Interaction of high-power waves with the plasma periphery of WEST and EAST tokamaks

Ion Cyclotron Resonance Heating (ICRH) in the H minority scheme can allow thermal ions to absorb up to half of the injected power. Meanwhile, ICRH heating can prevent heavy impurity accumulation similarly to broad ECRH [Sertoli 2017]. In order to benefit from ICRH in the core, waves must be coupled to the plasma from antennas whose location at the edge is subject to a trade-off between (1) efficient coupling and (2) impurity generation. This thesis presents mostly experimental observations made on these two aspects in EAST and WEST superconducting medium size tokamaks, on L-mode scenarios combining Lower Hybrid Current Drive from two launchers and D[H] minority heating by ICRH antennas. All ICRH antennas have been able to couple more than 1 MW per antenna, with indications of efficient wave absorption. The heating efficiency changes consistently with the total power injected. However in some cases without low-Z materials coating (Li, B), heating effect may only last several hundred milliseconds (time-scale for impurity transport up to core plasma) after powering ICRH. Lower Hybrid (LH) power helps improving ICRH coupling. Experiments reveal that fueling from the mid-plane not only helps to couple waves from nearby antennas like in other devices, but also has an impact on the scrape-off layer (SOL) density in regions that are not magnetically connected to the valves. Localized mid-plane nozzle valves allow in EAST better coupling compared to poloidally distributed valves. Core density control requirements for long-pulse operation, in particular in L-mode regime, however limit the amount of gas that can be injected. If injected from radially retracted points, gas can spread and help reducing sputtering yield on active antenna limiters by a combination of four effects: (a) reduce thermal effects by cooling down the SOL (b) improve coupling and therefore reduce the overall near-field amplitudes for a given RF power, (c) increase the electron mobility in the SOL and thus mitigate the excitation of parallel components of the electric field and (d) dilute impurities which are the main contributors to sputtering. During ICRH, impurities can contaminate the plasma to a level detrimental for the operation, e.g. 100% of ICRH power radiated in many L-mode discharges in WEST and H-mode discharges in EAST. Tungsten (W) production measured by extreme ultraviolet spectroscopy increases significantly when an ICRH antenna magnetically connected to W surfaces is powered, compared to a reference phase without ICRH. On some components such as antenna side limiters, the rise is larger than with a similar LH, NBI or ECRH power. The relative contribution of each object and physical process (RF-sheaths, fast ion ripple losses) to core contamination yet remains poorly known. Comparing antenna limiters with W-coating vs low-Z materials helped quantifying the role of these components. The core W content, in presence of divertor sources only, is correlated with the total injected power, either from ICRH or LH. Since 2018 the LH guard limiter tiles were W-coated. Their contribution to the core W content appears more important than divertor sources, particularly when the magnetically connected ICRH antenna is powered. Two-strap ICRH antennas magnetically connected to W components at the mid-plane already compromise high performance operations. In addition to low-Z materials in regions magnetically connected to antennas, using arrays of more than two straps toroidally would allow near-field's cancellation and RF-sheaths reduction. Other concepts (active limiters, TWAs) have been proposed but remain to be tested. (author)