Measurements of the Electromagnetic Emissions from theMUSES-C Ion Engine System*†

Kazutaka Nishiyama, Yukio Shimizu, Ikkoh Funaki, Hitoshi Kuninaka, Kyoichiro TokiInstitute of Space and Astronautical Science

3-1-1 YoshinodaiSagamihara, Kanagawa, Japan 2298510

+81-42-759-8288nishiyama@ep.isas.ac.jp

IEPC-01-112

* Presented as Paper IEPC-01-112 at the 27th International Electric Propulsion Conference, Pasadena, CA, 15-19 October, 2001.†Copyright © 2001 by Kazutaka Nishiyama. Published by the Electric Rocket Propulsion Society with permission.

Radiated electric field emissions from the prototype model of the Ion Engine System (IES) ofthe MUSES-C mission were measured in accordance to MIL-STD-461E. The average noiselevel exceeded the narrowband specification at frequencies less than 5 MHz. The microwavedischarge neutralizer generates a broadband noise and narrowband oscillations which have afundamental frequency of about 160 kHz and are accompanied by its harmonics up to the 5th.The leakage of the 4.25 GHz microwave for plasma production and its second harmonic were65 dB and 35 dB above specification, respectively. The X-band receiver onboard theMUSES-C measured the noise from the IES at the up-link frequency of 7.16 GHz through ahorn antenna. This susceptibility test proved that the microwave discharge ion thruster willnever interfere the deep space microwave communication.

Introduction

In satellites driven by electric propulsion, considerableamount of electric power generated in space is put intodischarge plasma. There are many cases in whichexposing the current path to the space is inevitable, andthis will become large electromagnetic noise source.There is a review by Sovey et al. [1] on the results ofground-based and flight tests on ElectromagneticInterference (EMI) in electric propulsion systems whichdates back to the 1980's. As for ion propulsion systemsdeveloped after the SERT-I project, which is consideredto be the first ion thruster flight, all of them includingthe Japanese ETS-III [2],[3], ETS-VI [4], GermanRIT-10 [5] and British T5 [6] have undergone ground-based EMI tests. Although it is usual practice to followthe U.S. military standard MIL-STD-461 for EMIdocumentation of components onboard satellites, suchrequirements have not been fully satisfied by anyelectric propulsion systems (whether they be ionthrusters or other type) developed up until now. In thecase of NASA’s NSTAR ion engine used by the DS1probe, which is presently operating in interplanetaryspace, an EMI measuring device (Plasma WaveDetector) is onboard as one of several environmentmeasuring devices [7].

An ion engine system (IES) for the MUSES-C asteroidsample return mission is now under development at theInstitute of Space and Astronautical Science (ISAS),Japan[8]. The spacecraft will be launched in the winterof 2002 on an ISAS' M-V rocket from the KagoshimaSpace Center located on the southern tip of theJapanese island of Kyushu. MUSES-C’s EMIspecifications applied to all onboard components are amodification of the radiated emission (RE02)specification of MIL-STD-461C. The MUSES-C/IESwill be the first ion propulsion system using microwavedischarges, and its electromagnetic compatibility(EMC) with spacecraft has not been confirmed untilnow. Though it was estimated that the microwave of4.25 GHz generated by traveling wave tube amplifierscould leak to some extent, what sort of electromagneticnoise is radiated and from where exactly, werequestions that remained to be answered. Accordingly,we planed two RE02 measurements: one involving bothan ion engine and neutralizer and another one for justthe neutralizer.

The degradation of the sensitivity of the X-bandreceiver (XRX) is caused when the radiated emissionfrom IES is larger than the thermal noise level in theuplink frequency of the MUSES-C’s XRX. In the REspecification of the MUSES-C deep space probe,

spurious radiations in the frequency range of ± 5 MHzwith centers of 7.16 GHz and 6.36 GHz is limited to -6,+34 dBµV/m, respectively. The former is for the XRXsignal band and the latter is for its image band. Since itis difficult to measure such weak noise with standardEMI test equipments, a compatibility test using the IESand the XRX was necessary in order to assure that theIES satisfied the requirements. Hence, an interferencecheck which used a RF/IF signal processing part of theXRX as a high-gain amplifier was carried out after theIES’s EMI test. There is a general concern about theattenuation and phase change of the communicationwave propagating through the plasma plume. Althoughthis is also one type of EMC problem between theelectric propulsion and the communication systems ofthe satellite, we consider this not be the case forMUSES-C, in which thruster plume density is too lowto degrade the communication link, according to theresult of experiment and analysis [9].

