A Magnetically Enhanced Inductive Discharge Chamber for...

2130 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 5, OCTOBER 2008

A Magnetically Enhanced Inductive DischargeChamber for Electric Propulsion Applications

J. E. Foster and Eric D. Gillman

Abstract—A magnetically enhanced inductive discharge wasinvestigated for electric propulsion applications. The high plasmadensity produced by the source makes it attractive as an ionsource for a gridded ion thruster or possibly a standalone am-bipolar thruster. The discharge plasma is produced by a compact“stovetop” spiral antenna that is similar to that used in plasmaprocessing sources. Operation on argon to pressures as low as1 mtorr was demonstrated at powers ranging from 100 to 250 W.Ion current as high as 1 A was extracted from a 10-cm-diameterdevice. Plasma properties and ion production efficiency are re-ported and commented upon.

Index Terms—Antenna, ion engines, ionization, permanentmagnets, plasma discharge, radio frequency (RF).

I. INTRODUCTION

RADIO-FREQUENCY (RF)-driven plasma sources haveseen success in electric propulsion applications as em-

bodied in the RIT series of ion thrusters [1]. Recently, an RFion thruster originally intended for north–south station keepingwas used for orbit raising to salvage a communication satellite(ARTEMIS) accidentally placed in the wrong orbit [2]. RF-driven plasma production offers many distinct advantages overconventional dc plasma sources presently being used in electricpropulsion devices such as ion and Hall thrusters. These dcengines feature the hollow cathode as the electron source forion production and beam neutralization. Hollow cathodes aresubject to physical and thermochemical wear that ultimatelylimits their lifetime. A long-duration wear test conducted byJPL of the Deep Space 1 flight spare yielded much insightinto the lifetime of these devices. The wear test, which wasvoluntarily terminated, suggests a cathode lifetime in excessof 30 kh [3]. In addition, recent NASA GRC and JPL lifemodeling and recent improvements such as the introduction ofthe graphite keeper suggest lifetimes of up to 50 kh [4]–[6].

Electrodeless systems such as RF and microwave plasmathrusters do not suffer from such wear-out mechanisms. Theionizer stage in these systems is limited primarily by the life-time of the power supply itself. Microwave power supplies havedemonstrated on-orbit lifetimes of over 140 kh [7]. Flight de-monstration of the capability and potential of microwave ionthrusters has been validated on the Hayabusa asteroid rendez-

Manuscript received October 5, 2007; revised April 4, 2008 andJune 18, 2008. First published October 7, 2008; current version publishedNovember 14, 2008.

The authors are with the Department of Nuclear Engineering and Radio-logical 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 onlineat 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 elec-tric propulsion systems utilized on deep space missions. TheHayabusa thrusters to date have accumulated over 25 000 h ofin-space operation time (distributed among four thrusters). RFion thruster systems hold the advantage that handling andpropellant purity requirements are considerably relaxed, result-ing in potential cost savings. In addition, RF and microwavesystems have less parts, making the use of these systems froman engineering and development point of view less risky. Unlikemicrowave thrusters, RF systems do typically utilize a conven-tional hollow cathode for beam neutralization. The neutralizer,however, is not immersed in a hostile plasma environment suchas that of a discharge cathode which undergoes constant ionbombardment; therefore, the neutralizer is expected to undergoonly limited wear provided that it is not intersected by off-axisbeam ions.

In contrast to traveling wave tube efficiencies, lower fre-quency RF power supplies have higher electrical efficiencies,typically comparable to dc sources. Therefore, RF-driventhrusters from an efficiency standpoint can fill the need for long-life thruster systems in the low-to-medium power range (fewhundreds of watts to several kilowatts). Conventional RFthruster systems employ the classic inductive discharge con-figuration [1]. Here, an external RF-driven coil wrapped alongthe outer diameter of a dielectric discharge chamber is used tocouple power to the gas to produce and sustain a plasma dis-charge by transformer action. In this regard, in these “barrel”sources, power is deposited into a skin depth typically on the or-der of centimeters in spatial extent (collisionless case). Densityand power dissipation are highest in this skin depth layer. Thisfollows directly from the solution to the wave equation wherethe skin depth represents the spatial decay constant of a dampedwave in a conductive medium [11]. In recent years, the movetoward high-density plasma sources for plasma processing ap-plications has made significant advances in the improvement ofinductive plasma sources. A review of such advances may befound in the work of Hopwood [12]. These advances includemovement away from the “barrel”-type inductive sources to theplanar, “stove top” and azimuthal coil designs, some of whichfeaturing multipole confinement and Faraday shielding to mit-igate sputtering effects due to capacitive coupling [13]. Theplanar coil designs allow for very large area low aspect ratioconfigurations [14]. Such a source can be used as an ionizerchamber for an ion thruster allowing for very compact thrusterdesigns. The planar coil design allows for not only scaling tolarger thruster diameters, but the relative low aspect ratios maymitigate some issues associated with vibration qualification

