Low-frequency density fluctuations in a tokamak plasma

Publisher Website
Google Scholar
Abstract
CO2 laser scattering and correlation techniques are used to study low‐frequency density fluctuations in plasmas in the Alcator C tokamak. Using crossed‐beam correlation techniques, these fluctuations are found to have largest amplitudes near the plasma edge. In discharges with mean plasma densities n̄≳2×1014 cm−3, the normalized fluctuation amplitude is ñ/n≂0.5 and peaks at the limiter radius; however, at lower plasma densities, ñ/n peaks in the plasma interior at a normalized minor radius r/a≂0.75 (where a is the limiter radius). The radial and poloidal wave vectors of the fluctuations are comparable, with mean wavenumbers k̄∼20 cm−1. At any given k, the observed frequency spectrum is broad. For plasma densities n̄≲1.5×1014 cm−3, the poloidal group propagation velocity vθ of the fluctuations is measured to be approximately 2×105 cm/sec in the electron diamagnetic drift direction, while at plasma densities n̄≳2×1014 cm−3, vθ≂9×104 cm/sec in the ion diamagnetic drift direction. The nature of these fluctuations is also studied in discharges exhibiting a boundary‐layer instability (i.e., ‘‘marfes’’).