THE T6 HOLLOW CATHODE: MEASUREMENTS AND MODELING

Transcription

1 34th AIAA Plasmadynamics and Lasrs Confrnc Jun 2003, Orlando, Florida AIAA th AIAA Plasmadynamics and Lasrs Confrnc, Orlando, Jun THE T6 HOLLOW CATHODE: MEASUREMENTS AND MODELING Mark W. Crofton Th Arospac Corporation, El Sgundo, California and Iain D. Boyd & Univrsity of Michigan, Ann Arbor, Michigan ABSTRACT Bhavior of th xnon dispnsr hollow cathod is known to b complx, xhibiting high through-anod ion mission, unusual particl nrgy distributions, and high plum rosion rats. A flight-typ T6 hollow cathod was charactrizd using svral diagnostic mthods. Masurmnts involvd a rtarding potntial analyzr, quadrupol mass spctromtr, and Langmuir prob. Far-fild nrgy, flux, plasma potntial, and charg stat distribution data wr obtaind for plum ions at various oprating points. Th plum xpansion was modld by xplicitly including both lctrons and havy particls in a combind Particl In Cll and dirct simulation Mont Carlo approach. Th simulations wr in good agrmnt with masurd ion nrgy distributions, which pakd at low nrgy in comparison to most othr studis. Comparisons of ion mission and plasma potntial profils wr rlativly poor. I ck I F I h NOMENCLATURE INTRODUCTION Hollow cathods ar of gnral tchnological importanc. On application is in th ara of spaccraft propulsion, whr thy srv as critical componnts providing an fficint sourc of lctrons for lctric thrustr systms. Xnon hollow cathods can produc singl-point failurs in griddd ion ngin and Hall ffct thrustrs, and thy ar an important factor rgarding rosion of th scrn grid and othr componnts in ion ngins. During opration at th high mission currnt rquird for high-powr ion propulsion systms, th orific and any componnts in th plum rod rapidly. 1 Ions of sufficint nrgy and flux to caus significant rosion hav bn obsrvd in plum xprimnts. 2-4 Masurmnts of th ion kintic-nrgy distribution in th far-fild hav bn prformd on thrustr cathods with a rtarding potntial analyzr (RPA) or an nrgy analyzr, 2-4 in addition to th rcnt T5 work that utilizd a quadrupol mass spctromtr (QMS) and an RPA. 5 Th arly rsults rvald that a significant ion flux was mittd and that th ion nrgy distribution function could b vry broad and pakd at high nrgy. Ions ar abundantly formd and mittd through-anod with nrgis as much as svral tims highr than V ck vn though th anod potntial is abov th cathod by V ck. Unfortunatly, RPA and nrgy analyzr dvics do not distinguish btwn xnon ions with diffrnt charg stats if thir nrgy to charg ratios (E/z) ar idntical. Th mchanism by which th high-nrgy ions aris has not bn stablishd, but two principal hypothss hav bn prviously put forward. On mchanism invoks th formation of a potntial hill a fw mm downstram from th orific. 3,6 Although consistnt with som of th data, no clar undrstanding had mrgd of th mans by which a hill of sufficint hight could b formd. An altrnativ mchanism has bn postulatd whrby th currnt dnsity at th A ampr d kpr-xtractor sparation E kintic nrgy E f lctric fild, V/cm F flow rat, mg/s cathod kpr currnt, Ampr Faraday cup currnt cathod hatr currnt, Ampr m particl mass, atomic mass units q ion charg (=z) T tmpratur, K T i ion tmpratur, V T lctron tmpratur, V V ck cathod kpr potntial, rfrncd to th cathod body, Volts V r rpllr potntial of th RPA, Volts z ion charg numbr angl btwn RPA normal and plum axis angl btwn QMS normal and plum axis Rsarch Scintist, Laboratory Oprations/Spac Matrials Laboratory, M5-754, P.O. Box 92957, Los Angls, CA Mmbr AIAA. &Profssor, Dpartmnt of Arospac Enginring, 1320 Bal Avnu. Associat Fllow, AIAA. 1 Copyright 2003 by Th Arospac Corporation. Publishd by th, Inc., with prmission.