0093-3813/$25.00 © 2008 IEEE

FOSTER AND GILLMAN: INDUCTIVE DISCHARGE CHAMBER FOR ELECTRIC PROPULSION APPLICATIONS 2131

as well. The planar spiral inductive discharge is capable ofproducing very high plasma densities (well into 1012 #/cm3)with low plasma potentials (<20 V) at reduced pressures. Un-like the barrel inductive sources, the planar configurations areamenable to multipole confinement schemes utilizing perma-nent magnets. The use of such arrangement improves not onlydensity at reduced pressure but also uniformity and dischargeefficiency [15]. Because the plasma heating and productionoccur within a few skin depths downstream of the antenna,the issue of eddy current induction/heating of the grids canbe completely avoided by simply utilizing discharge chamberdepths larger than the skin depth. In addition, the compactnessof the coil mitigates the need for extensive shielding. It wouldappear that the planar spiral source is the natural evolutionarystep for inductive ion sources for electric propulsion. Based onthe promising attributes of the flat spiral antenna as well as thedemonstrated performance of the sources in plasma processingsystems up to 300 mm in diameter with high uniformity, aninvestigation was initiated to assess the feasibility of using a flatspiral-type inductive source for electric propulsion applications.Performance attributes such as high density, uniformity, andcompactness have led us to investigate the planar spiral antennaionizer as an inductive source specifically for ion thrusterapplications as well as a possibly standalone plasma source forambipolar diffusion-type ion thruster systems [16], [17].

In this paper, the variation in plasma density, electron temper-ature trends, and extractable ion current as a function of powerand pressure in a magnetically enhanced inductive source isdiscussed. Estimates of ion production efficiency are calculatedand compared with compatible data acquired in the open liter-ature. The applicability of an inductive ion source featuring theplanar spiral antenna geometry for electric propulsion applica-tions is also commented upon.

II. BACKGROUND AND EXPERIMENTAL PROCEDURE

A. Basic Physics

Inductive discharge breakdown and discharge maintenanceare processes due to transformer action between the an-tenna and the secondary coil, the plasma, as described byFaraday’s law

∇× �E = −d �B

dt. (1)

Here, an electric field E is produced in the discharge chamberby the action of the time-varying magnetic field B produced byan RF current circulating in the planar coil. At low pressures,the penetration depth of the electric field depends on the density

δ =√

me

e2μons(2)

where me is the mass of the electron (9.11 × 10−31 kg), e isthe elementary charge of an electron, μo is the permeabilityof free space (1 × 10−7 H/m), and ns is the plasma density.Here, the low-pressure limit is defined at conditions where the

Fig. 1. Schematic depiction of planar compact antenna and induced electricfield.

excitation frequency exceeds (∼10× or greater) the electron-neutral collision frequency. The operating RF frequency usedin this paper is 13.56 MHz. An electron-neutral collision fre-quency equivalent to this excitation frequency occurs around25 mtorr. Measurements were acquired at pressures consider-ably below this value. For reference, assuming that collisionlessconditions prevail, at a plasma density of 1011#/cm3, the skindepth is nearly 2 cm. For the discharge to operate efficiently,the discharge chamber depth must therefore be at least a skindepth or so deep; therefore, very shallow discharge chambersare possible. Fig. 1 schematically shows a compact planar coilin action. The time-varying RF currents circulating throughthe antenna produce a time-varying magnetic field. This RFmagnetic field produces the electric field that sustains the dis-charge. Power is therefore transferred across the quartz vacuumwindow into the plasma essentially by transformer action, withthe plasma acting as the secondary winding.

It is well known that “magnetic bucket” configurations canextend the operating range of a plasma source to include lowerpressure while still maintaining high uniformity [18]. In thework presented here, a ring cusp magnetic section is integratedwith a planar inductive source to improve plasma density at re-duced pressures. The magnetic circuit reduces diffusion lossesto the wall and extends the residence time of the electrons in theheating zone. These contained hot electrons improve the overallionization efficiency. It should be pointed out that, even forplasma processing applications, very high electrical efficienciesfor planar inductive sources have been reported. For example,Coultas and Keller report 80%–90% electrical efficiencies [19].Here, efficiency is defined as the ratio of plasma power (RFpower absorbed by the discharge) to the total input electricalpower.

For ion thruster applications, it is expected that some back-sputtered material from low-level grid erosion or backsputteredmaterial from test facility sputtering could deposit onto the RFquartz coupling plate. Such deposition would be minimal inspace provided that the grids are properly aligned and gapped.The effect of the coating would be that of attenuation of the


power actually deposited into the plasma. The problem ofcoatings on the dielectric window for inductive sources canbe addressed through the use of a Faraday shield which canact as a baffle or shadow shield to prevent coatings fromoccurring on the window. The Faraday shield therefore playsthis role in addition to the role of shorting out capacitive fields.Hopwood et al. have demonstrated the utility of the Faradayshield to defeat the coating problem in metal vapor plasmas[20]. Here, the deposition vapor rates of ∼40 nm/min wereinvestigated in the presence of a Faraday shield. These metalvapor deposition rates were considerably higher than thosewhich would be expected from space or ground testing [21].The amount of attenuation expected for a system without theFaraday shield depends on the ratio of the conductive filmthickness to the skin depth. For the case of an electromagneticplane wave incident on a metal, the penetration depth is on theorder of the wavelength in the conductor

