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 injector (NBI) fast-ion losses using a scintillator plate that emits light when fast ions impinge on it. Yttrium aluminum garnet activated by cerium (YAG:Ce) is used as the scintillator material in MAST-U, which has a decay time in the range of 100 ns. This, in combination with an avalanche photo-diode (APD) camera at sampling rates up to 4 MHz, has made it possible to detect fast-ion loss fluctuations at frequencies 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 15- $\mu $ m grain graphite, similar to the divertor tiles. The probe is installed on an in-vessel rotary and reciprocating system actuated with bellow-based feed-throughs and a pair of stepper motors that make it possible to operate remotely on a shot-to-shot basis. The rotary actuator adapts the probe orientation to the magnetic field pitch and the reciprocating actuator adapts the probe radial distance to the separatrix. An infrared camera monitoring the temperature on the graphite cap showed that the temperature increase during a shot is lower than 20 °C. The impurities added to the plasma by the FILD probe have been monitored using spectroscopy measurements. The operation of the MAST-U FILD provides a source of lessons learned for the design of future reciprocating diagnostics such as the ITER FILD.