2 orific (on th ordr of 10 4 A cm -2 ) rsults in ion acclration via a magntohydrodynamic (MHD) ffct. 2,7 A third hypothsis concrning th formation of high-nrgy ions has bn proposd for vacuum arc plasmas, whr acclration of ions also occurs in th dirction away from th cathod and E/q can b much gratr than th applid voltag. In this cas a gas dynamic modl is invokd that dscribs ion acclration as drivn by prssur gradints and lctron-ion friction. 8,9 According to this approach, a small potntial hump may xist as a consqunc of plasma acclration rathr than as its caus. In th gas dynamic modl th ion nrgy distribution is indpndnt of charg stat. An xtnsiv litratur xists on th subjct of mittd ion nrgis from vacuum arcs. Th pak plasma dnsitis ar an ordr of magnitud or mor highr than applis to hollow cathods. A scond distinguishing charactristic is that th gas supporting th discharg is drivn out of th lctrods thmslvs. Spatially-rsolvd xprimntal masurmnts of lctric potntial and th ion vlocitis nar a hollow cathod orific hav prviously bn prformd. 10 A prominnt potntial hill was not found in ithr th Langmuir prob or lasr-inducd fluorscnc masurmnts. Th masurd vlocity distribution nar th hollow cathod tst articls dpndd strongly on th cathod typ and/or oprating point, including plum-mod vs. spot-mod opration. W hav prviously studid th T5 cathod, 5,11,12 a flight-typ xnon hollow cathod originally dvlopd for us as th main cathod of a low powr ion ngin, th UK Th T6 cathod is dsignd for substantially highr throughput and powr input capability, in accordanc with its various applications. 14 As in our prvious studis, a rtarding potntial analyzr (RPA) and a quadrupol mass spctromtr (QMS) wr both applid. In addition, Langmuir prob masurmnts wr mad in th plum. Th computational modl dvlopd to prform T5 cathod simulations has now bn usd to simulat th plum bhavior of th T6 cathod, with comparisons prsntd btwn modl rsults and xprimntal data. Th prsnc of abundant doubly-chargd ions was discovrd through th QMS masurmnts. 5 Th highr charg stats will contribut significantly to th rosiv powr of th hollow cathod plum, and b dominant ovr X + at rlativly low flux lvls. In addition, X 2+ prsnc in abundanc dmonstratd a highr fraction of ionization than had prviously bn assumd. QMS masurmnts of ion nrgy distribution showd, at last for som T5 oprating points, littl if any charg stat dpndnc. 11 This rsult providd vidnc for th potntial hill modl. Th fact that larg 2 currnt incrass at a givn flow rat did not corrlat wll with high-nrgy, broad distributions did not support th MHD hypothsis. Most prvious hollow cathod charactrization studis hav incorporatd a larg anod shll and a scondary discharg btwn th cathod and this anod to simulat th ion ngin nvironmnt. Such an approach complicats th study of th hollow cathod plum. Th hollow cathod was opratd in a standalon configuration in th prsnt study. Rcnt vidnc has suggstd that th diffrnc btwn th two configurations can b dramatically diffrnt in trms of mittd ion nrgy distributions. 15 As a thrustr componnt hollow cathods ar much simplr to build and oprat than th systms in which thy ar incorporatd. Hollow cathods will mak attractiv stand-alon ion thrustrs if th ion mission currnt and dvic fficincy can bcom mor comparabl to xisting ion thrustr systms. Improvd undrstanding of hollow cathod bhavior will aid in its dvlopmnt as an optimizd thrustr componnt and potntially as a stand-alon ion thrustr. EXPERIMENTAL A xnon hollow cathod was installd in a 75-cmdiamtr vacuum chambr, pumpd by a 1000 l/s (on nitrogn) turbomolcular pump and a 12,500/4,500/1,000 l/s (hydrogn/watr/xnon) TMP150 cryopump (CVI) mountd on a 10-in. conflat flang. Th cryopump could b radily isolatd from th chambr by an 8 in. lctropnumatic gat valv. Th bas prssur with no xnon flow was about Torr. At th maximum xnon flow rat of 1.08 mg/s, th background prssur indicatd by an ion gaug positiond far from th cryopump was Torr (cathod off), aftr applying a standard snsitivity corrction for xnon. Th ion gaug was locatd wll abov and bhind th hollow cathod orific, at a distanc of mor than 40 cm. Th cathod was installd on a rotatabl tabl, with a QMS positiond bhind a fixd bam skimmr, and th RPA also viwing th plum nar th orific (s Fig. 1). Th rang of rotary motion for th hollow cathod was = 50 to +135 dg. (masurd with rspct to th axis of th QMS). Th rotary tabl was mountd on a linar stag with 12 cm of fr translation rang along th prpndicular to th skimmr axis. Th 5-mm aprtur of th groundd skimmr was about 50 mm downstram from th kpr orific. Th ntranc of th QMS was about 50 cm furthr downstram, and alignd with th hollow cathod orific and bam skimmr. Diffrntial pumping was utilizd to lowr th background prssur in th rgion btwn QMS and skimmr. In th prsnt study, th QMS was always opratd with dtctor on and ionizr turnd off.

