Thermo-mechanical Assessment of the JT-60SA Fast Ion Loss Detector

Scintillator based fast-ion loss detectors (FILD) are used in virtually all major tokamaks and stellarators to study the fast-ion losses induced by magnetohydrodynamic (MHD) fluctuations. FILDs are charged particle collectors that work as magnetic spectrometers, making use of the magnetic field within the machine to collimate and disperse the escaping ions onto a scintillator plate, providing velocity-space measurements of the losses with alfvenic temporal resolution [1]. In particular, JT-60SA will operate in scenarios with a large fraction of fast-ion pressure and a FILD detector will be used to assess both the direct wave-particle interaction of fast-ions with different MHD instabilities, and the particle resonances with externally applied 3D fields. The JT-60SA FILD will be mounted on a reciprocating arm to approach the plasma for measurements, up to the radial position of the tokamak stabilizing plate. Under this condition, the probe head will be subject to relatively high heat fluxes. Assessing its thermal behaviour is crucial to define suitable operational cycles, consisting of several insertions (for measuring) and retractions (for cooling down) during long plasma pulses. As the scintillator temperature must be kept below 200 °C for proper performance [2], this assessment allows to maximize the signal obtained from FILD without jeopardizing its structural integrity. In this contribution, the preliminary design of the JT-60SA FILD is presented and its thermal behaviour is characterized by means of extensive finite element simulations. This is essential for determining the optimal parking position, which is required to properly cool down the system. Also, this study allows to optimize the operational cycles (number of measurement deployments and their duration within a complete plasma pulse) for different plasma scenarios.