnets, the neodynium magnets already se I don't think we will be able te gain much more than any ether magnets at least that are available now. The question about the degree of ionization, if you look at Berkner's paper which is really the only thing we have te go by, because of the fact that the cross sections are net known for many of the cross sections we need for neu­tralization, you find that only 30% gets you almest te where you have te be as far as neutralization. Fifty

740 Plasma Neutralizers

percent is overkill but would be nice, 100% is certainly not needed. We have achieved at about 2 milliTorr keep­ing the pressure down on the order of about 40% ioniza­tion, so we've already exceeded that at least in xenon. The heavier gas has some ether advantages for fusion applications but these are things that will probably be discussed and argued and debated for a long time to come. I can only say that Andrew's (Holmes) suggestion about putting back the magnets on the face is a very good one. We did not do that purposely because we wanted to test it out for the Berkeley beam source, and we were given specific instructions not to put any mag­netic fields at the entrance and exit and we also allowed for about 125 square centimeter openings at both ends besides no magnets. So, that was a worst case situation. I agree that putting magnets on these ends would probably increase the lifetime of the plasma and reduce the power requirements greatly.

Hopman: Sure. I want to comment on one point. If you just fed power into your neutralizer and produced a plasma, and you want to reduce the pumping to have a high gas efficiency like in Andrew Holmes' proposal, then your neutralizer with a length of about one meter will have on both sides the pipes or slots with lengths of over 2 meters. In these slots, you do not have plasma but you have gas. You have to look at the ove­rall gas target density over the full five meter length, although the plasma target is only 1 meter long. This factor makes you require a higher degree of ionization than if you would have no reduction of your gas. As far as I am aware, no one has made a study of optimum lengths of your pumping looking at these two target den­sities. But you have to be careful.