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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. [ABSTRACT FROM AUTHOR]

The dynamic Stark effect is a phenomenon in which photon(s) associated with an oscillating electric field are absorbed or emitted with the photon associated with an electronic transition. This multiphoton process leads to the formation of satellites in the spectrum at integer multiples of the frequency associated with the dynamic electric field. Utilizing the dynamic Stark effect the time varying electric field in a driven plasma sheath can be determined from the time-averaged and phase resolved emission spectra. Currently two methods are used to calculate the emission spectrum associated with an atomic system in the presence of a dynamic electric field: the quasi-static method and the Floquet method. The quasi-static method assumes that time varying field varies slowly enough that DC Stark effect theory can be used to generate integrated Stark profiles over the period of a time varying field. The Floquet method treats the dynamic multi-photon process and presents a more accurate treatment of the Stark profile dependence on transient field characteristics, particularly at higher frequencies. The methodology and applicability of the quasi-static and Floquet methods will be discussed with respect to sheaths driven in the RF range of frequencies. This work presents experimental measurements of line profiles in the RF sheath for both hydrogen and helium discharges in the presence of a DC magnetic field. The RF sheath electric field parameters are determined utilizing a generalized dynamic Stark effect model and a novel line shape analysis package. [ABSTRACT FROM PUBLISHER]