Collective microwave scattering diagnostic on the H-1 heliac

A multichannel microwave scattering diagnostic has been developed and installed on the H-1 heliac. The purpose of the new diagnostic is to study small-scale plasma fluctuations in H-1, which are believed to be responsible for the loss of particles and energy from the plasma. The diagnostic is a 132 GHz, four-channel superheterodyne system. The transmitter and receiver antennas (consisting of horns and focusing bispherical mirrors) are located inside the vacuum vessel of H-1. A radial resolution of Δr/a∼0.2 is achieved. The scattering volume is positioned in the density gradient region at r/a∼0.6. At present, the system is aligned to measure fluctuations in the poloidal wave number range from approximately 10 to 25 cm^-1. The use of the heterodyne detection system allows the fluctuation propagation direction to be determined. The low frequency bandwidth of the system is 1 MHz. The instrument sensitivity is about P_s/P_i∼10^-6.