Ion densities in a high-intensity, low flow nitrogen-argon plasma

The plasma density in an expanding thermal plasma was determined using planar Langmuir probe measurements. The are plasma was operated at low flow (500 standard cm(3) per minute). It is shown that the decrease of density with increasing distance from the nozzle of the are in an argon plasma can be explained by diffusion away from the expansion axis. The determined decay length is 10 cm. In the case that nitrogen is injected in the are, the plasma density is lowered considerably due to charge exchange and dissociative recombination in the expansion. Because of the low electron density (10(17) m(-3)) at a partial nitrogen flow larger than 10%, the dissociative recombination becomes slow. The main loss process of N-2(+) ions in this case is diffusion away from the plasma axis. The effective decay length found in the nitrogen plasma is 9 cm. (C) 1997 American Institute of Physics.