Density depletion caused by thermal instability in the electron cyclotron resonance heating plasmas

Summary form only given. An unexpected electron density depletion was observed in experiments involving electron cyclotron heating (ECH) of toroidal devices. Both the horizontal and vertical line average density decreased by approximately 15% during the heating pulse period. This was an anomalously large value since the empirical transport code calculations predicted only a 2% decrease in the line average density. A thermal instability is proposed to explain the anomalous electron density depletion. Considering the fact that the background plasma has an elevated electron temperature and a localized heat source located near the electron cyclotron resonance layer, it is shown that density striations having a scale length electron cyclotron resonance 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 a 10 ms ECH pulse, which causes about 50% electron temperature elevation. An 1% temperature fluctuation excited by the thermal instability leads to about a 20 V/m fluctuation. Such a large electrostatic field can effectively cause anomalous plasma diffusion via E/spl I.oarr/ /spl times/ B/spl I.oarr/ drift and account for the observed 15% electron density depletion.