Mitigation of RF sheaths via an ICRF antenna with electrically insulating sidewalls *

A single strap, high-power (~150kW), RF (2.4MHz) antenna was used to study RF sheaths in a magnetized helium plasma with plasma parameters n_e ~ 10¹⁸ – 10¹⁹ m^-3, T_e ~ 1 – 10 eV, and B₀ ~ 0.1 T. The experiments were conducted on the Large Plasma Device (LAPD). Three experiments were carried out on the Large Plasma Device (LAPD) using different plasma-facing materials on a single-strap ICRF antenna.  These experiments demonstrated that electrically isolating the antenna walls from the bulk plasma can significantly reduce near-field rectification. The three different enclosure sidewall materials included copper, MACOR (electrically isolating), and MACOR over copper (MACOR-copper). In the case of the MACOR-copper sidewalls, the non-conductive MACOR material was exposed to the bulk plasma but a layer of copper was added below to allow for image currents to flow. All three of the experiments had similar plasma density, temperature, and background magnetic field. In the case of the copper enclosure, RF rectified potentials and associated formation of convective cells were observed and reported¹.  In the experiments with MACOR and MACOR-copper enclosures, RF rectification was significantly reduced. Additionally, these experiments showed no evidence of convective cell formation. Although the results from the MACOR experiment are reminiscent of the results obtained in ASDEX-U with the 3-strap antenna optimized to reduce image currents on the antenna limiters², the MACOR-copper experiment seems to suggest that insulating plasma-facing materials have a stronger impact on potential rectification than a lack of image currents at the antenna boundary.