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This paper presents the technology of fabrication of T-shaped gates for GaAs transistors using optical lithography and a unique method of directional angular deposition of thin aluminum films. The minimal foot length of the T-shaped gate formed using the developed technology was Lg = 25 nm. GaAs HEMT with a T-shaped gate formed using directional angular deposition demonstrated the drain-source saturation current Idss = 140 mA/mm, the gate-drain breakdown voltage Ugdbreak = 9 V, and the cutoff voltage Ucoff = 2 V.

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.

Dedicated experiments have been carried out at the TEXTOR tokamak in biasing-induced improved confinement regimes using Langmuir probe and GPI measurements. The results show the first evidence for the intimate interaction among sheared flows, eddy structures, Reynolds stress, zonal flows, and ambient fluctuations during a transition to an improved confinement. The sheared flows impose dynamic effects on turbulence structures, resulting in generation of Reynolds stress and zonal flows and eventually the suppression of background fluctuation levels via nonlinear energy transfer processes. The latter may therefore trigger a transition to the H mode. © 2013 American Physical Society.

Gas puff imaging (GPI) [S. J. Zweben, D. P. Stotler et al. , Phys. Plasmas9, 1981 (Year: 2002)10.1063/1.1445179; R. J. Maqueda, G. A. Wurden et al. , Rev. Sci. Instrum.74, 2020 (Year: 2003)10.1063/1.1535249] is a powerful diagnostic that permits a two-dimensional measurement of turbulence in the edge region of a fusion plasma and is based on the observation of the local emission of a neutral gas, actively puffed into the periphery of the plasma. The developed in-vessel GPI telescope observes the emission from the puffed gas along local (at the puff) magnetic field lines. The GPI telescope is specially designed to operate in severe TEXTOR conditions and can be treated as a prototype for the GPI systems on next generation machines. Also, the gas puff nozzle is designed to have a lower divergence of the gas flow than previous GPI diagnostics. The resulting images show poloidally and radially propagating structures, which are associated with plasma blobs. We demonstrate that the local gas puff does not disturb plasma properties. Our results indicate also that the neutral gas emission intensity is more sensitive to the electron density than the electron temperature. Here, we present implementation details of the GPI system on TEXTOR and discuss some design and diagnostic issues related to the development of GPI systems in general.

The mechanism governing the impact of the mass isotope on plasma confinement is still one of the main scientific conundrums facing the magnetic fusion community after more than thirty years of intense research. We have investigated the properties of local turbulence and long-range correlations in hydrogen and deuterium plasmas in the TEXTOR tokamak. Experimental findings have shown a systematic increasing in the amplitude of long-range correlations during the transition from hydrogen to deuterium dominated plasmas. These results provide the first direct experimental evidence of the importance of multiscale physics for unraveling the physics of the isotope effect in fusion plasmas.

Recently, a European transport project has been carried out among several fusion devices for studying the possible link between the mean radial electric field (E r ), long-range correlation (LRC) and edge bifurcations in fusion plasmas. The main results reported in this paper include: (i) the discovery of low-frequency LRCs in potential fluctuations which are amplified during the development of edge mean E r using electrode biasing and during the spontaneous development of edge sheared flows in stellarators and tokamaks. Evidence of nonlocal energy transfer and the geodesic acoustic mode modulation on local turbulent transport have also been observed. The observed LRCs are consistent with the theory of zonal flows described by a ‘predator–prey’ model. The results point to a significant link between the LRC and transport bifurcation. (ii) Comparative studies in tokamaks, stellarators and reversed field pinches have revealed significant differences in the level of the LRC. Whereas the LRCs are clearly observed in tokamaks and stellarators, no clear signature of LRCs was seen in the RFX-mod reversed field pinch experiments. These results suggest the possible influence of magnetic perturbations on the LRC, in agreement with recent observations in the resonant magnetic perturbation experiments at the TEXTOR tokamak. (iii) The degree of the LRCs is strongly reduced on approaching the plasma density-limit in tokamaks and stellarators, suggesting the possible role of collisionality or/and the impact of mean E r × B flow shear on zonal flows.

