Nonthermal electron cyclotron emission from TFTR supershot plasmas

The Ohmic targets of supershot (Strachan et al., 1987) plasmas in the Tokamak Fusion Test Reactor (TFTR) are often characterized by the presence of nonthermal electron cyclotron emission (ECE) from a runaway or slideaway electron population. This nonthermal ECE decays away within the first 100-150 ms after the start of neutral beam injection (NBI) as the electron density rises and the loop voltage drops. After this usual slideaway-induced, nonthermal ECE is quenched a new nonthermal feature develops at frequencies just below both the optically thick, thermal emission of the ECE fundamental ordinary mode, and the second harmonic extraordinary mode. This paper presents an analysis of the ECE data which indicates a possible source of this nonthermal emission feature. In addition, correlation of the peak intensity of the nonthermal feature with various plasma parameters for 240 TFTR discharges from 1988 provides some insight toward identifying possible physical conditions which give rise to the feature.