Quasi-periodical kinetic instabilities in minimum-B confined plasma

Abstract

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.

Diagnostic techniques of minimum-B ECR ion source plasma instabilities

“…Some experimental campaigns probing the unstable plasma regime have been performed in the time-averaged mode using HPGe and cadmium-tellurium (CdTe) semiconductor detectors. 18,19,40 These studies have revealed that the shape of the temporally averaged bremsstrahlung spectrum does not significantly change between the stable and unstable plasma regimes. The result can be explained by the short duration (at the level of microseconds) of the instability burst and the properties of the detector system.…”

“…38, Izotov et al demonstrated the direct measurement of the energy distribution of axially escaping electrons from a stable ECR plasma using the ion source spectrometer magnet-originally designed for the m/q separation of multiply charged ions-as an electron energy analyzer. The technique has been used also in other studies for investigations of both stable and unstable ECR plasmas 39,40 in the time-averaged mode, i.e., by measuring the average electron current for each electron energy step. As plasma instabilities are characterized by the abrupt current transients, this time-averaged method provides only limited information of the expelled electrons.…”

“…(3) The time-averaged bremsstrahlung energy spectrum is not significantly influenced by the onset of instabilities, and thus, it cannot be used to determine the transition from stable to unstable plasma. 40…”