S2-3高周波プラズマカソードの研究開発
Research and Development of Radio Frequency Plasma Cathodes
渡邊裕樹，竹ヶ原春貴
Hiroki Watanabe and Haruki Takegahara
首都大学東京Tokyo Metropolitan University
1. Introduction
Hall thrusters are frequently used worldwide torealize the orbit control of geostationary satellites, aswell as for the orbital transfer of space probes1).
Hollow cathodes (H/Cs) are regularly employed aselectron sources in Hall thrusters to ionize thepropellant using electric discharges and to neutralize
the ion beam leaving the thrusters. Conventional
hollow cathodes use a porous tungsten insert that is
impregnated with an emissive mix of barium andcalcium oxides, and alumina for thermionic emission.
To prevent poisoning the thermionic emitter, which
can shorten the lifetime of the hollow cathode, contact
between the insert and the active gas should beavoided, and the type of the operation gas entering thecathode, which is commonly shared with the thruster
propellant, is restricted. In addition, the lifetime of thehollow cathode is considered to be restricted by oxide
depletion from the insert. As a result, it is difficult forHall thrusters with hollow cathodes to realize longterm operation, and they should be controlled strictlyfrom prelaunch to end-of-life.
We focused on a plasma cathode employing a radio
frequency (RF) discharge as a simple and robustelectron source to liberate Hall thrusters from theexperiencing the shortcomings of the hollow cathode.
The RF plasma cathode (RF/C) is an electricaldischarge device producing RF plasma that emits
electrons from its boundary. Although the plasma
cathode requires additional discharge power toproduce electrons, it does not have the drawbacks
associated with the thermionic emitter. In this study,
we evaluated the discharge characteristics and thethrust performance together with the combination of aHall thruster and the RF plasma cathode.
2. Experimental Apparatus and Procedure
Figure 1 shows a schematic representation of theRF plasma cathode, which consists of a discharge
vessel, an orifice plate, an induction coil, and an ion
collector. The cylindrical vessel is made of alumina,
and its inner diameter and length are 40 mm and 80mm, respectively. Copper wire whose diameter is 5mm is wrapped around the vessel as the induction coil.
The orifice plate is made of graphite, and there is a2-mm diameter orifice at the center of the plate forFig. 1 Schematic of the RF plasma cathode.

electron emission. Then electrons are emitted from theorifice, an equivalent quantity of ions then has to becollected at the ion collector in the cathode tomaintain quasi-neutrality in the RF plasma. Thus, thegraphite collector is necessary during the steady
operation of the cathode.
All of the experiments were conducted in a vacuum
chamber, whose diameter and length were 1.6 m and3.2 m, respectively. Xenon was employed as theoperation gas for the cathode and the propellant forthe thruster. The induction coil in the RF plasma
cathode was energized at 13.56 MHz by a RF
generator via a matching circuit. For an experiment toevaluate the electron-emission characteristics of theRF plasma cathode, a plate anode was locateddownstream of the cathode. The anode was biased
positively by the DC power supply to extract electronfrom the cathode. The anode current was measured asa function of the RF power and cathode mass flowrate. A 1-kW class anode-layer-type Hall thruster
developed at Tokyo Metropolitan University wasemployed to evaluate the thrust performance together
with the combination of the Hall thruster and the RF
plasma cathode. The thrust and discharge current were
measured as a function of the discharge voltage andanode mass flow rate. To compare the RF plasma
cathode and the hollow cathode, thrust performance ofthe combination of the Hall thruster and the hollow
cathode was also evaluated.
3. Results and Discussion
Figure 2 shows the anode current of the RF plasma
cathode as a function of the RF power at various
cathode mass flow rates. The anode current is theelectron current emitted from the cathode. The data

Fig. 2 Anode current of the RF plasma cathode as a functionof RF power at various cathode mass flow rates.

presented in Fig. 2 show that the anode current
proportionally increased as the RF power increased.
On the other hand, the effect of the cathode mass flowrate on the anode current at a constant RF power issmall. An anode current of 3.3 A was obtained at an
RF power of 140 W, a cathode mass flow rate of 0.3
mg/s, and an anode voltage of 58 V. The anodecurrent is sufficiently high to operate the 1-kW classHall thruster.
Normal ion beam extraction and neutralization
were experimentally confirmed for the combination of
the Hall thruster and the RF plasma cathode. Figure 3
and 4 show the discharge current and thrust for thetwo cathodes as a function of the discharge voltage atvarious anode mass flow rate, respectively. The
discharge current is not affected by the difference ofthe cathode type. The result indicates that the plasma
produced in the acceleration channel and the quantityof ions leaving the thruster is same for the hollow
cathode and the RF plasma cathode. In contrast to therelationship between the discharge current and thecathode type, the thrust for the hollow cathode isslightly lower than that for the RF plasma cathode foran anode mass flow rate of 1.0 mg/s, as shown in Fig.
4. For an anode mass flow rate of 2.0 mg/s, the thrust
for the hollow cathode is approximately the same tothat for the RF plasma cathode. For an the anode mass
flow rate of 2.9 mg/s, the thrust for the hollow
cathode is 1 mN higher than that for the RF plasma
cathode. We consider that the changes in the cathodeto-ground voltage that are due to the differences in thecathode type are the reasons for which the thrustslightly depends on the differences in the cathodetype.
4. Conclusion
To prevent the Hall thrusters from experiencing thedrawbacks of the hollow cathode, we focused on theplasma cathode employing the RF discharge as asimple and robust electron source and evaluated theFig. 3 Discharge current for the two cathodes as a functionof discharge voltage at various anode mass flow rates.

Fig. 4 Thrust for the two cathodes as a function of discharge
voltage at various anode mass flow rates.

discharge characteristics and thrust performance
together with the combination of the Hall thruster andRF plasma cathode. The results can be summarized asfollows:(1) An anode current of 3.3 A was obtained at an RF
power of 140 W, a cathode mass flow rate of 0.3
mg/s, and an anode voltage of 58 V. The anodecurrent is sufficiently high to operate a 1-kW classHall thruster.
(2) Normal ion beam extraction and neutralization
were confirmed for the combination of the Hallthruster and the RF plasma cathode. Based on thisresult, the feasibility of the RF plasma cathode as
an electron source for Hall thrusters in ground tests
was confirmed.
References
1) D. M. Goebel, I. Katz, Fundamentals of Electric
Propulsion Ion and Hall Thrusters, JPL Space Scienceand Technology series, Wiley & Sons, 2008.