THERMAL-INSTABILITY EXCITED IN THE ELECTRON-CYCLOTRON-RESONANCE HEATING OF PLASMAS

A thermal instability is studied as an explanation of the anomalous electron density depletion observed during the electron-cyclotron resonance (ECR) heating of plasma in a tokamak device for an impurity study experiment (ISX-B). Considering the fact that the background plasma has an elevated electron temperature and a localized heat source located near the ECR layer, we show that density striations having scale length 10 cm and larger can be excited via the thermal instability. The amplitude of the striations can grow by more than one e-folding during the ECR pulse period of 10 ms, which causes about 50 % electron temperature elevation. A 0-1 % temperature fluctuation excited by the thermal instability leads to about 50 V m-1 field fluctuation. Such a large electrostatic field can effectively cause anomalous plasma diffusion via the E x B drift and account for the observed 15 % electron density depletion.