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

Abstract

A diagnostic neutral beam system has been developed for the Madison symmetric torus (MST) reversed-field pinch. The system is primarily used: (1) for measurement of the majority ion equilibrium and fluctuating velocity and temperature by Rutherford scattering (RS); (2) for measurement of the impurity ion velocity and temperature, both equilibrium and fluctuating, by charge-exchange recombination spectroscopy (CHERS); and (3) for magnetic field measurement via motional Stark effect (MSE). The system consists of two neutral 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 atoms scattered from the plasma ions is measured with two 12-channel, 45° electrostatic energy analyzers equipped with a hydrogen stripping cell. A second injector creates a 30 keV, 4 A hydrogen beam, which is used for the CHERS and MSE diagnostics. In each injector ions are extracted from a plasma created by an arc discharge source and, after acceleration and focusing, 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 design 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 the MST axis and a high neutral current density (0.4 equivalent A/cm2). 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 analyzer designs and first results of their tests on the MST.