δ =√

2μσω

(3)

where μ is the absolute magnetic permeability of the medium,σ (in ohms per meter) is the electrical conductivity, and ω isthe RF frequency. If the medium is linearly isotropic, then μ =κm · μo, where κm is the relative permeability. In general, forferromagnetic materials, μ is nonlinear. The medium or coatingis also taken to be a linear and isotropic conductor so that σis the constant of proportionality between the current densitydriven by the high-frequency electric field. At 13.56 MHz, thewavelength inside carbon, a typical grid material, with an elec-trical conductivity of 61 000 Ω · m−1, is on the order of 1 mm,suggesting that attenuation effects should be minimal as theratio of skin depth to 1 μm, a thickness typically observed afterlong-duration ion thruster wear testing [21], is on the order of1000. Lower frequency operation, 1 MHz for example, wouldresult in even less attenuation. RIT thrusters, which operatearound 1 MHz and have been wear tested as well as flight tested,demonstrate that the deposition of metal coatings typically ob-served on ion thruster walls on the dielectric discharge chamberis not an issue [22].

B. Experimental Setup

The experimental investigation took place in a gaseous elec-tronics reference cell. The test cell was specifically designed asa reference apparatus for reproducible experiments in plasmaprocessing research [23]. The test cell was constructed of stain-less steel ultrahigh vacuum components. The inner chamberdiameter is 25.1 cm. The height of the chamber is 22 cm. Thebell jar features a symmetric pumping manifold that is designedto reduce angular variation in pumping speed at pressures aboveapproximately 13 Pa. Additional details regarding the bell jarmay be found in [23]. The apparatus is evacuated with a turbo-molecular pump with a pumping speed of 400 L/s backed by aconventional mechanical pump. Ultimate base pressures around10−7 torr are achievable with this configuration. During normaltesting, gas is input using a precision mass flow controller.The gas is actually fed into the bell jar, which in turn flows

Fig. 2. GEC test cell. Note the inset depicting discharge operation withoutmagnetic enhancement. RF-compensated Langmuir probe is also depicted.

into the source region by diffusion. In this case, the pressureinside the source should reflect the pressure in the bell jar asmeasured using ionization and thermocouple gauges attachedto the facility. At a given flow rate, test cell pressure is adjustedusing a variable aperture throttle valve located upstream of theturbomolecular pump. For the experimental results presentedhere, argon gas was used. The throttle valve upstream of theturbopump was set at 12% open. Fig. 2 shows the GEC bell jar.

Copper tubing, i.e., 0.32 cm in diameter, was fashioned intoa 10-cm planar–spiral coil antenna. The antenna was locatedadjacent to a 2.5-cm-thick quartz window. RF power is coupledinto the discharge chamber across this window. The antennaitself is coupled to a 500-W 13.56-MHz RF power supplythrough a Pi-configured matching network [32]. The matchingnetwork can be set to automatically match the impedance of thepower supply to the load. Typically, during a test, the impedanceis matched and then the autotuner is shut off. Once matched,the load absorbs most of the input power over a wide powerand pressure range. In all cases, the reflected power was lessthan 5% of the input power. The inset in Fig. 2 shows theinductive plasma source nominally operating without magneticconfinement.

As shown in Fig. 3(a), located just downstream of the quartzcoupling window was a mild steel cylinder or sleeve, henceforthcalled the magnetic sleeve, which contained a series of threesamarium cobalt magnet rings of alternating polarity. Two ofthe magnet rings were located on the inside surface of thecylinder section. The magnetic sleeve was used to confine theplasma and improve the ionization efficiency of the discharge.Because the magnetic sleeve is made of ferromagnetic material,its presence may also enhance the RF magnetic fields locally aswell [24]. The magnetic sleeve, which was fashioned from sheetstock, was not rolled into a continuous cylinder, rather a gap(∼1 cm) was left to allow for gas diffusion. To improve the uni-formity of the ring cusp circuit due to the presence of this gapin the magnetic sleeve, metal straps, held magnetically in place,were used to jumper portions of the sleeve. The jumpers makegood magnetic contact but poor electrical contact at 13.56 MHz.This minimizes eddy losses in the magnetic sleeve in contrastto a continuous cylinder. A magnetic flux plot computed for thesleeve geometry is shown in Fig. 3(b). The on-axis magnetic


Fig. 3. (a) Schematic depiction of the experimental setup. (b) Magnetic flux plot for magnetic sleeve used to enhance discharge confinement.

field ranged from approximately 30 G near the window to60 G at the midplane to 30 G at the exit plane. Located justdownstream of the cylindrical section (essentially terminatingthe exit plane, ∼40 mm downstream of the quartz window) wasa 10-cm-diameter bias-able electrode. This electrode was typ-ically biased around −38 V to collect ion saturation current toassess ion flux exiting the discharge chamber. Inserted onto thecenterline of the apparatus was a cylindrical Langmuir probe.The probe tip was 1 cm long and 0.38 mm in diameter. It wasused to acquire plasma density and electron temperature at the

middle of the discharge. Thin sheath analysis was applied to theinterpretation of the probe current–voltage (I–V ) characteris-tic. Ion number density was determined from the ion saturationportion of the curve. For the operating conditions investigatedhere, the ion-neutral mean free paths were on the order of 1 cm,which is smaller than the characteristic length scale of the dis-charge (10 cm). Under such conditions, collisions in thepresheath increase the presheath voltage required to satisfy theBohm condition for the ions, which can lead to uncertainty inthe ion saturation current by as much as a factor of two [25].


