Frequency-resolved multiple k scattering diagnostic

Abstract

In conventional Thomson scattering diagnostics S(w,k) is measured with either w or k fixed; this paper describes a technique which enables the frequency spectrum to be determined along three different density fluctuation k vectors simultaneously. Such a technique is invaluable in the diagnosis of irreproducible plasmas, since the complete dynamic form factor for the scattered light S(w,k) can be measured instantaneously. Experiments on the Culham Plasma Focus, with its short-lived (10–50 ns) dense plasma phase, demonstrate the advantages of the method. Three sets of collection optics were orientated to view an identical scattering volume with scattering k’s parallel, orthogonal, and at 45° to the plasma Z axis. The scattered light from each system was relayed by optical fibers to a Czerny–Turner spectrometer with a triple input slit. A gated optical multichannel analyzer was used to record the three dispersed spectra produced by this arrangement. The results demonstrate the gross variations in the density fluctuations parallel and perpendicular to the current flow. Examples of some of the detailed analyses of the results in terms of a two-component plasma with different Te, Ti, ne, V drift, etc., are given.