1799 IEPC-93-19 7

DIAGNOSTIC EXPERIMENT AND NUMERICAL ANALYSIS OFONE-DIMENSIONAL MPD FLOWFIELDS

H. Tahara, T. Tsubaki", Y. Kagaya*, Y. Tsubakishita" and T. Yoshikawa*Faculty of Engineering Science, Osaka University

Toyonaka, Osaka, Japan

Abstract flowfields have been made by many researchers - '7 , inwhich the electric field is determined by the point of the

A quasisteady magnetoplasmadynamic (MPD) arcjet sonic or the magnetosonic singularity. These calculatedwith 30-cm-class long electrodes, i.e., with a long results are significant for optimum design of MPD arcjetacceleration zone almost realizing a one-dimensional chambers and for understanding of the discharge featurechannel flow, was developed to examine current and the acceleration mechanism. Particularly, thedistributions in the discharge chamber. It was found that streamwise discharge current distribution is importantthe current patterns depended strongly on gas species and because the processes of heating and acceleration aredischarge current levels regardless of electrode length and determined positively by the current pattern. 2-4

6

that the feature on current conduction below the theoreticalcritical current was divided into two cases, depending on In the present paper, discharge current distributionsmonatomic and molecular gases. These experimental are examined using an MPD arcjet chamber with longresults agreed with analytical ones. Furthermore, in the electrodes, i.e., with a long acceleration zone almosttransition to a quasisteady current pattern at discharge realizing an one-dimensional channel flow. Thestart the travel of a current sheet was observed. The experimental results are compared with analytical ones.velocity for each gas increased linearly with the dischargecurrent although it was much smaller than the quasisteady II. Experimental Apparatusexhaust velocity evaluated by a pendulum method. Ata constant discharge current, the motion of the current The coaxial MPD arcjet newly designed, as shown insheet depended only on molecular weight. Fig.1, is provided with a straight anode 50 mm in diameter

280 mm deep. The long electrode with the longI. Introduction acceleration zone almost realizes an one-dimensional

channel flow. The anode made of copper is dividedThe quasisteady magnetoplasmadynamic (MPD) arcjet axially into 14 anode parts, each of which is electrically

is a promising propulsion device which utilizes principally insulated to one another. The current entering eachelectromagnetic acceleration of the interaction between anode segment is measured with a Rogowski coil, asdischarge current of kiloamperes and azimuthal magnetic shown in Fig.2, to examine current fractions on the anode,field induced by the discharge current. Various i.e., to infer the current pattern in the interelectrode region.propellants. such as light gases of H, and He, heavy Also, the cylindrical cathode 9.5 mm in diameter x 250gases of Ar, Xe and SF,, and waste ones in space mm long, respectively, are made of thoriated tungsten.stations etc, can be used because the thrust depends onlyon the discharge current and does not basically on The MPD arcjets are provided with two auxiliarypropellant species for electromagnetic acceleration, electrodes at floating potential. The floating electrodesHowever, in the accelerator channel of the discharge are useful in relaxing current concentration near the anodechamber, complicated chemical reactions including and cathode bases at the initial state of the discharge.dissociation and ionization are expected to occur together Gases are injected with a cathode slit / anode slit ratio ofwith acceleration process. Therefore, it has been 50/50 into the discharge chamber through a fast actingrecognized that the performance characteristics on the valve (FAV) fed from a high pressure reservoir. The risedischarge voltage, thrust, thrust efficiency and electrode time and width of the gas pulse, measured with a fasterosion depend on propellant species.'-" ionization gauge, are 0.5 and 6 msec, respectively. The

mass flow rates are controlled by adjustment of theOne or two dimensional analyses of MPD arclet reservoir pressure and the orifice diameter of the FAV.

