We aim to provide a first direct measurement on WEST of Ion Cyclotron Radio Frequency (ICRF)-enhanced plasma potentials responsible for increased sputtering and enhanced heat fluxes to the ICRF antenna limiters. A dedicated probe head equipped with three emissive probes and two Langmuir probes was built at MIT and installed on WEST in September of 2023. We will use these probes to investigate the dependence of enhanced plasma potentials on ICRF antenna electrical settings and edge plasma parameters. The measurements will provide validation data to quantitatively benchmark RF simulation tools used to make predictions for future machines.
ICRF heating is one of the auxiliary heating methods capable of achieving thermonuclear fusion-relevant temperatures in magnetically confined plasmas and is envisioned as an important component of next-generation fusion devices like SPARC and ITER. However, since the first ICRF experiments, strong parasitic interaction between the waves and the boundary plasma was found leading to edge power losses, impurity sputtering, and enhanced heat fluxes to the antenna. This makes it challenging to operate ICRF systems in a machine with full-metal walls. One of the dominant mechanisms at play is the enhancement of DC plasma potentials on field lines connecting to ICRF antenna limiters due to sheath rectification.
Using our new reciprocating emissive probes, we aim to document these enhanced plasma potential structures in the WEST tokamak (France) and study their dependence on edge plasma parameters. Nowadays, a few modeling tools in the world can address RF sheath rectification with realistic ICRF antenna geometry via appropriate sheath boundary conditions. One main objective for our work is to provide data to quantitatively benchmark these tools against plasma potential measurements. This would eventually allow us to confidently run these codes to design ICRF systems and predict their performance in future machines.
This work is supported by US DoE award number DE-SC0014264.
