Speaker
Description
When the density at the Ion Cyclotron Range of Frequencies (ICRF) antenna limiter’s edge falls below the lower hybrid (LH) resonance density (S = 0 in Stix dielectric tensor), the slow wave (SW) can propagate in front of the antenna. This wave carries large parallel electric fields that can strongly enhance the sheath potentials on the antenna limiters, thereby increasing the sputtering yield of ions striking the wall. Due to the presence of a cold plasma resonance and the SW’s short wavelength, this regime poses a major modeling challenge. Such conditions may be encountered in ITER [Colas et al., JNM (2025)] or with other foreseen ICRF systems such as the WEST travelling-wave array [Ragona et al., FED (2025)].
To document this low-density regime, WEST experiments were carried out in conditions where the SW was propagative in front of an active ICRF antenna. Using a reciprocating emissive probe connected magnetically to the antenna, we measured for the first time the DC plasma potential, VDC, over a radial scan of the LH resonance layer. VDC peaks when the density at the antenna limiter’s edge is near the LH resonance density, but never exceeds typical values of a few hundred volts. From the plasma-wall interaction standpoint, this regime can be highly favorable: as the particle fluxes are lower and the sheath potentials similar to standard operating conditions at the same antenna voltage, the local tungsten sources at the ICRF antenna and all other outer-wall objects become nearly undetectable. By contrast, sputtering in the divertor is primarily determined by the sputtering yield. Core impurity contamination is likewise significantly lower when the antennas are located far from the separatrix, both with and without ICRF power, and the radiated power fraction is reduced. Despite modest coupled powers at high clearance, good ICRF heating is maintained as the edge density drops below S = 0, and no deleterious operational consequences are observed across all relevant core plasma metrics when operating the antenna from this low-density region.
To help interpret these measurements, the 2D SW sheath interaction model by Myra and D’Ippolito [PRL 2008] was generalized to include arbitrary magnetic field incidences, additional dielectric tensor components, and both resistive and capacitive contributions to the sheath RF impedance. This simplified model is qualitatively consistent with the measured VDC trends if the incoming SW electric field is kept fixed while the density or wall inclination angles are varied.