Imaging interferometers for analysis of Thomson scattered spectra

Abstract

Polarization interferometers have some potential efficiency advantages for imaging Thomson scattering spectral analysis. In this article we present a number of designs for high-efficiency imaging polarization interferometers for Thomson scattering spectral analysis. The use of high-efficiency crystal polarizing beamsplitters (both displacement and angle) results in low-loss complementary passbands (no edge losses), simple imaging systems, and wide field of view. The efficiency and relative merits of both multiple-filter and dispersive-type configurations are being assessed before installation on the JT-60U ruby-laser Thomson scattering system. Light is transferred from the viewing port via a linear array of optical fiber bundles which will be imaged through the interferometer onto the photocathode of an intensified charge coupled device camera. Because of the broadband nature of the Thomson light, the optical delays required to Fourier analyze the spectrum are quite small. This leads to compact multicolor or dispersive systems based on combinations of Wollaston and Savart splitters and traditional waveplates.

Active and passive spectroscopic imaging in the DIII-D tokamak

“…Bremsstrahlung imaging [1], which extrapolates well to future high density devices, has provided wide field-of-view measurements of core MHD [2][3][4], and imaging of Doppler-shifted D α from re-neutralized beam ions has provided high spatial resolution fast ion profile information [5,6]. Further developments, such as those presented in this paper as well as others being developed elsewhere [7][8][9][10], are helping to provide detailed 2D measurements of neutral beam deposition and core kinetic profiles of everything ranging from carbon impurity density to electron density, temperature and flow velocity. Fortuitously, these high resolution measurements of phenomena and equilibrium parameters are becoming available just in time for validation of the increasingly detailed codes required to make believable predictions for future devices such as ITER.…”

“…Spectroscopic imaging as discussed here uses a narrowband interference filter combined with a fast-framing camera to obtain 2D, wide field-of-view (FOV) images of the intensity in an isolated wavelength band of interest. Recently, innovations on this simple approach have been developed that, through essentially interferometric techniques, greatly increase the obtainable spectroscopic information-so-called 'coherence imaging' [7][8][9][10].…”