Data processing for fast signals of Thomson scattering and application in the high repetition frequency laser system on LHD

Recently, high speed multi-channel digitizers are widely used as detectors of the polychromator signals in Thomson scattering measurement systems. It becomes possible to distinguish the stray light or noise components from the real signals of scattered light due to the detection of the transient signals. In the existing Thomson scattering system on the Large Helical Device (LHD), the charge integration type ADC system (FAST-BUS) is used. Three switched-capacitor-type digitizers, CAEN V1742B, each of which has 32 channels and sampling frequency of up to 5 GS/s, also started to be used for the electron temperature, Te, and density, ne, measurement at 18 spatial positions while the number of the all spatial positions is 144. For the measurement of Te and ne during fast phenomena in the LHD plasmas, a new laser which has high repetition frequency up to 20 kHz was operated in the last experiment campaign of LHD. Another type of the switched-capacitor-type digitizers, TechnoAP APV81G32, are tested. The number of channels is 32 in one board and the sampling frequency is 1 GS/s. The dead-time in these digitizers can be reduced to lower than 50 us for the measurement with the repetition rate of 20 kHz. The Te profiles at 64 positions are evaluated from the data of these digitizers.High Temperature Plasma Diagnostic Conferenc