Discharge Modes and Characteristics of Hollow Cathode

Transcription

1 Discharge Modes and Characteristics of Hollow Cathode EPC-7-5 Presented at the 3 th nternational Electric Propulsion Conference, Florence, taly Shunsuke Sakai *, Toshiyuki Katayama, Junichiro oyagi, and Haruki Takegahara Tokyo Metropolitan University, Tokyo, 9-5, Japan bstract: t is known that several modes exist in hollow cathode operation, e.g. the plume mode, spot mode and diffuse mode which is discovered and researched in TMU study. n the diffuse mode, the plasma forms intense luminescence in not only the volume between electrodes but also the whole region in space chamber. The characteristics of discharge modes are examined in this paper including the frequency oscillatory behavior. The current oscillation of frequency range from tens of khz to several MHz is confirmed to the anode and keeper discharge. t is suggested that anode and keeper current frequencies increase at the transition range of the plume mode to the diffuse mode. h f Cha R m& V V K = half amplitude of oscillation = frequency of oscillation = anode current = chamber current = keeper current = return current = propellant flow rate = anode voltage = keeper voltage Nomenclature. ntroduction RETY of electric propulsion systems are studied now, and they are applied as the main propulsion or the V auxiliary propulsion for attitude/orbit control in satellites. The hollow cathode is indispensable to emit electrons for electric propulsion systems such as electron bombardment ion thrusters and hall thrusters because of ionization of neutral particles or neutralization of ion beam to space potential. However, existence of several discharge modes has left physical process of discharge unknown. Plasma parameters, internal xenon pressure, cathode temperature and electromagnetic interference (EM) has been researched before now -3. Patterson and Fearn suggested the influence of the many different parameters on the noise and oscillatory behavior of anode and keeper voltage. Nishiyama and Kuninaka suggested keeper current noise and oscillations of microwave cathode and hollow cathode as a part of EM measurements 5. However, current noise and oscillations of hollow cathode in wide operation range and the diffuse mode, revealed in our libratory, have not been measured. * Graduate Student, Dept. erospace Engineering, sakai@astak3.tmit.ac.jp Graduate Student, Dept. erospace Engineering ssistant Professor, Dept. erospace Engineering, j-aoyagi@astak3.tmit.ac.jp Professor, Dept. erospace Engineering, hal@astak3.tmit.ac.jp The 3 th nternational Electric Propulsion Conference, Florence, taly

2 This paper describes the noise and oscillations of anode, keeper, and return current at the diode configurations of the hollow cathode. Relativity of frequency and discharge mode is suggested.. Hollow Cathode and Discharge Modes. Hollow Cathode Hollow cathode has coaxial multiplex structure, and the operating gas is fed to the internal volume. Then the gas collides with electrons emitted from the insert. dditionally, hollow cathode is the electron source of indirectly heated type. Therefore it is very advantageous in respect of a life time compared with the electron source of direct heated type such as filament cathode. ccordingly, the hollow cathode is indispensable as electron source for spacecraft which needs the operation without maintenance situation for a long time. nsulator Keeper Foil typical hollow cathode structure is illustrated Tip in Fig.. The hollow cathode tube was closed at the nsert e downstream end by a tip having a central orifice of Xe Gas Flow Xe e e + e diameter.5 mm. cylindrical porous tungsten Xe Xe e + e e dispenser (nsert) is located within the tube which Xe + touches the tip on its downstream end. t is Tube e impregnated with a low work-function material, such as barium oxide. The cathode tube is surrounded with a tantalic heater winding. These are Heater insulated with a ceramic material completely, Figure. Schematic diagram of hollow cathode. though this heater winding is fitted. B. Discharge Modes The physical process in the hollow cathode discharge is very complicated, and is not clarified completely yet. s the complicated factor, the existence of several discharge modes are known, for example the plume and spot mode, which differ greatly in characteristics as shown in Fig. 7. These modes depend critically on the anode current and the operating propellant flow rate. The characteristics of the plume mode are to indicate high anode voltage at a low anode current. Then the discharge is very luminous between the cathode and the anode (Figs. and 3 - (a)). Note that the experimental configuration is shown in Fig. 3 - (d). On the other hand, the anode voltage in the spot mode is constant at low even to extent a large current, and there is no luminosity (Figs. and 3 - (b)). n addition, it was revealed in TMU experimental configuration that discharge characteristics in the area of the propellant flow rate of 3 sccm are greatly different from the other operating area as shown in Fig. 7. s the anode (sccm.9mg/sec) node Voltage, V (a) (b) Tip Orifice (c) (d) Configuration node Current, Figure. llustration of discharge modes 7. Figure 3. ppearance of discharge modes 7. The 3 th nternational Electric Propulsion Conference, Florence, taly