Taking this uncertainty into account along with the uncertaintyin the electron temperature, the uncertainty in the ion densityis estimated to be at least 50%. These measurements, however,yield insight into trends in density as a function of power andpressure. At each operating condition, seven Langmuir probemeasurements were made and the extracted plasma parameterswere averaged. Because the plasma potential oscillates harmon-ically at the RF frequency, the potential difference betweenthe probe tip and the plasma potential changes with time. Thisgives rise to a distorted Langmuir probe I–V characteristic. Tooffset this effect, an inline filter and reference probe is used tocorrect the I–V characteristic. By increasing the impedance ofthe probe tip to ground at the RF frequency and harmonics, theprobe tip will follow the oscillations in the plasma. As long asthese oscillations are less than kTe/e in magnitude, the analysisof the traces follows that of a probe in a dc plasma [26]. Here, Te

is the electron temperature, and k is Boltzmann’s constant. Theinline filter and associated circuitry achieve this end. Althoughall attempts were made to insure that probe RF compensationwas adequate, the presence of finite RF sheath voltages cangive uncertainty in the electron temperature, typically makingit higher. The probe was located approximately 20 mm belowthe quartz window. In all cases, the probe was at least one skindepth away from quartz plate. A photograph of the probe with-out the discharge magnetic insert installed is shown in Fig. 2.Corrections to the probe’s I–V characteristic to account formagnetic effects on ion motion were neglected as the ionLarmor radius at the tip location was much larger than theprobe radius. In the case of the electron collection, if the Larmorradius is not significantly larger than the probe radius, then themagnetic effects cannot be completely neglected. Estimatingthe transverse electron velocity with the mean thermal speed,the electron Larmor radius was on the order of 1 mm at theprobe location (B ∼ 60 G). The probe radius was 0.19 mm,making the ratio of the Larmor radius to probe radius on theorder of 5 for a typical case. While collected electron currentmagnitude may be attenuated, the exponential variation in theelectron current, however, should not be affected by themagnetic field. In this regard, the electron retarding regionshould yield a representative value of the electron temperature[27], [28].

For comparison purposes, the discharge was operated withand without the magnetic sleeve at similar powers and pres-sures. It is interesting to note that, without the magnetic sleeve,discharge initiation required higher background pressures. Forexample, a pressure of approximately 20–30 mtorr was re-quired for discharge to appear over an input power range of100–250 W when no magnetic sleeve was used. As is wellknown (Paschen’s Law), the required initiation power tended toincrease with decreasing pressure. The discharge with the mag-netic enhancement initiated at pressures that were significantlylower around 10 mtorr. As shown to be the case in dc multipolesources, the reduced pressure initiation is surmised to be dueto better electron confinement [29]. Breakdown occurs whenproduction rates match loss rates. The magnetic field reducesthe loss rates, thereby allowing discharge initiation to occur atlower pressures. At the lower pressures, the losses are reduceddue to the presence of the magnetic field at the boundaries. In all

cases, the initiation of a discharge was observable at low pow-ers, typically <10 W. Here, a self-sustaining plasma is formed.Once formed, it is possible to maintain the inductive dischargeto pressure slightly below 1 mtorr with the magnetic sleeve.Without the magnetic sleeve, the discharge operation below3 mtorr became unstable. At each change in flow, the pressureis allowed to stabilize before data were acquired. In the casespresented here, the reflected power was less than 5% and didnot vary appreciably as a function of flow and pressure.

III. RESULTS

This investigation aimed to look at the ion production capa-bility of a planar inductive source retrofitted with a magneticmultipole jacket to enhance ionization efficiency particularly atlow pressures. The source achieved this end, allowing for a ten-times reduction in operating pressure, thereby making it possi-ble for operation in the target range of 1–2 mtorr. The jacketalso served to confine the plasma, making the configurationmore thrusterlike. In contrast, typical plasma processing reac-tors do not utilize plasma limiters as it would be a contam-ination source. The performance of the source was assessedby characterizing trends in plasma properties such as plasmadensity and electron temperature at the center of the source. Anupper limit on the maximum extractable ion current was as-sessed by collecting ion saturation current at a biased electrodelocated at the exit plane of the source. With this information,expected ion production costs could be estimated. The datapresented have argon as the working gas. It is expected thatthe performance should improve considerably with subsequenttesting utilizing xenon, owing to the lower ionization potential.For example, in the development of the RIT 10 thrusters, it wasfound that, to attain discharge performance (W/A) similar toxenon, it was necessary to operate the discharge chamber onargon at much higher pressures (∼2× in cases) [30].