* Research Engineer, Member JSASS/AIAA* Graduate Student of Osaka University The main power-supplying pulse forming network+ Assistant Engineer, Member JSASS (PFN), which is capable of storing 62 kJ at 8 kV, delivers++ Lecturer, Member JSASS a single nonreversing quasisteady current of maximum 27# Professor. Member JSASS/AIAA kA with a pulse width of 0.6 msec. A vacuum tank 0.6

IEPC-93-197 1800

Figures 3-7 show the current fractions distributed on

GAS PORTS the segmented anode. The current fractions on the

S anode segment No.15 are highest for all gases; i.e.. the

1 4 51 8 0 911113114115. s current concentrates near the downstream end.

iTH- E Furthermore, the current fractions of No.15 for moleculari . gases are much larger than those for monatomic gases

regardless of particle weight, and in other words for

monatomic gases the current is distributed in the

50mm' intermediate region of the anode although for molecular

gases the current hardly exit in the region. These are

Fig.1 Cross section of MPD arcjet chamber with explained from MPD flowfield analyses as follows."' The

long electrodes ionization process of molecular gases is slower than thatlong electrodesof monatomic ones owing to a time lag due to dissociation

process at low current levels. Therefore, the ionization

process of molecular gases starts near the arcjet exit.

L-C LADDER CIRCUIT Also, the current fractions of No.15 decrease with

increasing discharge current, and the current is distributed

MATCHING (IGNITRON in more upstream segments. Hence, these features on

RESROGOWSKICOIL current conduction are found to depend strongly on gas

1O O .44uF species and discharge current levels.

+ - r I I 1 I I I In previous papers', we reported current distributions0kq DROGOWSKI COIL3 - in a short-electrode MPD discharge chamber. The MPD

C k .44MF arcjet had a anode 50 mm in diameter x 70 mm long and

CURRET CI I - I T T , ,Ia a cathode 45 mm long. The anode diameter is equal toPROBE _ that of the long-electrodes MPD chamber used for the

.I R TTTl i i present study. The current patterns were measured withMPD ARCJET

Fig.2 Electrical circuit for measurement of current

fractions on anode. 100He 0

5kA80-

m in diameter x 5.75 m in length, where the arcjet is 60

fired, is evacuated to some 10 3 Pa prior to each 40

discharge. 20

Discharge current is measured with a Rogowski coil 0 1 2 3 4 5 6 78 9 101112131415

calibrated with a known shunt resistance. Voltage 100

measurement is performed with a current prove (Iwatsu 80 Ok

CP-502). which detects the small current bled through a 60

known resistor (10 kQ) between the electrodes. 40

Il. Experimental Results and Discussion 20S0 1 2 3 4 5 6 7 89 101112131415

Quasisteady Current Distributions m100so 15kA80

The present experiments are conducted with five

gases of helium, argon, hydrogen, nitrogen and mixture of

nitrogen and hydrogen simulating fully decomposed 40

hydrazine at discharge currents of 5, 10 and 15 kA. The 20

mass flow rate for each gas is set up at a corresponding 0 Ti 3 5 67 9101 12131415critical current of about 10 kA, which is derived Segmented Anode Number

theoretically from the rule of minimum input power or

Alfvdn s critical ionization velocity.Fig.3 Current fractions on anode for helium.

2

1801 IEPC-93-197

Ar rri=1.37g/s 10 Nz rh=0.74g/S

5kA

60-40-

10-0 23 4 5 678 9 10111'l2131415

S60- ~60-.40- .a 40-

.~20- z f 20-

0 "60

6100 .tv 100cc 15kA cc ' 1kA

60- 60-

40, 40-

2 0 r - r-M r-i M r fl~ 20-n

C T 34 56 78 9 1 112131415 3 4 5 6-4 7 8 9 101112131415Segmented Anode Number Segmented Anode Number

Fig.4 Current fractions on anode for argon. Fig.6 Current fractions on anode for nitrogen.

Hi rhQ40g/ 100Nz+2Hz mh=0.4t4g/S

100 S, k8 5kA : 5k

60 60-

20-: 20-

1_ _ _ _ _ _ _ _ _ _ _ _ 2 3 4 5 6 8 9 10111 131415 10

lOkA o.

