. -

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EXHAUST 'FLOW AND PROPULSION CHARACTERISTICS O F A PULSED M P D ARC THRUSTER Q by CHARLES J. MICHELS NASA L e w i s R e s e a r c h C e n t e r C leve land , Ohio and THOMAS M . YORK Pennsy lvan ia S ta t e Un ive r s i ty Un ive r s i ty P a r k , Pennsylvania

/'

BETHESDA, M D. /APRIL 17-19, 1972

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Note: This paper wadable a t A I A A New York office fur S I X months: thereafter, photopririt Co~ies are available a t photocopy prices from AIAA Library, 750 3rd Avenue, New York, New York 10017

EXHAUST FLOW AliD PROPUlSION CKARACTERISTICS OF A PULSED W D ARC TIiilJSTER

Charles J, Michels Levis Research Center

Cleveland, Ohio

and

Thomas M. York Pennsylvania S t a t e Univers i ty Univemi ty Park, Pennsylvania

Abstract

Experimental i n v e s t i g a t i o n of t h e n e a r - f i e l d , niegawatt, s ing le-shot exhaust is presented f o r t h e self-field and a u x i l i a r y f i e l d cases (0-2 T). P l a s m h p c t pressure and number dens i ty are c o r r e l a t e d t o provide v e l o c i t y p r o f i l e s (3x19' t o 7x104 mlsec) , t h r u s t (10 t o 120 N ) , impulse ( 3 t o 16 N-sec) and mass account ing. The d a t a a e r e e with Hzgel's s e l f - f i e l d theory for t h e c a s e where t h r u s t is produced e n t i r e l y by electromagnet ic force. i n c r e a s e s with a u x i l i a r y f i e l d .

The da ta show t h a t t h e t h r u s t monato.iically

In t roduct ion

A ' b e t t e r understanding of a c c e l e r a t i o n mecha- nisms and propulsion c h a r a c t e r i s t i c s of MPD-Arc t h r u s t e r s o p e r a t i n g from subki lowatt t o megawatt power l e v e l s is needed t a eva lua te t h e i r usefulness f o r propulsion. This paper presents t h e r e s u l t s of an experimental i n v e s t i g a t i o n of t h e n e a r - f i e l d megawatt MPD-Arc exhaust flow.

Measurement of exhaust impact pressure was undertaken t o more d i r e c t l y determine t h r u s t e r - r e l a t e d performance. The e a r l i e s t impact pressure measurements were descr ibed i n re ference 1. T h i s work i d e n t i f i e d t h e s t a r t i n g t r a n s i e n t s t h a t domi- 2ated t h e pulsed d ischarge event f o r a t l e a s t 100 nicroseconds p r i o r t o any quasi-s teady plasma flow in t h e exhaust. These measurements showed a l a r g e ampli tude i n i t i a l p ressure pulse followed by lower order pressure s i g n a l s t h a t were n o t well under- s tcod . toward d iagnos is of t h e pressure i n t h e " a f t e r " t i m e per iod, i.e., afteP t h e i n i t i a l pu lse has passed t h e probing s t a t i o n . been assoc ia ted with r e l a t i v e l y cold and weakiy- ionized flow, while t h e "af te r" region of flow is now e s t a b l i s h e d ( 2 ) re f u l l y ion ized plasma. A NASA supported e f f o r t Pesul ted i n t h e drvelop- ment of a t a i l o r e d pressure probing u n i t f o r use i n t h e r a r e f i e d plasma exhaust flow of t h e "af te r" time period. Early measwements with this probe on c e n t e r l i n e i n t h e exhaust duc t , 30 cm downstream from t h e anode f a c e probe u n i t are descr ibed in r e f e r e n c e I).

The present e f f o r t was d i r e c t l y pr ia lar i ly

The i n i t i a l pulse has

The new impact pressure d a t a obtained with t h i s probe a r e combined with e a r l i e r reported ex7erimen- t a l e l e c t r o n dens i ty and temperature provide c a l c u l a t e d values of t h e exhaust ve loc i ty . I n t e g r a t i n g t h e new pressure d a t a over t h e exhaust apea y i e l d s a c a l c c l a t e d ins tan taneous t h r u s t and impulse b i t . The impact pressure and d e n s i t y da ta 1r0 further analyzed t o determine t h e instantaneous

t o

mass flow r a t e in t h e plasna exhaust. An account- ing is given of mass apportioned t o t h e i n i t i a l co ld gas flow, t h e s t a r t i n g t r a n s i e n t flow, and t h e plasma flow por t ions of t h e e*aust. The r o l e of propuls i n theori.esg6 s T ) f o r t h e s e l f - f i e l d and a u x i l i a r y magnetic f i e l d ( * ) modes of operat ion of t h e t h r u s t e r a r e discussed.

arameteps and conparison w i t h

Apparatus

Capaci tor Bank and Switch System

The a r c was energized by a 10 k i l o j o u l e capaci- t o p bank, descr ibed in d e t a i l in re ference 5 . The discharge was i n i t i a t e d a f t e r t h e bank switch was closed and aLpc cur ren t was allowed t o develop t o its peak value. Then (21 lisec a f t e r bank f i r i n g t ime) a crowbar switch was closed, f o r c i n g c u r r e n t t o decay monatonicaily w i t h time. The L/R decay time ranged from 250 t o 350 microseconds depending on arc res i s tance . most l i n e a r decay of a r c curpent f o r 500 micro- seconds a f t e r crowbarr ins time. t ime period t h a t t h e d a t a was gathered. Typical vo l tage and cur ren t waveshapes f o r t h e two peak cur ren t cases i n v e s t l e a t c d (11.2 and 2 0 . 0 kA) a r e shown in f i g u r e 1. D i s t i n c t l y d i f f e r e n t waveshapes a r e evident For each of t h e t h r e e values of a u x i l - i a r y magnetic f i e l d (0, 1. and 2 T ) appl ied in t h e magnetic nozz le conf igura t ion .

