AN IMPROVED SMALL CLOSED DRIFT THRUSTER WITH BOTH CONDUCTING AND DIELECT RIC CHANNELS A.I.Bugrov, A.D.Desitskov, H.R.Kufmn, V.K.Khrchevnikov, A.I.Morozov c, V.V.Zhurin d Moscow Institute of Rdioelectronics, Moscow, Russi Kufmn & Roinson Inc., Fort Collins, CO, U.S.A. c Kurchtov_s Russin Reserch Center, Moscow, Russi d Front Rnge Fkel Inc., Fort Collins, CO, U.S.A. Astrct An improved sttionry plsm thruster (SPT) ws developed for opertion t power level of 00-60 W. The thruster ws tested with oth conducting (grphite) nd dielectric (orosil) dischrge chnnels. Integrl, or overll, chrcteristics were otined with oth chnnel mterils using Ar nd Xe s the working gs. Ioniztion procсesses tke plce with the sme intensity with oth chnnel mterils. However, the dischrge current t otherwise similr operting conditions is higher y out (20 30)% with the conducting chnnel compred with the dielectric one. This phenomenon is explined in the pper. Introduction Sttionry plsm sources sed on the sttionry plsm thruster (SPT) re widely used oth in spce propulsion nd technologicl pplictions. In recent yers, there hve een requirements in oth pplictions for smll, light plsm sources tht cn operte t power of out 00 50 W with low levels of erosion. An originl pproch sed on the SPT [] ws used for the ion source. The generl design pproch used for this ion source ws descried previously [2]. The improved version descried herein uses n improved mgnetic field tht results in reduced ion-em cosine losses nd reduced erosion of the dischrge chnnel. Both conducting (grphite) nd dielectric (orosil) dischrge chnnels were used in this source. The source is shown in Fig.. Its dimensions re: - externl dimeter is 55 mm; - length is 34 mm; - inside dimeter of the chnnel t the exit is 24 mm; - chnnel width t the exit is 4 mm; - chnnel length, node to exit, is 4 mm.
2 Fig.. - Smll ion source: - dischrge chmer wlls; 2 - node; 3 - mgnetic coil; 4 - mgnetic pth; 5 - insultor. - Mgnetic field distriution, Hr, in Guss t the men rdius of the dischrge chnnel. The cthode-neutrlizer for most of the tests ws hot filment (HF)( = 0 mk! ) mde of tungsten wire with dimeter of 0.28 mm. The HF ws locted 35 mm from the source exit plne. The mgnetic field distriution is shown in Fig.. The testing ws crried out in vcuum chmer with diffusion-pump cpcity of 2x0 3 l/s. The cylindricl chmer hs length of 2.5 m nd dimeter of 0.8 m. A hollow cthode (HC) ws used for thruster performnce nd endurnce tests. The xenon flow through the HC ws 0. mg/s during those tests.the vcuum chmer hd thrust scle with lser motion indictor. The thrust error ws less thn 3%. The chmer hs proe ctutor tht permits em profiles to e otined t vrious distnces from the exit plne. The plsm flow prmeters such s directed ion current nd ion flow divergence were otined with proe hving two collecting surfces oriented norml to ech other nd with the re of ech surfce equl to 0.28 mm 2. One surfce of the proe ws norml to the plsm flow to mesure the directed ion current. The second surfce of the proe ws oriented prllel to flow. The vlue of the ion current to the second surfce chrcterized the chotic current in the vcuum chmer nd ws utilized in the clcultion of flow divergence. The directed energy of the ions ws mesured y seprte three-electrode, retrding-potentil proe with n externl dimeter of 6 mm. Integrl chrcteristics of the sources Conducting chnnel, rgon working gs. In this test series the dischrge chnnel ws mde of grphite, with the chnnel electriclly isolted from the node. The potentil of the chnnel during opertion ws close to tht of the cthode. The tests showed tht the source cn operte without overheting the dischrge chnnel t dischrge voltge, U d 400 V nd rgon mss flows, m! =(0.42 0.7)mg/s (4-23.6 sccm). The mximum dischrge power, W, ws 70 80 W. Incresing the dischrge voltge up to 600 V t the sme pplied power resulted in the decrese of mss flow to (0.29 0.40)mg/s (9.8-3.5 sccm). The integrl plsm flow chrcteristics re given in Figs. 2 nd 3. The volt-mpere chrcteristics in Fig. 2 re verticl, showing generlly efficient opertion nd good utiliztion of the working gs.
