Experiments in the tokamaks TEXTOR, CASTOR, T-10 and ISTTOK, as well as in the reversed field pinch RFX have provided new and complementary evidence on the physics of the universal mechanism of E×B velocity shear stabilization of turbulence, concomitant transport barrier formation and radial conductivity by using various edge biasing techniques. In TEXTOR the causality between transport reduction and induced electric fields in the edge has been for the first time clearly demonstrated. The high electric field gradients have been identified as the cause for the quenching of turbulent cells. A quantitative analysis of the measured transport reduction is in good agreement with theoretical predictions. The scaling of plasma turbulence suppression with velocity shear has been established, revealing the density-potential cross-phase as a key element. Reduction in poloidal electric field, temperature, and density fluctuations across the shear layer lead to a reduction of the anomalous conducted and convected heat fluxes resulting in an energy transport barrier that is measured directly. In CASTOR the biasing electrode is placed at the separatrix in a non-intrusive configuration which has demonstrated strongly sheared electric fields and consequent improvement of the global particle confinement, as predicted by theory. The impact of sheared E×B flow on edge turbulent structures has been measured directly using a comprehensive set of electrostatic probe arrays as well as emissive probes. Measurements with a full poloidal Langmuir probe array have revealed quasi-coherent electrostatic waves in the SOL with a dominant mode number equal to the edge safety factor. In T-10 edge biasing is clearly improving the global performance of ECR heated discharges. Reflectometry and heavy ion beam probe measurements show the existence of a narrow plasma layer with strong suppression of turbulence. On ISTTOK, the influence of alternating positive and negative electrode and (non-intrusive) limiter biasing has been compared. Electrode biasing is found to be more efficient in modifying the radial electric field Er and confinement, limiter biasing acting mainly on the SOL. In the RFX reversed field pinch it has been demonstrated that also in RFPs biasing can increase the local E×B velocity shear in the edge region, and hence substantially reduce the local turbulence driven particle flux mainly due to a change in the relative phase between potential and density fluctuations.

http://iopscience.iop.org/article/10.1088/0741-3335/45/5/308/pdf

Turbulent transport reduction by E×B velocity shear during edge plasma biasing: recent experimental results

The spatial distribution of collisionless ions collected by a spherical object of radius much larger than the Debye length, in a flowing plasma, is calculated using a particle-in-cell code. The results provide the first rigorous theoretical calibration of a `Mach probe' in a plasma with negligible magnetic field. They are also applicable, for example, to spacecraft-plasma interactions. Ion to electron temperature ratios 0.1<Ti/ZTe<10 are explored. Qualitative differences are observed between the results for low and high Ti/ZTe, arising from ion dynamics in the wake. However, a practically universal form for the upstream to downstream current density ratio dependence on flow velocity is observed for Ti/ZTe3.

http://iopscience.iop.org/article/10.1088/0741-3335/44/9/313/pdf

Ion collection by a sphere in a flowing plasma: I. Quasineutral

Azimuthal flow is driven in the edge of the Large Plasma Device (LAPD) [W Gekelman, {\itshape et al}, Rev Sci Instr {\bfseries 62}, 2875 (1991)] through biasing a section of the vacuum vessel relative to the plasma source cathode As the applied bias exceeds a threshold, a transition in radial particle confinement is observed, evidenced by a dramatic steepening in the density profile, similar to the L- to H-mode transition in toroidal confinement devices The threshold behavior and dynamic behavior of radial transport is related to flow penetration and the degree of spatial overlap between the flow shear and density gradient profiles An investigation of the changes in turbulence and turbulent particle transport associated with the confinement transition is presented Two-dimensional cross-correlation measurements show that the spatial coherence of edge turbulence in LAPD changes significantly with biasing The azimuthal correlation in the turbulence increases dramatically, while the radial correlation length is little altered Turbulent amplitude is reduced at the transition, particularly in electric field fluctuations, but the dominant change observed is in the cross-phase between density and electric field fluctuations The changes in cross-phase lead to a suppression and then apparent reversal of turbulent particle flux as the threshold is exceeded

Modifications of turbulence and turbulent transport associated with a bias-induced confinement transition in LAPD

This paper summarizes the status of the COMPASS tokamak, its comprehensive diagnostic equipment and plasma scenarios as a baseline for the future studies. The former COMPASS-D tokamak was in operation at UKAEA Culham, UK in 1992–2002. Later, the device was transferred to the Institute of Plasma Physics of the Academy of Sciences of the Czech Republic (IPP AS CR), where it was installed during 2006–2011. Since 2012 the device has been in a full operation with Type-I and Type-III ELMy H-modes as a base scenario. This enables together with the ITER-like plasma shape and flexible NBI heating system (two injectors enabling co- or balanced injection) to perform ITER relevant studies in different parameter range to the other tokamaks (ASDEX-Upgrade, DIII-D, JET) and to contribute to the ITER scallings. In addition to the description of the device, current status and the main diagnostic equipment, the paper focuses on the characterization of the Ohmic as well as NBI-assisted H-modes. Moreover, Edge Localized Modes (ELMs) are categorized based on their frequency dependence on power density flowing across separatrix. The filamentary structure of ELMs is studied and the parallel heat flux in individual filaments is measured by probes on the outer mid-plane and in the divertor. The measurements are supported by observation of ELM and inter-ELM filaments by an ultra-fast camera.