In addition to radiated emissions, Conducted Emissions(CE) from the IES power processing unit (IPPU), forthe ion beam acceleration with a power consumption of1.2 kW and with three thruster units operating via thespacecraft power bus line, were also investigated. Also,the CE-02 measurement of the noise in the power linecurrent was envisaged using a prototype model (PM) ofthe IPPU and a commercial power supply as anunstabilized power bus simulator. Measurements ofconducted noise on the screen grid line and neutralizerline from IPPU were also carried out in order toexamine correlation with the conducted noise on the busline, as well as the noise source.

Experimental Arrangement

In this paper three EMI test results summarized inTable 1 will be reported. Antenna configurations andthe bandwidth of a spectrum analyzer in all experimentsare according to specifications in the latest edition ofMIL-STD-461E. In the RE02 measurements, severaltypes of antennas, which were selected depending onthe frequency range, a preamplifier, a preselector and aspectrum analyzer were utilized. These antennasinclude an active monopole, a biconical one, and twoconical log spiral antennas of different sizes dependingon the frequency range. Horn antennas, spectrumanalyzers, a digital voltmeter were employed in theinterference test with the XRX.

Table 1. Summary of MUSES-C/IES EMI Tests.IES RE02 &X-bandreceiverinterference

NeutralizerRE02

IES CE02

Environment Anechoicchamber

EMIchamber

IESendurancetest room

Chamber Uprightcylindricalglasschamber

T-shapedglasschamber

Metalchamber

Pumpingsystem

1 cryogenicpump

1 oildiffusionpump and 1rotary pump

8 cryogenicpumps

Ion source PM - PMNeutralizer PM PM PMPPU GSE GSE GSE / IPPU

(PM)Microwavegenerator

FM GSE GSE

Propellantfeed system

GSE GSE GSE

Thrustercontroller

ITCU (PM) Personalcomputer

Workstation

Frequency 14 kHz ~10 GHz,7.16 GHz

14 kHz ~1 GHz

100 kHz ~30 MHz

Radiated emission from the IES and interferencewith the X-band receiverThe anechoic chamber and the measurement room atISAS are shown schematically in the Figure 1. Thisfacility has radio wave absorbers attached to all wallsurfaces, as well as the ceiling and the floor, withelectromagnetic shields. This chamber is used forantenna pattern measurements, EMC tests betweenobservation equipments and telemeters or radartransponders of either scientific satellites or rockets,and for tests involving observation equipment withwireless transmission functions. In the RE02measurement and XRX interference test, a PM of theIES thruster control unit (ITCU), a PM of microwaveoscillator and a flight model (FM) of a traveling wavetube amplifier (TWTA), a PM of µ10 ion source and aPM of neutralizer were used. However, a PM of anIPPU which was not available due to its tight scheduleand propellant feed system which seemed to haveclearly no relation to EMI were replaced with ground

support equipments (GSE) comprising a mass flowcontroller and commercial high voltage DC powersupplies.

Radiated emission from the IES can be divided into twocategories: 1) Emission having its origin at the IPPU,such as switching noise or ripples; 2) Noise emanatingfrom the plasma acting as a “load”, which appearswhen the IPPU is operated in combination with the ionsource and the neutralizer. The former can be easilyreproduced and tackled through tests using resistiveloads simulating ion thruster heads. There was nodifference in conducted noise emission on the screen-and decel- grid lines between the case using a GSE andthe case using an IPPU, except apparent switchingnoises of IPPU at the switching frequency and itsharmonics. Therefore, if we concentrate on the noiseoriginated from the plasma, the RE measurement resultin the GSE operation can be considered same as themeasurement result of IES with EMI optimized flightmodel (FM) of the IPPU.

It is difficult to measure electromagnetic emission fromthe ion engine system due to the metallic tank for theusual operation on the ground. Also, a glass chamberconsisting of a cryopump, a cylindrical glass wall andan IES-mounting metal flange was prepared. An ionthruster and a neutralizer were hang from the top flange,and the chamber was evacuated using the cryopump.The IES was operated at Xe flow rates of 1.85 SCCMfor the main discharge and 0.4 SCCM for theneutralizer, which correspond to 80 % of the nominalthrust level, due to the limited exhaust capability of thecryopump. With this less flow rate, the higherbackground pressure in the glass chamber increased theion beam current to over 130 mA which corresponds tothe nominal thrust level of 7 mN.

