S2-4
Radio Frequency Plasma Cathode for High-Power Ion and Hall Thrusters
大電力イオンエンジンおよびホールスラスタ用高周波プラズマカソード	 
Hiroki Watanabe, Natsumi Torii, Satoshi Kasagami and Haruki Takegahara
渡邊裕樹, 鳥井夏実，笠上聡志，竹ヶ原春貴
Tokyo Metropolitan University
6-6, Asahigaoka, Hino, Tokyo 191-0065, Japan
首都大学東京 〒191-0065	  東京都日野市旭が丘6-6
Radio frequency (RF) plasma cathode has been developed as an electron source for ion and Hall thrusters
in place of a hollow cathode. The electron emission characteristics of the RF plasma cathode wereexperimentally evaluated in the range of 10 to 400 W of RF power. It is confirmed that the electron
emission current of 7 A was achieved at the RF power of 400 W with 13.56 MHz and the xenon mass flowrate of 0.2 mg/s. The effect of frequency of RF generator on the current is small whereas the inner diameterof the discharge vessel strongly affects the current. The efficiency of the 2-cm RF cathode is higher thanthat of the 4-cm RF cathode, because ion loss in the vessel wall decreases as the inner diameter decreases.

1. Introduction
There is an interest in using high-power ion andHall thrusters to reduce launch vehicle size and tofacilitate in-space transportation of large masses insupport of the human space exploration, because of
their high specific impulse. The next generation
thrusters are being developed for the in-space
propulsion in the range 5 to 100 kW with lifetime inexcess of 10,000 hours in U.S.A. and European
countries [1]. An electron source called “cathode”,
which is used to ionize the propellant and toneutralize ion beam leaving the thrusters, isimportant component for the thrusters, because its
performance militates against the thruster efficiency
and the lifetime of thruster system. An electroncurrent emitted from the cathode increases as thethruster’s input power increases. Therefore, thedevelopment of high current cathode with lifetimein excess of 10,000 hours is necessary for thedevelopment of high-power ion and Hall thruster.
A hollow cathode is regularly employed as theelectron source for ion and Hall thrusters. The
hollow cathode uses a porous tungsten insert that isimpregnated with an emissive mix of barium andcalcium oxides, and alumina for thermionicemission. To prevent an insert poisoning, which canshorten the lifetime of the hollow cathode, contact
between the insert and the active gas should beavoided, and the type of operation gas entering thecathode, which is commonly shared with thethruster propellant, is restricted. Additionally, thehollow cathode lifetime is considered to berestricted by oxide depletion from the insert. As aresult, ion and Hall thrusters with the hollow
cathodes have some difficulties in long-term
operation, and they should be controlled strictlyfrom a prelaunch to an end of life.
Thus, in order to liberate ion and Hall thrustersfrom the limitations of hollow cathode, a radio
frequency (RF) plasma cathode was studied as theelectron source for the thrusters. In previous study,
the RF plasma cathode (RF/C) achieved an electronemission current of 2.6 A [2]. The current value is
not enough for high-power ion and Hall thrusters.
Therefore, the electron emission characteristics of
the RF/C in high input RF power are evaluated.
2. Experimental Apparatus and Procedure
Figure 1 shows the schematic representation of
the RF/C, which consists of a discharge vessel, aninduction coil, an ion collector and housings. Two
cathode sizes were fabricated and evaluated. One
was the 2-cm RF/C, and the other was the 4-cm
RF/C. The value shows an inner diameter of acylindrical discharge vessel. There was an orifice at
the center of end of the vessel for electron emission.
When electrons are emitted from the orifice, ionsthat are equal to them have to be collected at the ion

Fig. 1. Schematic of RF plasma cathode.

Fig. 2. Experimental Setup
collector to maintain quasi-neutrality at the plasma.
Thus, the collector, which is inserted into the vessel,
is necessary for the RF/C during steady operation.
The cylindrical tungsten collector shown in Fig. 1
was inserted into the vessel along its wall to fill theroles of ion collection, and the collector had anaxial slit for propagation of electromagnetic field.
Figure 2 shows the experimental setup. The RF/C
and an anode were set in a vacuum chamber. The
anode was located at 50 mm downstream of thecathode, and it was biased positively by a DC
power supply to extract electrons from the cathode.
On the other hand, the collector was connected toground potential. When electrons are emitted fromthe cathode by the potential difference, an electron
current is carried to the anode and an ion current iscarried to the collector: the electron current to theanode was called “anode current” in this paper. The
anode current of each RF/C was measured as afunction of the RF power and frequency.
3. Results and Discussion
Figure 3 shows the anode current at the xenon
mass flow rate of 0.2 mg/s as a function of the RFpower, cathode size and frequency of RF generator.
The anode current is proportional to the RF power
regardless of the cathode size and the frequency asshow in Fig. 3. In 4-cm RF/C, this proportional
relationship was confirmed from 10 to 400 W. The
anode current of 7 A was achieved at RF power of
400 W with 13.56 MHz, the xenon mass flow rateof 0.2 mg/s. At same RF power, the anode currentof the 2-cm RF/C is higher than that of 4-cm RF/C,
and the effect of frequency on the anode current
was small. In space applications, it is desirable forelectron sources to emit a higher electron current ata lower input power. Therefore, the 2-cm RF/C ishigher performance than the 4-cm RF/C.
When electrons are emitted from the orifice, ionsthat are equal to them have to be collected at the ioncollector. Thus, the amount of electrons emitted
from the RF/C is limited by the amount of ionscollected in the ion collector. In the RF/C, theFig. 3. Anode current at xenon mass flow rate of 0.2
mg/s as a function of RF power, cathode size andfrequency of RF generator.
internal wall of the discharge vessel is notcompletely covered by the ion collector as shown inFig. 1, because of propagation of electromagnetic
field. Thus, ions produced within the cathode is not
only collected onto the ion collector surface, butalso lost in the vessel wall. The ion loss in thevessel wall is not preferable for electron emission
from the cathode. Since the ratio of the ioncollection area to the surface area of the vesselinternal wall at the 2-cm RF/C is 1.2 times as highas that at the 4-cm RF/C. Therefore, the ion loss inthe vessel wall decreased at the 2-cm RF/C. The
reduction of ion loss in the vessel wall is one of thereasons for the high performance of the 2-cm RF/C.
4. Conclusion
The electron emission characteristics of the RF/C
were evaluated in the range of 10 to 400 W of RFpower. It is confirmed that the electron emission
current of 7 A was achieved at the RF power of 400
W with 13.56 MHz and the xenon mass flow rate of
0.2 mg/s. The effect of frequency of RF generator
on the emission current is small whereas the vessel
inner diameter strongly affects the emission current.
The efficiency of the 2-cm RF/C is higher than thatof the 4-cm RF/C, because ion loss in the vessel
wall decreases as the inner diameter decreases. This
result shows that the electron emission performanceof the RF/C is still not enough for the cathode ofhigh-power ion and Hall thrusters. Thus, more
efforts require improving the performance of theRF/C as future work.
References[1] David Manzella and Kurt Hack: Proc. 50th Joint
Propulsion Conf., 2014, AIAA 2014-3718.
[2] H. Watanabe, et al.: Trans. JSASS Aerospace Tech.
Japan, 10 (2012), pp. Pb_37-Pb_42.