p-mhd-02-1113

7/29/2019 P-MHD-02-1113

1/6

1

American Institute of Aeronautics and Astronautics

AIAA 2002-1113

MAGNETO-PLASMA-DYNAMIC THRUSTERS FOR

SPACE APPLICATIONS

M. Andrenucci*, V. Antoni**, M. Bagatin**, C.A. Borghi, M.R. Carraro, A. Cristofolini,R. Ghidini, F. Paganucci*, P. Rossetti*, G. Serianni**

* Centrospazio, Pisa Research Consortium, Via A. Gherardesca 5, 56014 Pisa** RFX Consortium, Association Euratom-ENEA on Fusion, Corso Stati Uniti 4, 35127 Padova

Department of Electrical Engineering, University of Bologna, Viale Risorgimento 2, 40136 Bologna

ABSTRACT

The paper describes the first results of an experimen-tal investigation on the critical regimes of a gas-fed,

applied field Magneto-Plasma-Dynamic thruster (also

said Hybrid Plasma Thruster, HPT). A hollow cath-ode with a 20 mm diameter is placed in the center ofa cylinder with an inner diameter of 200 mm, whichacts as anode. Eight straps divide the region inside thecylinder into a central chamber and a peripheralchamber. The thruster is powered by a pulse with anelectrical current up to 8 kA for 2.5 ms. Tests were

performed using argon with a mass flow rate of 660mg/s, both with and without an externally appliedmagnetic field. Values of the field on the thruster axiswere of 40 and 80 mT.

Time and space behavior of electron temperature,density and floating potential have been measured bymeans of a system of 7 aligned electrical probes, in-serted into the thruster plume. The balanced tripleprobe configuration has been utilized. It is shown that

the external magnetic field flattens the density profileand increases the fluctuation levels of electron tem-perature and density.

The radiation emitted by the discharge plume, hasbeen observed by means of an optical multi-channelanalyzer. The observations refers to a time interval of0.5 ms. In the spectrum, lines of neutral argon and ofsingly ionized argon ions, are present. The lines ofthe metals of the electrodes are not observed. The in-

tensity of the ion lines increases both with the en-hancement of the applied voltage of the pulse andwith the use of the external magnetic field. The aver-age value of the electron temperature derived fromthe line intensity, is in accordance with the averagevalue measured by the probe method.

INTRODUCTION

Magneto-plasma-dynamics (MPD) thrusters are cur-rently under investigation as a possible high-power

electric propulsion system for space missions. Theaddition of a magnetic field has proved to increase

the performance of these devices. However, criticalregimes, observed when the power is increased, limitat present the performance. In the critical regime,

large fluctuations in cathode and anode voltage aremeasured.

The research projectExperimental Investigation on a

Magneto-Plasma-Dynamics Thrusters for Space Ap-

plications, aimed to understand the deterioration ofthe thruster performance, the origin of these largefluctuations, and the role of them, has been set up.Three research units are co-operating to the project.These are Centrospazio of Pisa Research Consortium,RFX Consortium of Padova of Association Euratom-.ENEA on Fusion, and Department of Electrical En-

gineering of University of Bologna.

In this paper the first results of the investigation men-tioned above, are presented. These refer to measure-

ments obtained by means of an array of electricalprobes inserted into the plasma plume and to meas-urement radiation emission of the plasma done bymeans of an optical multi-channel analyzer.

EXPERIMENTAL DEVICE

The experimental device is shown in Fig.1. It is anaxis-symmetric MPD thruster with an applied mag-netic field (Hybrid Plasma Thruster - HPT).1,2,3,4 TheHPT, developed at Centrospazio, consists of a hollowcathode (copper, 20 mm diameter, 50 mm length) ly-

ing on the thruster axis, through which 70-90% of agaseous propellant is injected into the dischargechamber, and an anode, consisting of a cylinder cen-tered on the axis (aluminum, 200 mm inner diameter,

180 mm length). Eight straps, made of copper, dividea central chamber from a peripheral chamber. 10-30%

of the propellant is injected through eight peripheralhollow electrodes (copper, 12 mm in diameter each).These electrodes can be used as auxiliary cathodes topre-ionize the peripheral propellant by means of asecondary discharge between these cathodes and theanode. The peripheral electrodes were not activated

during the tests described herein. During the experi-ments described in this paper argon is utilized as pro-

pellant. A 70-turns coil surrounding the thruster, cangenerate a magnetic field up to 100 mT on thethruster axis.