This c i r c u i t r y allowed an a l -

It is dur ing t h i s

Arc Chamber System

A c ross -sec t iona l v i e w 'of t h e a r c chamber is shown i n f i g u r e 2 . A supercmduct ing magnet is ".sed t o supply an a u x i l i a r y magnetic f i e l d a t t h e a r c chamber which can be var ied from 0 t o 2.0 T. An i ron f i l i n g s map of t h e magnetic f i e l d is also s h o w . 1 cn vide, 2 cm long. and 1 nrn th ick . The anode is a 4.? cm i n s i d e diameter copper r ing .

The cathode is a tungsten ribbon measuring

:titroeen ads was introduced i n t o t h e arc chamber by a high speed e m v a l v e t h a t was operated by an electromagnet ic a c t u a t o r . A l l t e s t s were run with t h e same puff MSS a t a peak n i t r o g e n co ld flow r a t e of 3 yhsec. sure in the a r c chanSer was measured by a commcr- c i a l l y a v a i l a b l e p i e z o e l e c t r i c pressure t ransducer in d prrvioi ls experincnt . o r i i i c e eg\idtions icr s teady flow were used t o c a l c u l a t e rhe W S S flaw r a t e f o r a l l t h e t e s t s of t h i s rewort. FrOn t h e r r m s i e n t pressure records i t was found tha t stable f low occurred a f t e r 650 microsecmds. Ihe a r c was s t a r t e d a t t h a t time.

The t r a n s i e n t cold f low gas pres-

That pressure and t h e

1

Thereaf te r , t h e t r a n s i e n t plasma flows for a few hundred microseconds i n t o t h e evacuated glassware s e c t i o n .

A sequence c o n t r o l l e r dctuiltes gas puff i n j e c - t i o n , delay f o r gas d i s t r i b u t i o n , bank switch

d c l o s u r e , crowbar w i t c h c losure , and thcn data ga ther ing " s t a r t " t imes. cycled every 4 minutes.

Thc system can be rc-

P i e z o e l e c t r i c Pressure Pro&

Basic cons idera t ions r e l a t i n g t o t h e measurement of pressure fy,$)flowing plasma have bean previous- l y reported. c a l l y developed(3) f o r t h e presen,t exhaust condi- t ions. element, r e l a t e d support s t r u c t u r e , and e l e c t r i c a l c i r c u i t r y $:ere desipped f o r t h e magnitude and dura- t i o n of t h e pressure s i g n a l s a n t i c i p a t e d and a matched sensing u n i t was u t i l i z e d as a b u i l t - i n simultaneous accelerometer . Two probing u n i t s with 0.75 and 1.25 cm diameter sens ing s u r f a c e s and 2.0, 2.5 cm o.d. housing r e s p e c t i v e l y , were constructed. Cal ibra t ion was c a r r i e d out with a simple shock tube. In both cases , with 1.8 m of c o a x i a l cable , an output of 4 v o l t s per atmosphere was achieved; with a matched a m p l i f i e r (xlO), an output of 40 v o l t s per atm r e s u l t e d . def ine t rends in t h e pressure d a t a , an e l e c t r o n i c low-pass f i l t e r (Spectrum Analog Elec t ronic F i l t e r Type H-18) was'used a t t imes f o r t h e 0.7 clii probing u n i t a s it demcnstrated an a c t i v e , well-defined h igher frequency Stress o s c i l l a t i o n a f t e r impact of t h e pressure f r o n t . s l i g h t delay of t h e f i l t e r (2.10 usec).

A new probing u i t was s p c c i f i -

The geometry of p i e z o e l e c t r i c sens ing

In order t n more p r e c i s e l y

Correc t ions were m d e f o r t h e

/A' A r i g i d mounting of t h e probe support f i t t i n g and subsequent comparison of simultaneous records from t h e pressure s e n s i n e and b u i l t - i n accelerom- e t e r u n i t s revealed pressure s i g n a l s an order of magnitude higher than those due t o undes i rab le probe a c c e l e r a t i o n s . Howe'rer, extraneous s i m a l s due t o thermal e f f e c t s and probe hea t ing proved t o be s u b s t a n t i a l . Experimental eva lua t ion ind ica ted t h a t r a d i a n t energy f l u x from both t h e source and l o c a l p l a s m r a d i a t i o n produced i n s i g n i f i c a n t e f f e c t s . However, thermal conduction t o t h e probe from t h e plasma proved t o be dominant. layer of v i n y l electrical i n s u l a t i n g t a p e (Scotch Brand No. 22, 3M Mfg. Co.) covering t h e s i d e s and sens ing s u r f a c e of t h e probe el iminated t h e thermal d r i f t without degrading pressure s e n s i t i v i t y or l i n e a r i t y of response.