3 Th RPA consistd of a Faraday cup (Kimball Physics, FC-72A) with four closly spacd grids at th input. Th ntranc and third grid wr groundd, and th fourth or innrmost was biasd ngativ to rjct plasma lctrons and to supprss th loss of scondary lctrons. Th scond grid opratd at th rtarding potntial, V r. Th accptanc ara of th ntranc aprtur was 1.0 cm 2, and ach of th grid aprturs was covrd with a fin tungstn msh. Th aprtur was locatd 21 cm from th cathod orific. To chck for diffrncs of ion nrgy distribution with charg stat, a variabl potntial was applid to a rtarding lctrod positiond just downstram from th groundd xit plan of th skimmr. By tuning th QMS to th transmission pak for a particular charg stat, approximat RPA data could b obtaind with dtction of a singl ion spcis. positiond just downstram from th nozzl in an nclosd configuration. A fraction of th xnon flow can scap on th priphry. Th hollow cathod had not kpr 5.0 mm 29 mm OD mm 19 mm OD Orific ( mm ) tantalum tub (7 mm OD) 5.0 mm X Ta tip Dispnsr (2 mm ID x 5 mm OD) - + V r. -30V LP RPA X Figur 2. Schmatic cross sction of th nozzl/kpr configuration, approximatly to-scal. Th hatr, radiation shild, and othr itms ar not shown. QMS Figur 1. Schmatic of th xprimntal stup, top viw. A Langmuir prob constructd with 1-mm tungstn wir and shildd with cramic insulator was mountd vrtically in th plum. Th prob was placd at = 90 dg. Th linar translation stag was usd to vary th axial cathod to prob distanc. A 2-mm lngth of th tungstn wir was xposd to th plum. Th hollow cathod containd an imprgnatd tungstn dispnsr with an insid diamtr of 2.0 mm, outsid diamtr of 5.0 mm, and lngth of 20 mm, which acts as a chmical factory to rlas barium to th surfac at an appropriat rat to achiv low work function and long lif. Th dispnsr must b at approximatly 1000 C for th cathod to oprat normally. Th orific of th hollow cathod, machind out of solid tantalum, was 0.9 mm in diamtr 1.0 mm long, with a downstram half-angl chamfr of 45 dgrs (s Fig. 2). Th cross-sctional ara of th T6 orific in comparison to th T5 was 20 gratr. A kpr lctrod with 5-mm-diamtr aprtur was 3 bn opratd prviously. For th prsnt study th dvic was run at a numbr of diffrnt oprating points (s Tabl 1). Th cathod to anod voltag, V ck, was always undr 30V. V ck dpnds on th cathod tmpratur, flow rat, I ck (which influncs cathod tmpratur), and prsumably th condition of th dispnsr, siz of th orific and othr dtails of th gomtry. At high flow rat and tmpratur, such as may b obtaind aftr startup bfor th cathod hatr is shutdown, V ck can b blow 10V. Th cathod discharg currnt was nvr st highr than 20 A in this study. Spot mod opration was not obsrvd in this study, and would b difficult to produc du to th chamfrd orific. Each oprating point is blivd to hav bn a plum mod, although bam mod opration is also possibl. 7 MODEL DESCRIPTION Modls of hollow cathod plums hav bn dvlopd by Parks t al 16 and by Williams and Wilbur. 17 In Rf. 16, a fluid modl of th lctrons combind with an assumd profil for th ion dnsity was usd to modl a mrcury hollow cathod. By

4 assuming anomalously low lctrical conductivity (rducd by a factor of about 1000), good agrmnt was obtaind for masurmnts of plasma potntial and lctron tmpratur. Th modl undr dvlopmnt in this work sks to go byond that dscribd in Rf. 16 by xplicitly modling both th lctrons and th havy particls (ions and nutrals). Du to th low dnsity natur of th hollow cathod plums, a kintic, particl approach is mployd to simulat th xnon ions and nutral atoms. A dtaild fluid modl of th lctrons is also mployd. Th ions and nutrals ar tratd using a combination of th Particl In Cll mthod (PIC) 18 for transporting th ions in lctrostatic filds, and th dirct simulation Mont Carlo mthod (DSMC) 19 for prforming collisions and transporting th nutral atoms. Momntum transfr and charg xchang collisions ar th only collision mchanisms implmntd at this stag. Th spatial distribution of ions givs th lctron numbr dnsity undr th assumption of charg nutrality. Solution of th lctron continuity quation thn provids th lctron vlocity, v. Th lctron momntum quation is givn by: 20 ( t m n v )+ m n ( v )v = n E p + R (1) whr m is th mass of an lctron, n is th lctron numbr dnsity, v is th lctron vlocity vctor, E is th lctric fild, p is th lctron prssur, and R is th friction trm. It is furthr assumd that th lctrons bhav as a prfct gas (p = n kt ), and that th friction trm is givn by: n j R = (2) whr j is th currnt dnsity, and is th lctrical conductivity. Assuming a stady stat, nglcting th inrtial