Evidence for drift waves in the turbulence of reversed field pinch plasmas

摘要

A detailed characterization of the high-frequency range of the fluctuation spectrum in reversed field pinch plasmas is presented, revealing a variety of new features distinct from global tearing modes and the cascade that they are thought to drive. The anisotropic broadband spectrum of the fluctuating electric field is measured. The power in the fluctuating kinetic energy (1/2) m(i)n(i)(V) over tilde (2)((E) over tilde xB0), previously measured to be smaller than the magnetic energy in the tearing-mode-unstable frequency range, becomes greater than and diverges from the magnetic energy above 60-80 kHz. The lack of equipartition at high frequencies coincides with the measured signatures of the independent fluctuation activity broadly consistent with the drift-wave fluctuations. Statistical coherence measurements reveal the mode activity that is compressive with a large amplitude in the vicinity of strong density gradients and with a phase speed comparable to the electron drift speed. There is a distinct high-frequency correlation between the fluctuations of density and the parallel magnetic field. Elevated coherences associated with this fluctuation feature return more quickly after a sawtooth event than the corresponding coherences associated with tearing activity.