A s i n g l e

Resul t s and Discussions

Cold Flow Tota l Pressure

The piezo-pressure probe was used t o measure

A t t h e da ta "cold" gas flow i n t h e duct. The propel lan t was i n j e c t e d without s t a r t i n g t h e arc . ga ther ing times and s r a t i a n used in t h i s r e p o r t , t h e cold gas propel lan t s tagnat ion p essure is an o r d e r of magnitude l e s s (about 6 N / m maximum) than t h e measured p r i s s u r e s fo r t h e powered case. Supplementary measurements i n d i c a t e s t a t i c pressure! a r e approximately one-tenth of t h i s s tagnat ion pressure.

5 . .

Exhaust Pressure P r o f i l e s

As out l ined i n r e f e r e n c e 1, t h e sequence of events a t a given s t a t i o n in t h e exhaust f o r a s i n g l e s h o t megawatt-level a r c source is:

( a ) Exhaust l i g h t a r r i v a l (about 70 vsec a f t e r a r c i n i t i a t i o n a t 30 cm).

(b) A few microseconds l a t e r , a narrow (20 t o 70 usee) t o t a l p ressure pulse of n e u t r a l gas F s s e s , most probably b l a s t wave r e l a t e d .

( c ) Arc c u r r e n t exhaust (plume) a r r i v e s t e n s of microseconds l a t e r , a t t h e a p p r o x i m t e time t h a t plasma is first de tec ted .

( d ) A flowing plasma is noted.

T h i s sequence can be c o r r e l a t e d with time v a r i a - t i o n s of t h e exhaust impact pressure shown i n f i g u r e 3 f a r ' a s t a t i o n 30 cm d o n s t r e a m from t h e anode. diameter piezo probe f o r t h e case where t h e peak c u r r e n t is 11.2 kA and f o r var ious a u x i l i a r y mag- n e t i c f i e l d s , 0 , 1.0 and 2.0 t e s l a . The f i g u r e shows t h e time-varying impact pressure a t t h r e e d i f f e r e n t r a d i i (0, 2 , and 4 cm).

The pressure was measured with t h e 0.75 cm

Each t r a c e shown is t h e mean value of t w o super- imposed t r a c e s and has been smoothed so t h a t t h e i n t e r n a l stpess o s c i l l a t i o n s of t h e piezo probe a r e not shown. Some shot t o s h o t v a r i a t i o n i n d a t a records e x i s t s as was shown i n t h e raw d a t a (two- s h o t over lays) of r e f e r e n c e 4.

The d a t a presented in f i g u r e 3 show one common fea ture ; t h e t o t a l p ressure appears as an i n i t i a l l a r g e amplitude pulse (8-10,000 N / m , 20 t o ?O "sec wide) with lower and varying pressure (1-2000 N/m?) t h e r e a f t e r f o r 200 nicroseconds. pulse , a s i n e a r l i e r u o r k ( l ) d e s c r i b i n g t h i s >he- nomenon, i s most p r o b a b l y a t t r i b u t a b l e t o a b l a s t wave genera t ing weakly ion ized gas. n a t u r e of t h e b l a s t wave-like per iod remains t o be determined. a pinch experiment. The present r e p o r t is con- cerned pr imar i ly with t h e period of t ime a f t e r t h e i n i t i a l pulse; t h a t is, t h e plasma flow period.

All t h e d a t a of f i g u r e 3 , a f t e r t h e i n i t i a l p ressure f r o n t show a drop and subsequent recovery t o another maximum impact pressure. The recovery time, T1, is defined a s t h e t ime t o reach 63 per- cent ( e - f o l d i n g t ime) of t h e peak plasma f low impact pressure. I t i s . t h e t ime t o e s t a b l i s h a per iod of quasi-s teady plasma exhaust flow. I lists values of t h e recovery time.

?

The i n i t i a l

The exac t

A s i m i l a r e f f e c t has been noted('O) in

Table

Table I - Recovery Times

T1( used B(T) ImX( kA)

2 50 2 10 200 225 200 175

11.2 11.2 11.2 20.0 20.0 20.0

2

i

i

! I i ! I

I l

! i I

! I

I

1

! 1

i

i

I !

Radial P r o f i l e s

11.2 kA Case - T h e r a d i a l v a r i a t i o n of t h e peak v a l m t m i t i a l pu lse ( b l a s t wave p a r t ) a s shorn i n f i g u r e 3 is much t h e Same a s in reference 1 ( f i g . 13) . The dependence of peak value on a u x i l i a r y magnetic f i e l d is also much t h e same. The new information concerns t h e rad iaT pressure v a r i a t i o n YS t ime f o r t h e lasma flow port ion of t h e exhaust flow. for the% case (fie. 3 ( a ) ) , t h e plasma flow pressure profi1.e decays w i t h r a d i u s , t h e maximum pressure being an c e n t e r l i n e . By con- t r a s t , f o r t!ie B = 1 . 0 T case ( f i g . 3 ( b ) ) , t h e plasma flow pressure increases with radius . B E 2.0 T, t h e plasma flow pressure p r o f i l e is a l s o seen t o i n c r e a s e with r a d i u s , but w i t h genera l ly l e s s pronounced amplitude change than . for B = 1.0 T. T h a s genera l ly reduced t h e amplitude of t h e plasma pressure p r o f i l e s . For both magnetic nozz le c a s e s , t h e pressure on c e n t e r l i n e is less than f o r t h e s e l f - f i e l d case. i n t h e euhaust on c e n t e r l i n e was f i r s t descr ibed in terms of number dens i ty i n re ference 5 (through a Thomson s c a t t e r i n g measurement). fest a s a reduct ion i n pressure on c e n t e r l i n e . peak value of plasma flow pressure is genera l ly less than h a l f t h e peak value of t h e b l a s t wave pressure pulse. Af te r t h e peak value of plasma flaw pressure , t h e pressure decays with time a s does t h e a r c curpent. s tatic pressure prof i . les a r e a t l e a s t an order of magnitude s m a l l e r than t h e corresponding impact p ressure .