The HPT thruster is mounted on a thrust stand insidethe Centrospazio IV3 vacuum chamber, which main-

7/29/2019 P-MHD-02-1113

2/6

2


tains a back pressure of the order of 10 -4 mbar duringthe pulse. The electric power for the HPT is generatedby a Pulse Forming Network (PFN), which supplies

quasi-steady current pulses during 2.5 ms. The pro-pellant is injected by two gas feeding systems, one forthe central cathode and the other for the peripheral

cathodes, based on fast acting solenoid valves, whichprovide gas pulses with a long plateau after few milli-seconds from valve activation3. When a steady state

mass flow is reached, the discharge takes place byswitching on an ignitron which closes the electric cir-cuit. The magnetic field solenoid is powered by a DCgenerator, switched on few seconds before the dis-charge. Fig. 2 shows typical current and voltage sig-nals of the HPT thruster.

Fig. 1. Schematic drawing of the thruster assembly.

5.0

4.0

3.0

2.0

1.0

0.0

DischargeCurrent[kA]

4.03.53.02.52.01.51.00.50.0

Time [ms]

140

120

100

80

60

40

20

0

AppliedVoltage[V

]

4.03.53.02.52.01.51.00.50.0

Time [ms] Fig. 2. Current and arc voltage as functions of time.

DIAGNOSTICS DESCRIPTION

The probe system consists of 7 aligned graphite elec-trodes, 8 mm apart from each other, and housed in aboron-nitride case5. The probes were used in a five-

pins balanced triple probe configuration to avoid

phase shift problems and to obtain simultaneousmeasurements of electron density and temperature. Inorder to obtain measurements of both average value

and fluctuations of electron temperature, electrondensity and plasma potential, the signal bandwidthwas 500 kHz and the sampling frequency was 10

MHz. The central electrode was placed at 80 mmfrom the thruster outlet, along the thruster axis. Withrespect to this position, other measurements were per-

formed by moving the probe by 115 mm in the direc-tion perpendicular to the axis of the thruster.

Optical observations have carried out been by means

of an Optical Multi-Channel Analyzer (OMA system)consisting in a Meshell 900 detector which integrates

a CCD system and a spectroscope6. The maximal

time resolution is of 100 ns and the maximal wave

length resolution is equal to /900, where is thewave length considered expressed in nano-meter (at500 nm the resolution is 0.56 nm). A collector hasbeen connected to the OMA system through a fiber-

optic. The collector is focused on the axis of theplasma plume at a distance of 70 cm. The focus is 8cm away from the thruster outlet. The region of theplasma observed has a circular cross section of a di-ameter of 1 cm. The scheme of the optical system isshown in Fig. 3.

Fig. 3. Scheme of the optical system.

THRUSTER PERFORMANCE

Tests were performed at a mass flow rate of 660 mg/sof argon (600 mg/s injected through the central cath-ode, 60 mg/s through the peripheral electrodes). Testswithout applied magnetic field and with applied in-

duction fields of 40 mT and of 80 mT where done.Fig.s 4 and 5 show the measured electrical character-istics and thrust. For each shot, discharge current andvoltage were measured by averaging on a time inter-

val of 100 microseconds in the middle of the pulse.The thrust value was obtained by a ballistic method

which measures the total impulse (momentum) of thethrust for each shot. The thrust is obtained by divid-

Collector

HPTPlasma

70 cm

8 cm

7/29/2019 P-MHD-02-1113

3/6

3


ing the measured impulse by the pulse duration. Thusthis value represents an average thrust of each shot.Each data point in Fig.s 4 and 5 was obtained as an

average of the relevant values obtained from four-fiveshots at the same nominal condition (i.e. VPFN: charg-ing voltage of the pulse forming network). Current

and voltage measurements showed a good repeatabil-ity (the uncertainty is within the marker dimension).Error bars on thrust measurements include both the

standard deviation and the measurement uncertainty(about 10% of the thrust value)

2, 3,4.

