Electrical output from closed loop MPD experiment using auxiliary ionization

AdvancedEnergyConversion. Vol.6, pp. 25-28. PergamonPress, 1966. Printedin Great Britain

ELECTRICAL OUTPUT FROM CLOSED LOOP MPD EXPERIMENT USING AUXILIARY IONIZATION W. B. BIENERT, W . H. YOUNG a n d E. N. ZAVODNY* (Received 20 October 1964)

A~traet--The paper describes measurements of the gas conductivity in an MPD generator at a gas temperature of 1000°K. Helium seeded with cesium was ionized by a d.c. electrical arc which increased the conductivity by four orders of magnitude above the thermal equilibrium value. In the presence of a magnetic field an open circuit voltage of 62V was measured compared to 65V theoretical and a maximum short circuit current of 0.03A. The resulting V - I curve was linear indicating that no enhancement due to magnetically induced ionization occurred. ELECTRICAL o u t p u t was recently o b t a i n e d f r o m a c l o s e d - l o o p m a g n e t o p l a s m a d y n a m i c ( M P D ) g e n e r a t o r o p e r a t i n g at a gas s t a g n a t i o n t e m p e r a t u r e o f 1000°K. I n o r d e r to o p e r a t e at this low temperature, the ion density in the w o r k i n g fluid was increased several orders o f m a g n i t u d e a b o v e its t h e r m a l e q u i l i b r i u m value b y a d.c. electrical discharge u p s t r e a m o f the M P D duct. T h e tests were c o n d u c t e d with a closed l o o p M P D facility (Fig. l) using helium seeded with cesium as the w o r k i n g fluid [1]. The helium was circulated at flow rate u p to 8-0 gm/sec CESI UM BAFFLES

FLOW VALVE

FLO~ VALVE

COMPRESSOR

.......

®

1!

E] I

E i , MAGNET t i I,.....-Ti \Y /~ .~, ~, ..[/'} ~

CESIUM MAIN SEPARATOR HEATER i Ed ~ l I ] CESIUMBOILER I ) MAIN COOLER ~ ........... \ / ................................................. ~,...,~ ............................ ..,.,,,.® ............"1 -. "

"

=

HELIUM .................... CESIUM

~-]

FLOWMETER ~

HELlUM BOTTLE

CESIUMTANK

FIG. 1. MPD flow diagram. by a reciprocating compressor. Helium, p r e h e a t e d to 800°K, was seeded with 0-1-0.5 at. o f cesium a n d the mixture was further h e a t e d t o 1000°K a n d e x p a n d e d t h r o u g h a nozzle into the M P D duct. T h e latter is a slightly divergent flow channel o f rectangular cross section c o n t a i n i n g six pairs o f electrically insulated electrodes (Fig. 2). The gas leaving the duct is * Martin Company, Nuclear Division, Baltimore, Maryland. 25

W. B. BIENERT,W. H. YOUNGand E. N. ZAVODNY PREIONIZATION

I 1 b I +lOOVOLTS

11.5CM

FIG. 2. MPD channel with preionization

4

electrodes.

cooled to 4OO”K, the cesium seed material separated out, and the helium recirculated. Typical thermodynamic operating conditions are : Helium density (g/ms) Cesium seed concentration (at. %) Stagnation temperature (“K) Mach number Gas velocity (mjsec) Channel cross section (cm) Magnetic flux density (Webers/m2)

25 (3.8 x 1024atoms/ms) 0.1-0.5 950-1050 0.75-0.85 1300-1500 1 x 2.3 O-2.9

The equilibrium ion density at 1000°K as given by the Saha equation is approximately 3 x 101s ions/m3 and the corresponding gas conductivity lo-5 mho/m. This gas conductivity is not sufficient to produce any measurable output voltage since the internal resistance of the plasma (- 107 fi2)is large by comparison with the unavoidable leakage resistance of the duct insulation (- 5 x lo4 sz>. However, when a low power d.c. arc (100 V, 2.8 A) was struck upstream of the duct, a small percentage of the ions of the arc were carried with the gas and raised the conductivity in the duct by several orders of magnitude. The output obtained from the MPD duct under these conditions is summarized. Open circuit voltage (V) Measured Theoretical Short circuit current of one electrode pair (A) Output power-4 electrode pairs (W) Effective gas conductivity (mhojm) Magnetic tIux density (Webers/m2)

62 65 0.03 1.50 0,16 2.47

The effective gas conductivity was calculated from the nearly constant slopes of the voltage current characteristic which is shown in Fig. 3 for four different electrode pairs. The linearity of the F-Zcurve indicates that the gas behaved like an ohmic conductor and no magnetically induced ionization was detected. Since the electron mobility [2] under the present test conditions was approximately 2.4 m2/V set, the measured value of the gas conductivity indicates an ion density of 5 x 1017 m-3. However, independent measurements of the conductivity without magnetic field will be necessary to verify this estimate. In a separate series of experiments the steady d.c. arc was replaced by a pulsed capacitor discharge. The lo-psec time constant of the capacitor circuit is short compared to the flight time of the gas between the ionizing and the output electrode. The output signal appeared

