Characteristics of Electron Density Fluctuations in Heliotron E Measured Using a Wide Beam Laser Phase Contrast Method

Abstract

Electron density fluctuations in Heliotron E were measured using a wide beam laser phase contrast method in a frequency range of 2.5≤ f ≤500 kHz and a wavenumber range of 0.1 ≤ k ≤1.3 rad/mm. Two branches of S ( k , f ) spectra were detected, which we have called a high speed branch and a low speed branch because of a difference in their measured phase velocities. Their characteristics were studied from spatial distributions, correlation with magnetic fluctuations, phase velocities, fluctuation levels and theoretical considerations. The high speed branch is consistent with a Magneto Hydro Dynamic (MHD) mode driven by a resistive interchange instability and mainly existed inside the limiter, and the low speed branch is explained as a microturbulence driven by a drift instability and mainly existed outside the limiter. The implications of these findings for studies of plasma confinement in Heliotron E are discussed.