trm on th lft hand sid of Eq. (1), and introducing th plasma potntial - = E, a gnralizd Ohm's law is obtaind: j = + 1 ( n n kt ) (3) Th charg continuity condition with sourc trm S du to ionization: j = S (4) is thn solvd to obtain th plasma potntial. This quation is writtn as a Laplac quation with wak 4 sourc trms and is solvd using an Altrnating Dirction Implicit (ADI) schm. Th lctron nrgy quation is givn by: nkt + n t 2 2 m T + je 3 vnk m ( v ) kt + p v = (5) i ( T TH ) nnaci i whr m i is th ion mass, is th total lctron collision frquncy, is th lctron thrmal conductivity, and T H is th havy particl tmpratur. Again assuming a stady stat, and nglcting th collision trm as small: T = T + { j E + n ( v ) kt 2 (6) m + p v + 3 nk( T TH ) + nnaci i} m i whr j is obtaind from Eq. (3) aftr th plasma potntial is calculatd. Equation (6) is again a Laplac quation with wak sourc trms that is solvd using th ADI approach. Th transport cofficints ar valuatd using th basic dfinitions from Rf. 20: n 2 = (7) m k nt = (8) 1 + i m 2 whr = i + n, i is th ion-lctron collision frquncy, n is th nutral-lctron collision frquncy, and ths frquncis ar valuatd for th xnon systm using cross sctions providd in Rf. 20. Th modl calculations bgin at th xit of th orific nozzl. Th flow conditions ar stimatd from th masurd mass flow rat and currnt in addition to making assumptions for th spcis tmpraturs. In th prsnt work, an lctron/ion tmpratur of 1.5 to 2.0 V is mployd whil th nutrals ar assumd to b at th cathod tmpratur of 1300 K. Th plasma dnsity obtaind in this mannr is consistnt with a dtaild modl of th hollow cathod insrt and orific rgions dvlopd by Domonkos. 21 Boundary conditions ar also rquird for plasma potntial and lctron tmpratur in th solutions of th currnt consrvation (Eq. (4)) and lctron nrgy (Eq. (6)) quations. Th potntial of th kpr is st along th surfacs of th kpr and th flow inlt, and zro-gradint conditions ar mployd along all othr boundaris. For lctron tmpratur, an isothrmal

5 condition is usd along th inlt plan and zro-gradint conditions ar applid vrywhr ls. Figur 3. Dtail of computational grid showing th orifickpr and plum rgions. Th computational domain xtnds to 0.30 m along th axis from th orific and 0.25 m radially from th cntrlin. A portion of th grid is shown in Fig. 3 and mploys 110 by 90 non-uniform, rctangular clls. A tim-stp of s is mployd, which is smallr than th invrs plasma frquncy. Th simulation typically rachs a stady stat aftr about 25,000 itrations and final rsults ar obtaind by avraging ovr a furthr 20,000 itrations. About 500,000 particls ar mployd in ach simulation. Th simulations ar primarily prformd for oprating points 4 and 10. Assssmnt of th rsults is prformd through comparisons with th masurd data for currnt profil and ion nrgy distribution obtaind with th RPA instrumnt. Ths data ar rcordd during th simulation at th sam location as in th RPA xprimnts (about 21 cm from th cathod). momntum transfr to nutrals by th upstraming ions. In a rcnt PIC-DSMC simulation of th T6 cathod intrior, thortical support was providd for ths mass transport mchanisms. 22 Th tst chambr prssur is proportional to th mass flow rat xhaustd by th cathod. According to this masur, th xpllant flow rat of th T5 changd abruptly following a kpr currnt adjustmnt, and rturnd slowly to its original quilibrium valu. A chang in th T5 hatr currnt during th startup cycl rquird a similar priod of tim to rturn to quilibrium, howvr th chang was always gradual in this cas, as may b xpctd from a thrmal phnomnon. Th chambr prssur would initially fall, possibly bcaus th prssur lvation in th xnon gas bhind th nozzl is constraind by th prssur in th cold gas rsrvoir of th supply lin. With th mtring systm supplying constant flow into th rsrvoir, vntually th nozzl backing prssur must ris nough to rproduc th original nozzl flow rat prior to th prturbation. If th nozzl backing prssur changs slowly compard to th tmpratur (du to th larg siz of th rsrvoir and low flow rat), an initial T -1/2 dpndnc in th tst chambr prssur would b xpctd, whr T is th stagnation tmpratur. 