Electrical Output from Closed Loop MPD Experiment Using Auxiliary Ionization

27

150-tzsec after pulsing the arc. This value is in excellent agreement with the thermodynamic measurement of the gas velocity. The long tail of the output signal can be explained by the existence of a nonflat velocity profile within the duct. This profile tends to diffuse the sharp pulse of ions carried by the gas. <> ELECIRODENO.3 o ELECTRODENO.4 D ELECTRODENO.5

60 ~ ~ 50 ~

~

40

~.'~A ~

VELOCITY

I, I0MISEC

SEEDRATE

0.~0AT. K GAUSS

30

20 10

oJ 0

4

8

12

16

20

24

28

32

CURRENT(MILLIAMP)

FIG. 3. Voltage-current curves for four electrodes using d.c. Ion Source. The high ratio of measured to theoretical open circuit voltage is a result of good electrical insulation between electrodes. During most previous experiments using nonequilibrium ionization, the plasma resistance was comparable to the leakage resistance between electrodes. As a result, the generated voltage was internally shorted and only a fraction of the theoretical open circuit voltage was measured [1, 3]. Under the present test conditions, no voltage sheaths at the electrodes were expected since the current density was well below the thermionic saturation current of the molybdenum electrodes [4] (J (thermionic) = 0.4 A/cm ~ vs. J (measured) = 0.05 A/cm2). The V - I curves shown in Fig. 3, which represent the output from four different electrode pairs, have essentially identical slopes, indicating thal the plasma conductivity did not vary along the channel. This result agrees with theoretical predictions that the recombination rate is small at low ion densities and high gas purity. [5.] The absence of magnetically induced ionization in this experiment can be explained. Without pre-ionization, the resistance of the plasma (107 ~) was high compared to the interelectrode leakage resistance (5 × 104 f~), resulting in a distribution of electric fields equal to that of the short circuited continuous electrode generator. This geometry does not lead to a high electron temperature [6]. In the present experiment, the theoretical limit for the attainable electron temperature was only 1070°K. With pre-ionization, the interelectrode leakage resistance was small compared to the duct resistance and theoretically a rise of the electron temperature to 1500-2000°K possible. However, the ion density which corresponds to this electron temperature does not greatly exceed the ion density generated by

28

W.B. BIENERT,W. H. YOUNG and E. N. ZAVODNY

t h e a u x i l i a r y i o n source. A n y i n c r e a s e o f t h e c o n d u c t i v i t y d u e to m a g n e t i c a l l y i n d u c e d i o n i z a t i o n w o u l d t h e r e f o r e be small. REFERENCES [1 ] Research Program on Closed Cycle MPD Electrical Power Generation with Non-equilibrium Ionization, Contract Nonr-3866(00), MND-3141, Martin Company, Baltimore, Maryland. 12J M. E. TALAAT,Magnetoplasmadynamic Electrical Power Generation with Non-equilibrium Ionization, Advd Energy Conversion, 3, 595-611 (1963). [3] B. C. LINDLEY and R. BROWN, Experiments with a Helium-Cesium Loop, Paper 108, International Symposium on M H D Electrical Power Generation, Paris, France (1964). [4] W. B. BmNERT,Experiments with Heated Electrodes in a Helium-Cesium Discharge Tube, to be published. [5] S. C. BROWN, Basic Data on Plasma Physics. Wiley, New York (1959). [6] H. HURWlTZ, JR, G. W. SOTTOI~ and S. TAMOR, Electron Heating in Magnetohydrodynamic Power Generators, AIAA Journal, 32, 1237-1243 (1962). R~sum6---Cet article d6crit des mesures de la conductivit6 du gaz dans un g6n6rateur magn6toplasmodynamique (MPD), pour une temp6rature du gaz de 1000°K. De l'h61ium ensemenc6 de c6sium est ionis6 au moyen d'un arc 61ectrique en courant continu. I1 en r6sulte une augmentation de conductivit6 d'un facteur 104 par rapport h la valeur h l'6quilibre thermique. En pr6sence d'un champ magn6tique, on a mesur6 une tension en circuit ouvert de 62 V - - ~tcomparer ~t la valeur th6orique de 65 V - - et un courant de court-circuit maximum de 0,03 A. La caract6ristique V-I est lin~aire, ce qui indique qu'il ne se produit pas d'instabilit6 due ~ l'ionisation induite par le champ magn6tique. Zusammenfassung--Diese Abhandlung beschreibt Messungen der Gas-Leit f~ihigkeit in einem MPDGenerator bei einer Gastemperateur von 1000°K. Mit Caesium geimpftes Helium wurde mit einem elektrischen Gleichstrom-Lichtbogen ionisiert, der die Leitf'~ihigkeit um 4 Gr6ssenordnungen fiber den thermischen Gleichgewichtswert erhfht. In Gegenwart eines magnetischen Feldes wurde eine Leerlanfspannung yon 62 V gemessen, verglichen mit dem theoretischen Erwartungswert yon 65 V, und ein maximaler Kurzschlusstrom yon 0,03 Amp. Die erhaltene Spannungs-Stromcharakteristik war linear und zeigt damit an, dass keine Erh6hung infolge von magnetisch induzierter Ionisation eintrat.