- ~ 60-~60- I- - 40,

40- 20

Z 0 n0" 0 1123 4 5 6 7 8 1 0 1 1 1 2 1 3 4 1 51 0 1 1 1 2 13 1 4 1 5L

20: 78 9101112131451123145 Zi10

8: l5kA 80. ~k80.-600.

H. 40.

20- 20-01 0~~ 123 4 56 7 8901121314152 34 567 8 910112131415 Segmented Anode Number

Segmented Anode Number Fig.7 Current fractions on anode for mixture of

Fig.5 Current fractions on anode for hydrogen. nitrogen and hydrogen.

3

IEPC-93- 19 7 1802

(a) 5 kA. (a) 5 kA.

(b) 10 kA. (b) 10 kA.

------------

(c) 15 kA. (c) 15 kA.

Fig.8 Spacial current patterns in MPD discharge Fig.9 Spatial current patterns in MPD dischargechamber with short electrodes for helium. These chamber with short electrodes for hydrogen.patterns were measured with a BP probe. These patterns were measured with a BO probe.

a B 0 probe. The typical enclosed current contours for Transition to Quasisteady Current Patternthe short-electrode arcjet chamber are drawn in Figs.8 and at Discharge Start9, where the number of the given line to the dischargecurrent. It is shown that the current patterns are divided Electrical breakdown is expected to occur near theinto two classes, particularly, at the low current level. For upstream end in the discharge chamber just after applyingthe monatomic gases such as He and Ar, the discharge a high PFN-charging voltage between the electrodes.occurs mainly inside the discharge chamber, and the The rise time of the discharge current is about 40-50current flows almost uniformly over the side surface of the gi sec. In the transition to a quasisteady current patterncathode. 'For the molecular gases such as H2 and N2, in discharge start, the current waveforms are observed onthe given contours are withdrawn downstream, and most anode segments as shown in Figs.10-12. It is inferredof the discharge current enters the regions of the cathode that a current sheet, which we call the current spiketip and the end surface of the anode. As the discharge observed, is generated on the upstream segments andcurrent increases, the current patterns for the molecular travels downstream on the electrodes. After the travel ofgases are similar to those for the monatomic gases; that the current sheet, a quasisteady current distribution isis, the zones of current conduction spread widely from the created. Generally, a practical MPD propulsion system isinside of the arcjet chamber to the downstream region. provided with an auxiliary electrode supplied an ignitionAccordingly, these features on current conduction for the pulse at the upstream end of the discharge chamber.3 Inshort-electrode MPD chamber agree with those for the these systems the travel of current sheets is expected tolong-electrode arcjet chamber; i.e., the electrode length occur as well as the present experimental system.does not influence them, as expected from several MPDflowfield analyses. The velocity of the current sheet can be evaluated

from the delay time of the current spike observed on

4

1803 IEPC-93-197

H2 5kA rh=040g/s Ar 10kA ri=137q/s

LO TOTAL n TOTAL

_ z ULL Z--.1 15 W . 15 S

8 2n-L --- . ----- 10 [-^ S; --- / - . -- 10 uJ

o H

10 W o

2 -2

0 1 0 1TIME.msec TIME. sec

Fig. Current waveforms on anode segments for Fig. 1 Current waveforms on anode segments forhydrogen at 5 kA. argon at 10 kA.

H2 lOkA rm=040g/s in

bi O H

"T 6 A Nz-2H2-" ° Nv

---- TOTAL r W aHew a He

Io I Ar& W0 Z Z

W 15 J aZ U

14 <aLL 2

10 Z A

U Au l ---- 6 01

V 5 10 '152 CURRENT,kA

S TIMErse Fig.13 Current sheet velocities vs dischargecurrent characteristics.

