The effect of ecrh on the dite plasma boundary

Abstract

During the past year electron cyclotron resonance heating (ECRH) has been applied for the first time to the DITE tokamak. A new edge diagnostic, operable both as a Langmuir probe and a retarding field analyser, has been used extensively to investigate the variation of plasma boundary parameters during ECRH heated discharges. Arrays of Langmuir probes embedded in a pumped and inner bumper limiter have also been operated during additionally heated discharges. Measurements have been made over a wide range of ECRH input powers, as a function of global plasma density and for many values of the toroidal magnetic field. The behaviour of the edge plasma in response to the additional heating is observed to be strongly dependent on the major radial position at which resonant absorption of the ECRH pulse occurs. Radial profiles of electron density and temperature have been obtained, together with observations of the time response of these edge parameters to the ECRH pulse. Characteristic lengths in the boundary are observed to remain approximately unchanged throughout heated discharges. In general, the edge particle flux exhibits a well defined, reproducible behaviour, increasing during ECRH to ~1.5 times its ohmic value with rise times ≤ 2 ms. The percentage increase in the edge temperature is smaller with rise times ≤ 10 ms. Reasonable global power balance is obtained using data from the limiter Langmuir probes in both ohmic and ECRH phases of a typical discharge.