A filter bank system for scattered spectrum analysis in collective Thomson scattering diagnostic on JT-60

In JT-60, a collective Thomson scattering (CTS) diagnostic system based on a pulsed CO2 laser (wavelength 10.6 μm, beam energy 15 J, pulse width 1 μs) has been developed to measure ion temperature and velocity distribution of fast ions to demonstrate the feasibility of measurements of confined alpha particles. A high power pulsed CO2 laser and heterodyne receiver system using a quantum-well infrared photodetector (QWIP) has been developed and installed in the diagnostic room. In order to resolve frequency distribution of pulsed scattered light (1 μs), we developed a wideband filter bank system (0.4–4.5 GHz) for scattered spectrum analysis in JT-60 CTS diagnostic. The detector bandwidth and frequency resolution are both determined by the calculation results of expected scattered spectrum. In order to realize wideband multichannel detection, cascade connected seventh order simultaneous-Chebyshev (elliptic) bandpass filters were adopted. Signal to noise ratio based on the noise equivalent power of the QWIP receiver and laser pulse length are also evaluated.