Heat Flux Measurements in the Hall Thruster Plume

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1 Hat Flux Masurmnts in th Hall Thrustr Plum IEPC Prsntd at th 29 th Intrnational Elctric Propulsion Confrnc, Princton Univrsity, Octobr 31 Novmbr 4, 25 G. Makrinich * and A. Fruchtman Holon Acadmic Institut of Tchnology, Holon 5812, Isral S. Shitrit, G. Applbaum and J. Ashknazy ** Sor NRC, Yavn 818, Isral Abstract: A hat flux prob has bn dvlopd for th purpos of masuring th hat flux dnsity in th plum of th Hall thrustr. Th masurmnt is basd on comparing th rats of hating and cooling of th prob during its xposur to and isolation from th plasma flow. In th cas of a hlicon plasma sourc this is accomplishd asily by turning on and off th plasma flow. Th masurd hat flux into th ngativly-biasd prob is in a vry good agrmnt with th calculatd hat flux carrid by th impinging plasma ions, indicating that thr ar no nrgtic nutrals. This mthod will b mployd for th plum of th Hall thrustr and is xpctd to provid an stimat of th hat flux carrid by nrgtic nutrals. A C K i M M W m m p δ dt + /dt dt - /dt Nomnclatur = th collction ara of th prob = thrmal capacity of th prob matrial = ion nrgy at th prshath-shath boundary = ion mass = mass of an atom of th prob matrial = lctron mass = prob mass = nrgy transfr cofficint = drivativ of th prob tmpratur with rspct to tim, whn th prob is xposd to th plasma = drivativ of th prob tmpratur with rspct to tim, whn th prob is isolatd from th plasma * Rsarch Scintist, Scincs Dpartmnt, gnnady@hait.ac.il Profssor, Scincs Dpartmnt, fnfrucht@hait.ac.il Graduat Studnt, Propulsion Physics Laboratory. Rsarch Assistant, Propulsion Physics Laboratory. ** Had of Elctric Propulsion Projct, Propulsion Physics Laboratory, josph@sor.gov.il 1 Th 29 th Intrnational Elctric Propulsion Confrnc, Princton Univrsity, Octobr 31 Novmbr 4, 25

2 I i n S n T Φ u B i rc out n rad = ion currnt to th prob = plasma dnsity at th prshath-shath boundary = bulk plasma dnsity = th lctron tmpratur = th diffrnc btwn th floating potntial and th prob potntial = Bohm vlocity = total hat flux into th prob = hat flux into th prob du to th kintic nrgy of th impinging ions = hat flux into th prob du to th rlasd rcombination nrgy of th ions = hat flux out of th prob du to laving lctrons = hat flux into th prob du to th impinging nutrals = hat flux into th prob du to radiation I. Introduction Masurmnt of hat flux is potntially an important tool for undrstanding various procsss in th Hall thrustr. Thr ar svral tchnius for masurmnt of hat flux: th watr cooling prob 1, th tmpratur dpndd rsistanc 2, th gradint of th tmpratur along th body that on part of it is insrtd into th plasma 3, th hat flux compnsation 4, th chang in tmpratur of th targt 5, and th tmpratur-tim drivativs 6-8 (TTD). Th TTD masurmnt is basd on comparing th rats of hating and cooling of th prob during its xposur to and insulation from th plasma flow. In this papr th rlativly simpl TTD tchniu for masuring th hat flux from plasmas of argon, xnon, hlium, oxygn and nitrogn gass into a molybdnum plat. Th masurd hat flux into th ngativly-biasd prob is in a vry good agrmnt with th calculatd hat flux carrid by th impinging plasma ions, indicating that thr ar no nrgtic nutrals. This mthod will b mployd for th plum of th Hall thrustr and is xpctd to provid an stimat of th hat flux carrid by nrgtic nutrals. In Sc. II w dscrib th hat flux prob and th mthod of masurmnt. In Sc. III w dscrib th hlicon plasma sourc that was usd for th masurmnts and th xprimntal stup. In Sc. IV w prsnt a thortical modling of th hat flux and compar th masurmnts with th thory. Th rsults and th comparison of thory and xprimnt ar prsntd in Sc. V. II. Hat flux masurmnts Th Hat Flux Prob (HFP) shown in Fig. 1 consists of a molybdnum flat plat with glud chroml-aluml thrmocoupl. Th prob is of dimnsions 17.5mm 19mm and thicknss of.25mm. It is positiond on th axis of th vacuum chambr at a distanc of 3cm from th hlicon sourc, and its plan is paralll to th flow. Th masurmnts of th molybdnum plat tmpratur ar rgistrd in ral tim by a digital voltmtr connctd to a computr. Th hat flux is found from th rlation: dt dt + = Cmp dt dt, (1) whr dt + dt and dt dt ar th drivativs of th tmpratur with rspct to tim, C and m ar th thrmal capacity and th mass of th p molybdnum flat plat. Th drivativs dnotd by plus and minus signs ar masurd whn th prob is xposd to and isolatd from plasma, rspctivly. Both drivativs ar calculatd at th sam tmpratur, and th calculatd hat flux is found to b indpndnt of th varying plat tmpratur. Figur.2 shows a typical tim tmpratur charactristic whr th incrasing part of th tmpratur is masurd whn our hlicon plasma sourc oprats and th dcrasing part whn th sourc is switchd off. Figur 3 shows th drivativs Figur 1. Hat Flux Prob 2 Th 29 th Intrnational Elctric Propulsion Confrnc, Princton Univrsity, Octobr 31 Novmbr 4, 25

