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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 5, OCTOBER 2008
A Magnetically Enhanced Inductive Discharge Chamber for Electric Propulsion Applications J. E. Foster and Eric D. Gillman
Abstract—A magnetically enhanced inductive discharge was investigated for electric propulsion applications. The high plasma density produced by the source makes it attractive as an ion source for a gridded ion thruster or possibly a standalone ambipolar thruster. The discharge plasma is produced by a compact “stovetop” spiral antenna that is similar to that used in plasma processing sources. Operation on argon to pressures as low as 1 mtorr was demonstrated at powers ranging from 100 to 250 W. Ion current as high as 1 A was extracted from a 10-cm-diameter device. Plasma properties and ion production efficiency are reported and commented upon. Index Terms—Antenna, ion engines, ionization, permanent magnets, plasma discharge, radio frequency (RF).
I. I NTRODUCTION
R
ADIO-FREQUENCY (RF)-driven plasma sources have seen success in electric propulsion applications as embodied in the RIT series of ion thrusters [1]. Recently, an RF ion thruster originally intended for north–south station keeping was used for orbit raising to salvage a communication satellite (ARTEMIS) accidentally placed in the wrong orbit [2]. RFdriven plasma production offers many distinct advantages over conventional dc plasma sources presently being used in electric propulsion devices such as ion and Hall thrusters. These dc engines feature the hollow cathode as the electron source for ion production and beam neutralization. Hollow cathodes are subject to physical and thermochemical wear that ultimately limits their lifetime. A long-duration wear test conducted by JPL of the Deep Space 1 flight spare yielded much insight into the lifetime of these devices. The wear test, which was voluntarily terminated, suggests a cathode lifetime in excess of 30 kh [3]. In addition, recent NASA GRC and JPL life modeling and recent improvements such as the introduction of the graphite keeper suggest lifetimes of up to 50 kh [4]–[6]. Electrodeless systems such as RF and microwave plasma thrusters do not suffer from such wear-out mechanisms. The ionizer stage in these systems is limited primarily by the lifetime of the power supply itself. Microwave power supplies have demonstrated on-orbit lifetimes of over 140 kh [7]. Flight demonstration of the capability and potential of microwave ion thrusters has been validated on the Hayabusa asteroid rendezManuscript received October 5, 2007; revised April 4, 2008 and June 18, 2008. First published October 7, 2008; current version published November 14, 2008. The authors are with the Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48105 USA (e-mail:
[email protected];
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2008.2001975
vous mission [8]. These microwave thrusters join the NSTAR (Deep Space 1) [9] and PPS1350 (SMART 1) [10] as electric propulsion systems utilized on deep space missions. The Hayabusa thrusters to date have accumulated over 25 000 h of in-space operation time (distributed among four thrusters). RF ion thruster systems hold the advantage that handling and propellant purity requirements are considerably relaxed, resulting in potential cost savings. In addition, RF and microwave systems have less parts, making the use of these systems from an engineering and development point of view less risky. Unlike microwave thrusters, RF systems do typically utilize a conventional hollow cathode for beam neutralization. The neutralizer, however, is not immersed in a hostile plasma environment such as that of a discharge cathode which undergoes constant ion bombardment; therefore, the neutralizer is expected to undergo only limited wear provided that it is not intersected by off-axis beam ions. In contrast to traveling wave tube efficiencies, lower frequency RF power supplies have higher electrical efficiencies, typically comparable to dc sources. Therefore, RF-driven thrusters from an efficiency standpoint can fill the need for longlife thruster systems in the low-to-medium power range (few hundreds of watts to several kilowatts). Conventional RF thruster systems employ the classic inductive discharge configuration [1]. Here, an external RF-driven coil wrapped along the outer diameter of a dielectric discharge chamber is used to couple power to the gas to produce and sustain a plasma discharge by transformer action. In this regard, in these “barrel” sources, power is deposited into a skin depth typically on the order of centimeters in spatial extent (collisionless case). Density and power dissipation are highest in this skin depth layer. This follows directly from the solution to the wave equation where the skin depth represents the spatial decay constant of a damped wave in a conductive medium [11]. In recent years, the move toward high-density plasma sources for plasma processing applications has made significant advances in the improvement of inductive plasma sources. A review of such advances may be found in the work of Hopwood [12]. These advances include movement away from the “barrel”-type inductive sources to the planar, “stove top” and azimuthal coil designs, some of which featuring multipole confinement and Faraday shielding to mitigate sputtering effects due to capacitive coupling [13]. The planar coil designs allow for very large area low aspect ratio configurations [14]. Such a source can be used as an ionizer chamber for an ion thruster allowing for very compact thruster designs. The planar coil design allows for not only scaling to larger thruster diameters, but the relative low aspect ratios may mitigate some issues associated with vibration qualification
0093-3813/$25.00 © 2008 IEEE
FOSTER AND GILLMAN: INDUCTIVE DISCHARGE CHAMBER FOR ELECTRIC PROPULSION APPLICATIONS
as well. The planar spiral inductive discharge is capable of producing very high plasma densities (well into 1012 #/cm3 ) with low plasma potentials (
