Feasibility study of Passive Optical Emission Spectroscopy for the electric field measurements in IShTAR

Direct, non-intrusive, measurements of electric fields are essential for understanding the RF-sheath physics. This is especially true in the case of the ICRF antenna-plasma edge interaction in fusion devices. The rectification of the RF-fields near the plasma-facing components of an antenna leads to the development of DC electric fields that accelerate the ions from the plasma towards the antennas' plasma-facing components, enhancing physical sputtering and release of impurities. IShTAR is a device dedicated to the investigation of the plasma-antenna interactions in tokamak edge-like conditions. It has a simplified geometry and enables an easy access and fast modifications, which makes it a suitable environment to develop diagnostics for electric field measurements. This paper presents the observed Stark effect on He I spectral line profile, with passive optical emission spectroscopy. To be able to fully control the operating parameters, at this initial stage, the measurements are conducted on a simple DC-biased electrode rather than the ICRF antenna. Measured line profiles are compared with the analytical models of the Stark effect in magnetised helium plasma that, as a result of the good fit, provide the electric field strength.