The spatial structure of the lowest ion-acoustic instability modes in the high-current discharge of CW argon ion lasers

Abstract

The cross-sectional structure of the two lowest ion-acoustic instability modes in the positive column of a cylindrical, high-current discharge, traditionally used as an active medium in CW ion gas lasers, has been investigated. The diameter and length of the discharge column in our experiments were 16 mm and 1 m, respectively, and the discharge current was 250–350 A. Under these conditions, the observed mode frequencies were ν1 = 190 kHz and ν 2 = 390 kHz. To obtain the spatial structure of these modes, time-resolved integral projections of spontaneous argon-plasma emission were measured in planes normal to the discharge axis. From the power spectra of the measured projections, the structures of the modes ν1 and ν 2 were reconstructed using computerized tomography algorithms. Based on available theoretical models of the instability, the modes have been identified as the (1, 1) and, tentatively, the (1, 2) modes of oscillation.