0

20

40

60

80

100

120

140

160

0 2000 4000 6000 8000

Current (A)

Arc

Voltage(V)

600-60 mg/s; 0 mT

600-60 mg/s; 40 mT

600-60 mg/s; 80 mT

Fig. 4. Electrical characteristics (660 mg/s of argon).

0

1

2

3

4

5

6

78

9

10

0 1000 2000 3000 4000 5000

Current (A)

Thrust(N)

200-20 mg/s ; B=0 mT

200-20 mg/s ; B=40 mT

200-20 mg/s ; B=80 mT

Fig. 5. Thrust as a function of the current

(660 mg/s of argon).

PROBE MEASUREMENTS

The mean parameters characterizing the plasma of theplume and their fluctuations were measured by meansof the probe system.

Mean Parameters

To relate the thrust to the plasma main parameters,the electron density and temperature, averaged in a

time interval of 0.1 ms around t= 1 ms from the dis-

charge initiation have been analyzed for different in-put powers (i.e of and of magnetic fields. In Fig. 6.a

the electron density on the truster axis (r= 0) and at adistance of 115 mm from the axis (r= 115 mm), isshown for different values ofVPFN. It appears that the

density, which is typically of the order 1020

m-3

, has apeak centered on the axis. When the input power wasincreased, the difference between the values on the

axis and outside the axis resulted slightly increased,indicating a tendency to peaking with VPFN. The ef-fect on the density of the external magnetic field Bext= 40 mT is shown in Fig. 6.b. The presence ofBextreduces the electron density on the axis and increasesit at r= 115 mm. The profile of electron density re-sulted to be flattened. This tendency increased at highVPFN. The ratio between the density value at r= 0 andr= 115 mm is shown in Fig. 7 as a function ofVPFN.

The values range from 6 to 5 without Bext and from2.5 to 1.5 withBext.

Fig. 6. Electron densities on the thrust axis and at a dis-

tance of 115 mm from the thruster axis without magneticfield (plot a) and with a magnetic induction of 40 mT

(plot b), as functions of VPFN.

Fig. 7. Ratio of the electron densities on the thrust axis and

at 115 mm from the axis without magnetic field and with amagnetic induction of 40 mT as a function of VPFN.

The electron temperature showed a different depend-ence onBext. In Fig. 8.a the values ofTe at the two dif-ferent radial positions of the observations, withoutBext, are plotted as a function of VPFN. Typical values

range from 3 to 5 eV with a slight increase ofVPFN.The temperature profiles are rather flat with small dif-

7/29/2019 P-MHD-02-1113

4/6

4


ferences between values on the axis and outside of it.Fig. 8.b shows that the external magnetic field causedan increase ofTe. This assumes values of 6 9 eV. In

Fig. 9 the ratio between Teat r= 0 and r= 115 mmfor tests with and without external field are shown.

Hence withoutBext the density profile has a peak cen-

tered on the thruster axis and with the presence of anexternally applied magnetic field the temperature pro-file is flatter. Moreover the effect of an externalBextisto increases the temperature and to reduce the den-sity.

Fig. 8. Electron temperatures on the thruster axis and at adistance of 115 mm from the thruster axis without mag-

netic field (plot a) and with a magnetic induction of 40 mT(plot b), as functions of VPFN.

Fig. 9. Ratio of the electron temperatures on the thrusteraxis and at 115 mm from the thruster axis without mag-

netic field and with a magnetic inductionof 40 mT, as a function of VPFN.

Fluctuations

The study of the electrostatic fluctuations has been

addressed to characterize the regimes with and with-out external field and the onset of the critical regimesat higher power. These regimes are characterized by a

high level of fluctuations of the electrode voltage, andby a decrease of the electron density close to thecathode.