Radiated emission from the neutralizerThe experimental setup to measure radiated emissionsfrom the neutralizer is shown in Figure 2. Theelectromagnetic shield characteristics of the EMIchamber at ISAS is better than the anechoic chamberdescribed before, and EMI measurements of smallonboard equipments are usually conducted in this room.Because the cylinder glass chamber was too large to becarried in the EMI chamber, it was not possible tocarry out the IES radiation noise measurement here. A30-mm-square punching steel plate, acting effectivelyas an anode, was placed 20-mm downstream from the

neutralizer. As a microwave supply system, a GSEwhich consisted of an oscillator and a solid-stateamplifier was used. The neutralizer was installed in thesmall T-shaped glass chamber, evacuated with an oildiffusion pump, and an electron current equivalent tothe one of nominal IES operation was extracted.

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAA

AAAAAAAAAAAA

Cryogenic Pump

OscillatorTWT Amplifier

Microwave Supply Unit with Cooling Fanand Clean Booth

ITCU

AAAAAAAAAAAAAAAA

AAAAAAAAAAAA

PC-1(CMD)

PC-2(TLM)

Neutralizer

Measurement Room with Shields

Anechoic Chamber

Cooling Water Xe Tank

Glass ChamberHe Compressor

Antenna

EMI Measurement System

Wave Absorber

Bus Simulator X-band Receiver

Ion Thruster

DC Power Supply,Xe Feed System

Figure 1 - Experimental setup to measure radiatedemissions from MUSES-C/IES and to check theinterference with the X-band receiver.

AAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAA

Oil Diffusion & Rotary Pumps

MicrowaveAmplifier(GSE)

Measurement Room with Shields

EMI Chamber

Mass FlowController

T-Shaped Glass Chamber

Antenna

EMI MeasurementSystem

PC

AAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAA

AAAAAA

DC Power Supply

Anode Neutralizer

Cooling Water

Figure 2 - Experimental setup to measure radiatedemissions from the neutralizer.

Conducted emission from the IESThis measurement was performed in a room equippedion engine endurance testing capabilities, which was notshielded from external electromagnetic noise. Themeasurement of the conducted noise can be simplycarried out only by installing current probes for the

wirings, and the effect of the external noise was notobserved whatsoever. Though the wirings used werecommon vinyl lines and were not flight-qualified, thishad no effect on the measurement results, even if theywere replaced by well-shielded coaxial cables.Accordingly, the cable type was considered to beindependent in the properties of the conducted noise.The measurement was carried out at 3 points: screencurrent line, neutralizer current line, and hot line of thespacecraft bus simulator. Because the PM of the IPPUhad not been well tuned considering the EMIcharacteristics yet and had its own switching noiseindependent of the load characteristics, themeasurement with the IES operating with a GSE inorder to distinguish plasma-oriented noise, was alsocarried out.

Experimental results and discussion

RE02 background noiseConcerning radiated emission measurements, inaccordance to RE02 specification of MIL-STD-461C,background noise was observed at the onset of theexperiment, when the ITCU and GSE including acomputer, vacuum pumps, and DC power supplieswere all kept switched on but not in operating condition.This noise factor was subtracted from the data from theoverall IES case as well as for the case of theneutralizer operating independently. Both narrowband(NB) and broadband (BB) E-field emissions weremeasured simultaneously, but only the former data arereported in this paper. The units for the vertical axis ofthe graphs of the RE02 results are given in dBµV/m.Figure 3a shows much more noise in the anechoicchamber than in the EMI chamber at frequencies lessthan 1 GHz, due to the superior electromagnetic shieldcapability of the EMI chamber. In the anechoicchamber, the external noise was more remarkable thanthe noise generated by the equipments inside. In bothfacilities there were a lot of frequency bands in whichbackground noise deviated the restriction of MIL-STDshown by the polygonal line in Figure 3a and are notideal for noise measurement. However, good shieldingcharacteristics were shown for frequencies over UHF,often used for telecommunications, and in those nonoise exceeded the internal noise of measuringinstruments over 1 GHz. Only the result of the anechoicchamber is shown in Figure 3b. The straight line whichshows monotonous increase in Figure 3b provides theMIL-STD-461C specification, and the two addednotches indicate an uplink frequency of 7.16 GHz for

MUSES-C’s XRX and its image frequency of 6.36GHz.

a)

b)Figure 3 - RE02 Background noise and EMIspecification of MUSES-C spacecraft.

a) 14 kHz ~ 1 GHzupper: in the anechoic chamber.lower: in the EMI chamber.

b) 1 GHz ~ 10 GHz in the anechoic chamber.