A t

Increas ing t h e a u x i l i a r y magnetic f i e l d t o 2.0

The "hole" (or reduced d e n s i t y )

I t is now mani- The

Although n o t presented, t h e

20 kA Case - Figure 4 shows t h e temporal var ia - t i o n of t h e exhaust impact .pressure a t 2 = 30 cm f a r t h e 20 kA peak c u r r e n t case. Data a r e shown f o r t h e same a u x i l i a r y s a g n e t i c f i e l d s and r a d i a l p o s i t i o n s a s in f i g u r e 3. The genera l c h a r a c t e r i s - t ics descr ibed f o r f i g u r e 3 a r e a l s o ev ident a t t h i s h igher peak cur ren t . The primary d i f fe rences are i n t h e f a c t t h a t t h e pressures a r e approximate- l y twice a s l a r e e f o r t h e 20 kA case and t h e recov- e r y t ime, T1, is l e s s .

Exhaust Veloci ty Calcu la t ions

The v e l o c i t y of t h e b l a s t Wave port ion of t h e exhaust was c a l c u l a t e d and descr ibed in re ference 1. This v e l o c i t y was determined by t h e time of f l i g h t of t h e pulse pas t two measuring s t a t i o n s i n t h e duct . A t 2 = 30 cm, t h e v e l o c i t i e s were about 2x104 m/sec f o r t h e 11.2 kA, B = 0 case. a p p l i e d n a p e t i c f i e l d t h e r a d i a l p r o f i l e s of t h e v e l o c i t y became inverted. The reader is refer red t o re ference 1 for more d e t a i l s .

With

The pressure d a t a of t h e present r e p o x can be combined with t h e number dens i ty d a t a of re ference 2 t o provide a c a l c u l a t e d v e l o c i t y a t each i n s t a n t of time f o r t h e p lasna flow port ion of t h e exhaust. The c a l c u l a t i o n assumes complete i o n i z a t i o n and t h a t ? = p V 2 , vhere ? is t h e impact pressure, p is t h e d e n s i t y , and V is t h e ve loc i ty . An exac t anal- y s i s of t h e pressure-veloci ty r e l a t i o n s h i p in t h e flowing exhaust plasma must include d e t a i l e d plasma s h e a t h and gasdynamic e f f e c t s . Meaningful evalua- t i o n of such higher order i n t e r a c t i o n s would re- q u i r e more instrument prec is ion and experiment re- p e a t a b i l i t y than is present ly ava i lab le . C o r r e c t i o s

ro me pressure d a t a because of flow a n g u l a r i t y , v e l o c i t y g r a d i e n t s , etc., are es t imated t o be n e e l i g i b l e w i t h i n t h e accuracy of t h e experiment. The v e l o c i t i e s c a l c u l a t e d p r i o r to 150 micro- Yeconds a r e not considered t o be a p a r t of t h e .rue "blowing" plasma flow phase of t h e exhaust

s i n c e t h e p a r t i a l l y ion ized b l a s t wave-like decay pressure may over lap t h e plasma flow pressure a t t imes e a r l i e r than 150 microseconds. This makes t h e c a l c u l a t i o n of v e l o c i t y d i f f i c u l t t o i n t e r p r e t .

V d o c i t y P r o f i l e s for t h e Se l f -F ie ld Case - The r e s u l t s of t h e v e l o c i t y c a l c u l a t i o n f o r t h e two d i f f e r e n t c u r r e n t cases, 1 1 . 2 kA and 20.0 kA, are presented in f i g u r e 5 . t i o n s are shown only f o r two r a d i i ( r = 0, and r = 2 cm) because number d e n s i t y d a t a a t r = 4 cm was below t h e instpument d e t e c t a b i l i t y l i m i t

The v e l o c i t y i n t h e plasma flow port ion of, t h e exhaust a t about 250 microseconds is 3 . 5 ~ 1 0 m/sec. This is an o r d e r of magnitude l a r g e r than t h e b l a s t wave v e l o c i t y ( s e e re f . 1). A s t e a d y flow a t t h i s v e l o c i t y would correspond t o a s p e c i f i c impulse of about 3500 seconds.

In f i g u r e 5(a), ca lcu la-

~ m - ~ ) .

For t h e 20.0 kA case shown in f i g u r e 5(b) . pres- s u r e and number d e n s i t y d a t a were a v a i l a b l e for t h r e e r a d i a l l o c a t i o n s (r = 0, 2, and 4 cm). In genera l , t h e v e l o c i t i e s a r e l a r g e r in magnitude and t h e exhaust dura t ion is longer (450 usec). v e l o c i t y is lower toward t h e duct edge and de- creases f a s t e r . General ly , t h e v e l o c i t i e s agree with t h e o r e t i c a l c a l c u l a t i o n s descr ibed by Hiigel ( r e f . 7, f i g . 3 ) .

T h e

Fur ther a n a l y s i s of t h e tempoml v a r i a t i o n s i n t h e s u s t a i n e d plasma flow is n o t cons idered , warranted a t p resent because t h e 15 cm duct may n o t be adequate t o avoid wal l i n t e r a c t i o n s . The t h r u s t e r is scheduled t o opera te i n a much l a r g e r free exhaust f a c i l i t y i n t h e n e a r f u t u r e .

