Initial operation of the scintillator-based Fast-Ion Loss Detector rotary and reciprocating system in MAST-U
The first scintillator-based Fast-Ion Loss Detector (FILD) was installed in MAST-U as part of its main upgrade and it was commissioned during the first experimental campaign. FILD works as a magnetic spectrometer, directly measuring neutral beam injection (NBI) fast-ion losses using a scintillator plate that emits light when fast ions impinge on it. The scintillator material used in MAST-U has a decay time in the range of the nanoseconds. This, in combination with an avalanche photo-diode camera at sampling rates up to 4 MHz, has made it possible to detect fast-ion loss fluctuations up to 2 MHz, an unprecedented measurement for any scintillator-based FILD. The probe is protected from the plasma loads with a cap made of the same fine-grain graphite as the divertor tiles to avoid distinct impurities in the plasma. The probe is installed on an in-vessel rotary and reciprocating system actuated with bellow-based feedthroughs. The rotary actuator makes it possible to adapt the probe orientation to the magnetic field pitch and the reciprocating actuator adapts the probe radial distance to the separatrix. A pair of stepper motors makes it possible to operate each actuator remotely in a shot-to-shot basis. A radial scan of the probe has made it possible to characterise the main plasma parameters that affect the FILD measurements. Besides, an infrared camera monitors the temperature on the graphite cap to assess its integrity while bolometry measurements monitor the plasma radiated power to infer the impurities added by moving FILD closer to the plasma. The operation of the MAST-U FILD provides a source of lessons learnt for the design of future reciprocating diagnostics like the ITER FILD.