RE02 14 kHz ~ 1 GHzLow-frequency noise spectrums under 1 GHz duringthe IES operation are shown in Figure 4. Here,although the background noise is already deducted fromthe measured data, in the frequency band wherebackground noise is originally large, such noise can notbe totally removed. Figure 4a shows noise emission inthe condition of plasma discharge of both the ion sourceand the neutralizer. There was no noise larger than thebackground noise and it means that radiation from theIES in discharge mode is negligible. Once the highvoltage DC power supply was turned on and the ion

beam acceleration was started, a broadband noise witha tendency for the intensity to increase at lowerfrequencies, and which is apparently larger thanbackground noise as shown in Figure 4b, was generated.This deviates from the restriction by 30 ~ 40 dB atfrequencies under 5 MHz. Even though there are anumber of sharp upsurges and drops in the 2 ~ 4 MHzfrequency range, it is not clear whether this is due to theIES operation since the background noise reaches itsmaximum level around this frequencies and its randomfluctuation may influence the result.

Figure 5 shows the results of the neutralizer operationsin the EMI chamber. This set of data has comparativelysmaller effects of the background noise. It is not clearwhy the emission by the neutralizer discharge by itself,as shown in Figure 5a, is at maximum 20 dB largerthan the emission by both main and neutralizerdischarges as shown in Figure 4a. Figure 5b shows thenoise when the electron current is extracted to the anodefrom the neutralizer at a bias voltage about 30 V. Thisnoise spectrum resembles the spectrum emitted by theIES during the beam acceleration as shown in Figure4b, indicating that most of the low-frequency noiseunder 5 MHz is originated from the neutralizer plasma.Similar results for neutralizer noise have also beenreported for the T5 ion thruster [6] in combination witha Kaufman-type ion source and a hollow cathodeneutralizer. It will be added that both devices adopted aplasma production mechanism completely differentfrom the one in the MUSES-C/IES’.

a)

b)Figure 4 - RE02 Narrow band noise from the ionengine system.14 kHz ~ 1 GHz (with background noise subtracted)

a) The neutralizer and main discharge.b) The thruster at 7 mN.

a)

b)Figure 5 - RE02 narrowband noise from the neutralizer.14 kHz ~ 1 GHz (with background noise subtracted)

a) Neutralizer discharge.b) Electrons are extracted.

CE-02 100 kHz ~ 30 MHz for power line and noisein screen and neutralizer current linesThe low-frequency radiated emission below 5 MHzsuggests that conducted noise, which has a similarspectrum, influences other equipments on the spacecraftby propagating through four feeding lines includingscreen, accelerator, neutralizer, and return lines fromthe IPPU to the thruster heads, by way of theunstabilized spacecraft power line to which the threeIPPUs are connected. Incidentally, not only the usualCE-02 measurement on the power line but alsomeasurement of the spectrum of alternating currentcomponents, which are included in the current-to-screengrid and from neutralizer, were carried out. Theneutralizer and the decelerator grid were wired at thesame potential in such IES.

Figures 6 ~ 8 show the spectrum of the conducted noiseon screen- and neutralizer-current in the case of usingthe GSE of DC power supplies instead of an IPPU. Theunits in the vertical axis of the graphs for the CE02results are given in dBm, and its reference (0 dBm)corresponds to a current amplitude of 447 mA in thismeasurement, with the current probe gain and thespectrum analyzer’s input impedance. Note that a -20dB difference in this figure indicates an amplitudedecrease by one order of magnitude. Figure 6 andFigure 7 show the change of low-frequency noisesunder 300 kHz which were measured in the screencurrent line and the neutralizer current line in plasmadischarge mode and in beam acceleration mode. It isproven that the noise of the neutralizer current is verylarger than that of the screen current in beamacceleration mode, though the noises in plasmadischarge mode are small for both. This fact is so easyto notice just in monitoring neutralizer current historywith a multi-channel digital recorder that a 5-Hz filterfunction should be enabled to eliminate the noise whoseamplitude reaches 70% of the neutralizer current. Onthe neutralizer line, coherent oscillations which have afundamental frequency of 160 kHz and its harmonicsup to the fifth.Condition of the oscillation generation and thecharacteristic frequency are dependent on xenon flowrates and neutralizer currents. The oscillation was notobserved only in the conducted noise on the neutralizerline shown in Figure 7 but also radiated emissionspectrum measured by operating the neutralizer byitself as shown in Figure 5b. Although the narrowbandnoise peak found in the radiated emission from the IES

shown in Figure 4b has a slightly higher frequency ofabout 200 kHz, it may be regarded as fundamentallythe same phenomenon. The oscillation reappeared in thesame fashion with additional switching noise from theIPPU other frequencies when an IPPU instead of GSEswas used, and it was also the same case when theoperation of only the neutralizer was performed. Thisimplies that the neutralization processes in the ion beamand the power supply characteristics are unrelated toboth the oscillation and the low-frequency noise.Results concerning a more detailed investigation of thecorrelation between neutralizer operating conditionsand the oscillation, as well as its generation mechanism,will be reported later in this paper.

