BURSTS OF ELECTRON-CYCLOTRON EMISSION DURING ELMS AND HIGH-BETA DISRUPTIONS IN TFTR

Large ELM events in TFTR are often accompanied by short, intense burst s of electron cyclotron emission. A unique combination of two fast gra ting polychromator instruments located at different toroidal positions is used to measure the emission and characterize these bursts, which exhibit strong toroidal asymmetries. Bursts are compared to those whic h occur early in the thermal quench phase of high beta disruptions, th e only other time that bursts occur with comparable robustness. In the se disruptions, bursts are localized to the vicinity of the ballooning mode, a medium toroidal mode number (n = 10-20) precursor, localized toroidally, poloidally, and radially, which triggers the disruption. R apid loss of particles and sudden rapid cooling occurs. In both cases, bursting can be explained not in terms of excitation of enhanced emis sion but rather in the reduction of absorption of thermal emission. Bu rsting is consistent with a modification of the electron distribution function f(e) due to a rapid energy or particle exchange between hot e lectrons and cold electrons from the edge, momentarily reducing the ve locity gradient of f(e) in the thermal region. This model is qualitati vely and quantitatively consistent with experiment, predicting for exa mple emission enhancement factors of approximate to 10.