23 Prssur (Torr) x A, 0.12 mg/s 8A, 0.12 mg/s 8A, 1.1 mg/s (dividd by 5) 4A, 0.12 mg/s, hatr on 4A, 0.12 mg/s, no shutdown RESULTS AND DISCUSSION Masurd Rsults In our prvious study of th T5 cathod, tst chambr prssur variation following a significant chang in cathod oprating point was consistnt with a variabl nozzl flow impdanc. Assuming thrmal quilibrium with th vacuum chambr walls and givn that th flow systm maintains a constant mass flow rat into th cathod rsrvoir, th obsrvd instantanous drop in chambr prssur upon switching on th T5 discharg was proposd to aris from a corrsponding incras of flow impdanc. This impdanc could aris from two mass transport ffcts: th upstram migration of ions through th orific, drivn by th local lctric fild, and collisional Tim (sc) Figur 4. Tst chambr prssur variation following turn-on of th T6 kpr discharg, for oprating points shown. In most cass th discharg was switchd off again aftr 300 sconds. Whn th T5 kpr discharg was switchd on or off, th chang in th background prssur of th vacuum chambr was larg and immdiat, followd by a slow rturn to th original lvl, on a tim scal of th ordr of 10 minuts. This obsrvation indicatd that th initial ffct was nonthrmal. Th subsqunt drift toward quilibrium and rturn to th starting chambr prssur

6 could b thrmal or dtrmind by th flow rat and rat of chang of nozzl backing prssur. Bhavior of th vacuum chambr background prssur was vry diffrnt for th T6 cathod. Changs of kpr currnt lvls did not produc Currnt (arb. units) Cas 10, Phi = 0 Cas 8, Phi = 0, baslin shift & x m/z (amu) Figur 5. Mass spctral scans for two cass, showing th contrast in X 2+ /X + ratio. th larg, abrupt changs obsrvd with th T5. Figur 4 illustrats th T6 rsults. Initial changs of background prssur wr of opposit sign to th T5 cas and varid littl at high flow sttings, spcially if I ck was not st high. Th timscal for variations that did occur was on th ordr of 10 minuts. Exponntial dcay of th prssur was obsrvd following discharg shutdown, without a stp function at turn-off. Howvr, th cas of high currnt and low flow rat (I ck = 8.0A, F = 0.12 mg/s) xhibitd a larg, slow ris that was nithr smooth nor closly rpatabl. Prvious masurmnts on a T6 hollow cathod hav rvald a complx variation of its back (intrnal) prssur with currnt and flow lvls. 7 A mchanism involving th j B forc and a z pinch ffct was proposd as th caus of th lvatd back prssur. Th z pinch mchanism prsums a runaway incras in th j B forc blow som critical flow rat or orific prssur, 7 and may provid an altrnat or supplmntal mchanism to xplain th cathod bhavior. Th hypothtical flow impdanc proposd in th T5 cas is not abl to xplain th T6 data bcaus th background prssur changs ar of opposit sign and th chambr prssur did not rturn to th starting point within a rasonabl timfram. Flow impdanc ffcts can b much smallr undr most of th conditions of this study, as much as 400 lowr for cas 1 vs. a typical T5 oprating point with th sam flow rat bcaus th T6 cross-sctional ara is 20 gratr. Th most likly xplanation for th T6 data sms to b lvation of vacuum chambr outgassing rats whn th 6 cathod discharg is on. A hat lamp insid th chambr was abl to produc a similar ffct whn switchd on and off. In our prvious study of th T5 hollow cathod, 5 th mass spctromtr indicatd a rapid initial ris in both X 2+ and X + flux with incrasing kpr currnt. A similar rsult was obtaind with th T6, and th variability of X 2+ :X + ratio is illustratd by Fig. 5. Th two cass shown hav idntical flow rats, but th X 2+ :X + ratio is much highr at I ck = 20A than at 8A. Similar rsults wr obtaind at othr oprating points, with I ck lvls scald according to th flow rat. Ion currnt mittd from th T6 hollow cathod was collctd as a function of th RPA rpllr voltag. Th data of Fig. 6 wr obtaind for svral oprating points (s Tabl 1). Ths data indicat, as xpctd, that collctd Faraday cup currnt riss with discharg currnt. Th flow rat is a mor influntial paramtr than kpr currnt in dtrmining th RPA profil. At th 0.50 mg/s flow rat, th main ffct of incrasing I ck is to push up th starting point of th RPA profil whil largly prsrving its shap. Th currnt lvls at V r =0 obviously hav a nonlinar dpndnc on I ck, a rsult that is spcially obvious in comparing th low-flow cas to th othrs. For th and 0.50 mg/s cass with 4.0A kpr currnt, th low-flow (cas 2) RPA currnt was much gratr as xpctd bcaus of its rlativly high kpr voltag. Th low-flow RPA profil is also distinguishd by a long tail and a transition rgion that occurs at rlativly high V r. Faraday Currnt (microamprs) Rtarding Potntial, V r 4.0A, mg/s 8.0A, 0.50 mg/s 12.0A, 0.50 mg/s 4.0A, 0.50 mg/s Figur 6. RPA currnt as a function of rpllr voltag at various oprating points. Th quantity!i F /!V r approximats th ion nrgy distribution. Whn ach ion nrgy distribution function (IEDF) is computd from th RPA profils of Fig. 6, th mg/s oprating point stands out from th othrs. Whil th IEDF width dcrasd slightly with incrasing currnt for F = 0.50 mg/s, it broadnd dramatically whn th flow was rducd to mg/s (s Fig. 7). With 30% highr V ck and on-fourth th