Figure 6 - CE measurement result for screen grid lineover the frequency range 0 to 300 kHz.

Figure 7 - CE measurement result for neutralizer lineover the frequency range 0 to 300 kHz.

The noise tails of the screen current and neutralizer athigh frequencies up to 5 MHz are both shown in Figure8. As for neutralizer there is almost no noise over 4MHz. Figure 8 shows that the screen current containssmall amount of noise in the 500 kHz ~ 4 MHzfrequency range. Incidentally, this finding cannot beestimated just by inspecting Figure 6, which shows the

decrease at relatively higher frequencies right below300 kHz.

Figure 8 - CE measurement results for screen grid lineand neutralizer line over the frequency range 0 to 5MHz.

Though there was conducted noise correlative withradiated noise in the wires connected with the ionthruster head, noise caused by the engine could not beobserved in the results concerning CE02 measurementsof the simulated spacecraft power line to the IPPU, asshown in the lower data of Figure 9. The upper trace ofFigure 9 shows the result when operating the IPPUconnected to resistors as dummy loads replacing thethruster head. However, significant differences in bothresults even in the low frequency region where the REand CE appear were not found. This is probablybecause most of the power consumption by the IPPU ismade by the beam current with much less noise, and thenoise power of the neutralizer is too small to influencethe bus power. By raising the frequency resolution ofthe spectrum analyzer, it turned out that the power linecurrent noise consists of switching noise of the IPPU atfrequencies of 100 kHz and its very high orderharmonic components. However, at present,irrespective of whether a resistive or plasma load isconsidered, the noise level exceeds the MUSES-Cspecification limits, which require emissions lower than40 dBµA below 3 MHz, and lower than approximately20 dBµA for the 5 ~ 17 MHz frequency range. It wasjudged that elimination of the switching noise leak willbe easy when manufacturing the FM of the IPPU, sincecircuit design and mounting considering the EMIcharacteristics had not been completely carried out inthe PM of the IPPU. Besides, the MUSES-Cspecification was too strict by 20 dB in comparisonwith MIL-STD, which was just satisfied by the PMIPPU, and can be relaxed to the same level. Since the

test clarified the fact that the noise from IES plasmadoes not affect the spacecraft power bus, the EMCtuning of the flight model is possible with the IPPU byitself without fitting to other IES components.

Figure 9 - CE02 measurement result for IPPU’s powerline over the frequency range 100 kHz to 30 MHz,compared with the result in resistive load operation.

More details on the low-frequency neutralizer noiseIn order to understand the characteristics of the noiseemission from the neutralizer at low frequencies,further parametric studies were carried out. Figure 10shows two typical time histories of the neutralizercurrent recorded with a digital memory recorder at arelatively slow sampling time of 50 µs. At a xenon flowrate of 0.5 SCCM there was noise with an amplitude aslarge as the 140 mA direct current component, asshown in Figure 10a. Nonetheless, it decreased inoverall amplitude to 1/3 of the noise level shown in theupper trace, at an excessive flow rate of 1.0 SCCM asshown in Figure 10b.

0 20 40 60 80 1000

50

100

150

200

250 140 mA, 33.9 V, 10 W, 1.0 SCCM

Neu

tral

izer

Cur

rent

(m

A)

Time (ms)

0

50

100

150

200

250

b)

a)

l = 15 mm, d = 4.0 mm, Sampling Time = 50µs

140 mA, 36.1 V, 10 W, 0.5 SCCM

Figure 10 - Time domain waveforms of the neutralizercurrent.a) Noise in nominal operation.b) Less noise at a larger mass flow rate.