3 current increases, the point where the anode voltage rapidly increases is appeared. When the anode current increases further, the luminescence phenomenon by discharge is observed not only between the cathode and the anode but also into the whole vacuum chamber as shown in Fig. 3 - (c). This mode, named diffuse mode at TMU, will be one of the discharge mode of the hollow cathode. n this mode, the emission current from the hollow cathode does not correspond to the anode current. n this paper, the diffuse mode was treated as one of the discharge mode of hollow cathode.. Experimental Facilities and Procedures Experimental schematics are shown in Fig.. The hollow cathode used in this study is made by Vecco-ontech Co., Ltd, HCN5 (orifice diameter:.5 mm). The cathode was mounted within a cryo-pumped vacuum chamber of. m diameter and 3.3 m length. The ultimate vacuum was about Pa (5. -7 Torr). The stainless steel anode plate of 3 mm square was similarly mounted 5 mm downstream from the keeper with the same axis. Power supplies were used for keeper and anode discharge, made by Stabilizer Co., Ltd, NSTC-NT7 (~7V, ~, keeper), NH (~V, ~, anode). ll power supplies were operated with constantcurrent regulation. Three current probes (DC~MHz) were used for measurement of keeper current, anode current, and return current R. These current waveforms were concurrently displayed on a digital oscilloscope (sampling rate of MHz). n addition, current 5mm Cha, which flows through a vacuum chamber, was Hollow Cathode Space Chamber node measured by current probe in the diffuse mode, because Cha flows only in the diffuse mode. The anode voltage and each current waveform Cha Keeper against the anode current were measured. Operating conditions are following: R Oscilloscope Current Probe Xenon flow rate m&.sccm~sccm node current.5~ V V V K V Keeper current 3m (constant) Cathode-node distance 5mm Orifice Tip-keeper distance.5mm Frequency spectrums of each current oscillation Figure. Experimental schematics. were taken by software, Origin ver.5. V. Results and Discussions.sccm sccm sccm Figure 5 shows discharge modes in this study. 3sccm 5sccm sccm sccm sccm t the high current regions of sccm and 3sccm, the diffuse mode was confirmed. Luminous phenomenon, current waveforms, and frequency spectrums in the 3sccm ((a)-(a) in Fig. 5) are shown in Fig.. s the anode current increased, (a3) (a) luminescence was intense, current amplitude was large and frequency was high. nd typical (a) (b) (a) frequency of anode and keeper current was approximately corresponding. t is highly possible (b3) (b) that the cause of anode and keeper current (b) oscillations was same. n addition, typical frequency as a function of anode current is shown Fig Frequency increased especially at the transition node Current, range of plume mode to diffuse mode. There are several theories about the cause of this oscillation Figure 5. Discharge modes in this study. phenomenon e.g. current-driven ion acoustic 3 The 3 th nternational Electric Propulsion Conference, Florence, taly node Voltage, V (sccm.9mg/sec)

