Theory of a Cw Supersonic Nitrogen Recombination Laser

Quasi-two dimensional model calculations show the possibility to realize a cw atomic nitrogen laser, pumped by the recombination process in rapidly expanded plasma flows. Sufficient gain for lasing is demonstrated for stagnation conditions of 13 000 ,< To< 24 000 K and 2 < po6 30 kPa. Location and size of the gain regimes in the flow direction depend on the plasma starting condition. At a wavelength of 0.91 um values of small signal gain up to 0.6 m-l can be attained. The model is described and some selected results are presented. The effect of the electron heat conduction is dis cussed. 1 . In t roduct ion The i d e a t o pump a l a s e r by a r ap id recomcombination of expanding plasmas, some b ina t ion of e l ec t rons i n t o t h e h ighly exy i e ld ing very promising r e s u l t s . (A more c i t e d e l e c t r o n i c s t a t e s of a n e u t r a l o r r e c e n t compilat ion may be found i n Ref. 6 ) . i o n i c spec i e s is r a t h e r o ld . ' Meanwhile t h e Also many experiments have been performed mechanism of many pulsed l a s e r s has been and t h e ex i s t ence of populat ion inve r s ions explained by t h i s process. The suggest ion has been proven repea ted ly . But only Campt o opera te a recombination l a s e r by exp l o i t i n g t h e r ap id cool ing r a t e s , and hence recombination r a t e s , i n a supersonic plasma expansion flow has been made nea r ly a s long Though basing on a d i f f e r e n t physic a l p r i n c i p l e , i t has always been tempting t o regard such a plasmadynamic l a s e r a s t h e n a t u r a l ex tens ion o f t h e powerful i n f r a r e d gasdynamic l a s e r s i n t o t h e v i s i b l e o r even u l t r a v i o l e t wavelength region. Since recomb ina t ion i s a genera l phys ica l mechanism t o p r e f e r e n t i a l l y populate high l y i n g e lect r o n i c l e v e l s , t h e method should be app l i c ab l e t o pump l a s e r t r a n s i t i o n s i n almost any ion i zab le element o r molecule and thus open a wide spectrum of l a s e r wavelengths. Reviews and more d e t a i l e d d i scus s ions may be found i n Refs. 4 and 5. Q u i t e a few theo r e t i c a l models have been developed t o desc r i b e t h e populat ion dynamics by t h e re b e l l e t a l . ' have been a b l e t o a c t u a l l y demonstrate 1 m s l a s e r a c t i o n i n an argpn plasma, expanded from a pulsed MPD a r c source. However, i n an a t t e q p t by Hoffmann and ~ i i ~ e l ~ t o extend t h e s e r e s u l t s t o cw opera t ion a t equal plasma parameters abso rp t ion was found i n t h e l a s e r t r a n s i t i o n s . Today it i s not known which s teady s t a t e processes prevent t h e recombination l a s e r a c t i o n dnd t o what ex t en t e x i s t i n g models do not desc r ibe t h e r e a l physics accu ra t e ly enough. I n t h i s paper a f u r t h e r t h e o r e t i c a l model i s descr ibed and some ca l cu l a t ed r e s u l t s a r e presented f o r a n i t rogen plasma a s a p o t e n t i a l l a s e r candidate. The model consists of two main p a r t s t o compute t h e r e combination and e x c i t a t i o n k i n e t i c s simultaneously wi th t h e gasdynamics along t h e