Frequency spectra of conducted neutralizer noise areshown in Figure 11 for several operating conditions.Nomenclatures used are as follows.VD : Voltage between the neutralizer and the anode.ID : Electron emission current.Pµ : Microwave power for plasma discharge.&m : Xenon flow rate.

l: Anode distance from the neutralizerd: Neutralizer orifice diameterBroadband noise below 5 MHz and narrowband noiseby coherent self oscillation with harmonics can be seen.In the case with just a plasma discharge withoutelectron extraction taking place, conducted emissionwas not observed, as shown in Figure 11a. Afterincreasing the electron current to about 15 mA,broadband noise and the noise level saturated at thecurrent of 70 mA were generated. Comparing Figure11b and Figure 11c, it is seen that increasing xenonflow produces a decrease in broadband noise andsuppresses the oscillation. Comparison of Figure 11band Figure 11d suggests that it is impossible tosuppress the coherent oscillation, although thebroadband noise can be reduced by decreasing themicrowave power.

Next, a scaling of the fundamental frequency of thecoherent wave, ranging over 150 ~ 200 kHz wasinvestigated by changing parameters such as &m , d andl. Table 2 summarizes the frequency of appearance ofsuch oscillations when d and l are changed, and showsthat when the ratio of d to l is large, the oscillation isinfrequent. We may consider d as a characteristiclength describing the diameter of the plasma columnblowing out of the neutralizer, and l as a characteristiclength describing the plasma column length. Strictlyspeaking, a plasma column has diameter distribution inaxial direction, and increasing l may also increase thecolumn diameter. In that sense l and d are not perfectlyindependent parameters.

0 ~ 1 MHz 0 ~ 5 MHz

a) VD = 0 V, ID = 0 mA, Pµ = 10 W, &m = 0.5 SCCM.

0 ~ 1 MHz 0 ~ 5 MHz

b) VD = 42.0 V, ID = 140 mA, Pµ = 10 W, &m = 0.5SCCM.

0 ~ 1 MHz 0 ~ 5 MHz

c) VD = 33.0 V, ID = 140 mA, Pµ = 10 W, &m = 1.0SCCM.

0 ~ 1 MHz 0 ~ 5 MHz

d) VD = 62.1 V, ID = 140 mA, Pµ = 5 W, &m = 0.5SCCM.

Figure 11 - Noise spectra of neutralizer current.l = 20 mm, d = 4.5 mm, 10 dB/div.

Table 2. Coherent oscillation occurrence probabilityand parameters relating to the shape of plasma plume.

d (mm) / l (mm) 10 15 20 304.0 y ý ý ý

4.5 þ ý ý ý

5.0 - y ý ý

5.5 - - þ ý

6.0 - - - y

ý: Oscillation observed at various currents and flowrates.y: Oscillation observed at some currents and flowrates.þ: Oscillation never occurred. - : Not tested.

Although strong oscillations clearly provide a sharppeak, a weak oscillation tends to have a broad spectrumand its representative frequency is not obvious. This isthe cause of the ambiguity of the peak frequency and isexpressed in terms of error bars in the figures below.Figure 12 shows typical variations of oscillationintensities and fundamental frequencies as functions ofxenon flow rates. The intensity usually has a peak atcertain flow rate and decreases at larger flow rates, asshown in Figure 12a. Figure 12b illustrates a weakdependence of the frequency on the flow rate and isinclined to decrease as the flow rate increases.

Figure 13 provides the fundamental frequencyvariations with respect to neutralizer current. Theneutralizer current was swept from 100 to 300 mA withan orifice diameter and xenon flow rate fixed at 4.5 mmand 0.5 SCCM, respectively. The frequency alwaysincreases by increasing the current. Four anodedistances l were chosen as a parameter, and it is shownthat the larger the value of l is, the lower the value ofthe frequency, even though such dependency is ratherweak.

Figure 14 shows the frequency change as a function ofthe orifice diameter d. The frequency is smaller when dis larger. In spite of the change of d from 4.0 to 6.0 bya factor of 1.5, the factor by which the accompaniedchanges lies between 1.3-1 and 1.1-1, which can beinterpreted as a nearly inversely proportional relationbetween f and d. With fixed flow rate, a 1.5 timeslarger orifice makes the neutral pressure in theneutralizer discharge chamber 2.25 times higher.Because the relationship between the flow rate and thefrequency f has a negative slope, as given in Figure 12,

if the pressure is maintained constant, f and d will showa more evident inversely proportional relation.

0.2 0.4 0.6 0.8 1.0140

160

180

200

220

b)

a)

l=30, d=4.0, 200mA l=20, d=4.5, 200mA l=20, d=4.5, 300mA l=20, d=4.0, 140mA l=20, d=4.0, 180mA

Osc

ilatio

n F

requ

enc

y (k

Hz)

Xenon Mass Flow Rate (SCCM)

0.2 0.4 0.6 0.8 1.0-70

-60

-50

-40

-30

Osc

ilatio

n A

mpl

itud

e (d

Bm

)

Figure 12 - Typical variation of oscillation intensitiesand fundamental frequencies as functions of xenon flowrates. Pµ = 10 W.

