Opticalcoherence-basedtechniquesfor motional Stark effect measurements of magneticfield pitch angle

Abstract

The motional Stark effect measurement of magnetic field pitch angle in tokamaks is a mature and powerful technique for estimating plasma current density in tokamaks. However, its range of applicability is limited by the requirement that and manifolds are spectrally sufficiently well separated (high magnetic fields, high beam energies) to ensure adequate net polarization for a successful measurement. This paper proposes alternative schemes based on the optical coherence properties of the Stark multiplet that are somewhat more versatile than the standard method and better suited to measurements on low-field toroidal confinement devices. An interference filter is used to transmit the Stark multiplet to a polarimeter (which uses a single photoelastic plate) that modulates the light temporal coherence and/or its first spectral moment. This light is subsequently processed using a novel electro-optically modulated solid-state interferometer that is sensitive to low-order spectral moments. The modulation of these quantities conveys information about the orientation of the light polarization and hence the magnetic field pitch angle.