4 =., Fig. 5-(a) =5., Fig. 5-(a) - Current, Half mplitude, Half mplitude, 5 =., Plume to Diffuse Fig. 5-(a3). =., Fig. 5-(a) 5 5 Figure. Characteristics of plume mode to diffuse mode in 3sccm. instability 5,. lmost theories pointed out the proportional relationship between frequency and the square root of ion temperature, and the inverse relationship between frequency and plasma column radius or length. n the 3sccm flow rate, it is thought that ion temperature increased in the diffuse mode because frequency increased and plasma column radius or length was not small clearly compared with the plume mode (Fig. Luminous Phenomenon). Figure shows characteristics of transition of plume mode to spot mode at the sccm ((b)-(b) in Fig. 5). Luminous region was gradually slight as the anode current increased. t the point of Fig. 5 - (b) and (b3), vibration of luminescence was observed. node current waveforms changed greatly at the point of Fig. 5 - (b) to (b). There was large amplitude and low frequency (about.7~. khz) oscillation of anode current waveforms at the point of Fig. 5 - (b) and (b3) (this range frequency spectrum is not listed). t is difficult to evaluate its low frequency oscillation, because there is low frequency region in power source noise bandwidth. Typical frequency of anode and keeper current was corresponding such as Fig.. But there was no change of the typical frequency against the increase of the anode current compared with Fig.. t is thought that ion temperature does not increased, or ion temperature decreased with shrinking of plasma radius..k 5.k.k 5.k.k.k 5.k.k 5.k.k Current Frequency Spectrum Keeper Current Frequency Spectrum Luminous Phenomenon Current Waveforms node Current Frequency Spectrum Keeper Current Frequency Spectrum Frequency, khz node Current, Figure 7. Frequency as a function of anode current. The 3 th nternational Electric Propulsion Conference, Florence, taly 3sccm, = 3m

5 =3., Fig. 5-(b) =., Plume to Spot Fig. 5-(b) - Current, 3 =5., Plume to Spot Fig. 5-(b3) 5. =., Fig. 5-(b) Figure. Characteristics of plume mode to spot mode in sccm. Figure 9 shows frequency as a function of propellant flow rate at the =.. The frequency decreased as the propellant flow rate grew. t is thought that the propellant gas between keeper and anode plate increased as propellant flow rate grew. So energy of ion transferred to neutral particle, and ion temperature decreased. s a result, the frequency decreased. But there were several typical frequencies in some regions. t is difficult to estimate the frequency and ion temperature as one to one correspondence. n the diffuse mode, it was suggested that the measured current was out of balance. Fundamentally, anode current and keeper current are equal to return current (Eq. ()). = + R K () But in the diffuse mode, this equivalent was not equality. Figure shows the anode, keeper, return, and anode plus keeper current waveforms in the plume and diffuse mode. n the plume mode, Eq.() was equality, on the other hand, in the diffuse mode, Eq.() was not equality. n the diffuse mode, current of flow to vacuum chamber Cha was measured. Figure shows anode, keeper, and the chamber current waveforms and frequency spectrum. Frequency of chamber current oscillation was slightly different from anode and keeper. t is possible that vacuum pump or other facilities influenced the chamber current Half mplitude, k 5.k.k 5.k.k.k 5.k.k 5.k.k node Luminous Phenom enon Current Frequency Spectrum Keeper Current Frequency Spectrum Current Waveforms Frequency, khz Propellant Flow Rate, sccm Figure 9. Frequency as a function of propellant flow rate. 5 The 3 th nternational Electric Propulsion Conference, Florence, taly Half mplitude, =., = 3m

