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An important step in understanding the Radio Frequency sheaths in the vicinity of Ion Cyclotron antennas in magnetically confined fusion plasmas is to benchmark the existing theoretical models against real antenna operation. The data needed is the local DC electric field that develops in the sheaths surrounding the plasma-facing structures of the antenna. In this contribution we present our most recent progress on the development of a spectroscopic diagnostic, which delivers direct and local measurements of the electric field across the sheath. The method uses polarization spectroscopy where the electric fields are optically determined from the Stark shift of the emission spectra of excited helium atoms. The sheath electric fields obtained with this diagnostic scheme are measured in the vicinity of the ICRF antenna in IShTAR, a test facility designed to mimic the tokamak edge plasma parameters and focused on studying the ICRF antenna-plasma interaction. [ABSTRACT FROM AUTHOR]

IShTAR is a linear magnetized plasma test facility dedicated to the investigation of RF wave/plasma interaction. The IShTAR ICRF system consists of a single strap RF antenna. When using the antenna for plasma production without an external plasma source, it is shown that the plasma is either produced in front of the antenna strap or inside the antenna box depending on the antenna parameters. Here, we present experimental and numerical investigation of the plasma initiation parametric dependencies. Detailed pressure and RF power scans were performed in helium at f = 5.22MHz and f = 42.06 MHz. The experiment shows the parameter ranges for which the plasma is produced in front of the strap, or inside the antenna box. These ranges are validated by simulations with the RF dinity model, and by theoretical predictions. [ABSTRACT FROM AUTHOR]