ICRF-specific W sources: Advances in minimization in ASDEX Upgrade and near-field based extrapolations to ITER with W-wall

Experimental data from the 2022 ASDEX Upgrade (AUG) helium (He) campaign reveals that a lower tungsten (W) content can be achieved during operation of the 3-strap antennas with the W-coated limiters. By fine-tuning of electrical feeding of the 3-strap antennas, the core W content was reduced by about a factor of 2 compared to that during operation of the 2-strap antennas with boron coated limiters at the same power and target plasma. This is in contrast to what is observed in deuterium (D) plasmas, where both types of antennas perform similarly in terms of the W release. A higher significance of remote W sources in He than in D is one plausible explanation.

Special AUG experiments in D plasmas, with a density in the SOL reduced to the level relevant for some of the predicted profiles for ITER, show similar characteristics of the near-fields and the sputtering even when the slow wave is propagative. Nevertheless, it is advised for ITER to tailor the SOL profiles by the foreseen local gas injection close to the antenna, to avoid the conditions when the lower hybrid resonance position is approximately aligned with the position the leading edges of the plasma facing components.

Based on near-field calculations for the AUG 3-strap and the ITER ICRF antennas using the HFSS code under the same conditions, the experimentally validated RF-sheath rectified voltages of the AUG 3-strap antenna were scaled to the ITER ICRF antenna. Using conservative assumptions, the estimations of the ICRF-specific W sources for ITER were then made for the D-T case with neon seeding. The extrapolation shows that by carefully choosing the electrical feeding, the increment of the W sputtering rate during ITER ICRF operation at up to 20 MW, can be kept (in the worst case) below 10 % of the total W wall rate without ICRF, as well as below 25 % of the W rate at the outboard wall without ICRF.