Quasi-periodical kinetic instabilities in minimum-B confined plasma

We present the results of an experimental investigation of quasi-periodical kinetic instabilities exhibited by magnetically confined electron cyclotron resonance heated plasmas. The instabilities were detected by measuring plasma microwave emission, electron losses, and wall bremsstrahlung. The instabilities were found to be grouped into fast sequences of periodic plasma losses, separated by ∼100 µs between the bursts, followed by 1–10 ms quiescent periods before the next event. Increasing the plasma energy content by adjusting the plasma heating parameters, in particular the magnetic field strength, makes the instabilities more chaotic in the time domain. Statistical analysis reveals that the energy released in a single instability event depends on the magnetic field strength and microwave power but not on the neutral gas pressure. The effects of these ion source parameters on the instability characteristics are explained qualitatively by considering their influence on the electron energy distribution. A correlation is found between the energy dissipated in an instability event and the recovery time of the periodic bursts, i.e., a large amplitude instability leads to a long recovery time of the electron energy distribution.