Enhancement and Suppression of Density Fluctuations around Electron Drift Frequency in Heliotron E Plasmas Measured Using CO₂ Laser Phase Contrast Method

Enhancement and suppression of electron density fluctuations around the electron drift frequency was observed in Heliotron E. This behavior corresponded to changes in the density profiles. Density fluctuations around this frequency were enhanced when the density profile was peaked in the inner region, and suppression was associated with the flattening of the density profile. An examination of the frequency, wavenumber, and collisionality regimes suggested that the most likely candidate for the instability responsible for driving these fluctuations is the collisionless circulating electron mode.