• Energy Research

Aiming at high-power and long-pulse operation up to 1000 s, some improvements have been made for both 2.45 GHz and 4.6 GHz lower hybrid (LH) systems during the recent 5 years. At first, the guard limiters of the LH antennas with graphite tiles were upgraded to tungsten, the most promising material for plasma facing components in nuclear fusion devices. These new guard limiters can operate at a peak power density of 12.9 MW/m2. Strong hot spots were usually observed on the old graphite limiters when 4.6 GHz system operated with power >2.0 MW [B. N. Wan et al., Nucl. Fusion 57 (2017) 102019], leading to a reduction of the maximum power capability. With the new limiters, 4.6 GHz LH system, the main current drive (CD) and electron heating tool for EAST, can be operated with power >2.5 MW routinely. Long-pulse operation up to 100 s with 4.6 GHz LH power of 2.4 MW was achieved in 2021 and the maximal temperature on the guard limiters measured by an infrared (IR) camera was about 540 °C, much below the permissible value of tungsten material (∼1200 °C). A discharge with a duration of 1056 s was achieved and the 4.6 GHz LH energy injected into the plasma was up to 1.05 GJ. Secondly, the fully-active-multijunction (FAM) launcher of 2.45 GHz system was upgraded to a passive-active-multijunction (PAM), for which the density of optimum coupling was relatively low (below the cut-off value). Good coupling with reflection coefficient ∼3% has been achieved with plasma-antenna distance up to 11 cm for the new PAM. Finally, in order to eliminate the effect of ion cyclotron range of frequencies (ICRF) wave on 4.6 GHz LH wave coupling, the location of the ICRF launcher was changed to a port that is located 157.5° toroidally from the 4.6 GHz LH system and is not magnetically connected.

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.