Fig. 10. Te and ne as functions of time (in each plot t

ranges from 0 to 5 ms) for VPFN= 1400 V. The upper plotsare done for Bext= 0, the lower ones for Bext= 40 mT.

Fig. 11. RMS of Te and ne as functions of time (in each plot

t ranges from 0 to 5 ms) for VPFN= 1400 V. The upper plotsare done for Bext= 0, the lower ones for Bext= 40 mT.

In Fig. 10 the time evolution of the electron density

and of the electron temperature in two different shotswith and without external magnetic field, are shown.In the signals a high level of fluctuations is present.The time evolution of the corresponding Root Mean

Square (RMS) of the signals, obtained by averagingover 0.01 ms time intervals, is shown in Fig. 11. The

utilization of an externally applied magnetic field

causes an increase of the absolute level of RMS forTeand a reduction of the RMS of the electron density. InFig.s 12 and 13, the mean values of the RMS normal-ized to the mean electron density and electron tem-perature are plotted as functions of applied voltage.

Each point is obtained averaging over an interval of0.1 ms around t = 1 ms.

The normalized RMS for electron density fluctuations

is about twice that for temperature fluctuations. Theutilization of an externally applied magnetic fieldcause an increase of the normalized RMS. Higherpowers result in larger RMS of the temperature fluc-

tuations. The normalized RMS values slightly dependon the radial location.

7/29/2019 P-MHD-02-1113

5/6

5


Fig. 12. Average the RMS value of ne normalized to theaverage ne as a function of VPFN in two two different posi-

tions, and Bext= 0 (plot a), and Bext= 40 mT (plot b).

Fig. 13. Average the RMS value of Te normalized to the

average ne as a function of VPFN in two two different posi-tions, and Bext= 0 (plot a), and Bext= 40 mT (plot b).

OPTICAL MEASUREMENTS

The radiation emitted by the plasma of the thrusterplume has been measured by means of the opticalsystem shown in Fig. 3. The observation interval was

of 0.5 ms starting at the start up of the PFN pulse, af-ter 0.5 ms, and after 1 ms. Shots with VPFN = 900,1100, 1200, 1300 and 1400 V, without externally ap-

plied magnetic field, and with VPFN = 900, 1200 and1400 V forBext = 40 and 80 mT, have been done. Inorder to decrease the influence of the backgroundnoise, each shot with a defined condition has beenrepeated three times and the measured signals havebeen averaged.

Spectra of the radiation observed during typical testsare shown in Fig. 14. The figure refers to two testsforBext = 0 andBext = 40 mT respectively. The obser-

vation time interval is between 1 and 1.5 ms from thepulse start up. In each measured spectrum, lines emit-ted by neutral argon atoms and by singly ionized ar-

gon ions are observed. The argon ion lines lie in therange 400-650 nm and the neutral argon lines lie inthe range 650-900 nm. This has been observed during

all tests. Lines of metals of the electrodes have notbeen detected.

The intensity of the ion lines is much higher than that

of the neutral lines. Moreover the intensity of ionlines is increased both by increasing the PFN voltageand the externally applied magnetic field. Duringtests at different VPFN and Bext, the variation of theline intensity emitted by the neutral argon atoms wasmuch lower than that of the ion lines. These events

indicate that the number of ions is much greater thanthe number of neutral atoms and that the plasma is

nearly approaching full ionization.

Fig. 14. Spectrum of the radiation emitted by the plasmafor VPFN= 1200 without external magnetic field a (plot a),

and for Bext= 40 mT (plot b).

An analysis of the absorption of emitted radiation was

made at electron temperatures and densities of theorder of magnitude obtained by probe system. The

dominant mechanism of line broadening is Dopplerbroadening. The plasma in the wave-length range ofthe ion lines, results to be optically thick with an ab-sorption length of few millimetres. The radiation ob-served comes from the first layers of the plasmaplume surface. In the wave length range of the lines

of neutral argon the plasma is optically thin and theradiation detected comes from the whole region of theplasma crossed by the optical axis.

