Direct Measurement of ICRF-Enhanced Plasma Potentials Using Reciprocating Emissive Probes on the WEST tokamak

A new reciprocating probe head containing emissive and Langmuir probes was installed and commissioned on the WEST tokamak. The main scientific goals for the emissive probes are (i) to provide a first direct measurement on WEST of ICRF-induced plasma potentials responsible for enhanced sputtering and heat fluxes to ICRF antenna limiters, and (ii) to provide data to quantitatively benchmark RF simulation tools equipped with the sheath boundary condition and used to make predictions for future machines. An extensive experimental survey was conducted under various electrical settings of the 2x2-strap array (coupled power, toroidal phasing, left-right power balance) and plasma parameters (SOL density, edge safety factor). Plasma potentials exceeding 100 V were routinely measured when the probe was magnetically connected to the top of an active ICRF antenna. A correlation between the RF voltages at the antenna feeding ports and rectified potentials is typically observed, with the potentials decreasing as the coupling improves at prescribed RF power. This is especially visible in cases with intense MHD activity, where the antenna voltage and sheath potentials oscillate together over fast time scales. The complex structure of the measured plasma potential profiles will be discussed and compared with the Langmuir probes' floating potential measurements.