Veloci ty P r o f i l e s f o r t h e Auxi l ia ry F i e l d Case - The in f luence of t h e a u x i l i a r y magnetic f i e l d on t h e plasma flow v e l o c i t y is shown in f i g u r e 6. The d a t a ape presented f o r one r a d i u s ( r = 4 cm) be- cause even though t h e pressure d a t a a r e a v a i l a b l e for o t h e r r a d i i , t h e number dens i ty was below t h e d e t e c t a b l e l i m i t (lx1013 par t ic les /cm3) . In f i g u r e 6(a) (11.2 kA peak c u r r e n t ) , t h e v e l o c i t y f o r t h e B 2.0 T f i e l d case is l e s s than t h e velo- c i t y f o r t h e B = 1.0 T f i e l d case. It should be noted t h a t even though the a r c power is g r e a t e r f o r t h e 2.0 T case, t h e a r c c u r r e n t probably is n o t s u f f i c i e n t t o provide an optimum condi t ion f o r t h i s mass input . noted when t h r u s t is analyzed in l a t e r s e c t i o n s of t h i s repor t .

This condi t ion is more dramat ica l ly

For t h e 20 kA peak cur ren t case ( f i g . G(b)) , t h e v e l o c i t y i n c r e a s e s with a u x i l i a r y magnetic f i e l d , t h a t is with arc power. equiva len t steady-flaw s p e c i f i c impulse for t h e d a t a of f i g u r e 6(b) are on t h e order of 2700 seconds f o r t h e B = 0 case. 5300 seconds f o r t h e B = 1.0 T case , and 6500 seconds f a r t h e B = 2.0 T case,

A t recovery time, T1, t h e

Thrust and Impulse B i t

The b e n e f i t s of pressure probing i n t h e plasma

3

flow port ion of tho exhaust become most eOident when t h e impact pressure meaSUPementS versus rad ius and time ape i n t e g r a t e d over t h e exhaust area t o provide a c r i t i c a l propulsion parameter, namely t h r u s t . the probe a l low instantaneous t h r u s t determina- t i o n s , a d i f f i c u l t f e a t f o r most reac t ion t h r u s t measuring systems.

-

The e x c e l l e n t high frequency response of

The t h r u s t v a r i e s with t ime during a t y p i c a l s ing le-shot event . t o n s ) is caused by cold gas propel lan t flow out t h e exhaust nozz le prior t o arc s t a r t - u p . Then t h e r e is a time-varying but l a r g e r t h r u s t as t h e a r c is i g n i t e d . wave-like t r a n s i e n t in t h e flow. Both t h e cold gas thrusv and t h e b l a s t neve-l ike t h r u s t c m be con- s i d e r e d s t a r t i n g t r a n s i e n t s . In reference 1 it was pointed out t h a t if the a r c powerinE t ime is s h o r t . t h e impulse provided by t h e s t a r t i n e t r a n s i e n t t h r u s t w i l l dominate over t h e impulse due t o t h e later plasma flow por t ion of t h e t h r u s t cycle. Attempting t o apply s teady s t a t e plasma a c c e l e r a t o r theory t o t h a t s h o r t pu lse case obviously would re- s u l t in ~ W O P S . Following t h e s t a r t i n g t r a n s i c n t t h r u s t t h e r e is a t ime varying t h r u s t caused by t h e

lssma flow por t ion of t h e exhaust. This port ion k h - s t i n g time can be r e l a t e d mope c l o s e l y t o s teady t h r u s t e r performance.

The i n i t i a l t h r u s t ( a few new-

This is a s s o c i a t e d with t h e b l a s t

Instantaneous t h r u s t versus time f o r t h e c a s e of a peak cur ren t of 11.2 kA is shown i n f i g u r e 7. The peak t h r u s t values vary from 25 t o 40 newtons, depending on a u x i l i a r y f i e l d case. Although input power l e v e l s i n c r e a s e w i t h increas ing magnetic f i e l d , t h r u s t does not i n c r e a s e monatonically w i t h a u x i l i a r y magnetic f i e l d . t h r u s t f o r t h e plasma flow port ion of t h e exhaust is shown t o y i e l d impulses of 3.4 t o 6.1 d i - scc . In s h a r p c o n t r a s t t o t h e 11.2 kA case, t h e i n s t a n - taneous t h r u s t vs t ime d a t a f o r 20 kA peak cur ren t is monatonically i n c r e a s i n g with t h e value of t h e a u x i l i a r y magnet f i e l d . A s t h e peak a r c power in- creases from 3.0 t o 7.2 megawatts, the peak t h r u s t increases from 73 t o 120 newtons. i n c r e a s e s with magnetic f i e l d from 8 t o 1 6 mN-sec.