3 Prob tmpratur, o C , Tim, s Figur 2. Tim tmpratur charactristic of th molybdnum hat flux prob opratd in an argon plasma. Th potntial is 7V. Figur 3. Th drivativs of th prob tmpratur with rspct to tim vrsus th tmpratur. of th prob tmpratur with rspct to tim vrsus th prob tmpratur. Th constant diffrnc btwn th two curvs dmonstrat that th hat flux dos not dpnd on th prob tmpratur. Th choic of th physical and mchanical proprtis of th HFP dpnds on th conditions of prob opration. In th nxt sction w dscrib th hlicon plasma sourc that was usd for th masurmnts. III. Exprimntal stup Th xprimntal systm is schmatically shown in Fig.4. Th plasma sourc is composd of a vacuum chambr, a mass flow controllr, solnoids that gnrat an axial magntic fild, radio-fruncy gnrator with matching units, and an antnna. Th plasma is gnratd insid a Figur 4. Exprimntal stup Pyrx tub, 52cm in lngth, 1 cm outsid diamtr and 2.5mm thicknss. Th radio-fruncy gnrator radiats at MHz and th powr varis hr btwn 2W to 3W. Th antnna is a hlix of six turns of 35 cm lngth and 1.5cm diamtr. Th zro cross sction of axis z is locatd on th plac connction a Pyrx tub with a vacuum chambr. An antnna is locatd btwn z = 8cm and z =43cm. Th magntic fild intnsity in a hlicon plasma sourc in th xprimnts hr is 12G. Th vacuum chambr and Pyrx tub ar pumpd to a bas prssur of Torr. Employing a flat Langmuir prob of diamtr 1 mm, w hav masurd th plasma dnsity and lctron tmpratur. Th Langmuir prob was protctd by L-filtr (L=.46mH). Th hat flux prob that was dscribd in prvious sction can oprat as th Langmuir prob. In ordr to incras th plasma dnsity in th stainlss stl chambr outsid of th hlicon sourc, whr th masurmnts wr takn, a magntic fild of intnsity 1G and 15G was mployd. Th hat flux into th prob is IV. Thory + = i + rc out + rad n (2) 3 Th 29 th Intrnational Elctric Propulsion Confrnc, Princton Univrsity, Octobr 31 Novmbr 4, 25

4 Hr is th dissipatd kintic nrgy of th ion flux, th rcombination ion hating, th cooling i part whn lctron lav th surfac of th prob, rad th radiation hat flux from th plasma and n th dissipatd nutral nrgy. For our condition of collisionlss plasma th radiation and dissipatd nutral nrgy ar small, so that w nglct thm. In particular, w writ rc out 1/2 M i = Iiδ ( Ki + T ln Φ) 2 π m (3) Th trm in th parnthss is th diffrnc btwn th voltag drop btwn th plasma and th prob, Φ is th voltag drop btwn th floating potntial and th prob. Also K = T 2 is th ion nrgy at th prshath I M,m boundary. Hr is th ion currnt,t (V) th lctron tmpratur, th ion and lctron masss. For i binary collisions th nrgy transfr cofficint is givn by 7,1 i δ = 4MMW ( M + M ) 2 W (4) whr M W is th mass of prob atom. Th ion currnt is dtrmind from Bohm shath critrion and givn by I = n u A (5) i S B whr n S th plasma dnsity at th shath boundary is.6 n, n bing th bulk plasma dnsity, u B th Bohm A vlocity, th ara of ion currnt collction by th prob. If this ara grows with th incras in voltag w tak it into account in th calculation. V. Rsults Th hat flux of various gass was masurd by molybdnum and ELERO (an alloy composd of 7% tungstn, 3% coppr) probs. W also masurd th lctron dnsity and tmpratur and th plasma and th floating potntials. Taking into account also th rcombination nrgy of ions and th molybdnum work function, w calculatd th hat flux according to Es (2)-(5). Th agrmnt btwn thory (solid lin) and xprimnt (dottd) that is shown in Figs. (5)-(8) is vry good. Hat flux, mw cm Potntial, V -8-1 Figur 5. Molybdnum prob and a xnon plasma, Figur 6. Molybdnum prob and an argon 16-3 T = 4.3 V, n = m plasma, T = 4.9 V, n = m 4 Th 29 th Intrnational Elctric Propulsion Confrnc, Princton Univrsity, Octobr 31 Novmbr 4, 25 Hat flux, mw cm Potntial, V -8-1

