Thermo-mechanical limits of a magnetically driven fast-ion loss detector in the ASDEX Upgrade tokamak

Abstract

A real-time control system is being developed for a magnetically driven Fast-Ion Loss Detector (FILD) at the ASDEX Upgrade tokamak. The insertion of the diagnostic head will be adjusted in real-time to react to changes in the graphite head temperature, plasma position and appearance of MHD instabilities. The graphite probe head of the detector is exposed to an intense heat flux (located ∼3–5 cm from separatrix). The control algorithm performance is constrained by: the graphite head sublimation temperature, the ultimate stress limit, the reaction time of the controller and the retraction time. In this work, the temperature and thermal induced stress distribution on the probe head are assessed to determine what temperature-related magnitude is the limiting factor. The heat flux at the probe head has been estimated using the time-averaged parallel heat flux measured at the divertor target via infrared thermography. A field line tracing algorithm determines which regions of the probe head receives a weighted heat flux due to shadowing (self-induced or from other structures) and the incidence angle of the field lines. A finite element model is used to simulate the temporal evolution of the graphite head temperature and to obtain the induced thermal stress. The temperature spatial distribution at the probe head is validated against measurements of the probe head for different FILD systems showing a good agreement. These measurements have been obtained from visible cameras with an infrared filter. The model concludes that the maximum stress (∼100 MPa) does not overcome the graphite mechanical limit (170 MPa) and that the probe head is not affected by fatigue. Therefore, the graphite sublimation temperature (2000 °C) is set as the limiting factor of the new control system.