Mitigation of RF sheaths with insulating side walls

A single strap, high-power ($\sim $150kW), RF (2.4MHz) antenna was used to study RF sheaths in a magnetized helium plasma with plasma parameters n$_{\mathrm{e}}$ \textasciitilde 10$^{\mathrm{18}}$ -- 10$^{\mathrm{19}}$ m$^{\mathrm{-3}}$, T$_{\mathrm{e}}$ \textasciitilde 1 -- 10 eV and B$_{\mathrm{0}}$ \textasciitilde 0.1 T. The experiment was conducted on the Large Plasma Device (LAPD). This presentation will draw a comparison between two experiments carried out at the LAPD with different antenna strap enclosures. The two different enclosure consisted of different materials for enclosure side walls- copper and electrically insulating, macor. Both experiments had similar plasma density, temperature, magnetic field and fast wave amplitude. In the case of the copper enclosure, formation of convective cells as a result of plasma potential rectification was observed and reported$^{\mathrm{1}}$. In the experiments with the macor enclosure we observe a lack of plasma potential rectification as well as no evidence of convective cells. The results are reminiscent to the results obtained in ASDEX-U with the 3-strap antenna optimized to reduce image currents on the antenna limiters$^{\mathrm{2}}$. $^{\mathrm{1}}$ M. Martin \textit{et al}, Phys. Rev. Lett. \textbf{119}, 205002 (2017) $^{\mathrm{2}}$ V. Bobkov \textit{et al}, Nucl. Fusion \textbf{56}, 084001 (2016)