Collective scattering technique for sub-millimeter wavelength fluctuation detection in a helicon plasma

Summary form only given. A possible explanation for the high RF absorption efficiency of helicon sources is the damping of the slow wave, the so called "Trivelpiece-Gould" wave, at the plasma edge in a helicon discharge. Because of the extremely short wavelength of the slow wave (less than 1 mm) direct measurements of the slow wave are not possible with standard electrostatic probes. We report on the development of a simple, portable diagnostic capable of directly measuring sub-millimeter wavelength fluctuations in helicon sources. The technique is based on collective scattering principles in which a plane polarized electromagnetic wave scatters off an electron density fluctuation in the plasma with a fraction of scattered light being detected at an angle /spl theta/ relative to the direction of the incident wave. Similar measurements are routinely performed in fusion plasmas. Our quasi-optical diagnostic, operating at 288 GHz, will be capable of measuring fluctuation wavelengths from .63 to 6.3 mm (k/sub /spl perp//=100-10 cm/sup -1/). As shown by Kline et al., waves with k/sub /spl perp///spl sim/10 cm/sup -1/ are routinely observed in HELIX. The scattered signal is measured with a homodyne detector with sufficient time resolution to detect waves in the frequency range of 1 kHz to 50 MHz.