A novel spectral filtering technique to improve incoherent Thomson scattering diagnostics*

Incoherent Thomson scattering, or simply TS, diagnostics are ubiquitous in the plasma physics community and have had unparalleled success in providing direct measurements of plasma electron temperature (T_e) and density (n_e). Many plasma experiments use polychromators with three to eight wideband spectral channels to spectrally resolve measurements of Thomson scattered radiation. Theoretically, increasing the number of spectral channels in a polychromator can increase the T_e measurement range and decrease overall measurement uncertainties. However, in practice, there is a direct trade-off between the number of spectral channels and the signal-to-noise ratio of the measurement.

 A novel filtering technique called Thomson Scattering Spectral Multiplexing (TSSM) has been developed at the Helically Symmetric eXperiment (HSX) in order to circumvent the limitations imposed by existing polychromator systems. TSSM implements an optimized set of multiple-band dichroic filters with two spectral passbands per filter instead of only one passband. Comparisons of the expected measurement uncertainty between a standard TS approach and TSSM show that spectral multiplexing not only increases the measurement range of T_e but also reduces the average measurement uncertainty and the deviation in the uncertainty over the increased range of T_e. Calculations, calibration data, and experimental results for the TSSM system on HSX are presented.