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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.

Reduced particle losses have been observed during the operation of an m/n = 6/2 resonant magnetic perturbation in the TEXTOR tokamak. The influence of the perturbation field on multi-scale turbulence and turbulent transport has been surveyed in detail across the perturbation-induced reduction of edge transport. The results indicate that with magnetic perturbation (MP), both the large-scale zonal flows and small-scale drift-wave turbulence are significantly reduced. At high MP currents, a reduction of edge transport can be realized due to primarily the decline of small-scale ambient turbulence and turbulent transport in the ergodic zone, where the turbulence eddy size is largely decreased. Investigation on the dynamic process of fluctuation quantities during the perturbation current ramp-up phase further shows that geodesic acoustic mode zonal flows and their nonlinear interaction with background turbulence decrease incessantly with increasing perturbation current. The transport reduction takes place only after small-scale turbulence starts to be strongly reduced by the MP when the MP strength reaches a certain threshold value.

Existence of high electric fields near an RF antenna launcher causes a number of parasitic phenomena, such as arcing and impurity release, which seriously deteriorate the performance of an Ion Cyclotron Range of Frequencies (ICRF) heating scheme in fusion devices. Limited accessibility of the near antenna region in large-scale fusion experiments significantly complicates the associated experimental studies. The IShTAR test facility has been developed with the requirement to provide a better accessibility and diagnosability of plasmas in the direct vicinity of an ICRF antenna. The purpose of this work is to give a detailed description on the experimental setup and the available diagnostics. Furthermore, the paper will demonstrate the capability of the experiment to study phenomena near an ICRF antenna launcher which are relevant for large-scale fusion ion cyclotron resonance heating systems.

Biasing multiple Langmuir probes arranged in an array through one voltage source presents a more compact, reliable and convenient way to bias than using an individual source for each probe. This approach however is the subject of ground loops and several methods have been discussed to avoid them. The high frequency isolation transformer is considered to be a best way to break ground loops since it does not require an additional power supplies and, in general, any other electric circuit. The presented results demonstrate a considerable suppression of the ground loop noise using this technique. The following limitations should be considered in this approach: only an AC voltage bias should be applied, the relatively high internal resistance of the transformer should be taken into account, the frequency of the AC bias signal should correspond to the transforner bandwidth.

A multichannel spectroscopic diagnostic based on the Stark effect on helium lines was developed and implemented in IShTAR (Ion Cyclotron Sheath Test ARrangement) to measure the spatial distribution of electric fields across the radio frequency sheaths of the ion cyclotron antenna. Direct measurements of the DC electric fields in the antenna sheaths are an important missing component in understanding the antenna-plasma edge interactions in magnetically confined fusion plasmas since they will be used to benchmark theoretical models against real antenna operation. Along with the high-resolution Czerny-Turner monochromator and a detector with an intensifier, the hardware relies on the 2 chained set of linear-to-linear fiber bundles that provide seven optical channels capable of resolving an 8.4 mm region in the vicinity of the antenna's box. The diagnostic is supported with local helium gas puff, enabling it to operate in nonhelium plasmas. Spatially resolved electric field was measured for two discharge configurations, one with and one without the ICRF antenna. The results show a clear difference in the shape of the DC electric field's spatial profile for the two cases studied, with the elevated values when the ICRF antenna was operating. This demonstrates the ability of the diagnostic to measure even small relative changes in the intensity of the electric field.

IShTAR, Ion cyclotron Sheath Test ARrangement, is a linear device dedicated to the investigation of the edge plasma-ICRF (Ion Cyclotron Range of Frequencies) antenna interactions in tokamak edge-like conditions and serves as a platform for a diagnostic development for measuring the electric fields in the vicinity of ICRF antennas. We present here our progress in the development of an optical emission spectroscopy method for measuring the electric fields which concentrates on the changes in the helium spectral line profiles introduced by the external electrical field, i.e., the Stark effect. To be able to fully control the operating parameters, at the first stage of the study, the measurements are conducted on a planar electrode installed in the centre of the plasma column in IShTAR's helicon plasma source. At the second stage, the measurements are performed in the vicinity of IShTAR's ICRF antenna.

A fast-swept Langmuir probe capable to be biased at a high voltages has been constructed and successfully operated at the VINETA-II magnetic reconnection experiment. The presented circuit has two main features beneficial for fast transient parameter changes in laboratory experiments as, e.g., plasma guns or magnetic reconnection: the implementation simplicity and the high voltage sweep range. This work presents its design and performance for time-dependent measurements of VINETA-II plasmas. The probe is biased with a sinusoidal voltage at a fixed frequency. Current − voltage characteristics are measured along the falling and rising slopes of the probe bias. The sweep frequency is fsweep= 150 kHz. The spatiotemporal evolution of radial plasma profiles is obtained by evaluation of the probe characteristics. The plasma density measurements agree with those derived from a microwave interferometer, demonstrating the reliability of the measurements. As a model plasma system, a plasma gun discharge with typical pulse times of 60 μ s is chosen.

Different transport mechanisms in a magnetized radio frequency plasma discharge in the IShTAR device are compared. The total cross field particle transport systematically shows the best agreement with the turbulent diffusion. Also, the ion mobility dominated transport could substantially contribute to radial losses. The relative role of parallel and perpendicular losses in the overall particle confinement is also compared. The total perpendicular particle losses are comparable or even larger than the parallel ones, imposing a practical limitation on achieving high density plasma simply combining several helicon antennae sequentially arranged along the glass tube. For the same reason, reducing the diameter of a source tube, in an attempt to achieve a higher plasma density, could be not very efficient when perpendicular particle loss is dominant.Different transport mechanisms in a magnetized radio frequency plasma discharge in the IShTAR device are compared. The total cross field particle transport systematically shows the best agreement with the turbulent diffusion. Also, the ion mobility dominated transport could substantially contribute to radial losses. The relative role of parallel and perpendicular losses in the overall particle confinement is also compared. The total perpendicular particle losses are comparable or even larger than the parallel ones, imposing a practical limitation on achieving high density plasma simply combining several helicon antennae sequentially arranged along the glass tube. For the same reason, reducing the diameter of a source tube, in an attempt to achieve a higher plasma density, could be not very efficient when perpendicular particle loss is dominant.