Single beam, single detector diagnostic for plasma density profiles across a magnetic field gradient

Abstract

A scheme is proposed whereby the density profile of a tokamak or mirror plasma is mapped to nonlinear sidebands of a diagnostic wave, so that the sideband frequency spectrum is an image of the density profile traversed by the wave. The diagnostic relies on a monochromatic, high-frequency (omega>~omegace), pulsed wave source to determine the density profile across a magnetic field gradient. In principle, the technique offers time-dependent direct density profiles having high spatial resolution. However, the feasibility of the diagnostic is limited by the intense power and the rapid rise time required of the pulsed diagnostic wave. For parameters relevant to fusion plasmas, and for the particular scattering geometry suggested here, the technique requires, roughly, 30 mJ pulses of 2 nsec duration. The required power can be reduced by a factor of roughly 10^4 by applying signal averaging techniques. The rise time of the pulsed source must be short in comparison to one period of the diagnostic wave.