7 gas dnsity, a substantially lvatd rducd fild, E f /n, will xist at this oprating point. Th ris in rducd fild may produc highr lctron and ion tmpraturs and a broadnd ion nrgy distribution. 12,24 Th fullwidth-half-maximum of th distribution is about 25V for cas 2, and much lss for th othr cass shown. A significant numbr of ions with E/q xcding 100V ar prsnt. For most of th T6 cass shown in Fig. 7 th width of th distribution is considrably lss than thos obtaind in th T5 study. 11 In th T5 simulations th most influntial paramtr rgarding th IEDF was th lctron tmpratur. In contrast, at a singl flow stting th valu of I ck has only a minor ffct on V ck and th IEDF width A, mg/s 8.0A, 0.50 mg/s 12.0A, 0.50 mg/s 4A, 0.50 mg/s of data was obtaind for cas 2, with slightly lowr plasma potntial and highr T. Faraday Currnt (microamprs) A 8A 12A 20A, warming up 20A, fully warmd up 20A, phi = 90 f(e) Rtarding Potntial, V r Figur 8. RPA currnt as a function of rpllr voltag at various oprating points, F = 1.08 mg/s. Rtarding Potntial, V r Figur 7. Ion Enrgy Distribution Functions for slctd oprating points at normal incidnc; f(e) = -!I F /!V r. RPA profils for various oprating points with F = 1.08 mg/s ar plottd in Fig. 8. Th rlativly low pak nrgy of th = 90 dg. distribution is consistnt with charg xchang formation or larg angl ion scattring off nutral xnon. At small th ions ar mor nrgtic. This was also th cas in th T5 study. As in that study, low flow rat and high discharg voltag ar associatd with rlativly high-nrgy distributions. Th profil at I ck = 20A was found to slowly shift toward lowr nrgy. Th warming up and fully warmd up profils in Figs. 8 and 9 wr obtaind svral hours apart. Th shap of th lattr profil is anomalous, as I F riss ahad of th stp transition rgion. No significant diffrnc was obsrvd in rspctiv oprating paramtrs dspit th larg shift in pak nrgy. RPA profils and ion nrgy distributions ar shown in Figurs 10 and 11, rspctivly, for a singl oprating point that is approximatly cas 4 (V ck = 17.8; th kpr potntial was significantly lowr upon rturning to this oprating point for furthr study). Langmuir prob data obtaind undr cas 4 conditions ar plottd in Fig. 12. Plasma potntial, floating potntial, and lctron tmpratur all dcras significantly in th downstram dirction. A similar st 7 f(e) A 8A 12A 20A, warming up 20A, fully warmd up 20A, phi=90 dg Rtarding Potntial, V r Figur 9. Ion Enrgy Distribution Functions for slctd oprating points at normal incidnc, xcpt whr indicatd; f(e) = -!I F /!V r. Comparison of Simulatd and Masurd Rsults Comparison of plasma potntial and lctron tmpraturs for cas 2 show that th masurd T xhibits gratr variation and th potntial lss variation than th simulation (s Fig. 13). Th rapid chang in plasma potntial prdictd by th modl was not obsrvd in th masurmnt. If a high potntial gradint xists nar th kpr, it vidntly xtnds lss than 3 mm out from th kpr xit plan.

8 In Figs comparisons ar shown btwn simulation and xprimnt for cas 2, 4, and 10 IED functions, rspctivly, on th cntrlin ( = 0). Excllnt agrmnt was obtaind for both width and pak position in ach cas, with T = 1.5 V for cas 4 and T = 2.0 V for cass 2 and 10. Th most significant discrpancy is that th simulation did not rproduc th high-nrgy tail of th xprimntal distribution for cas 2. A similar comparison for cas 10 with = 90 dg. and th sam T as = 0 dg., also did vry wll (s Fig. 17). Potntial (V) Plasma Potntial Floating Potntial T Elctron Tmpratur (V) phi = 0 dgrs phi = 15 dgrs phi = 30 dgrs phi = 45 dgrs phi = 75 dgrs Distanc (mm) Figur 12. Summary of masurd Langmuir prob rsults for cas 4. Th distanc was masurd from prob cntr to th downstram kpr fac. Faraday Currnt (ua) Rtarding Potntial, Vr Figur 10. RPA currnt as a function of rpllr voltag at various viwing angls, cas 4 (V ck = 17.8) phi = 0 dgrs phi = 15 dgrs phi = 30 dgrs phi = 45 dgrs phi = 75 dgrs Figur13. Comparison of masurd and simulatd plasma potntial and lctron tmpratur profils, cas 2. f(e) Rtarding Potntial, Vr Figur 11. Ion Enrgy Distribution Functions, cas 4 (V ck = 17.8) at various RPA viwing angls; f(e) = -!I F /!V r. Comparisons of simulatd vs. masurd angular ion mission profils for cass 10 and 4 (s Figs. 18 and 19, rspctivly) rval ordr of magnitud discrpancis. Th simulation ovr-prdicts th mission flux, and th xprimntal profil for cas 10 8 Figur 14. Ion nrgy distributions at th RPA position for cas 2 (I ck = 4.0A, F=0.125 mg/s), = 0 dg.

