A low-cost multichannel Thomson scattering system for CDX-U

A multichannel Thomson scattering system has been developed for the CDX-U spherical torus. The system is designed for 10 eV<T_e<400 eV, n_e>10¹⁸ m^-3, which includes typical and predicted central and edge conditions in CDX-U. The system uses two laser passes to double the scattered photons from a 5 J ruby laser. The beam path is vertical through the ∼66 cm (elongated) diameter of the plasma and is movable in the major-radial direction, enabling coverage of nearly 70% of the major-radial plasma extent. Twelve channels over the vertical minor radius provide ∼2.5 cm spatial resolution. The main collecting lens, located 45 cm from the laser beamline, provides high solid angle (ΔΩ∼0.01) light collection using a 15 cm diam lens. The system makes maximum usage of an optically fast (f/1.8) compact imaging spectrometer. An intensified charge coupled device with a GaAs photocathode provides quantum efficiency of ∼20% at 6943 Å. The combination of plasma access, multiple beam passes, high-throughput spectrometer, and high quantum efficiency detector provide for very high total photon statistics in a relatively simple and inexpensive system.