100 150 200 250 300120

140

160

180

200

220

240

l=10, d=4.5, 0.5SCCM l=15, d=4.5, 0.5SCCM l=20, d=4.5, 0.5SCCM l=30, d=4.5, 0.5SCCM

Osc

ilatio

n F

req

uen

cy (

kHz)

Neutralizer Current (mA)

Figure 13 - Fundamental frequency versus neutralizercurrent curves weakly dependent on the anode distancesl. d = 4.5 mm, &m = 0.5 SCCM, Pµ = 10 W.

100 150 200 250 300110

120

130

140

150

160

170

180

190

l=30, d=4.0, 0.5SCCM l=30, d=4.5, 0.5SCCM l=30, d=5.0, 0.5SCCM l=30, d=5.5, 0.5SCCM l=30, d=6.0, 0.5SCCM

Osc

ilatio

n F

req

uen

cy (

kHz)

Neutralizer Current (mA)

Figure 14 - Fundamental frequency versus neutralizercurrent curves dependent on the orifice diameters d. l = 30 mm, &m = 0.5 SCCM, Pµ = 10 W.

What is the cause of the coherent noise in themicrowave discharge neutralizer at 160 kHz? When acurrent is applied to plasma, oscillations at frequencieslower than the ion plasma frequency are often observed.Such phenomenon is generally known as current-drivenion acoustic instability. There are reports that in mostcase it is an oscillation in axial direction along with theelectron current, whose wavelength is twice the plasmacolumn length and whose phase velocity equals the ionsound velocity or the ion thermal velocity [10],[11],[12].Although there are several interpretations for theoscillation mechanism, it is generally thought that thereciprocal of the ion transit time τi , calculated by usingthe ion thermal velocity and the plasma column length l:

1 3τ i i p= kT m l

basically represents the oscillation frequency, where kis the Boltzmann constant, mp is the ion mass and Ti isthe ion temperature. Assuming Ti = 0.04 eV, a value of14 kHz is obtained. However, this value is muchsmaller than the experimentally observed value of 160kHz, and certainly too small to explain the oscillationby an longitudinal oscillation mode.

Nevertheless, there is a report by Crawford [13] inwhich the radial mode of an ion acoustic wave is mostprobable. He conducted experiments by using severalkinds of direct current discharge plasmas with hotcathodes and various diameter glass tubes, and foundlow frequency oscillations below 1 MHz, exactly as inour results. According to this publication, thefundamental frequency f is given by

f d kT m= ( .405 / )( / ) /2 1 2π γ e p

where d is the diameter, γ is the specific heat ratio (=from 1 to 3) and Te is the electron temperature. Theconstant 2.405 is the minimum number which makesthe Bessel function zero. Assuming Te = 3.5 eV and f =160 kHz, we obtain d = 7.7 (γ = 1) ~ 13.2 (γ = 3) mm,which is comparable to the real orifice size or slightlylarger. This and the experimentally obtained relation, fd= const., suggests that the radial mode of standing ionacoustic wave exists in the plasma column in front ofthe microwave discharge neutralizer.

RE02 1-10 GHz and X band receiver interferencetest results

Microwave leakage at frequencies of 4.25 GHz and 8.5GHz were detected when microwave dummy loadsreplaced the ion source and the neutralizer in theanechoic chamber. Still, their levels were below theMUSES-C limit by 10 dB and we did not specify theleakage position. The radiated emissions in thedischarge mode and beam acceleration mode were toostrong to measure with the nominal setup of themeasuring instruments. As shown in Figure 15 theobtained data is saturated at the two frequencies. The4.25 GHz peak has a spread which is originated fromthe spurious characteristics of the TWTA and not fromnon-linear phenomena of the discharge plasma. Theseemissions at two frequencies are caused by leakage ofthe supplied microwave power through the grid holes ofthe ion beam optics and the neutralizer orifice.

