Simulation of radio-frequency heating and fast-ion generation in Wendelstein 7-X

The next scientific operation phase of Wendelstein 7-X (W7-X) is scheduled to begin in late autumn of 2022 and will, for the first time, include experiments in which the ICRH (ion-cyclotron-resonance heating) antenna will be used. In addition to heating the plasma, this system will generate fast ions and thus offers a new way to assess fast-ion confinement in a stellarator such as W7-X. The first plasmas that will be used for the upcoming ICRH operation will be Helium-4 plasmas with a small Hydrogen minority on the order of about 10%. In tokamaks such plasmas typically offer good power absorption and are thus considered a safe way for gaining first experiences with the new antenna in W7-X. This assessment is confirmed by the SCENIC simulations carried out in this contribution that use profiles foreseen for the upcoming campaign as input. The simulations are carried out in the standard configuration of W7-X in low-beta (0.3% ≲ 〈β〉 ≲ 1%) plasmas. However, also scans over minority concentration and background-plasma density are performed. We find that the power absorbed by the Hydrogen minority directly from the radio-frequency wave is typically (provided that the minority concentration is not too high) on the order of about 90% with the rest going to the electrons. Very little power goes to the Helium-4 ions. Under the present simulation conditions only fast-ion energies up to about E ≈ 50 keV can be reached. Combining SCENIC and ASCOT simulations enables us to track lost particles through the scrape-off-layer to the 3D wall of W7-X and to compute wall loads caused by ICRH. The results show that the wall loads that can be expected from ICRH under the first operating conditions are benign.