RF Sheath Mitigation with Insulating Antenna Enclosures on the LAPD

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. Three experiments were carried out on the Large Plasma Device (LAPD) using different plasma-facing materials for ICRF antenna enclosure. These experiments demonstrated that electrically isolating the antenna walls from the bulk plasma can significantly reduce near-field rectification. The three different enclosure materials included copper, MACOR (electrically isolating), and MACOR over copper (MACOR-copper). In the case of the MACOR-copper side walls, the non-conductive MACOR material was exposed to the bulk plasma but a layer of copper was added below to allow for image currents. In the case of the copper enclosure, RF rectified potentials and associated convective cells were observed and reported [1]. In the experiments with MACOR and MACOR-copper enclosures, RF rectification was significantly reduced. Additionally, these experiments showed no evidence of convective cell formation.

Sheath control by insulating limiters has been explored by Majeski [2] et al and Myra [3] et al. The model presented by Myra et al treats the insulating wall and the plasma sheath as two electrical components in series with each other. Depending on the RF voltage, plasma parameters, and insulator parameters one can determine the potential drop across the plasma sheath vs. the insulator. In experiments done on the LAPD, as predicted by the model, the MACOR insulating layers are effective at mitigating large RF sheath potentials at higher plasma density while they are not as effective at lower plasma density.

[1]M. J. Martin et al., Physical Review Letters 119, (2017).

[2]R. Majeski et al., Fusion Engineering and Design 24, 159 (1994).

[3]J. R. Myra et al., Journal of Nuclear Materials 249, 190 (1997).