Fig.11 Current waveforms on anode segments forhydrogen at 10 kA.

of the current sheet depend only on molecular weight.

anode segments. Figure 13 shows the velocity of current IV. Conclusionssheets vs discharge current characteristics. The velocityfor each gas increases linearly with the discharge current A quasisteady magnetoplasmadynamic (MPD) arcjet

although it is much smaller than the quasisteady exhaust with 30-cm-class long electrodes, i.e., with a long

velocity estimated by a pendulum method. At a acceleration zone almost realizing a one-dimensional

constant discharge current, higher the velocity is as channel flow, was developed to examine current

smaller the molecular weight is. This is reasonable with distributions in the discharge chamber. It was found thata constant electromagnetic body force: that is. the motion the current patterns depended strongly on gas species and

5

IEPC-93-197 1804

discharge current levels regardless of electrode length and Engines and Their Technical Applications, Moscow, Russia.

that the feature on current conduction below the theoretical A2-15, 1993.critical current was divided into two cases, depending on

monatomic and molecular gases. These experimentalresults agreed with analytical ones. Furthermore, in thetransition to a quasisteady current pattern at dischargestart the travel of a current sheet was observed. Thevelocity for each gas increased linearly with the dischargecurrent although it was much smaller than the quasisteadyexhaust velocity evaluated by a pendulum method. Ata constant discharge current, the motion of the currentsheet depended only on molecular weight.

References

1. Tahara, H., Kagaya, Y. and Yoshikawa. T.,"Quasisteady Magnetoplasmadynamic Thruster with AppliedMagnetic Fields for Near-Earth Missions," J. Propulsionand Power, Vol.5, 1989, pp.713-717.2. Tahara, H., Takiguchi, F., Kagaya, Y. and Yoshikawa,T.," Performance Characteristics and Discharge Featuresof a Quasi-Steady Applied-Field MPD Arcjet." Proc.AIDAA/AIAA/DGLR/JSASS 22nd Int. Electric PropulsionConf., Viareggio, Italy, Vol.1, Paper 91-073. 1991.3. Yoshikawa, T., Kagaya, Y. and Tahara. H.," ContinuousOperational Tests of a Quasi-Steady MPD Arcjet System,"Proc. AIDAA/AIAA/DGLR/JSASS 22nd Int. ElectricPropulsion Conf., Viareggio, Italy. Vol.1, Paper 91-075,1991.4. Tahara. H., Kagaya, Y.. Tsubakishita. Y. andYoshikawa. T.," Performance Characteristics and DischargeFeatures of a Quasisteady Applied-Field Magneto-plasmadynamic Arcjet," IInd German-Russian Conf.Electric Propulsion Engines and Their TechnicalApplications, Moscow, Russia. A2-13, 1993.5. Matsumura, H., Tahara. H., Tsubakishita. Y. andYoshikawa, T.," Electromagnetic Particle-in-Cell Simulationof Plasma Thruster Exhaust Plumes," Proc. Int. Symp.Simulation and Design of Applied Electromagnetic System(ISEM-93), Sapporo, Japan, Paper No. F38. 1993.6. Tahara, H., Yasui, H., Kagaya. Y. and Yoshikawa. T.."Experimental and Theoretical Researches on Arc Structurein a Self-Field Thruster," AIAA Paper 87-1093. 1987.7. Sumida, M. and Toki, K.," Real-Gas Effect on theMagnetoplasmadynamic Arcjet." J. Propulsion and Power.Vol.7, 1991, pp.1072-1074.8. Tsubaki, T., Tahara, H., Kagaya. Y.. Tsubakishita. Y.and Yoshikawa, T.," Current Distributions of a Self-FieldMagnetoplasmadynamic Arcjet," Proc. Int. Symp. Simulationand Design of Applied Electromagnetic System (ISEM-93).Sapporo, Japan, Paper No. F39. 1993.9. Yoshikawa, T., Tahara. H., Kagaya. Y. andTsubakishita, Y.," Performance Characteristics and CurrentDistributions of Quasisteady MagnetoplasmadynamicArcjets," Innd German-Russian Conf. Electric Propulsion

6