The radiated emissions measured again without the pre-amplifier normally used, with a 20 dB attenuator, aresummarized in Table 3. The microwave leakage was 10dB larger in the beam acceleration than in the plasmadischarge at the electron cyclotron resonance frequencyof 4.25 GHz. It is shown that the leakage increases asthe plasma density is lower. As for the emission at thesecond harmonic frequency of 8.5 GHz, the leakage isindependent of the plasma density, and it is indicated

that there is originally no absorption by the plasma atthat frequency. In this experiment using the cylindricalglass chamber, the thrust level was throttled to 80 %,but it turned out to be a more severe testing conditionfrom the viewpoint of microwave noise emission, inwhich the plasma density was lower and noise leakageincreased. If we assume that the leaked electromagneticwave from the IES is a spherical wave, the leakedpowers can be calculated as 66 mW at 4.25 GHz and1.9 mW at 8.50 GHz. For effective utilization ofsupplied microwave power of about 40 W for plasmageneration in an ion source and a neutralizer, theselosses are sufficiently small and not problematic.

In order to provide a reference to reflect the radiatedsusceptibility (RS) specification of MUSES-C, E-fieldintensities at a distance of 1 m were calculated asshown in Table 3, given in V/m units. In the originalMUSES-C specification the required level had beendetermined as 5 V/m for high frequencies over 30 MHz.As an illustrative example, assuming the worst case inwhich a equipment is located at a distance of 0.3 mfrom thruster heads and three ion engines are operatedsimultaneously, the E-field intensity at that positionwould be calculated as

3 3 1 0 0 3 3 192. ( . . )× × = V m .

Thus the RS specification at 4.2 GHz was modified to20 V/m and the RE specification was waived for theIES. The philosophy of the RS specification at the RFfrequency for plasma production of microwavedischarge ion thrusters may be identical with othertransmitters onboard spacecraft.

Figure 15 - RE02 Narrow band noise from the ionengine system at 7 mN.1 - 10 GHz (with background noise subtracted)

Table 3. Microwave leakages at a distance of 1 m atoscillator frequency, its second harmonic and X-banduplink frequency of MUSES-C spacecraft.

4.25 GHz 8.5 GHz 7.16 GHzOperating at7 mN

130.4dBµV/m(3.3 V/m)

99.7 dBµV/m(0.097 V/m)

-48.72dBµV/m

Main andneutralizerdischarge

119 dBµV/m(0.89 V/m)

99.2 dBµV/m(0.091 V/m)

-48.82dBµV/m

The power density at the 7.16 GHz X-band uplinkfrequency was calculated from the results of theinterference test with the PM of the XRX. Table 4shows the change of the noise level detected by theXRX depending on the operating condition of the IES.Because a larger amount of noise was observed whenthere was no plasma or its density was smaller, it isthought that very weak spurious signals originating atthe microwave amplifier leaked from the ion source aswell as from the neutralizer at 7.16 GHz. The low gainantenna C (LGA-C) of the MUSES-C spacecraft,which is located most closely to the IES and is easilyaffected by radiated emissions, is designed to output anestimated noise power of -210.2 dBm/Hz consideringthe distance, the angle separation and the antenna gainpattern. This is sufficiently small compared to theinternal noise level (-173 dBm/Hz ) of the receivingsystem, and so it is concluded that no adverse effect isgiven to the XRX, even in the case when the three ionengines are operating at the same time with themaximal thrust levels.

Table 4. Noise levels observed with the X-bandreceiver at several IES conditions.

Ion source Neutralizer Noise levelBeam extraction Electron extraction 0.6 dBDischarge Discharge 0.5 dBDischarge Microwave

introductionwithout Xe gas

0.7 dB

Microwaveintroductionwithout Xe gas

Microwaveintroductionwithout Xe gas

1.1 dB

No operation No operation 0 dB(Backgroundnoise floor)

Conclusions

Comparing the results of EMI measurements using theIES of the MUSES-C asteroid sample return missionand the neutralizer operating independently, it wasfound that radiated emissions from the IES exceedingthe MIL-STD-461C originate mainly from theneutralizer. Radiated emission in the high frequencyrange above 1 GHz was not observed, except at theoscillator frequency and the second harmonic frequencyof the TWTA utilized for plasma production, whichleaks from the exit of both the ion source and theneutralizer. It is concluded that the operation of the IESdoes not interfere with the XRX of the MUSES-C,based on the findings of the test using the PM of theXRX. Although the IES needs no longer satisfy REspecification, the final check for electromagneticcompatibility between the IES and the spacecraft, notonly in steady-state operation but also in transientphenomena, such as high voltage breakdown betweengrids, will be carried out by the operating an ion enginein the glass chamber located in the vicinity of the flightmodel of the MUSES-C spacecraft.

Acknowledgments

The authors wish to thank Professor Z. Yamamoto andMr. Y. Kamata of ISAS, Mr. F. Fuke of NEC and therest of the MUSES-C communication subsystem stafffor their cooperation concerning this series of tests.

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