``Runaway'' Electrons and Cooperative Phenomena in B-1 Stellarator Discharges

Abstract

Examination of the temporal distribution, relative intensity, and maximum energy of x‐rays above 30 kev emanating from the B‐1 stellarator during a pulsed discharge has revealed a number of interesting phenomena. The x‐rays are assumed to be produced by ``runaway'' electrons when they strike the wall. Spuriously large pulse heights have been observed. The x‐rays abruptly appear early in the discharge, before the Kruskal instability can occur. Depending on the operating conditions, x‐rays can be observed at any time during the discharge except when the current is at the Kruskal limiting value. If the longitudinal electric field used for ohmic heating is ``crowbarred,'' x‐rays can appear afterwards in copious quantities; furthermore, the discharge current can decay in abrupt steps correlated in time with bursts of x‐rays. The hypothesis that the current in these steps is due entirely to runaway electrons is consistent with the data. These observations are taken to be evidence for the existence of cooperative phenomena, or collective motions which can affect both the confinement and the heating of a plasma. In addition, intense nonthermal microwave noise has been detected at times correlated with x‐ray emission.