Development of the motional Stark effect with laser-induced fluorescence diagnostic

Abstract

The motional Stark effect diagnostic (MSE) is presently a widely accepted technique for measuring the magnetic field pitch angle in high field (>1 T) plasma devices. A hydrogen neutral beam passing through a magnetic field perceives υ×B electric field, and its Balmer-α spectral emission is split and polarized by the linear Stark effect. The technique cannot be readily used at lower magnetic fields, due to loss of polarization fraction when lines of different polarization overlap due to line broadening which is on the order of the separation. This article describes the development of a technique to extend the capability of MSE to include lower fields (0.01 T and up) and the field magnitude as well as direction. The technique employs laser-induced fluorescence on a diagnostic neutral beam. The narrow-band laser and low energy spread neutral beam allow the observed linewidths to be significantly narrower than these observed from previously employed collisionally induced fluorescence systems.