A rotary and reciprocating scintillator based fast-ion loss detector for the MAST-U tokamak

The design and unique feature of the first fast-ion loss detector (FILD) for the Mega Amp Spherical Tokamak - Upgrade (MAST-U) is presented here. The MAST-U FILD head is mounted on an axially and angularly actuated mechanism that makes it possible to independently adapt the orientation [0°, 90°] and radial position [1.40 m, 1.60 m] of the FILD head, i.e., its collimator, thus maximizing the detector velocity-space coverage in a broad range of plasma scenarios with different q₉₅. The 3D geometry of the detector has been optimized to detect fast-ion losses from the neutral beam injectors. Orbit simulations are used to calculate the strike map and predict the expected signals. The results show a velocity-space range of [4 cm, 13 cm] in gyroradius and [30°, 85°] in pitch angle, covering the entire neutral beam ion energy range. The optical system will provide direct sight of the scintillator and simultaneous detection with two cameras, giving high spatial and temporal resolution. The MAST-U FILD will shed light on the dominant fast-ion transport mechanisms in one of the world's two largest spherical tokamaks through absolute measurements of fast-ion losses.