6 Current, 3 3sccm, =., =, R =.3 3sccm, =., =, R =7. K R Figure. Current waveforms in plume mode and diffuse mode. Current, K R Current, Cha 3sccm, =., Cha =.7, Cha.7 Half mplitude, k.k.k.k. k Current Waveform Frequency Spectrum Cha Figure. Current waveforms including Cha in diffuse mode Figure. node current frequency as a function of anode current and propellant flow rate..sccm sccm sccm 3sccm sccm 5sccm sccm 7sccm sccm 9sccm sccm Note that chamber current measured in this study was a part of all current which flows to vacuum chamber. t is impossible to measure all current flowed to vacuum chamber at present. t is necessary to enclose the hollow cathode with something such as ring probe 7. Figure shows 3D map of the frequency as a function of propellant flow rate and anode current. This map can describe several frequencies at one measuring point. n the low propellant, there are high frequency region which includes diffuse mode. nd frequency is low as propellant flow rate grows at the almost all of anode current line. Frequency of entire operating region of the hollow cathode is understood at a glance. V. Conclusions This paper reported the results of the experimental characterization of the hollow cathode. t has been shown that the current waveforms and frequency spectrum of the plume to spot mode and diffuse mode were affected by the propellant flow rate and the anode current. n the transition range of the plume to diffuse mode, frequency increased The 3 th nternational Electric Propulsion Conference, Florence, taly

7 as an anode current grew. But in the transition range of plume to spot mode, increase of frequency did not occur. t is possible that ion temperature influences the current oscillation phenomenon. 3D frequency map as a function of anode current and propellant flow rate is shown at entire operation of the hollow cathode. This map suggests the general representation of frequency of hollow cathode at diode configuration. Futures tasks are following;. Measuring current oscillation in high frequency range of several MHz. (n this paper, only low frequency range of tens khz to hundreds khz were measured.). Measuring current in different configuration such as an additional base plate or discharge chamber. (nfluence of alteration of configuration on discharge mode and frequency oscillatory behavior will be measured.) 3. Measuring ion temperature. (t is highly possible that there is the proportional relationship between frequency and the square root of ion temperature. nd frequency in transition region of the plume mode to diffuse mode increased as anode current grew. Thus, change of ion temperature in transition region of the plume mode to the diffuse mode will be measured.) ccording above experiments and this study, relationship between discharge mode and frequency oscillatory behavior, one of the physical processes of discharge, will be explained. n addition, discharge mode of hollow cathode used in ion thruster may be made known. References Shlhi,., Myers, R. M., and Turchi, P. J., Experimental nvestigation of a Hollow Cathode Discharge, 3 rd EPC-93-5, 3- Sep Jongeward, G., Mandell, M. J., and Katz,., Conductive Nature of Low Frequency (f<mhz) Electromagnetic Fields Generated by a Hollow Cathode Plasma Contactor, 3 th Joint Propulsion Conference, , ndianapolis, N, June 7-9, Polk, J., and Marrese, C., Temperature Distributions in Hollow Cathode Emitters, th Joint Propulsion Conference and Exhibit, --, Fort Lauderdale, Florida, July -,. Patterson, S. W., Malik,. K., and Fearn, D. G., Noise and Oscillatory Disturbances in the T on thruster Hollow Cathode, 35 th Joint Propulsion Conference and Exhibit, , Los ngeles, California, June -, Nishiyama, K., Shimizu, H., and Funaki, K., EM of Microwave Discharge Neutralizer and Hollow Cathode, bstracts of SS Space Transport Symposium (in Japan),, pp (in Japanese) Kuriki, K., and rakawa, Y., ntroduction to Electric Propulsion, University of Tokyo Press, 3. (in Japanese) 7 Matsuyama, J., Oniki, T., and Takegahara, H., nfluence of Operating Conditions on Hollow Cathode Discharge Modes, th nternational Symposium on Space Technology and Science, STS--b-7, Miyazaki, Japan, May 3-June,. Yoshida, H., Four-step Double Sheath Model of a Neutralizer Hollow Cathode, th Space Sciences and Technology Conference, -G, Fukui, Japan, Nov -,. (in Japanese) 7 The 3 th nternational Electric Propulsion Conference, Florence, taly