Radio frequency sheaths in an oblique magnetic field

Abstract. Recent advances in finite-difference time-domain (FDTD) modeling techniques allow plasmasurface interactions such as sheath formation and sputtering to be modeled concurrently with the physics of antenna near-and far-field behavior and ICRF power flow. Although typical sheath length scales (micrometers) are much smaller than the wavelengths of fast (tens of cm) and slow (millimeter) waves excited by the antenna, sheath behavior near plasma-facing antenna components can be represented by a sub-grid kinetic sheath boundary condition, from which RF-rectified sheath potential variation over the surface is computed as a function of current flow and local plasma parameters near the wall. These local time-varying sheath potentials can then be used, in tandem with particle-in-cell (PIC) models of the edge plasma, to study sputtering effects. Particle strike energies at the wall can be computed more accurately, consistent with their passage through the known potential of the sheath, such that correspondingly increased accuracy of sputtering yields and heat/particle fluxes to antenna surfaces is obtained. The new simulation capabilities enable timedomain modeling of plasma-surface interactions and ICRF physics in realistic experimental configurations at unprecedented spatial resolution. We will present results/animations from high-performance (10k-100k core) FDTD/PIC simulations of Alcator C-Mod antenna operation.

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“…[7], can also be incorporated into the model, and future code development efforts to incorporate such models in VSim are envisioned.…”

Section: Fdtd Methods For Em Waves In Plasmamentioning

confidence: 99%

Time-Domain Modeling of RF Antennas and Plasma-Surface Interactions

Jenkins

Smithe

2017

EPJ Web Conf.

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“…Since their first formulation, SBCs have been improved using more basic models of spatially-resolved magnetized RF sheaths over scales much smaller than antenna dimensions [6], [7]. The structure of SSWICH is modular and can incorporate these improvements progressively.…”

Section: Rf and DC Sheath Boundary Conditionsmentioning

confidence: 99%

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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“…Thus, Eq. (1) expresses a complex linear relationship between V 1 and I, which we apply to the nonlinear rf sheath to obtain an effective impedance at the driving rf frequency This is done by projecting the inphase and out-of-phase components of the current [3]. To do this, we write…”

Section: Introductionmentioning

confidence: 99%

“…For E t we have ) Jz ( V V t 1 t sh t t E and J = J n = i D n /(4 ). (The sign change between V sh and V 1 is explained in [3]). From these expressions we have the final result, to be employed as the new rf sheath BC in rf wave and antenna codes:…”

Section: Introductionmentioning

confidence: 99%

“…nearly tangent to limiter tips), this approach needs to be generalized, as discussed in Ref. [3] and in the following sections. The new sheath BC retains both the resistance and capacitance of the thin rf sheath layer, and it retains the electron and ion particle currents across the sheath, in addition to the displacement current kept previously.…”

Section: Introductionmentioning

confidence: 99%

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A generalized BC for radio-frequency sheaths

D’Ippolito

Myra

2015

AIP Conference Proceedings

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Abstract.A new radio-frequency (rf) sheath boundary condition (BC) is described and applied to the problem of far field sheaths. The new BC generalizes the one presently used in rf codes to include: (1) an arbitrary magnetic field angle, (2) the full complex impedance, (3) mobile ions, (4) unmagnetized ions, and (5) the magnetic pre-sheath. For a given wavepropagation (macro) problem, root-finding is used to match the impedance of the rf wave with that of the micro-sheath problem. For a model far-field sheath problem, it is shown that the structure of the (multiple) roots with the new BC is similar to that with the capacitive BC, but the location of the resonance changes when the full impedance is used.

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Radio frequency sheaths in an oblique magnetic field

Cited by 45 publications

References 43 publications

Time-Domain Modeling of RF Antennas and Plasma-Surface Interactions

Time-Domain Modeling of RF Antennas and Plasma-Surface Interactions

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

A generalized BC for radio-frequency sheaths

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