Microwave reflectometry on the Advanced Toroidal Facility to measure density fluctuations and their radial correlation lengths

A two-frequency correlation reflectometer has been designed, built, and operated on the Advanced Toroidal Facility (ATF) to measure plasma density fluctuations. This reflectometer uses quadrature phase detection to allow true phase measurement of the reflected microwave signal (probing beam). By measuring the phase fluctuations in the reflected probing beam, it is possible to estimate the amplitude of the density fluctuations. Simultaneous two-frequency operation makes it possible to measure the coherence between fluctuations at two radially separated cutoff layers, from which the radial correlation lengths and wave numbers can be estimated.

This reflectometer has been used to study the density fluctuations in the edge density gradient region in low-density ATF plasmas with electron cyclotron heating (ECH). These studies have revealed globally coherent turbulence having a radial correlation length $\sim$5 cm, a radial wave number k$\sb{r}$ $\sim$ 0 cm$\sp{-1}$, and a poloidal wave number k$\sb\theta$ $\sim$ 1 cm$\sp{-1}$. The fluctuation rms amplitude reaches a maximum ($\sim$5%) at the plasma edge and decreases with radius to a level of $\sbsp{\sim}{<}$1%. Simultaneous measurements with the reflectometer, heavy ion beam probe and fast reciprocating Langmuir probe, provided verification of the measured fluctuation amplitude.

A comparison of the results of these measurements with theoretical models shows that the pressure-gradient-driven resistive interchange instabilities are a likely cause of the observed turbulence.