BURSTS OF ELECTRON-CYCLOTRON EMISSION DURING DISRUPTIONS OF HIGH-BETADISCHARGES IN THE TOKAMAK FUSION TEST REACTOR TOKAMAK

Disruptions are sudden terminations of tokamak plasma discharges. Duri ng disruptions at high beta beta where beta = plasma pressure/magnetic pressure, short (order of mu s) and intense bursts of electron cyclot ron emission (ECE), an order magnitude above thermal levels, are obser ved in the second harmonic electron cyclotron frequency range, which c orresponds to 100s of GHz in the Tokamak Fusion Test Reactor tokamak. A unique combination of two, fast, 500 kHz, 20-channel grating polychr omator instruments, located at different toroidal positions, is used t o measure the emission and characterize these bursts. New insights int o the three-dimensional dynamics of these disruptions and the accompan ying bursts of ECE have been obtained. Bursts of ECE occur at the begi nning of the thermal quenches and exhibit strong toroidal asymmetries. Bursts are localized to the vicinity of the ballooning mode, a fast g rowing (few ms) medium toroidal mode number (n = 10-20) precursor, loc alized toroidally, poloidally, and radially, which triggers the disrup tions. Fast-particle losses occur with the explosive growth of the bal looning mode, followed by plasma/wall interaction. Bursts of ECE occur shortly afterwards, within 10s of mu s of the East particle losses. A n explanation of the bursting is presented which is consistent both qu alitatively and quantitatively, with observations predicting, for exam ple, radiation enhancement factors of approximate to 10. Bursting can be explained not in terms of enhanced excitation of emission but rathe r in the reduction of absorption of thermal emission. Bursting is cons istent with a modification to the electron distribution function f(e) due to a rapid energy or particle exchange between hot electrons and c old electrons from the edge, momentarily reducing the Velocity gradien t off, in the thermal region. Large edge localized mode events also ex hibit bursts of ECE due to a similar sequence of events. (C) 1997 Amer ican Institute of Physics.