5 Hat flux, mw cm Potntial, V Figur 7. Molybdnum plat and a nitrogn plasma, 15-3 T = 7 V, n = 8 1 m Hat flux, mw cm Potntial, V Figur 8. ELERO plat and a xnon plasma, 16-3 T = 5 V, n = m Hat flux, mw cm Ppotntial,V Figur 9. Molybdnum prob and a hlium 14-3 plasma. T = 14 V, n = 2 1 m Figur 1. Enrgy transfr cofficint (ETC) for molybdnum prob vrsus atomic mass of th impinging ions.. It is intrsting to not that th agrmnt btwn xprimnt and thory for nitrogn was obtaind only whn w assumd that th plasma is composd of N and not of atomic ions. Similar indications to molcular oxygn + 2 plasmas wr obtaind (not prsntd hr). Thrfor, this masurmnt provids us indirctly with important information about th plasma composition. A disagrmnt btwn thory and xprimnt is shown in Fig. 9. In ordr to xamin th discrpancy, w tak th masurd hat flux, and find, according to Es. (2) and (3), th nrgy transfr cofficint. Figur 1 shows th dpndnc of th nrgy transfr cofficint on th atomic mass of th impinging ion. Th solid lin shows th cofficint calculatd by E. (4) and th points ar th valus of th cofficint calculatd according to th masurd hat flux. Th cofficint for argon is in a good agrmnt with arly masurd rsults. 11 Excpt for hlium, thr is an agrmnt btwn th thortical and xprimntal rsults (nitrogn and oxygn ions ar assumd molcular, as discussd abov). Th hlium is shown not to fit E. (4). W suggst that th hlium atoms ar dpositd on th molybdnum surfac, and th hlium-hlium collisions caus an ffctiv incras of th nrgy transfr cofficint. 5 Th 29 th Intrnational Elctric Propulsion Confrnc, Princton Univrsity, Octobr 31 Novmbr 4, 25

6 VI. Conclusion Th mthod for masurmnt of th hat flux will b usd in th Hall thrustr masurmnt. Th diffrnc btwn th masurd and calculatd hat fluxs will b attributd to th nutrals. Acknowldgmnts Th authors thank Y. Raitss, A. Orn, and A. Warshavski for usful discussions. This rsarch has bn partially supportd by Isral Ministry of Industry (through Rafal) and by th Isral Scinc Foundation (Grant 59/99). Rfrncs 1 King, L. B., Gallimor, L. D., and Marrs, C. M., Transport-Proprty Masurmnts in th Plum of an SPT-1 Hall Thrustr, Journal of Propulsion and Powr, Vol. 14, No. 3, 1998, pp. 327, Samul, D. E., and Stvn, K. D., Invstigation of Thrmal Flux to th Substrat During Sputtr Dposition of Aluminum, J. Vac. Sci. Tchnol., A 6, 22, pp Bilis, I. I., Kidar, M., Boxman, R. L. and Goldsmith, S., Intrlctrod Plasma Paramtrs and Plasma Dposition in Hot Rfractory Anod Vacuum Arc, Physics of Plasmas, Vol. 7, No. 7, 2, pp. 368, Wintrs, H. F., and Horn, D., Enrgy Transfr to a Tungstn Lattic by Ion Bombardmnt, Physical Rviw B, Vol. 1, No. 1, 1974, pp. 55, Chavrs, D. G., Chang-Diaz, F. R., Irvin, C., and Suir, J. P., Momntum and Hat Flux Masurmnts in th Exhaust of VASIMIR Using Hlium Propllant, 28 th Intrnational Elctric Propulsion Confrnc, Papr No. IEPC-3-28, Toulous, Franc, March, Krstn, H., Stoffls, E., Stoffls, W. W., Ott, M., Csambal, C., Dutsch, H., and Hippr, R., Enrgy Influx from an RF Plasma to a Substrat During Plasma Procssing, J. Appl. Phys., Vol. 87, No. 8, 2, pp. 3637, Krstn, H., Dutsch, H., Stffn, H., Krosn, G. M. W., and Hippr, R., Th Enrgy Balanc at Substrat Surfac During Plasma Procssing, Vacuum, Vol. 63, 21, pp. 385, Makrinich, G., and Fruchtman, A., Th Hat Flux Masurmnts in a Hlicon Plasma Sourc, 21 ST IVS Gnral Confrnc, Sptmbr 11, Librman, M. A., and Lichtnbrg, W. D., Principls of Plasma Procssing, Wily, Nw York, Cuthbrtson, J. W., Motly, R. W., and Langr, W. D., High-Flux Sourc of Low-Enrgy Nutral Bams Using Rflction of Ions from Mtals, Rv. Sci. Instrum., Vol. 63, No. 11, 1992, pp. 5279, Compton, K. T., and Lamar, E. S., A Tst of th Classical Momntum Transfr Thory of Accommodation Cofficint of Ions at Cathods, Physical Rviw, Vol. 44, 1933, pp. 338, Th 29 th Intrnational Elctric Propulsion Confrnc, Princton Univrsity, Octobr 31 Novmbr 4, 25

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