https://iopscience.iop.org/article/10.1088/0741-3335/58/1/014015/pdf

Status of the COMPASS tokamak and characterization of the first H-mode

Azimuthal flow is driven in the edge of the Large Plasma Device (LAPD) [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] through biasing a section of the vacuum vessel relative to the plasma source cathode. As the applied bias exceeds a threshold, a transition in radial particle confinement is observed, evidenced by a dramatic steepening in the density profile, similar to the L- to H-mode transition in toroidal confinement devices. The threshold behavior and dynamic behavior of radial transport is related to flow penetration and the degree of spatial overlap between the flow shear and density gradient profiles. An investigation of the changes in turbulence and turbulent particle transport associated with the confinement transition is presented. Two-dimensional cross-correlation measurements show that the spatial coherence of edge turbulence in LAPD changes significantly with biasing. The azimuthal correlation in the turbulence increases dramatically, while the radial correlation length is little altered. Turbulent amplitude is reduced at the transition, particularly in electric field fluctuations, but the dominant change observed is in the cross-phase between density and electric field fluctuations. The changes in cross-phase lead to a suppression and then apparent reversal of turbulent particle flux as the threshold is exceeded.

Modifications of turbulence and turbulent transport associated with a bias-induced confinement transition in the Large Plasma Device

The COMPASS-D tokamak, originally operated by UKAEA at Culham, UK, will be reinstalled at the Institute of Plasma Physics (IPP) AS CR. The COMPASS device was designed as a flexible tokamak in the 1980s mainly to explore the MHD physics. Its operation (with D-shaped vessel) began at the Culham Laboratory of the Association EURATOM/ UKAEA in 1992. The COMPASS-D tokamak will have the following unique features after putting in operation on IPP Prague. It will be the smallest tokamak with a clear H-mode and ITER-relevant geometry. ITER-relevant plasma conditions will be achieved by installation of two neutral beam injection systems (2 × 300 kW), enabling co-and counter-injections. Redeployment of the existing LH system (400 kW) is also envisaged. A comprehensive set of diagnostics focused mainly on the edge plasma will be installed. The scientific programme proposed for the COMPASS-D tokamak installed in IPP Prague will benefit from these unique features of COMPASS-D and consist of two main scientific projects, both highly relevant to ITER-Edge plasma physics (H-mode studies) and Wave-plasma interaction studies. The COMPASS-D tokamak will offer an important research potential as a small, flexible and low-cost facility with ITER-relevant geometry.

/pdf/reinstallation-of-the-compass-d-tokamak-in-ipp-ascr-2xccjyclut.pdf

Reinstallation of the COMPASS-D tokamak in IPP ASCR

The ion current collected by a probe in a magnetized plasma is sensitive to the angle between its surface and the flow streamlines. This intuitive concept is the basis of the Gundestrup probe, a polar array of planar collectors mounted around an insulating housing. Probe theory for measuring flows has been developed on two fronts: Recent kinetic and fluid models, reviewed here, give similar predictions for the collected current within the range of applicability of the model assumptions. A comparison with measurements by a rotating Mach probe in the CASTOR tokamak (Czech Academy of Sciences Torus) [J. Stockel, J. Badalec, I. Ďuran et al., Plasma Phys. Controlled Fusion, 41, 577 (1999)] highlights the role of magnetization in ion collection at grazing angles of incidence between the probe surface and the magnetic field lines.

Edge flow measurements with Gundestrup probes

Upgrade of the COMPASS tokamak microwave reflectometry system with I/Q modulation and detection

Radio-Frequency design of a Lower Hybrid Slotted Waveguide Antenna

A rotating Mach probe is designed and used for measurement of plasma flows on the CASTOR tokamak. Measurements are performed at standard discharge conditions as well as in shots with edge plasma polarization. The one dimensional fluid model is developed to deduce the parallel and perpendicular Mach numbers from experimental data.

F. Žáček

Papers

Reinstallation of the COMPASS-D tokamak in IPP ASCR

Edge flow measurements with Gundestrup probes

Upgrade of the COMPASS tokamak microwave reflectometry system with I/Q modulation and detection

Radio-Frequency design of a Lower Hybrid Slotted Waveguide Antenna

Rotating Mach Probe for Ion Flow Measurements on the CASTOR Tokamak