The. t ime i n t e g r a l of t h o

Impulse b i t also

Mass Accounting

The cold f low of gas for a s i n g l e shot has been analyzed by measuring the pressure in t h e arc cham- ber and t h e mass flow has been c a l c u l a t e d from t h e sonic flow equat ions for mass flow through t h e 4.12 PO diameter anode. The t r a n s i e n t a r c chamber pres- sure is noted t o reach a peak value of 0.15 p s i a a t about 600 ysec a f t e r t h e 70 usee puff of N2 has been introduced i n t o t h e evacuated a r c chanber. This value i n d i c a t e s a f low r a t e of 3.0 gmslsec when t h e a r c is i n i t i a t e d . The measured arc cham- ber pressure f o r t h e cold flow case a f t e r peak p r e s s u r e is simply an exponent ia l decay of pressure with time. In f i g u r e 9 , mass flow r a t e versus t ime is s h o m f o r t h r e e cases. The upper curve is t h e i n d i c a t e d cold flow r a t e , determined by t h e above procedure, shown decaying with time. A t 200 wec after t h e d ischarge is i n i t i a t e d (850 usec a f t e r t h e i n i t i a l puff of gas was introduced i n t o t h e chamber) t h e flow r a t e is seen t o be about 2.0 gs/ sec. By t h e time t h e exhaust flow event would normally be O Y ~ F ( W O . ysec of arc i n i t i a t i o n ) t h e m a s s f low Pate is dom t o 1.5 gs/sec.

The remaining two C U P Y ~ S i n f i g u r e 9 represent

These ins tan taneous measured ion mass flow r a t e s dur ing t h e a c t i v e t h r u s t i n g plasma flow period. flow r a t e s were determined from temporal v a r i a t i o n s i n ion number d e n s i t y obtained from <m e a r l i e r Thomson s c a t t e r i n g experimental measurement(2) and t h e v e l o c i t i e s c a l c u l a t e d from pressure d a t a in t h e present paper. The flow r a t e s were c a l c u l a t e d f o r the 8 = 0 a u x i l i a r y f i e l d case, for 11.7 kA peak curren t and 20 kA peak cur ren t cases'. For t h e more optimum c a s e , 20 kA peak c u r r e n t , t h e mass f low r a t e s f o r 150 t o 225 vsec period a re i n good agree- Rent with cold mass flow rate, i n d i c a t i n e f a i r mass u t i l i z a t i o n . For t h e 11.2 kA peak c u r r e n t case t h e r e is a l a r g e d i f f e r e n c e between co ld flori mass f l c w ' r a t e and t h a t c a l c u l a t e d t o be occurr ing in t h e plasma flow port ion of t h e exhaust . c a t e s poor mass u t i l i z a t i o n and is another indica- t i o n of t h e off-optimum operat ion for t h i s l o w - cur ren t C Z S ~ . The t o t a l mass per s h o t is obtained by measuring t h e i n c r e a s e i n s t a t i c pressure when t h e cold flow of gas is allowed t o fill t h e aTIc chamber and 90 l i t e r glassware exhaust duct system t h a t has been closed off from t h e vacuum pump. This mass per s h o t is measured t o be 3000 micro- grams.

This ind i -

Using t h e above da ta , Table I1 r e p r e s e n t s t h e mass account ing f o r var ious time increments for a t y p i c a l shot .

Table I1 - Mass Accounting

(i = 0, 20 kA)

Event % of T o t a l Mass i n Mass Involved Micrograms

Cold gas fl.ow before a r c is w o n t ,

w e d )

"Blast wave" flow (meas- ured)

(meas- ,

33 1000

3 100

Plasma flow 8 port ion of ex- haust (meas- ured )

Unaccounted 1 5 for flow

"Bleedout" of 41 mass a f t e r powering cyc le

T o t a l mass measured 100%

-

250

Q50

i200

- 3000

The mass account ing s h w s t h a t 33% of t h e mass per s h o t is expended i n developing t h e proper ma59 flow rate ( 3 g/sec) before t h e a r c is i g n i t e d . The "bleedout" of mass a f t e r t h e powering cyc le i s 41% of t h e mass per shot . 3% of t h e mass is measured i n the "blastwave" p a r t and 8% is measured i n t h e p l a s m f l o w por t ion of t h e exhaust. This leaves 15% of the t o t a l mass unaccounted. The unaccounted mass is t h e measured d i f fe rence of t h e i n t e g r a t e d cold flow mass and t h e

During t h e powering cycle,

- i n t e g r a t e d plasma flow mass i n f i g u r e 9. resolved whether t h i s is due t o uncer ta in ty in t h e measurements o r due t o mass n o t properly u t i l i z e d i n t h e t h r u s t e r .

