Measurement of sheath potential in RF-biased flux tubes using a retarding field analyzer in Tore Supra tokamak

Kubič, M.; Gunn, J.; Heuraux, S.; Colas, L.; Faudot, E.; Jacquot, J.

doi:10.1016/j.jnucmat.2013.01.105

Cited by 24 publications

(45 citation statements)

References 13 publications

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“…A 2-hump poloidal variation appears, with local maxima near the poloidal extremities of the antenna box, and local minima near the equatorial plane. Such spatial structure was observed experimentally on TS [34] and AUG [35] for ion parallel energies. A 2-hump poloidal structure was also present for heat loads along TS antenna limiters [36] and W sputtering yield on AUG [37].…”

Section: DC Spatial Structure Near Antenna Limiterssupporting

confidence: 61%

“…Even in presence of propagating Fast Waves, this DC plasma conductivity was found necessary to produce a V DC spatial distribution peaked radially at the leading edge of the side limiters, as measured experimentally [34], [43]. The amplitude and radial width of the peak was sensitive to the badly constrained value of  DC .…”

Section: Plasma Biasing Far From the Antennasmentioning

confidence: 77%

“…These large parallel distances (e.g. 12m in [34]) contrast with the much smaller evanescence lengths for the SW invoked above to interpret the power unbalance experiments.…”

Section: Plasma Biasing Far From the Antennasmentioning

confidence: 78%

“…RF-induced DC modifications of the SOL are frequently observed very far away from the active ICRF antennas, on field lines magnetically connected near the antenna limiters [43], [34], [44]. These large parallel distances (e.g.…”

Section: Plasma Biasing Far From the Antennasmentioning

confidence: 99%

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SOL RF physics modelling in Europe, in support of ICRF experiments

Colas

Jacquot

et al. 2017

EPJ Web Conf.

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Abstract.A European project was undertaken to improve the available SOL ICRF physics simulation tools and confront them with measurements. This paper first reviews code upgrades within the project. Using the multi-physics finite element solver COMSOL, the SSWICH code couples RF full-wave propagation with DC plasma biasing over "antenna-scale" 2D (toroidal/radial) domains, via non-linear RF and DC sheath boundary conditions (SBCs) applied at shaped plasma-facing boundaries. For the different modules and associated SBCs, more elaborate basic research in RF-sheath physics, SOL turbulent transport and applied mathematics, generally over smaller spatial scales, guides code improvement. The available simulation tools were applied to interpret experimental observations on various tokamaks. We focus on robust qualitative results common to several devices: the spatial distribution of RF-induced DC bias; left-right asymmetries over strap power unbalance; parametric dependence and antenna electrical tuning; DC SOL biasing far from the antennas, and RF-induced density modifications. From these results we try to identify the relevant physical ingredients necessary to reproduce the measurements, e.g. accurate radiated field maps from 3D antenna codes, spatial proximity effects from wave evanescence in the near RF field, or DC current transport. Pending issues towards quantitative predictions are also outlined. ICRF antennas as active PlasmaFacing ComponentsThe phased strap arrays used to launch Ion Cyclotron Range of Frequency (ICRF, 30-80MHz) waves into magnetic fusion devices poorly radiate in vacuum: efficient coupling of the fast wave to the main plasma necessitates minimizing the distance from the straps to a critical peripheral density: the R-cutoff layer. In the Scrape-Off Layer (SOL) the antennas then behave as Plasma-facing Components (PFCs), subject to PlasmaSurface Interactions (PSI). As RF-field-emitting structures, ICRF antennas are active PFCs able to create RF-specific PSI and to modify locally their environment: enhanced ion energies, heat loads, erosion and density modifications have been observed locally. These phenomena are critical in the prospect of long-pulse machines with high-Z plasma-facing materials. Predicting the magnitude and spatial location of these processes, in relation with plasma parameters, launcher design and electrical settings, remains challenging. So far, realistic tokamak predictions relied on very simple models of oscillating double probes. A European project was undertaken to improve the available SOL RF physics simulation tools and confront them with measurements. This paper first reviews code improvements and associated basic research within the project. The available simulation tools are then applied to interpret experimental observations on various machines. From these results we exhibit qualitative properties common to the various devices, identify relevant physical ingredients necessary to reproduce the measurements, as well as pending issues towards quantitative predictions. I...

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Section: DC Spatial Structure Near Antenna Limiterssupporting

confidence: 61%

Section: Plasma Biasing Far From the Antennasmentioning

confidence: 77%

“…These large parallel distances (e.g. 12m in [34]) contrast with the much smaller evanescence lengths for the SW invoked above to interpret the power unbalance experiments.…”

Section: Plasma Biasing Far From the Antennasmentioning

confidence: 78%

Section: Plasma Biasing Far From the Antennasmentioning

confidence: 99%

See 2 more Smart Citations

SOL RF physics modelling in Europe, in support of ICRF experiments

Colas

Jacquot

et al. 2017

EPJ Web Conf.

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“…It is found that the local density perturbations can propagate at least several meters in AUG, but to which extend still requires further experimental investigations. In Tore Supra the average effects of RF potentials are indicated to be able to propagate 12 m along magnetic field lines [31].…”

Section: Potential Extrapolated To 3dmentioning

confidence: 99%

Modelling of the ICRF inducedE × Bconvection in the scrape-off-layer of ASDEX Upgrade

Zhang

Feng

Noterdaeme

et al. 2016

Plasma Phys. Control. Fusion

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Numerical analysis of the impact of an RF sheath on the Scrape-Off Layer in 2D and 3D turbulence simulations

Tamain

Colin

Colas

et al. 2017

Nuclear Materials and Energy

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International audienceMotivated by Radio Frequency (RF) heating studies, the response of the plasma of tokamaks to the presence of a locally polarized limiter is studied. In a first part, we use the TOKAM3X 3D global edge turbulence code to analyse the impact of such biasing in a realistic geometry. Key features of experimental observations are qualitatively recovered, especially the extension of a potential and density perturbation on long, but finite, distances along connected field lines. The perturbation is also found to extend in the transverse direction. Both observations demonstrate the influence of perpendicular current loops on the plasma confirming the need for an accurate description in reduced models. In a second part, we use the TOKAM2D slab turbulence code to determine the validity of using a transverse Ohm's law for this purpose. Results indicate that a local Ohm's law with a constant and uniform perpendicular resistivity appears at least as an oversimplified description of perpendicular charge transport in a turbulent Scrape-Off Layer

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Measurement of sheath potential in RF-biased flux tubes using a retarding field analyzer in Tore Supra tokamak

Cited by 24 publications

References 13 publications

SOL RF physics modelling in Europe, in support of ICRF experiments

SOL RF physics modelling in Europe, in support of ICRF experiments

Modelling of the ICRF inducedE × Bconvection in the scrape-off-layer of ASDEX Upgrade

Numerical analysis of the impact of an RF sheath on the Scrape-Off Layer in 2D and 3D turbulence simulations

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