Development of a multipoint Thomson scattering system for a large reversed field pinch experiment

A multipoint Thomson scattering system is under development for the measurement of Te and ne spatial profiles in RFX, a large (a=0.48 m, R=2 m, I≤2 MA) reversed field pinch experiment. In this system, the beam from a low divergence ruby laser (E=15 J, Δt=30 ns) is focused to cross the plasma in the equatorial plane. The scattering signal from 30 locations along a plasma diameter is collected by means of high transmission fiber optic bundles. An effective viewing dump was obtained by engraving sharp poloidal grooves in the first wall graphite tiles. The scattering light spectra are analyzed by means of three spectrometers, each including an F/3.4, aberration corrected, holographic grating and a 40-mm-diam MCP photomultiplier with a 10x10 anode array. These detectors use a high strip current, V-plate, electron multiplier with 105 electron gain and have a recovery time of less than 30 μs. In the 300-channel data acquisition electronics, the plasma light background may be sampled 100 ns before the scattering pulse. The system is designed in order to achieve a spatial resolution of 2.5 cm, useful to study small-scale correlation phenomena, and will also measure fast, submillisecond fluctuations of Te and ne by exploiting the two-pulse capability of the laser