Overview of recent ICRF studies and RF-related wave-field measurements on ASDEX upgrade

This manuscript overviews the latest developments and results from the ASDEX Upgrade ICRF team. The ICRF control system has been upgraded to allow the operation of active antennas with a controlled arbitrary phase difference between them. This upgrade, in combination with an extensive RF probe coverage inside the ASDEX Upgrade torus, makes it possible to study a global wave phenomenon that may result from the two antenna pairs' fields. The first results show that multiple active antennas form a superposition of the launched waves, with limited plasma shielding. A combination of high- and low-field side RF probes makes it possible to study the efficiency of various heating schemes such as the hydrogen minority or the 3-ion species scheme with a He-3 minority. The effect of the hydrogen minority concentration on the ICRF wave absorption and transmission through the core is best shown during conditioning discharges following a torus opening: as the hydrogen fraction is reduced from 80% to below 20%, the amplitude of the RF waves that reaches the high-field side probes is reduced by nearly two orders of magnitude indicating a strong absorption increase. The experimental results are supported by full wave FELICE code showing a similar absorption improvement as a function of the hydrogen fraction. The low-field side probes, on the other hand, do not show a sensitivity to the hydrogen fraction, which is expected as the low-field side wave amplitude is dependent on antenna coupling, not wave absorption. For the case of the 3-ion heating scheme, the high-field side probes detect a minimum in the wave amplitude as the wave absorption layer is radially scanned past the optimal position in the core. The minimum is interpreted as the optimal core absorption condition and matches the observed maximum in the core soft x-ray data. The optimal radial position of the wave absorption layer is also indirectly indicated via the appearance of strong ion cyclotron emission (ICE). The frequency of ICE is matched with the cyclotron frequency of RF-accelerated He-3 ions and the emission origin is placed similar to midway between the magnetic axis and the separatrix. In order to self-consistently assess the global ICRF wave behavior, it is necessary to account for the three dimensional (3D) nature of the toroidal tokamak plasma and the launching antenna structures. The ASDEX Upgrade ICRF team studies multiple aspects of how 3D effects influence launched ICRF wave fields and antenna performance. These include 3D plasma density perturbations in the plasma edge and the scrape off layer, either intrinsically generated by turbulence or externally imposed by resonant magnetic perturbation coils. The experimental study is supported with RAPLICASOL: a computational tool that solves for launched ICRF wave fields in the presence of realistic 3D plasma density profiles and antenna geometry. Additionally, the work of the ASDEX Upgrade ICRF team includes fundamental plasma physics and RF wave studies on the linear test device IShTAR: the areas of interest are RF sheath rectification and arc formation