3 360 2 360 2 320 280 3 4 320 280 3 4 U, В 240 U, В 240 200 200 60 60 20 0,4 0,6 0,8 I, A 20 0,4 0,6 0,8 I, A Fig. 2. Volt-mpere chrcteristics of the dischrge with rgon s the working gs: - dielectric chnnel ( - m! = 0.36 mg/s, 2 - m! = 0.44 mg/s, 3 - m! = 0.55 mg/s, 4 - m! = 0.63 mg/s); - conducting chnnel ( - m! = 0.42 mg/s, 2 - m! = 0.5 mg/s, 3 - m! = 0.63 mg/s, 4 - m! = 0.7 mg/s). HF cthode; vcuum chmer pressure, P, ws *0-4 Torr. Dielectric chnnel, rgon working gs. The integrl chrcteristics of the source with the orosil chnnel were otined t dischrge voltges, U d =(60 320)V, rgon mss flows of 0.36 0.63 mg/s nd W < 60 W. These chrcteristics re shown in Figs. 2 nd 3. The volt-mpere chrcteristics re gin verticl. From the ion em profiles in Fig. 3, the mximum ion current density is decresed nd the divergence is incresed for the orosil chnnel, s compred to tht for the grphite chnnel. Fig. 3. Ion em profiles with rgon working gs: - with dielectric chnnel nd- m! = 0.63 mg/s; - with conducting chnnel nd- m! = 0.7 mg/s. Both with HF cthode; P = *0-4 Torr, U d = 200 V, z = 5 cm. Detiled performnce prmeters. A vriety of performnce prmeters re shown in Tle for oth shnnel mterils nd oth working gses. The est comprisons etween conducting nd dielectric chnnels re shown for xenon,
4 where most of the prmeters re nerly the sme. The most significnt difference is in dischrge current, I d, where tht current is sustntilly lrger for the conducting chnnel. Tle Working gs Ar Ar Xe Xe Chnnel C BNC C BNC m!, мг/с 0.29 0.63 0.35 0.3 U d 600 200 200 200 J d 0.3 0.77 0.4 0.32 J i 0.27 0.69 0.24 0.2 J i /J d 0.87 0.896 0.58 0.62 J i /J m 0.39 0.46 0.92 0.90 J p /J m! 0.44 0.5.58.45 The energy distriutions of the em ions using the two chnnel mterils re shown in Fig. 4. The mostprole energy is out 0% lower for the conducting chnnel. 0,8 0,8 f, отн.ед. 0,6 0,4 f, отн.ед. 0,6 0,4 0,2 0,2 0 0 50 00 50 200 eu, эv 0 0 50 00 50 200 eu, эv Fig. 4. Energy distriutions of the em ions with rgon s the working gs: - dielectric chnnel, m! = 0.63 mg/s; - conducting chnnel, m! = 0.49 mg/s. Both with HF cthode-neutrlizer t vcuum chmer pressure, P, of *0-4 Torr, dischrge voltge, U d, of 200 V, nd t 5 cm from the thruster exit. Lifetime tests Life tests with durtions of 00 h were crried out using oth grphite nd orosil chnnels. After ech two hours of opertion, the source ws turned off for 5 0 minutes, fter which it ws restrted. While operting, the operting prmeters were held constnt t the vlues given in Tle 2. Tle 2 Chnnel U d J p m! BNC 200 0.77 0.63 C 200 0.79 0.7 The lifetime of the HF cthode-neutrlizer ws 9 2 hours. It ws replced when the vcuum chmer ws open. The ion source ws not clened during life test.the chnnel_s dimensions nd mss were checked efore nd fter the 00 h tests. The following results were otined during these tests:
Externl Chnnel Mteril Width of erosion Men erosion rte, Mss, g nd,mm mm/h BNC 7 0.5 0,057 C 6.2 0,0 Internl Chnnel Mteril Width of erosion Men erosion rte, Mss, g nd,mm mm/h BNC 5 6,7 0,06 C 9,6 0,50 5 Compring the experimentl results with dielectric nd grphite chnnels, the following conclusions cn e drwn: - sources with oth chnnel mterils operted stly nd relily in the selected rnges of dischrge voltge nd node mss flow t vcuum-chmer pressure of *0-4 Torr; - the mximum dischrge power without overheting source components ws slightly lower with the dielectric chnnel (60 W) thn for the grphite chnnel (70 80 W); - the ion current density on the source xis ws 0 20% higher with the dielectric chnnel thn with the grphite chnnel for the sme dischrge current; - the mximum divergence hlf-ngle with the dielectric chnnel is slightly less (/2 38 40 ) thn for the grphite chnnel (/2 40 42 ); - the men energy of em ions ws out 0 5% higher with the dielectric chnnel thn it ws for the grphite chnnel;- the men erosion rte of the externl dielectric chnnel wll is sustntilly less thn for tht of the grphite chnnel (0.2 versus.2 mm/h). Using the erosion rtes mesured in the 00 hr tests, lifetime of out 000 hr cn e projected for either orosil or grphite chnnel. However, mesured erosion rtes usully decrese sustntilly with operting time, so tht this liner projection cn e excessively conservtive. The erosion rtes of the inner nd outer wlls re pproximtely the sme for the grphite chnnel, ut sustntilly different with the orosil chnnel. This difference is elieved due to grphite eing n electricl conductor. Becuse the grphite is conductor, its potentil must e the sme over the entire chnnel length. The secondry electron emission t electron energies of out 20 ev is sent. There is lso no current trnsmission due to the ner-wll conductivity. As shown experimentlly, the potentil of the grphite chnnel is close to cthode potentil. Such potentil ttrcts ions nd destroys the ccelertion of ions long most of the chnnel length. Due to the symmetry of these processes in the rdil direction, the erosion must e out the sme for the externl nd internl wlls, nd the ion current density upstrem of the exit plne must e low. The sitution is different in the dielectric chnnel. The wll potentil vries long the length of the chnnel. At sufficiently high electron temperture, secondry electrons re emitted from the externl insultor, nd the nomlous Deye lyers ppers [3]. The externl wll is chrged negtively nd repulses ions towrd the internl insultor. Due to the orienttion of the ditic invrint [4], ner-wll conductivity doesn_t tke plce t the inner wll, the Deye lyer is the clssic one, nd there is symmetry to the oserved flow over most of the flow cross section. Due to the ove resons, erosion on the externl nd internl wlls of grphite chnnel will e prcticlly the sme. Erosion of the inner nd outer wlls of dielectric chnnel, however, will e quite different. As nother result, there will e difference in the ion em profiles for grphite nd dielectric chnnels, nd corresponding difference in thruster chrcteristics. Thruster chrcteristics of the sources with xenon. Dielectric chnnel.