9 is obviously mor flat than its thortical countrpart. Th shaps of th profils compar bttr for cas 4. Discussion Th only paramtr adjustd in th T6 simulations to-dat was T. Th T5 simulations showd that T and i xrtd th most influnc on th rsults. Th simulation is also snsitiv to assumptions concrning plasma potntial distribution nar th kpr aprtur. Whil furthr paramtr adjustmnts may improv th agrmnt btwn simulation and xprimnt, additional prob data in th orific-kpr rgion is clarly ndd to dtrmin th corrct paramtrs in this critical rgion and to stablish dfinitiv boundary conditions. In th potntial hill modl, nrgy distributions will dpnd strongly on charg stat (E ~ qv) and Figur 15. Ion nrgy distributions at th RPA position for cas 4 (I ck = 4.0A, F=0.50 mg/s), = 0 dg. largly coincid whn plottd vs. V r (V r = E/q). In contrast, th gas dynamic modl prdicts th kintic nrgy distributions will b similar for all ions. 8,25,26 Availabl T5 and T6 cathod data imply that th kintic nrgy distributions for X + and X 2+ ar similar. Th potntial hill modl givs primary importanc to plasma potntial and lctron tmpratur, whras in th gas dynamic modl th lctron-ion friction trm and ionlctron collision rats ar critical. Th simulation suggsts that all of ths paramtrs may contribut, howvr T dominats in th rgim invstigatd. It should also b notd that individual trms in th lctron nrgy quation usually contain mor than on paramtr (s Equ. 6), and T appars multipl tims. Th IED paks appard at quit low nrgy in this study, similar to a prvious rsult obtaind with th T6 cathod placd in a simulatd thrustr discharg chambr. 7 Rcnt lctrostatic analyzr masurmnts with a diffrnt cathod in a wll-simulatd thrustr discharg chambr found many ions at high nrgy, and vry strong dpndnc of th distribution on flow rat, currnt, and lin-of-sight, with rlativly low nrgy ions sn whn viwing th orific on-axis. 15 In th prsnt study, howvr, th highst IEDF is obtaind whn viwing th orific on-axis. It is not clar whthr th T6 cathod has a high propnsity to produc low nrgy ions (good for thrustr applications as an lctron sourc, bad for standalon micro-thrust applications) undr typical oprating conditions, or whthr this is a mattr of appropriatly simulating th thrustr discharg chambr. It is to b xpctd, howvr, that th influnc of th ion-thrustr chambr and its discharg will vry significantly influnc th nrgy spctrum of dtctd ions, du to th high discharg currnt, magntic fild, and containmnt Figur 16. Ion nrgy distributions at th RPA position for cas 10 (I ck = 2A, F=1.08 mg/s), = 0 dg. Figur 17. Ion nrgy distributions at th RPA position for cas 10 (I ck = 2A, F=1.08 mg/s), = 90 dg. 9

10 ffcts on ions and nutrals. Littl is known about highnrgy ion gnration in this nvironmnt. Figur 18. Angular ion mission profils at th RPA position for cas 10 (I ck = 2A, F=1.08 mg/s). Figur 19. Angular ion mission profils at th RPA position for cas 4 (I ck = 4.0A, F=0.50 mg/s). CONCLUDING REMARKS A charactrization study of th T6 hollow cathod has bn prformd. Far-fild nrgy, angular, flux, and charg stat data hav bn obtaind for mittd ions. Numrous oprating points of th T6 wr studid, rvaling som with unaccptably high ion nrgy for us insid a thrustr and othrs that ar mor bnign. Th ion nrgy distribution functions wr mphasizd in this study. Ths functions wr gnrally simplr and lss nrgtic than thos obsrvd in othr cathods prviously. Simulation with combind PIC and DSMC mthods has producd rsults in xcllnt agrmnt with th masurd ion nrgy distribution functions undr a varity of conditions, using only lctron tmpratur as an adjustabl paramtr. Significant discrpancis appar in th comparisons of angular distributions of ion mission, and in th potntial and lctron tmpratur gradints in th rstrictd plum rgion that was invstigatd. Bcaus rtarding potntial profils xhibit littl dpndnc on charg stat and lctron tmpratur has bn th most important paramtr in rproducing th T5 and T6 ion nrgy distribution functions, it is likly that a potntial hill acclration mchanism prdominantly causs ion acclration. Th influnc of othr paramtrs, particularly th ion-lctron collision frquncy in T5 simulations, may indicat that th mchanism is not purly potntial hill. Th prsnt study did not xplor th vry high ion nrgy rgim that has bn obsrvd for spcific cathods whn mountd in simulatd ion thrustr discharg chambrs, a mor complx situation. Ths rsults carry implications for vacuum arc plasmas, whr similar nrgy distributions ar obtaind from a discharg of about th sam!v, whr dbat has prsistd for dcads concrning th ion acclration mchanism. Tabl 1. Slctd oprating point data. Oprating Point Kpr Currnt, I ck Kpr Voltag, V ck Flow Rat, F (mg/s) Tst Chambr Prssur (Torr) " " "