It is not

Propulsion Parameters - Experiment and Theory

In order t o c l a r i f y t h e b a s i c regimes of thrus- ter opera t ion , the instantaneous t h r u s t da ta ( f i g . 8 ) were c o r r e l a t e d w i t h t h e ins tan taneous discharge cur ren t of f i g u r e 1. Tne r e s u l t s a r e presented i n f i g u r e 10 for t h e 20.0 kA peak c u r r e n t case; t h e v a r i a t i o n of instantaneous t h r u s t with a r c cur ren t is shown, w i t h a u x i l i a r y m a w e t i c f i e l d a s an a d d i t i o n a l paraae ter . Time during t h e discharge event is noted t o progress downward in f i g u r e 1 0 for each case , and accordingly, the mass f low r a t e is maximum a t t h e peak t h r u s t values and decreases with t ime ( f i g . 9 ) . Data e r r o ~ bars a r e not pre- sented , b u t could be 20 percent of t h e ind ica ted values . Also presented f o r re ference in f i g u r e 10 is t h e v a r i a t i o n of s e l f - f i e l d e lectromagnet ic t h r u s t versus c u r r e n t , ca cu la ted using t h e s tand- a r d a n a l y t i c f o r r n u l a t i ~ n . ~ ~ ~ ~ ~ ) The cathode s p o t s i z e used in t h e c a l c u l a t i o n was est imated from erosion p a t t e r n s and assumed constant . t h e curves shown are c l e a r l y not s t r a i g h t l i n e s (exponent ia l r e l a t i o n s h i p of t h r u s t t o c u r r e n t ) over t h e range of experimental curr-ent condi t ions. The.per iod of higher cur ren t flow with B = 0 is seen t o agree reasonably well with t h a t a n a l y t i c a l formulat ion. Per iods of agparent ly quasi-steady plasma flow a r e i d e n t i f i e d by t h e s o l i d curves. The da ta shown c l e a r l y emphasizes t h e r o l e t h a t an a u x i l i a r y magnetic f i e l d can play i n increas ing t h r u s t f o r a given value of discharge current . Spec i f i . ca l ly , t a k i n g a value of discharge c u r r e n t of 7 kA, t h e v a r i a t i o n of t h r u s t der ived from t h e i n t e g r a t e d exhaust measurements versus t h e m a p i - tude of t h e appl ied magnetic f i e l d is presented i n f i g u r e 11. crease with a u x i l i a r y magnetic f i e l d . i n c r e a s i n g value of appl ied magnetic f i e l d can be shown ( f i g . 1) t o increase t h e power l e v e l of t h e arc t h r u s t e r discharge; t h e v a r i a t i o n of discharge power with magnetic f i e l d is presented a s a secmd C U P V ~ i n f i g u r e 11. The r a t i o of t h r u s t t o pow=? is increas ing with magnetic f i e l d . Hence, t h e e f f i c i e n c y is a l s o increas ing with appl ied f i e l d . The e f f i c i e n c y is not ca lcu la ted because of un- c e r t a i n t i e s in t h e mass flaw r a t e discussed e a r l i e r .

However,

Thrust is noted t o monatonically in- However, t h e

The condi t ion of c r i t i c a l i t y for c u r r e n t and mass flow r a t e descr ibed i n reference 1 3 is

where e is t h e e l e c t r o n i c charge, Vi is t h e i o n i z a t i o n p o t e n t i a l , m is t h e mass of t h e p a r t i - c l e s and b f s a constant of the t h r u s t e r . The value of lz /n for the 20 kA s e l f - f i e l d co;e v a r i e s from 2 5 t o 200. In re ference 1 3 t h e criti- cal value of t h i s parameter was aiven a s between 60 and 150 f o r somewhat s i m i l a r propel lan ts and geom- e t r y . The value depends on whether instantaneous cold flow r a t e o r instantaneous plasma flow Pate is used and on t h e range of uncer ta in ty of the m e a s -

ured mass f l o w r a t e .

Conclusions

The r e s u l t s of an experimental i n v e s t i g a t i o n of t h e near - f ie ld negawatt NPD-Arc exhaust flow a r e presented. The t h r u s t e r was operated s ingle-shot and results were obtained with and without a u x i l - i a r y magnetic f i e l d . ments of impact pressure with a newly deslgned piezo pressure probe system and of plasma d e n s i t y determined i n e a r l i e r work. The d a t a wore analyzed t o determine propuls ion c h a r a c t e r i s t i c s such a s t h r u s t , impulse b i t , v e l o c i t y , and mass accounting. Conclusions fo r t h i s s tudy a r e as follows:

The data c o n s i s t s of measure-

1. New impact pressure da ta reconfirm t h e i n i t i a l s t a r t i n g t r a n s i e n t pressure pulse and add new r a d i a l and temporal p r o f i l e d a t a for t h e later occurr ing plasma flow p a r t of t h e exhaust. These impact pressures v a r i e d from l d 0 3 t o k103 N/m2 f o r t h e few hundred microseconds dura t ion of t h e plasma flow port ion f o r t h e s e l f - f i e l d and f o r B = 1.0 T and B = 2.0 T a u x i l i a r y f i e l d cases .

Calculated exhaust v e l o c i t y during uhe 2. plasma flow period v a r i e d from 2x104 t o 7x104 m/sec a d were func t ions of r a d i a l p o s i t i o n , a s w e l l a s a u x i l i a r y magnetic f i e l d . For t h e s e l f - f i e l d case , t h e experimental ly determined v e l o c i t i e s agree with Hiigel's theory f o r t h e case where a c c e l e r a t i o n is produced e n t i r e l y by electromagnet ic f o r c e .

3. For t h e s e l f - f i e l d case, although t h e t h r u s t v a r i e s with time under t h e inf luence of decaying c u r r e n t and mass florr r a t e , t h e r e is a region which approaches t h e steady-flow t h r u s t (propor t iona l t o c u r r e n t squared) .

4. Thrust is f o m d t o i n c r e a s e monatanical ly with a u x i l i a r y magnetic f i e l d . Impulse b i t was found t o range from 3 t o 1 6 N sec, dependent an magnetic f i e l d s t r e n g t h .

References

1. Michels, C. J. and York, T. M . , "Ressue Meas- vrernents i n t h e Exhaust of a m s e d Megarxtt MPD ARC Thrus te r , " Paper 71-196, Jan. 1571, ALAA, New York, N.Y.

2. Michels, C. J., Rose, J. R., and S i m , D. R., "Temporal S w e y of E l e c t r o n Number Density and FTe t ron Temperature i n t h e Exhaust o f a Megawatt MPD-Arc Thrus te r , " Fqer 72-209, Jan. 1972, AIAA, New York, N.Y.

3. York, T M., "Dp.mic Prsscure Transducer 3ystern for Pulsed masma F l o w Diagnosis," AERSr 71-3, June 1971, Fennsylvania S t a t e Univ., Univers i ty Park, Pa.

