Plasma light detection in the SPIDER beam source

摘要

The source prototype SPIDER in the Neutral Beam Test Facility at Consorzio RFX has been developed to demonstrate the target performances of the RF plasma source of ITER Heating Neutral Beam injectors. SPIDER diagnostics characterize the plasma in the source, optimize performances and provide signals for machine protection. Photodiodes measure the time evolution of lines of sight integrated visible radiation emitted by the plasma in the source. Some are equipped with interference filters to select specific spectral lines, e.g. those of cesium to control its evaporation, which assists the H- production, and of metal impurities, which might indicate wall damages. Other lines of sight are fed to grating spectrometers to measure the spectrum at a slower rate, either at low resolution for spectral survey of hydrogen Balmer lines and impurities or at high resolution for molecular Fulcher bands. The overall layout of the photodiode diagnostic and preliminary results during the first SPIDER operation are presented. The custom photodiode electronics is described in detail, including a low-noise variable gain amplifier, FPGA-based remote control and suitably shaped digital output for feedback control. Main characteristics are: linear response up to 10 V, 14 bit dynamic range and 1.2-135 kHz bandwidth depending on gain in range 100 k Omega to 5 G Omega. Measured light through the drivers has allowed to recognise plasma features and dependencies with RF power and filter field current, as well as dominant frequencies in signal fluctuations.