A. Plasma Density

Fig. 4 shows the variation in plasma density as a functionof pressure all the way down to 1 mtorr for cases with andwithout the magnetic enhancement. Note that the operationbelow 3 mtorr without the magnetic multipole sleeve was notpossible. As can be seen in the figure, at a fixed power, plasmadensity increases monotonically as a function of pressure. Therate of density increase with pressure depended on the inputpower. The lowest power case investigated, 100 W, indicates aconsiderably smaller slope than the higher power cases for themagnetically enhanced source. However, without the magneticconfinement, the rate of density increase with pressure at dif-ferent powers was approximately the same. Also worth noting,as expected, was that plasma density at fixed pressure increasedwith power. This increase was most notable at pressures above2 mtorr in the magnetically enhanced case. Below 2 mtorr,the difference in plasma density at 100 and 170 W was notappreciable, suggesting that at lower powers, at a fixed pressure,the ionization efficiency saturates. In general, if one considersthe RF breakdown characteristic for inert gases, the requiredbreakdown field tends to increase sharply with decreasing


Fig. 4. Plasma density variations as a function of chamber pressure at differentinput powers.

pressure particularly at collision frequencies well below theRF frequency [31]. This is due to the infrequency of electronionization collisions and collisional heating. Unlike the 250-Wcase, similarities in density at the low pressures at the 100-and 170-W cases indicate that plasma production at these twopowers may be dominated by capacitive rather than inductivefield-driven ionization. The data imply that at least at lowpressures, operating at 100 W rather than 170 W may be moreefficient, i.e., if the ion extraction takes place at the plane of theLangmuir probe.

Perhaps the most interesting aspect of this comparativeset of data is the dramatic increase in plasma density withthe magnetic enhancement. Not only does the magnetic fieldextend the operating range to lower pressures, but also thedensity increases at comparable pressures are significantlyhigher than the zero magnetic field case (∼0.5-G Earth field).Indeed, at 7 mtorr, the comparative density increase at 250 Wis over a factor of five. Also equally remarkable is the factthat the lowest power magnetically enhanced density dataare significantly higher than the highest power case with-out the enhancement. These data clearly show significantimprovements in density and plasma confinement with themagnetic enhancement. At the 250-W condition, the densityappears to saturate at pressures above 4 mtorr, indicatingdiminishing returns with increasing pressure at this powerlevel. The slight dip in density observed at 6 mtorr at the250-W condition, although within the uncertainty of the mea-surement, may be due to poorer matching at this operating con-dition. In summary, as can be seen in the figure, the magneticenhancement, at a given power, boosts the density considerably,thereby allowing for operation at reduced power and lowerpressure.

The plasma densities obtained in this work can be comparedwith a conventional GEC planar inductive source without mag-netic enhancement. Miller et al. [32] characterized inductiveplasma variations of a GEC cell down to 5 mtorr. The densitiesmeasured in that work are consistent with the zero magneticfield data determined here.

Fig. 5. Variation in the electron temperature with pressure and power.

B. Electron Temperature

Fig. 5 shows variations in the measured electron temperatureas a function of pressure. Plot 5 represents trends in electronaverage energy for the magnetic case. For comparison purposes,the data acquired without the magnetic sleeve are also pre-sented. Electron temperature measurements made without themagnetic sleeve are consistent with the GEC data reported atsimilar operating conditions [32]. As can be seen in the figure,the data acquired without the magnetic sleeve fell betweenthe magnetically enhanced 170- and 250-W conditions. Theno-magnet-case (no magnetic sleeve) measured temperatureswere found to be lower than the magnetic sleeve case with theexception of the 170 W with magnets condition. At pressuresbelow 4 mtorr, the discharge tended to become less stable,tending rather to jump into a capacitive mode as observedby a sudden drop in luminous intensity and plasma density.Typically, for electropositive discharges such as this one, theelectron temperature is a function of the background pressure.It is expected that the electron temperature should decreasewith increasing pressure. The relative insensitivity to pressureover the pressure range investigated may be attributed to therelatively long electron mean free paths which prevail un-der these conditions. Indeed, at 4 mtorr, the electron-neutralmomentum exchange mean free path is approximately 10%larger than the diameter of the antenna window. The magnitudeof the electron temperature in the no-magnet case tended todecrease with an increasing input power at a given pressure.This has been observed elsewhere in RF plasma discharges[33]. These data suggest an improved discharge efficiency withincreasing input power, indicating that more power is goinginto ionization (see Fig. 3) rather than going into raising theaverage electron energy. This is consistent with the improve-ment of inductive coupling as the plasma density increases. Theelectron temperatures measured here compared favorably with(within 10%–20%) existing electron temperature data at similaroperating conditions in a magnetic multipole inductive source[11], [35]. With the exception of the 170-W case, the electrontemperature acquired from the magnetically enhanced sourcetended to also be relatively insensitive to pressure changes.