6 Fig. 5. Ion em profiles with xenon s the working gs: - dielectric chnnel, m = 0.54 mg/s; - conducting chnnel, m = 0.7 mg/s. Both with HF cthode-neutrlizer t vcuum chmer pressure, P, of x0-4 Torr, dischrge voltge, U d, of 200 V, nd t 5 cm from the thruster exit. At n pplied power, W< 250 W the source opertes stly t xenon mss flows of 0.3-0.5 mg/s (3.- 5. sccm). Tests were crried out with hollow cthode (HC) s the cthode-neutrlizer nd with xenon mss flow through the HC equl to 0. mg/s. The neutrlizer mss flow ws included in the clcultions of thruster efficiency nd specific impulse. The ion current profile nd the thruster chrcteristics for this comintion re given in Figs. 5 nd 6. The men divergence hlf-ngle ws equl to 27.
7 250 200 0,8 W, Вт 50 F, г 0,6 00 0,4 50 00 200 300 400 500 0,2 00 200 300 400 500 2000 45 F/m, с 600 200 Эффективность, % 40 35 30 25 800 00 200 300 400 c 20 00 200 300 400 500 Fig. 6. Integrl chrcteristics of the source with dielectric chnnel operting on xenon: - dischrge power; - thrust; c - specific impulse; d - thruster efficiency, ll s functions of pplied voltge ( - m = 0.3 mg/s, - m = 0.4 mg/s, - m = 0.5 mg/s). By compring Fig. 5 with Fig. 3, one cn see tht the solute vlue of ion current density, ji, is shrply incresed with xenon in comprison with the similr result for rgon. The volt-mpere chrcteristics (not shown) were found to e lmost verticl. The thruster efficiency with dielectric chnnel hs mximum vlue of 39% t mss flow of 0.5 mg/s nd dischrge voltge of 350 V. d
Conducting chnnel. 8 250 200 0,8 W, Вт 50 F, г 0,6 00 0,4 50 00 200 300 400 0,2 00 200 300 400 35 F/m, с 400 000 600 00 200 300 400 Эффективность, % 30 25 20 5 0 00 200 300 400 c d Fig. 7. Integrl chrcteristics of the source with conducting chnnel operting on xenon: - dischrge power; - thrust; c - specific impulse; d - thruster efficiency, ll s functions of pplied voltge ( - m! = 0.35 mg/s, - m! = 0.5 mg/s, - m! = 0.7 mg/s). In this cse, the xenon mss flow through node ws equl to 0.35 0.7 mg/s for the rnge of dischrge voltges equl to 50 350 V. These tests were lso crried out with HC cthode-neutrlizer with xenon flow of 0. mg/s. The ion current profile nd the thruster chrcteristics re shown in Fig. 5 nd Fig. 7. The volt-mpere chrcteristics (not shown) were gin found to e lmost verticl.the thruster efficiency with grphite chnnel hs mximum vlue of 28% t mss flow of 0.7 mg/s nd dischrge voltge of 300 V. Conclusions An improved smll thruster hs een operted with oth conducting nd dielectric dischrge chnnels. On the sis of 00 hr tests, conservtive lifetime of t lest 000 hr is projected for oth chnnel mterils. For the dielectric chnnel, the thruster efficiency ws s high s 39% t specific impulse of out 800 sec. The thruster efficiency with the conducting chnnel ws more thn 0% less.the decresed efficiency with conducting chnnel ws ttriuted to incresed wll losses, y mechnisms descried in the pper.the superior performnce with dielectric chnnel confirms once more tht the excellent performnce of the SPT with dielectric chnnel wlls is dependent on the dielectric properties of those wlls.
9 References. Bugrov, A.D. Desitskov, H.R. Kufmn, V.K. Khrchevnikov, A.I. Morozov, nd V.V. Zhurin, U.S. Ptent 6,456,0, Sept. 2002. Other ptents pending 2. A.I Bugrov, A.D. Desitskov, H.R. Kufmn, V.K. Khrchevnikov, A.I. Morozov, nd V.V. Zhurin, IEPC-02-344, Psden, CA, Oct. 4-9, 200. 3. Reference for nomlous Deye lyer. 4. Reference for the ditic invrint.