Density fluctuation measurements in ATF using correlation reflectometry

Abstract

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 -5 cm, a radial wavenumber k, -0 cm-' and a poloidal wavenumber k, -1 cm-I. The rms amplitude of the fluctuations reaches a maximum of approximately 5% at the plasma edge ( p = 1, where p is the flux surface normalized radius) and decreases with decreasing radius to a level of 5 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, fiehe = l/k,L, or fi,/n, = l/k,L,, 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.

Geometrical optics interpretation of the phase runaway in microwave reflectometry

“…This so called ''phase runaway'' has been observed for many plasma conditions with almost all reflectometers. [1][2][3][4][5][6] It has been suggested that it is a Doppler shift of the probing radiation reflected from rotating cutoff layer disturbances. 2,4,5,7 Either these disturbances are asymmetric or a misalignment of the reflectometer axis with respect to the normal at the cutoff layer exists.…”

Microwave reflectometry for magnetically confined plasmas

“…This led to the reemergence of microwave reflectometry as a diagnostic with good resolution for long wavelength fluctuations. [83][84][85][86][87][88][89][90][91][92][93][94][95] In this paper, we review the application of microwave reflectometry to the measurement of density profiles and to the detection of density fluctuations in magnetically confined plasmas. The paper is organized as follows: Section II is a brief review of the theory of plasma waves that are of interest for reflectometry in magnetized plasmas, i.e., those in the electron cyclotron range of frequencies.…”

Surface Structure of N/Pd(100)-c(2×2) Determined by Tensor Low Energy Electron Diffraction Analysis

“…Many of the results reported here have been observed in experimental signals-indeed there is now a substantial body of evidence in the literature (e.g. [14][15][16][17][18][19][21][22][23]). However, it is beyond the scope of this paper to include an experimental example for each simulation result (space limitations notwithstanding) but where possible reference will be made to examples in the literature.…”

“…It has been widely suggested that asymmetry or misalignment between the launch and receive antennae and the plasma reflection layer may be responsible for the ubiquitous phase runaway effect observed in many experiments [14][15][16][17][18][19][20]. Indeed, recent simulations using 2D…”