4. Michels, C. 3. and York, T. M., "Flow Character- ist ics i n t h e Exhaust of a Pulsed Megawatt Gas Fed Arc," TM X-67931, 1971, NASA, Cleveland, Ohio.

5. Michels, C. J. and Signan, D. R., "Exhaust C h a r a c t e r i s t i c s of a t.!egawatt Nitrogen MPD- ARC Thruster," A I M Journal, Vol. 9 , No. 6 , June 1971, p. l l 4 4 - U 4 7

- 6. Jahn, R. G., Clark, K. E., Oberth, R. C. , and 10. York, T. M. and Stover , E. K., "Transient Elm

Turchi, P. J., "Accelerat ion Pat.terns in and l ient ine Characteristics i n a Pinched Quasi-Steady M?D Arcs, ATAA Journa l , Vol. 9, No. 1, Jan. 1971, pp. 167-172. New York, N.Y.

Plasma C o l m , " Faper 72-208, Jan. 1972, AIA&

! 7 . FIkel, H., Krucl le , G. , ami Pe te r s , T., "Inves- U. Michels, C. J., "Dynamic Valta&e-Current Char-

a c t e r i s t i c s of a Megawatt MPD-ARC Thrus te r , " t i e a t i o n s on i l a s i m Thrus te rs with T h e d and Self-:,ln@etic Accclerat ion," A I A A Journal , AIAA Journal , Vol. 9, No. 1, Jan. 1971, Vol. 5, No. 3, Ik. 1967, pp. 551-558. pp. 173-176. .i

8 . John, R . R . , Pennet t , S., and Connors, J. F., "Exp?rimcntnl Performance of a High S w c i f i c Xn&.se Arc Je t Engine," As t ronaut i rz Acta, Vol. ll, Mar.-Apr. 1966, pp. 97-105.

9. York, T. M., '*Stress Dynamics i n High Speed P i e z o e l e c t r i c &-sure Probes," Review of S c i e n t i f i c I n . e t r m e n t s , Vol. 41, No. 4, Apr.

1 2 . &gel, H., "Sclf-l-etic E f f c c t i n Arc je t .Engines" AIAA Journal, Vol. 6, No. 8, Aw. 1968, pp. 1573-1575.

13. MalLiaris, A. C., John, R. R. , Garrison, R . L., and Libby, D. R., " F z r f o m n c e of Quas i - Steady MF'D Thrusters at High Powers," &

Vol. 10, No. 2 , Feb. 1972,

CURRENT, i

ARC O- ? 165Vlcm o-

--.. ...,,. , ~

VOLTAGE, 1 I ' '.

ARC CURRENT. 3.86 XAlcm

ARC 0- VOLl AGE. 166Vlcm o-

O l W ? W r n s a , r x l

ARC CURR€NT. I kAicm

ARC O- VOLTAGE. 4X Vlcm o.

TIME, hllCROSECONOS flgure 1. - Current-voltme lracei 17-trace overlap).

FllAMLNT SUPPORT 4N0 ' HEAlEA LEAD POST '

Figure 2. - A r c chamkr.

6

i

Ibl 1 .01 AUXILIARY hlAGNt?lC FPLO CASf

I

I L I Z . O ~ A U Y I L I A R Y M A G N T I C FltlO C A Y .

i1gum3. -lmpatpnrl"npdllll111.21*p*r"rn"lcav!

-?

IO

2 5

Ut0

E s

5wtDff (b) 1.01 A U X l l l A R Y MAGNETIC flt10 CAS(.

0 8 . 0 0 8 . 1 . 0 T A B L O T 6.0 i" 0 8.0x10+4

la1 FOR PEAK CURRENT OF 11.2 kA

Ibl FOR PEAK CURRENT OF M kA.

Figure 5. - V e l a i t y proli las i se l l field case],

AA 0 Y

2 c 0 I I I I I la) KAK CURRENT OF 11.2 kA.

0 1w 2w 3w 4 w m TIME. pSEC

ib i PEAK CURRENT OF 20 kA

Figure 6. - V e l a i i y versus t ime iat R . 4 cm, 2-Mcm).

0 B 0 B-1.OT. IhlPULSE-6.1mN-SEC A 8=2 .01 , IMPULSE-3. lmN-SEC

0, IMPULSE = 3.4 mN-SEC

TIME, pSEC

Figure 1. - Instantaneous t h r u s t versus time lpeak cur ren t of 11.2 MI.

8

I

i E

EXPERIMENTAL { y.0 T A 2.07

TKORETICAL

!

Figure 11. -Thrust and pwier versus auxil- iarymagnetic field lcurrent - 8 MI.

1 u.u 0 10 20 M

INSTANTANEOUS CURRENT. I, XA

Figure 10. -Instantaneous thrust versus instantaneous cur- renf 120.0 kA peak current c m l .

9

0 6 - 0 , IhlPULSE’8.Omll-SEC 0 6- 1.01, IMPULSE; 12.6mN-SEC A B - Z . ( I T , IMPUI.SE-16.OmMSEC

Figure 8. -Instantaneous thrust versus time (peak current of 20 kAl.

i - ’,

:

MEASURED COLD CAS m hlEASURED PLASMA m, 11.2 kA CASE hlEASURED PLASMA A . 20 kA PEAK CURRENT CASE

Figure 9. -instantaneous propellant flav rate.

- - ; . .... - .

10