By means of the Boltzmann plot of excited states of

argon the electron temperature has been determined.Excited states of singly ionized argon ions have beenconsidered only as the intensity of neutral lines is too

0

10

20

30

40

50

60

70

80

400 450 500 550 600 650 700 750 800 850 900

Wavelength [nm]

Intensity[uW/cm^2nm

a)

0

100

200

300

400

500

600

400 450 500 550 600 650 700 750 800 850 900

Wavelength [nm]

Intensity[uW/c

m^2nm]

b)

7/29/2019 P-MHD-02-1113

6/6

6


low in comparison with the background signal. In or-der to consider the absorption of these lines their in-tensity have been corrected by means of the escape

factor. A typical Boltzmann plot obtained from theline intensity observed during a test with VPFN= 1200V, Bext = 40 and the observation time between 1 and

1.5 ms, is shown in Fig. 15. The electron temperaturederived is 2.2 eV.

Fig. 15. Boltzmann plot obtained from the ion lines for with

VPFN= 1200 V, Bext= 40 andt between 1 and 1.5 ms.

CONCLUSIONS

A research project aimed to investigate the conditionswhich causes the deterioration of the performance ofHPT thruster have been set up. The investigation iscarried out in the thruster device of Centrospazio ofPisa Research Consortium. The plasma conditions of

the thruster plasma plume have been observed bymeans of a probe system ad by means of optical

measurements realized through an optical channelanalyzer. The first results of the investigation are de-scribed.

Temperature and density profiles have different be-havior with power and externally applied magneticfield. An externally applied magnetic field flattens thedensity profile. Fluctuation analysis reveals that nor-malized RMS of density are larger than those fortemperature and that the external magnetic field

causes an increase of the fluctuation level.

Radiation spectra show the presence of ion and neu-tral atoms of argon. The line of ions are in the opti-

cally thick region and the line of neutrals are in anoptically thin region.

The intensity of ion lines is much higher than that ofneutral lines. The former increase by increasing thepulse voltage and the external magnetic field. Appar-

ently a small variation of the neutral lines by variingthe pulse voltage and the external magnetic field isobserved. From the ions lines the Boltzmann plot of

the singly ionized ions have been derived. Electrontemperatures of few eV have been obtained in agree-ment with the temperature obtained by the probe

measurements.

AKNOLEDGEMENTS

The Experimental Investigation on a Magneto-Plasma-Dynamics Thrusters for Space Applicationsproject in the frame of which this work has been

done, is founded by MIUR (Ministry of Universityand Research of Italy).

REFERENCES

1. V.B. Tikhonov et al.,Investigation on a New Typeof MPD Thruster, OR 21, 27th European Physical

Society Conference on Controlled Fusion andPlasma Physics, 12-16 June 2000, Budapest,

Hungary, 12-16 June 2000.2. V.B. Tikhonov et al.,Development and Testing of

a New Type of MPD Thruster, IEPC-01-123, 27thInternational Electric Propulsion Conference, Oc-

tober 14-19, 2001, Pasadena, CA.3. F. Paganucci et al., Performance of an Applied

Field MPD Thruster, IEPC-01-132, 27th Inter-

naa New Type of MPD Thruster, IEPC-01-123,27th International Electric Propulsion Confer-ence, tional Electric Propulsion Conference, Oc-tober 14-19, 2001, Pasadena, CA.

4. F. Paganucci, P. Rossetti, M. Andrenucci, Per-

formance of an Applied Field MPD Thruster witha Pre-ionization Chamber, this conference, paperAIAA 2002-1109.

5. G. Serianni, et al.,Plasma Diagnostics in an Ap-plied Field MPD Thruster, 27th Internation Elec-tric Propulsion Conference, IEPC-01-135,

Pasadina (2001).6. C.A. Borghi, M.R. Carraro, A. Cristofolini, and

R. Ghidini, Theoretical and experimental Inves-tigation on a Magneto-Plasma-dynamicThruster, this conference, paper AIAA 2002-1112.

-40

-35

-30

-25

-20

19 19.5 20 20.5 21 21.5

p-mhd-02-1113

Documents