A diagnostic neutral beam system for the MST reversed-field pinch

A diagnostic neutral beam system has been developed for the Madison symmetr ic torus (MST) reversed-field pinch. The system is primarily used: (1) for measurement of the majority ion equilibrium and fluctuating velocity and te mperature by Rutherford scattering (RS); (2) for measurement of the impurit y ion velocity and temperature, both equilibrium and fluctuating, by charge -exchange recombination spectroscopy (CHERS); and (3) for magnetic field me asurement via motional Stark effect (MSE). The system consists of two neutr al beam injectors, and two neutral particle analyzers. One injector creates a 20 keV, 4 A helium beam for RS. The energy spectra of the helium beam at oms scattered from the plasma ions is measured with two 12-channel, 45 degr ees electrostatic energy analyzers equipped with a hydrogen stripping cell. A second injector creates a 30 keV, 4 A hydrogen beam, which is used for t he CHERS and MSE diagnostics. In each injector ions are extracted from a pl asma created by an arc discharge source and, after acceleration and focusin g, neutralized in a gaseous target. A low ion perpendicular temperature at the plasma emission surface, achieved via plasma expansion cooling, results in a low (0.016 rad) intrinsic beam divergence. A hallmark of the beam des ign is the focusing ion optical system that consists of four multiaperture spherically curved electrodes. The geometric focusing, together with a low intrinsic beam divergence, provides a small beam size-5 cm in diameter-on t he MST axis and a high neutral current density (0.4 equivalent A/cm(2)). A beam injector is compact in size-30 cm in diameter and 70 cm in length-and weighs about 70 kg. In this article we present details of the beam and anal yzer designs and first results of their tests on the MST. (C) 2001 American Institute of Physics.