Development of a Fast Visible Light Measurement System for the Study of Ion Cyclotron Range of Frequency Waves in GAMMA 10/PDX Plasmas

A fast visible light measurement system was installed on GAMMA 10/PDX to measure plasma light fluctuations induced by ion cyclotron range of frequency (ICRF) (6360 kHz) waves. Time evolution and power spectrum were obtained and the relative amplitude of the fluctuations was the order of 0.1%. The difference of measured wavelength region was investigated by using three interference filters, but obtained time evolutions and the relative amplitudes were similar when we neglect a factor of difference. The RF pick-up noise is about two orders of magnitude smaller than the ICRF wave induced plasma signal, and the broad frequency band component is about an order of magnitude smaller than the signal. The latter arises from the shot-noise of the photon detection Poisson process, and an expression of the shot-noise, in which the effects of the frequency response of the system and the detector property are included, was obtained. The expression is confirmed experimentally using a stable light source. (C) 2021 The Japan Society of Plasma Science and Nuclear Fusion Research