A radio-frequency sheath boundary condition and its effect on slow wave propagation

D’Ippolito, D. A.; Myra, J. R.

doi:10.1063/1.2360507

Cited by 68 publications

(121 citation statements)

References 29 publications

(50 reference statements)

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“…However, salient sheath effects can be represented by a "sub-grid sheath boundary condition" [5], in which local capacitive and resistive circuit elements associated with the presence of the sheath are defined at the nodes of grid cells intersecting the material boundary. This formulation enables a local sheath potential to be defined at the boundary.…”

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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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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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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“…At the RF timescale, RF sheaths are often treated as thin dielectric layers of width  between the wall and the quasi-neutral plasma. The associated RF SBC was first formulated in [4] and reads…”

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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“…Inside this domain and as shown on figure 2, three fields are solved for: the RF parallel electric field E , an oscillating sheath voltage V RF (both varying as as exp(−iω 0 t) at the wave pulsation ω 0 ) and the DC plasma potential V DC . Sheaths at both ends of open flux tubes are described by RF and DC sheaths boundary conditions [17,18]. The non-linear I-V characteristic of the sheaths couples the RF and DC quantities.…”

Section: Sswich-sw Asymptoticmentioning

confidence: 99%

Sequential modelling of ICRF wave near RF fields and asymptotic RF sheaths description for AUG ICRF antennas

et al. 2017

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Abstract. A sequence of simulations is performed with RAPLICASOL and SSWICH to compare two AUG ICRF antennas. RAPLICASOL outputs have been used as input to SSWICH-SW for the AUG ICRF antennas. Using parallel electric field maps and the scattering matrix produced by RAPLICASOL, SSWICH-SW, reduced to its asymptotic part, is able to produce a 2D radial/poloidal map of the DC plasma potential accounting for the antenna input settings (total power, power balance, phasing). Two models of antennas are compared: 2-strap antenna vs 3-strap antenna. The 2D DC potential structures are correlated to structures of the parallel electric field map for different phasing and power balance. The overall DC plasma potential on the 3-strap antenna is lower due to better global RF currents compensation. Spatial proximity between regions of high RF electric field and regions where high DC plasma potentials are observed is an important factor for sheath rectification.

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A radio-frequency sheath boundary condition and its effect on slow wave propagation

Cited by 68 publications

References 29 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

Sequential modelling of ICRF wave near RF fields and asymptotic RF sheaths description for AUG ICRF antennas

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