Investigations of short-scale fluctuations in a helicon plasma by cross-correlation enhanced scattering

Abstract

Correlation enhanced scattering (CES) near the upper hybrid resonance has been applied for studying small-scale plasma density fluctuations excited by the rf fields in a helicon discharge. The turbulent fluctuations are diagnosed for conditions where the electron plasma frequency exceeds the electron cyclotron frequency considerably. The frequency and wave number spectra of the fluctuations are measured both in the plasma core as well as in outer region of the helicon discharge. The spectral measurements evidence the short-scale fluctuations to originate from a parametric decay instability. The low-frequency fluctuations obey the ion-sound dispersion relation while the lower sideband of the helicon wave frequency satisfies the Trivelpiece–Gould wave dispersion relation. In order to gain more insight in the experimental results and, in particular, to estimate the fluctuation level the backscattering process is analyzed both numerically and analytically for high-density plasma conditions. A fully electromagnetic model was developed that takes into account the radial density distribution of the plasma column as well as the antenna diagram of the rectangular emitting/receiving horn. Using this model the relative amplitude of ion-sound density fluctuations in the core of the helicon discharge is estimated as 11%. The role of nonlinear effects on formation of the scattering spectra due to the high fluctuation level in the plasma center is discussed. The findings also demonstrate that the CES diagnostic can be applied to diagnose fluctuations in spherical tokamak plasmas where the probing conditions resemble those of high-density helicon discharges.