Speaker
Description
This study is motivated by the observation of a macroscopic (~10 cm in scale) carbon tile failure in the ion cyclotron range of frequencies (ICRF) limiter structure during a hydrogen minority ion cyclotron heating scenario in D plasma on ASDEX Upgrade (AUG). In this discharge (#42762): the on-axis magnetic field is 2.65 T, the plasma current is 0.36 MA, the core plasma electron density is 5-6x10^19 m^-3, the hydrogen minority fraction is ~5%, and the ICRF operational frequency is 36.5 MHz. As the ICRF power increased from 0.15 MW to 3.3 MW between 1.30 s and 7.95 s, a part of the ICRF antenna protection limiter tile in Sector 12 (near the midplane) was seen to separate just before 8 s. The tile failure necessitated an unscheduled torus opening to replace the broken tile.
In order to estimate heat loads induced by lost energetic ions on the ICRF limiter structure, we employ ASCOT-RFOF. This code has earlier demonstrated its applicability in simulating ICRF heating and fast ion loss detector signals on AUG [1]. The initial ASCOT-RFOF simulation runs revealed a spatially concentrated heat load pattern, where the values range from several MW/m^2 to nearly zero across a distance of just a few cm. To resolve such narrow heat loads, the entire AUG outer wall surface mesh required an upgrade from the previous version with mesh triangles measuring ~10 cm across to a more refined mesh with triangles ~1 cm across. CAD data was used to generate a new ASCOT 3D wall mesh for all plasma-facing components of interest, in particular the four ICRF antennas with their limiters and Faraday screens. Applying the new high-resolution mesh and a high fast ion marker count (~10 million) to the studied plasma scenario reveals heat load values that reach 10 MW/m^2 (at highest ICRF power) in a highly concentrated tile surface region, just a few cm across. The tile regions experiencing these high heat loads are on the ‘right hand’ side of the antenna limiter (when viewed from the plasma), in the vicinity of the Faraday screen rods. The location is well matched to the sheared-off section of the midplane tile #6. The ASCOT output reveals additional hot spots located on the Faraday screen, near the tile surfaces, and on the ‘left hand’ side of the limiter. The simulated heat loads are currently being coupled to ANSYS, a thermo-mechanical analysis software, to estimate material stresses on the antenna limiter tiles. The overall computational loop that links plasma-generated fast ions with thermo-mechanical stresses experienced by the outer wall structures will be presented, in view of applying such a computational workflow to future fusion devices such as ITER.
For details of the ASCOT wall heat load simulation model, see the contribution of S. Sipilä et al. in this Technical Meeting.
[1] S. Sipilä et al., 2021 Nucl. Fusion 61 086026.