The 170-W data displayed the classic trend of monotonicallydecreasing the temperature with increasing pressure, saturatingat higher pressures. The difference between this behavior andthe 100- and 250-W data is not well understood, although it issuspected that capacitive effects may be higher in the 100-W(lower average plasma density) case and in the 250-W case(higher antenna voltage). The electron temperature at the lowpower condition was fairly constant at a high value of 7.6 eV.This may be indicative of an increased capacitive couplingat this low power condition. The electron temperature at the250-W case at the lower pressures was around 6 eV andactually increases somewhat above 5 mtorr. In the case ofthe magnetically enhanced system, the spread in temperaturewith power is significant in contrast to the unmagnetized GECdata. The difference in ordering and magnitude of the averageelectron temperature at a given pressure and varying powers atpresent is not well understood, although it is speculated that thecapacitive coupling plays a role in a likely complicated way asthe electrons are also interacting with the magnetic field in thepresence of the capacitive electric field.

C. Collected Ion Current

Of great merit to the magnetically enhanced source are theextractable ion current and the ion production efficiency. Likea dc ion thruster, the extraction of ions from an RF dischargeplasma requires a drain electrode to collect an equivalent num-ber of electrons. In the case of a dc ion source, the anode col-lects the electron current. In an RF ion source, a drain electrodeinserted into the discharge serves in this capacity. Without thedrain electrode, the plasma potential would simply shift withion extraction bias voltage in such a manner to confine theions. The ion current collected would be limited to only thatelectron current that is able to somehow escape to a groundpotential surface. For a well-designed discharge chamber, suchcurrent is very small. The magnetic sleeve served the roleof drain electrode. The magnetic sleeve could be electricallyisolated from ground. This surface was grounded during thesetests to collect the electron current. The maximum extractableion current is intimately related to the electron flux collectedprimarily at the cusps of the magnetic sleeve when grounded.This balance maintains quasi-neutrality in the plasma. Becauseof the high mobility of the electrons and the adequate surfacearea projected by the magnetic sleeve, the ion saturation currentcould actually be extracted from the exit plane of the source.Fig. 6 shows a plot of current from the magnetic sleeve toground and extracted ion current to bias electrode. For thesedata, the pressure was set at 1.4 mtorr, and the forward RFpower was 170 W with 5 W reflected. The bias on the end-plane electrode was fixed at about 38 V, corresponding to ionsaturation current. The drain electron current to the magneticsleeve was varied by adjusting the resistance of a variableresistor placed between the magnetic sleeve and ground. Fromthe figure, one observes a linear relationship between the col-lected electron current and the response at the biased end-planeelectrode, the ion current. These currents essentially match asthey should as indicated by the linear trendline added. The yintercept of the trendline is approximately 0.03 A, indicating

Fig. 6. Correlation between magnetic sleeve electron drain current and ex-tracted end-plane ion current.

Fig. 7. End-plane ion saturation current collected as a function of pressureand power.

a small electron leakage out of the source to ground potentialsurfaces other than the magnetic sleeve. This small offset ismost likely due to the flow of electrons to ground through theslot in the magnetic sleeve discussed earlier.

The extractable ion current was estimated by biasing a10-cm disk electrode, located 44 mm downstream of the RFwindow, to ion saturation (−38 V). The bias voltage of −38 Vwas sufficiently negative that the electrode remained in ionsaturation over the range of operating conditions presentedherein. These data were also acquired as a function of pressureand power. Fig. 7 shows these ion current variations. At a fixedpressure, the ion current increased with input power followingthe trends in plasma density with power. Also, as can be seenin the figure, it was possible to extract nearly 1 A of current at250 W and 3 mtorr with argon. Fig. 8 shows the ratio of theion current taken at 250 W to that acquired at lower powers. Intypical electropositive discharges, the ion flux scales with theplasma density and the square root of the electron temperature.Mathematically, the density scales as [11]

ns =Pabs

euBAeffεT. (4)


Fig. 8. Variation in the ion current ratio with pressure.

Here, Pabs is the power absorbed, uB is the Bohm velocity, Aeff

is the effective plasma loss area, and εT is the energy loss perion lost to the wall in volts. The loss area is determined by themagnetic circuit and the square root of the electron tempera-ture. One would then expect to first order the ratio of 250-Wion current to 100-W ion current to be approximately 2.5,following the power ratio. Instead, the ratio was measured to be40% higher (3× instead of 2.5×) at pressures above 3 mtorr.The ratio was found to increase as pressure was reduced,indicating that the lower power discharge was not as efficient.At 170 W, on the other hand, the ratio is expected to be 1.5, andindeed, over the broad range, it is in fact approximately 1.5. Thedata seem to indicate that, at the lowest pressures investigated,the lower power conditions, 100 and 170 W, are not as effi-cient as the 250-W operating point. In general, all inductivelycoupled discharges possess some inherent capacitive coupling.At lower powers in particular, the capacitive coupling becomesmore pronounced due to the fact that the plasma productionvia high-voltage sheath acceleration begins to dominate ratherthan production by inductive electric fields. Inductive couplingovercomes capacitive coupling as the plasma density increasesbecause the associated thinner sheath shields the plasma fromcapacitive coupling. Therefore, in this case, at the lower powersand lower pressures, it becomes increasingly difficult to couplethe power into the plasma inductively. Indeed, the magneticinsert section was implemented to improve such coupling.Because the capacitive contribution is not as efficient as theone whose inductive power fraction is considerably higher, thecurrent ratio can be expected to be higher at the lower powersand pressures. Again, this is due to the energy loss per ion lostto the wall term (εT ) in (4). This term is higher for capacitivedischarges because, relatively speaking, the power lost to thewalls in a capacitive discharge (sheath voltage ∼100 V) is largecompared to that in inductive discharges (∼10 V). This explainsnot only the deviations in the ratio at the lower powers but alsothe increase in the deviations appearing at the lower pressures.It may be possible to improve efficiency at the lower powers and

