The effects of thermal motion of neutrals on the non-potential instabilities in a weakly ionized sodium plasma

The results of recent experimental measurements of the differential cross-section for elastic scattering of electrons on sodium atoms were used to evaluate the electron steady-state distribution function in a weakly ionized, uniform and non-magnetized sodium plasma placed in a d.c. electric field. The field was assumed to be of moderate intensity, so that the thermal motion of the neutrals had to be taken into account in the evaluation of the distribution function. The resulting ‘modified Druyvesteinian function’ was applied to study the non-potential instabilities arising from the presence of the field in this particular plasma. Threshold drifts for both very slow and slow modes were obtained and the conditions for the onset of instabilities were discussed. It is shown that the thermal motion of the neutrals affects both critical drifts and the angles of propagation.