Bursts of electron cyclotron emission during ELMs and high beta disruptions in TFTR

Large ELM events in TFTR are often accompanied by short, intense bursts of electron cyclotron emission. A unique combination of two fast grating 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 which occur early in the thermal quench phase of high beta disruptions, the only other time that bursts occur with comparable robustness. In these 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. Rapid loss of particles and sudden rapid cooling occurs. In both cases, bursting can be explained not in terms of excitation of enhanced emission but rather in the reduction of absorption of thermal emission. Bursting is consistent with a modification of the electron distribution function f(e) due to a rapid energy or particle exchange between hot electrons and cold electrons from the edge, momentarily reducing the velocity gradient of f(e) in the thermal region. This model is qualitatively and quantitatively consistent with experiment, predicting for example emission enhancement factors of approximate to 10.