Updated HIT-SI3 Thomson Scattering Diagnostic

The Thomson scattering diagnostic on HIT-SI3 has been modified to enable measurements of electron temperature down to nearly 5 eV. This should allow for up to three simultaneous spatial measurements of T_e per discharge. These will be the first Thomson scattering measurements on the inductively driven HIT-SI3 spheromak. The modification involved installing new filters on the polychromators that bin the Doppler-shifted scattered light by wavelength. The filter pass-bands (and therefore the sampling bins) have been shifted closer to the laser line to increase sensitivity to the colder T_e distributions. Preliminary measurements with the new polychromators will be compared with the old polychromator setup to quantify the change in sensitivity. The aim of the new system is to enable both density and temperature measurements on HIT-SI3. Previous attempts at Thomson scattering measurements on the HIT-SI3 device indicated temperatures of 10 eV or less, however recent plasma performance achievements may allow for the new Thomson measurements to be made on spheromak plasmas of higher current.