Noise in collision-free alkali metal plasmas

Abstract

Oscillations in alkali metal plasmas generated by contact ionization have been attributed to unstable drift waves both in collision-free and collision-dominated plasmas. In the early experiments, however, the influence of the radial electric field Er in causing rotation of the plasma column was ignored. In later work with collisional plasmas, measured Er values were used to calculate frequencies in the rest-frame of the plasma and so demonstrate unstable collisional driftwaves.Measurements are reported on collision-free potassium and caesium plasmas. The radial electric field causes a Doppler shift comparable with the frequencies observed and often larger than the predicted frequency of the fastest growing drift wave. When this is taken into account the frequencies observed in potassium are within a factor of two of those predicted for the collisionless drift instability and the radial distribution of the density fluctuations shows a peak away from the edge of the column. Other features are consistent with the interpretation of the noise in potassium as due to the drift instability. In caesium edge effects predominate and the cause of the enhanced noise is not the drift instability.