Density fluctuation measurements in ATF using correlation reflectometry

A two-frequency correlation reflectometer has been operated on the Advanced Toroidal Facility (ATF) to measure plasma electron density fluctuations. This reflectometer uses quadrature phase detection to permit true phase measurement of the reflected microwave signal (probing beam). By measuring the phase fluctuations in the reflected probing beam, the amplitude of the density fluctuations can be estimated. Simultaneous two-frequency operation makes it possible to measure the coherence between fluctuations at two radially separated cut-off layers, from which the radial correlation lengths and wavenumbers can be estimated. This reflectometer has been used to study the density fluctuations in the edge gradient region of low density ATF plasmas produced by electron cyclotron heating. These studies have revealed globally coherent turbulence with a radial correlation length of up to approximately 5 cm, a radial wavenumber k_r ∼ 0 cm^-1 and a poloidal wavenumber k _θ ∼ 1 cm^-1. The rms amplitude of the fluctuations reaches a maximum of ∼ 5% at the plasma edge (ρ = 1, where ρ is the flux surface normalized radius) and decreases with decreasing radius to a level of ≲1%. Simultaneous measurements of the fluctuations with the reflectometer, the heavy ion beam probe and the fast reciprocating Langmuir probe provide consistent results. A comparison of the measurements with simplistic mixing length estimates, n_e/n_e = 1/k _θL_n or n_e/n_e = 1/k _rL_n, shows that these estimates are too high by factors of two to more than 100, while a comparison with a more detailed estimate for the pressure gradient driven resistive interchange turbulence yields reasonable agreement with the experimentally measured fluctuation characteristics.