Interferometric measurement of high-frequency density fluctuations in Madison symmetric torus

As a consequence of recent improvements to the far-infrared (FIR) interfero meter time response, from bandwidth of 10 kHz up to 1 MHz, high-frequency d ensity fluctuations can now be resolved on the Madison symmetric torus (MST ) reversed-field pinch. The phase measurement allows absolute calibration o f the fluctuation amplitude while eleven chords generate information on the spatial distribution. Density fluctuations up to 200 kHz are observed. A u nique feature of the MST interferometer is that six chords are toroidally d isplaced 5 degrees from the remaining five. Hence, besides information on t he poloidal structure, i.e., m = 1, toroidal information is also available. By computing the cross power, coherence, and cross phase between two toroi dally displaced chords, one can determine the toroidal rotation speed, disp ersion, and correlation length. (C) 1999 American Institute of Physics. [S0 034-6748(99)58001-5].