The distribution of neutral atoms and charge- exchange ions downstream of an ion thrustor

I n a p r a c t i c a l design of an e lec t r ic -propuls ion space vehicle, it is l i k e l y t h a t some components, such 3s a so la r ce l l array, w i l l be exposed i n some degree t o neu t r a l atoms and charge-exchange ions leaving t h e th rus to r s . The purpose of t h i s paper i s t o describe the energy and angular d i s t r ibu t ions of such p a r t i c l e s emerging from a 15 cm diameter mercury electron-bombardment th rus to r . An ana ly t i c a l approach was used to determine the p a r t i c l e e f f l lues . The r e s u l t s i nd ica t e t h a t the erosion and/ or coatings due t o charge-exchange ions or neu t r a l p a r t i c l e e f f luxes would be to l e rab le fo r severa l years i f components were loca ted outside the p r i mary ion beam and a t distances of 75 cm o r g rea t e r from the th rus to r . The value of 7 5 cm w a s chosen t o simplify the ana ly t i ca l so lu t ion . A t distances much c loser than 75 cm a more de t a i l ed study would be required because the thrus tor can no longer be accurately represented as a point. sourc?. Approxim a t e s ca l ing r e l a t i o n s are given t h a t should allow rough determination of p a r t i c l e e f f luxes f o r v a r i ous combinations of nominal operating parameters o ther than those used i n t h i s study. r ' n INTRODUCTION w E l e c t r i c propulsion i s becoming more a t t r a c t i v e f o r a va r i e ty of spacef l igh t missions as a r e sult of improvements i n th rus to r performance and component technology. For example, an in t e re s t ing design study has been made recent ly t o inves t iga te the f e a s i b i l i t y of e l e c t r i c propulsion systems for unmanned probes (ref. 1). problems found i n t h e course of such design s tudies concerns t h e r e l a t i v e placement of t h rus to r s with respec t t o t h e so l a r ce l l a r rays o r o ther spacec r a f t components ( r e f . 2 ) . If the spacecraft is designed for maximum compactness and minimum weight, it i s l i k e l y t h a t t he so l a r ce l l a r ray w i l l be exposed t o some amount of neu t r a l atoms and/or ions leaving t h e th rus to r . The major d i rec ted p a r t of t h e ion beam i s e a s i l y avoided. However, t he "edge" of t h e beam is not w e l l defined due t o the presence of charge-exchange ions i n t h e th rus to r exhaust. I n recent long dura t ion t e s t s , neu t r a l i ze r s were mounted i n regions outside the major d i r ec t ed ion beam and they experienced appreciable ion erosion ( r e f . 3). The ex ten t of t h e damage was found t o decrease as t h e r a d i a l d i s tance from the beam axis increased. Thus, t h e problem arises of defining regions i n which poss ib le n e u t r a l p a r t i c l e coatings and/or i on e ros ion w i l l be a t an acceptable l eve l . O f course, acceptable l eve l s must be determined from considering t h e mission duration, t he spectrum of p a r t i c l e s leav ing the th rus to r s , and the propert ies of t h e spacecraf t s t ruc tu re h i t by these part i c l e s . One of t h e poss ib le ment th rus tor . quan t i t i e s are small, accurate experimental measurements are very d i f f i c u l t . However, t he processes giving r i s e t o such ef f luxes a r e f a i r l y w e l l understood s o t h a t an ana ly t i ca l approach may be used. Although the r e s u l t s obtained a re not prec ise , they are adequate f o r i n i t i a l spacecraf t design calculat i ons . I n addition, they serve t o ind ica t e which aspects of the problem may deserve more de t a i l ed Because t h e magnitudes of these