Plasma light detection in the SPIDER beam source

The source prototype SPIDER in the Neutral Beam Test Facility at Consorzio RFX has been developed to de-monstrate the target performances of the RF plasma source of ITER Heating Neutral Beam injectors. SPIDERdiagnostics characterize the plasma in the source, optimize performances and provide signals for machineprotection. Photodiodes measure the time evolution of lines of sight integrated visible radiation emitted by theplasma in the source. Some are equipped with interferencefilters to select specific spectral lines, e.g. those ofcesium to control its evaporation, which assists the H ? production, and of metal impurities, which might indicatewall 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 formolecular Fulcher bands. The overall layout of the photodiode diagnostic and preliminary results during thefirstSPIDER operation are presented. The custom photodiode electronics is described in detail, including a low-noisevariable 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 de-pending on gain in range 100 k?to 5 G?. Measured light through the drivers has allowed to recognise plasmafeatures and dependencies with RF power andfilterfield current, as well as dominant frequencies in signalfluctuations.