pressures by improving the magnetic circuit, which, at present,is not optimized.

Also worth pointing out in Fig. 7 is that the ion current doesappear to saturate at the higher pressures, remaining relativelyconstant above some threshold pressure. Below this thresholdpressure, there is a precipitous drop in current with decreasingpressure. The location of the knee demarcating the current drop-off shifts with operating power, changing from ∼2.75 mtorr at100 W to 1.5 mtorr at 250 W. A goal of this work is to displacethe knees to a lower pressure using magnetic containment. It isexpected that the knees will also shift to a lower pressure if thesource were to be operated on xenon gas.

D. Ion Production Efficiency

Ion production costs of an ion thruster with beam extractionare defined as the ratio of input discharge power to extractedion current. By using the ion current data, it is possible to cal-culate the discharge ion production costs for the magneticallyenhanced discharge by dividing the absorbed RF power by thecollected ion current at the location of the disk electrode. Here,the discharge ion production cost is therefore defined as

ηdis =Pnet

Icoll. (5)

Here, Pnet is the RF power absorbed by the plasma, and Icoll

is the ion current collected at the disk electrode that terminatesthe exit plane of the source. Because gas flow into the sourceregion is, by diffusion from gas, fed directly into the bell jar,the pressure and, thus, the neutral density of the source arewell represented by the pressure of the bell jar itself. Thisarrangement in conjunction with the slot in the magnetic sleeveallows for direct control of the pressure in the source region.Therefore, one can achieve thrusterlike discharge conditions bysimply adjusting the bell jar flow rate so that chamber/sourcepressure matches the expected thruster pressure. In this manner,no flow corrections are necessary as they are in hollow cathodeion thruster discharge tests. In the case of a dc ion thrusterdischarge-only test, to simulate beam extraction, corrections arenecessary to account for reingestion of neutralized ions on thegrids, which affects the total chamber flow rate and pressure.Indeed, such reingestion gives rise to a larger effective dis-charge flow rate and an associated reduced discharge voltage.This can lead to lower calculated discharge losses. Brophy[34] describes the general procedure that accounts for thereingestion effects. In the RF source case, however, matchingactual thruster pressures is sufficient. Plasma production ratesand electron temperature are functions of pressure and poweralone.

Fig. 9 shows the variations in the discharge ion productioncosts as a function of pressure. The ion production costsincrease with decreasing pressure. This is a direct result ofreduced collisions and reduced inductive coupling as discussedearlier. Each curve has a distinct knee, which tends to shiftsomewhat to lower pressures with increasing power. At the verylow end of the pressure range investigated, the discharge is mostefficient at the highest discharge power, i.e., 250 W. At 6 mtorr,


Fig. 9. Estimated discharge ion production cost at the exit plane of the source.

the ion production cost was approximately 250 W/A at the250-W condition. At 1.3 mtorr, the discharge ion productioncost at 250 W was observed to increase to 290 W/A. Thesemeasurements were all taken with argon. Published data onmuch larger inductive sources investigated for plasma process-ing applications indicate comparable ion production efficiencyat 1 mtorr. Hopwood et al. [35] report ∼250 W/A at 1 mtorrand similar power levels. It is remarkable that the inductiveion source developed here has comparable performance tothe much larger 27-cm-diameter source developed by Coultasand Keller which also featured magnetic confinement [19].Typically, systems with a large surface area to volume ratioperform considerably poorer than larger sources with a lowersurface area to volume ratio [32]. The comparable performanceof this smaller source to the larger source is a consequenceof the magnetic circuit geometry. In addition, as mentionedearlier, it is expected that the operation on xenon should resultin significant increases in efficiency owing to the decreasedionization potential of this atom (12 eV compared to 15.8 eVfor argon) [30].

Ion production efficiency at the 170- and 250-W cases indi-cated comparable performance over a wide range at the higherpressures. Significant deviations are apparent, however, at thelower pressures. Again, the 250-W operating condition wasmore efficient. Apparently, at the lower pressures, RF couplingto the plasma improves with increasing power.