The electrostatic potential and density fluctuations have been measured at the edge of the TEXTOR tokamak by two toroidally distant Langmuir probe arrays. The geodesic acoustic mode (GAM) zonal flows (ZFs) are observed in potential fluctuations with a toroidal and poloidal symmetric structure. The GAM frequency, fGAM, changes monotonically with the local temperature and is close to the frequency-dispersion predicted by theories. Bispectral analysis shows clear nonlinear coupling between the GAM and broadband ambient turbulence. The GAM packet has a narrow radial extent with kr 0.5–0.7 cm−1 and exhibits explicitly a radially outward propagation. Furthermore, the radial correlation structure of GAMs and their radial propagation have been investigated in a wide range of parameters by varying plasma density and edge safety factor (5.0 ≤ q(a) ≤ 5.9). It is found that the magnitude of the GAM correlations reduces remarkably with the increase in the plasma density approaching the density limit, while the radial wavelength of GAMs only decreases slightly in higher density and larger q(a) discharges. With increasing plasma density, the radial propagating phase speed of GAMs is strongly reduced along with the drop in the local temperature. The results provide new evidence on the propagation properties of GAM ZFs.

Direct measurements of residual stress (force) have been executed at the edge of the TEXTOR tokamak using multitip Langmuir and Mach probes, together with counter-current NBI torque to balance the existing toroidal rotation. Substantial residual stress and force have been observed at the plasma boundary, confirming the existence of a finite residual stress as possible mechanisms to drive the intrinsic toroidal rotation. In low-density discharges, the residual stress displays a quasi-linear dependence on the local pressure gradient, consistent with theoretical predictions. At high-density shots the residual stress and torque are strongly suppressed. The results show close correlation between the residual stress and the Er × B flow shear rate, suggesting a minimum threshold of the E × B flow shear required for the k∥ symmetry breaking. These findings provide the first experimental evidence of the role of Er × B sheared flows in the development of residual stresses and intrinsic rotation.

The sensitivity of two numerical methods for the detection of zonal flows, especially geodesic acoustic modes (GAMs), through the movement of turbulence eddies is studied. One of them is the widely used time delay estimation method which determines a propagation time delay between two measurement points. The second technique is based on analyzing movements of the minimum in the autocorrelation function of a single measurement signal (ACFM method). The applicability of both methods is analyzed in detail in terms of time resolution, signal-to-noise ratio (SNR) and velocity modulation amplitude. It is found that both methods exhibit a broadband noise spectrum in the calculated velocity from inherent statistical reasons and they also underestimate the RMS velocity modulation amplitude by at least a factor of two, even under noise-free conditions. Their sensitivity to GAMs ceases below an SNR of about 2. These methods are applied to measurement signals obtained in Ohmic plasmas using correlation reflectometry, lithium beam emission spectroscopy (Li-BES) and probe diagnostics on the TEXTOR tokamak. The equivalence of the two methods is shown under these conditions experimentally. The availability of both methods considerably widens the possibility to detect zonal flows therefore the phase relationship between velocity modulations, related to GAMs, can be determined at both poloidally and toroidally well separated locations. A toroidally and poloidally symmetric m = 0, n = 0 velocity modulation and a coherent π/2 out of phase electron density modulation is found with odd poloidal mode number, as predicted by theory.

In this letter, we present direct evidence of eddy breaking and tilting events observed at the edge of the TEXTOR tokamak using a 2D gas puff imaging diagnostic. The occurrence of eddy breaking depends on the magnitude of the E r × B flow shearing rate. The results confirm the theoretical predictions about the influence of E r × B sheared flows in tilting and splitting turbulence eddies, and consequently, reducing the turbulent transport.

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.