Design of a multichannel polychromator for KTX Thomson scattering diagnostic

A polychromator is designed based on the transmission volume phase holographic grating spectrometer coupled with avalanched photon diodes through fiber bundle and has been successfully implemented on the Keda Torus eXperiment [Plasma Phys. Control. Fusion 56 (2014) 094009] Thomson scattering system. The polychromator is operated with a 1064 nm laser, and the designed spectral range is from about 1000 nm to 1100 nm. The spectral channels have been optimized with a five-channel design to lower the estimated error of the electron temperature and density within 5% and 3% respectively when the temperature is around 100 eV. With a calibration method about the ratio of the channel average responses using bremsstrahlung radiation, we obtain an electron temperature of 7 eV in a typical experiment with low plasma currents.