It is also possible to estimate the discharge efficiency ifthe ion extraction plane was closer to the RF antenna. Forexample, the ion current density can be estimated in the planeof the Langmuir probe. The probe was located approximately20 mm above the ion collection electrode. It has been shownthat the ion density profile for magnetically confined inductivedischarges is essentially flat [11]. In this regard, the centerlinecurrent density can be used to estimate a lower limit on ioniza-tion efficiency in this plane as well provided that the profile isflat. It is expected that the ion flux closer to the heating zoneshould result in higher currents and discharge efficiency. Thesecalculations were performed and shown in Fig. 10 for the bestperforming condition, 250 W. These data suggest that optimumplacement of the collection plate or ion grid, as well as magneticcircuit optimization, is a key to achieving high performance.

Fig. 10. Calculated ion production cost in the plane of the Langmuir probe.

As can be seen, the discharge efficiency all the way down to1 mtorr is less than 200 W/A. The discharge ion productioncosts calculated here are not inconsistent with optimized in-ductive sources developed for plasma processing applicationswhich have been reported to range from 100 to 300 W/A [36].These data suggest a pathway to even more efficient ion sourcesfor low power to medium power ion thruster applicationsprovided that the additional magnet mass does not appreciablyincrease the total thruster mass. The additional magnet masscould be offset by the reduced length of the discharge chargemade possible by the compact planar antenna geometry. If thesemeasured ion production costs can be realized in a thrusterconfiguration, then they could not only exceed the performanceof the barrel-type ion thrusters [37] but also would exceedthe performance of conventional hollow cathode sources suchas the previously investigated and comparably sized 8-cm ionthruster whose discharge production costs ranged between 250and 350 W/A [38].

IV. CONCLUSION

A magnetically enhanced inductive discharge was investi-gated for potential electric propulsion applications. The sourceis envisioned for service as the primary ionization stage forsmall diameter ion thrusters (<15 cm) or as a standalone plasmasource for an ambipolar accelerator. The inductive sourcefeatures a compact planar antenna that allows for very lowaspect ratio thruster configurations. A significant increase inplasma density was observed when the source was magneticallyenhanced using a magnetic sleeve. Ion production costs weremeasured as low as 250 W/A at low pressures on argon. It wasfound that, at low pressures (∼1 mtorr), ion production costimproved with increasing input RF power. This is speculatedto be related to the RF coupling efficiency. At low powers andpressures, capacitive coupling reduces ionization efficiency. AFaraday shield can minimize such coupling. It is not clear,however, that elimination of the capacitive coupling will bringabout improved performance at lower powers and pressures asthe capacitive fields, although inefficient from a plasma produc-tion standpoint, may be necessary for discharge maintenanceunder these conditions. Magnetic circuit optimization will benecessary to better utilize these fields as well as improveinductive coupling at the lower powers and pressures as well


as understand the capacitive effects. Projected ionization effi-ciency, ∼2 cm downstream of the antenna, was estimated usinga centerline Langmuir probe. It was found that ion productioncosts less than 200 eV/ion may be attainable over the pressurerange investigated which extended down to 1 mtorr on argon.The performance of the source on xenon is expected to yielda significant performance improvement. Based on the analysisof the data presented here, the preliminary projected dischargeperformance appears to be comparable to, if not, somewhatbetter than, ion thrusters of similar size such as the RIT 10 or theNASA 8-cm ion thruster. This is remarkable for an unoptimizedsource and therefore shows promise for future development.Future work will entail a spatial plasma mapping, magneticcircuit optimization, as well as the fabrication of a standalonesource. Once the operating envelope has been fully assessedof the standalone unit, concrete comparisons between it andexisting sources can be made. Ion energy profiles will also becharacterized at the exit plane of this device. While much ofthe initial focus of this paper has been on the operation of thedevice as an ion source for gridded ion thruster applications, ifthe exiting ion energies are controllable, then the device mayhave application as a standalone ambipolar thruster as well.Near term work will address this possibility.

ACKNOWLEDGMENT

The authors would like to thank B. Rogers, A. Covert,and B. Yee for their assistance in carrying out some of themeasurements that went into this work.

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John E. Foster received the B.S. degree in physicsfrom Jackson State University, Jackson, MS, in 1991and the Ph.D. degree in applied physics from theUniversity of Michigan, Ann Arbor, in 1996.

From 1996 to 1998, he was a Postdoctoral Re-searcher with the NSF Center for Plasma AidedManufacturing, University of Wisconsin–Madison.From 1998 to 2006, he was a Senior Research En-gineer with the NASA Glenn Research Center wherehe conducted research on electric propulsion devicessuch as ion thrusters. He is currently an Associate

Professor with the Department of Nuclear Engineering and Radiological Sci-ences, University of Michigan. His research areas of interests include plasmapropulsion and processes in low-temperature plasmas.

Eric D. Gillman received the B.S.E. degree in en-gineering physics from the University of Michigan,Ann Arbor, in 2007, where he is currently workingtoward the Ph.D. degree in the Department of Nu-clear Engineering and Radiological Sciences.

His primary research interests include basicplasma physics phenomena and their applications,specifically to aeronautical, astronautical, and spacesciences. He recently received a NASA Aeronau-tics Fellowship to perform research for his thesisdissertation on blackout mitigation for spacecraft

atmospheric reentry. His other areas of significant research interest include elec-tric propulsion, specifically dc, radio frequency, and microwave ion thrusters.

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