• Energy Research

For the first time, material analysis techniques have been applied to study the effect of ion cyclotron wall conditioning (ICWC) on probe surfaces in a metal-wall machine. ICWC is a technique envisaged to contribute to the removal of fuel and impurities from the first wall of ITER. The objective of this work was to assess impurity migration under ICWC operation. Tungsten probes were exposed in ASDEX Upgrade to discharges in helium. After wall conditioning, the probes were covered with a co-deposited layer containing D, B, C, N, O and relatively high amount of He. The concentration ratio He/C+B was 0.7. The formation of the co-deposited layer indicates that a fraction of the impurities desorbed from the wall under ICWC operation are transported by plasma and deposited away from their original location.

For simulation of plasma-facing component erosion in fusion experiments, an analytical expression for the ion velocity just before the surface impact including the local electric field and an optional surface biasing effect is suggested. Energy and angular impact distributions and the resulting effective sputtering yields were produced for several experimental scenarios at JET ILW mostly involving PFCs exposed to an oblique magnetic field. The analytic solution has been applied as an improvement to earlier ERO mod- elling of localized, Be outer limiter, RF-enhanced erosion, modulated by toggling of a remote, however magnetically connected ICRH antenna. The effective W sputtering yields due to D and Be ion impact in Type-I and Type-III ELMs and inter-ELM conditions were also estimated using the analytical approach and benchmarked by spectroscopy. The intra-ELM W sputtering flux increases almost 10 times in comparison to the inter-ELM flux. EURATOM 633053

In 2023, switching the material on the first wall of ITER to tungsten (W) was recommended. In magnetic Fusion devices, waves in the Ion Cyclotron Range of Frequencies (ICRF) interact with the Scrape-Off Layer (SOL) via RF-sheath rectification. This contribution re-assesses this phenomenon close to the ITER ICRF antenna, focusing on the ICRF-specific gross erosion of W from the antenna port sides. Our quantitative estimates rely on predictive multi-2D numerical simulations of the ICRF antenna environment using the SSWICH-SW code. They combine Slow Wave propagation from the antenna mouth to the SOL, the excitation of RF oscillations in the sheath voltages at the antenna port sides and a subsequent DC biasing of the SOL. Maps of the parallel RF electric field at the antenna mouth, from the antenna code TOPICA, excite the system. Our simulations cover more than four decades in the local densities near the antenna. Since both the sputtering and the local heat loads are proportional to the local particle fluxes, the most intense Plasma-Wall Interaction is found for high local density, with or without ICRF waves. In these conditions, larger margins also exist for coupling the ICRF power. We tested several operational trade-offs between these two constraints. The simulated target plasma contains 2% of neon ions. These are efficient at sputtering W, already at low accelerating voltages. Consequently, although the RF-sheath rectification sufficiently amplifies the local sputtering at the antenna port for a detection using visible spectroscopy, the ICRF-induced increment of the gross W production represents at worse 22% of the W source expected from thermal sheaths over the eighteen out-board mid-plane ports. An upper bound, independent of our main assumptions, is proposed for this enhancement factor. This moderate expected global increase questions the ability to detect ICRF-specific W contamination of the plasma core, even at the planned maximal ICRF power.

Use of new 3-strap ICRF antennas with all-tungsten (W) limiters in ASDEX Upgrade results in a reduction of the W sources at the antenna limiters and of the W content in the confined plasma by at least a factor of 2 compared to the W-limiter 2-strap antennas used in the past. The reduction is observed with a broad range of plasma shapes. In multiple locations of antenna frame, the limiter W source has a minimum when RF image currents are decreased by cancellation of the RF current contributions of the central and the outer straps. In JET with ITER-like wall, ITER-like antenna produces about 20% less of main chamber radiation and of W content compared to the old A2 antennas. However the effect of the A2 antennas on W content is scattered depending on which antennas are powered. Experiments in JET with trace nitrogen (N 2 ) injection show that a presence of active ICRF antenna close to the midplane injection valve has little effect on the core N content, both in dipole and in -90 °phasing. This indicates that the effect of ICRF on impurity transport across the scape-off-layer is small in JET compared to the dominant effect on impurity sources leading to increased impurity levels during ICRF operation. EURATOM 633053 US Department of Energy DE-AC05-00OR22725