Ion Acoustic Turbulence in Inductively Coupled Magnetized Plasmas

Abstract

Summary form only given. Low-frequency (LF) wave activity in the megahertz frequency range, which has been detected in various helicon plasmas excited by m=1 antennas, is thought to be a substantial factor in the discharge performance. In this report, LF turbulence is characterized comparatively in two different plasma sources excited at 13.56 MHz by m=0 antennas: in a conventional helicon plasma (HP) and in a magnetized inductively coupled plasma (MICP). As the magnetic field shape has a strong influence on plasma production in both devices, the role of this factor is specially attended. LF turbulence was found to be inherent for both devices and to demonstrate many similar features. The spectrum of turbulence has a wide peak around the lower hybrid frequency (in the range 0.5-1 MHz) and a narrow peak, independent of the magnetic field, around 150-200 kHz. In the HP, oscillations show quite long correlation in the azimuthal direction, along which they propagate with phase velocities somewhat higher than the ion-acoustic velocity. In the MICP, oscillations demonstrate similar behavior at lower magnetic field but become short-correlated at higher magnetic field. The magnetic field shape influences considerably on turbulence spatial distribution and, to a lesser extent, on turbulence intensity. Two possible mechanisms of turbulence excitation are examined; electron drift current drive and parametric instabilities. To estimate the effect of drift current, profiles of the plasma density, electron temperature and plasma potential were measured and experiments with a diamagnetic loop were performed. To ascertain the role of parametric instability, wave profiles at the fundamental frequency were measured together with the sideband and LF wave profiles, and their correlation was examined. Potentiality of both mechanisms to excite, either separately or jointly, the LF oscillations is analyzed theoretically, and the role of turbulence in the power absorption is estimated