11 Acknowldgmnts Th prparation of this manuscript was supportd by Th Arospac Corporation through its IRAD Program. IDB gratfully acknowldgs Dr. Matt Domonkos of NASA Glnn Rsarch Cntr for us of his hollow cathod analysis cods. Th hollow cathod was providd by D.G. Farn and N.C. Wallac of th Dfnc Evaluation and Rsarch Agncy (now Qintic). Rfrncs 1. Kamyama, I., and Wilbur, P.J., "Znith-Angl Distributions of Erosion Rats nar High-Currnt Hollow Cathods," AIAA Papr , July Latham, P.M., Parc, A.J., and Bond, R.A., "Erosion Procsss in th UK-25 Ion Thrustr," IEPC Papr , Oct Fridly, V.J., and Wilbur, P.J., "High Currnt Hollow Cathod Phnomna," J. Propulsion and Powr, Vol. 8 (3), 1992, pp Kamyama, I., and Wilbur, P.J., Masurmnts of Ions from High-Currnt Hollow Cathods Using Elctrostatic Enrgy Analyzr, J. Propulsion and Powr, Vol. 16, No. 3, 2000, pp Crofton, M.W., Prliminary Mass Spctroscopy of a Xnon Hollow Cathod, J. Propulsion and Powr, Vol. 16, No. 1, 2000, pp Kamyama, I., and Wilbur, P., Potntial-Hill Modl of High- Enrgy Ion Production nar High-Currnt Hollow Cathods, ISTS Papr 98-a-2-17, 21st Intrnational Symposium on Spac Tchnology and Scinc, Omiya, May Pattrson, S.W., and Farn, D.G., Th Gnration of High Enrgy Ions in Hollow Cathod Dischargs, IEPC Papr , Oct Yushkov, G.Y., Andrs, A., Oks, E.M., and Brown, I.G., Ion Vlocitis in Vacuum Arc Plasmas, J. Applid Physics, Vol. 88, No. 10, 2000, pp Wickrt, C., Th Expansion of th Cathod Spot Plasma in Vacuum Arc Dischargs, Physics of Fluids, Vol. 30, No. 6, 1987, pp Williams, G.J. Jr., Smith, T.B., Domonkos, M.T., Gallimor, A.D., and Drak, R.P., Lasr Inducd Fluorscnc Charactrization of Ions Emittd from Hollow Cathods, IEEE Transactions on Plasma Scinc, Vol. 28, No. 5, 2000, pp Crofton, M.W., and Boyd, I.D., Plum Masurmnt and Modling Rsults for a Xnon Hollow Cathod, AIAA Papr , July Boyd, I.D., and Crofton, M.W., Modling th Plasma Plum of a Hollow Cathod, to b publishd in J. Appl. Phys., Crofton, M.W., Evaluation of th Unitd Kingdom Ion Thrustr, J. Spaccraft and Rockts, Vol. 33, No. 5, 1996, pp , and rfrncs thrin. 14. Simpson, H.B., Wallac, N.C., Farn, D.G., and Klly, M.K., A Summary of th Qintiq Hollow Cathod Dvlopmnt Programm in Support of Europan High Powr Hall Effct and Griddd Thrustrs, IEPC Papr , March Farnll, C.C., Williams, J.D., and Wilbur, P.J., Charactristics of Enrgtic Ions Emittd from Hollow Cathods, IEPC Papr , March Parks, D. E., Mandll, M. J., and Katz, I., "Fluid Modl of Plasma Outsid a Hollow Cathod Nutralizr," Journal of Spaccraft and Rockts, Vol. 19, 1982, pp Williams, J. D. and Wilbur, P. J., "Elctron Emission from a Hollow Cathod-Basd Plasma Contactor," Journal of Spaccraft and Rockts, Vol. 29, 1992, pp Birdsall, C. K. and Langdon, A. B., Plasma Physics Via Computr Simulation, Adam Hilgr Prss, Bird, G. A., Molcular Gas Dynamics and th Dirct Simulation of Gas Flows, Oxford Univrsity Prss, Mitchnr, M. and Krugr, C. H., Partially Ionizd Gass, Wily, Nw York, Domonkos, M. T., "Evaluation of Low-Currnt Orificd Hollow Cathods," Doctoral Thsis, Dpartmnt of Arospac Enginring, Univrsity of Michigan, Sptmbr Crawford, F.T. and Gabril, S.B., Numrical Simulation of th Hollow Cathod Plasma using a PIC-DSMC Cod, IEPC Papr 03-27, March Millr, D.R., Fr Jt Sourcs, Atomic and Molcular Bam Mthods, ditd by G. Scols, Vol. 1, Oxford, Nw York, 1988, pp Von Engl, A., Ionizd Gass, Oxford, London, Davis, W.D., and Millr, H.C., Analysis of th Elctrod Products Emittd by dc Arcs in a Vacuum Ambint, Journal of Applid Physics, Vol. 40, No. 5, 1969, pp Tsuruta, K., Skiya, K., and Watanab, G., Vlocitis of Coppr and Silvr Ions Gnratd from an Impuls Vacuum Arc, IEEE Transactions on Plasma Scinc, Vol. 25, No. 4, 1997, pp