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

Abstract

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.

Numerical solutions of Maxwell's equations in 3D in frequency domain with linear sheath boundary conditions

“…(12) assuming ε sh = 1. We conclude that the time-domain sheath boundary conditions of [13][14][15] , when transformed to frequency domain, are nearly equivalent to the frequency-domain sheath boundary conditions considered here, with the sole difference being the value of ε sh . They are exactly equivalent when ε sh = 1, i.e.…”

“…ε E = iω µ 0 J ant (1) where E are the unknown electric fields near the antenna, ω is the angular antenna frequency, ↔ ε is the cold plasma dielectric tensor given in eqs. (15,16). Near any surface, we can decompose E into a tangential component E t and a normal component…”

“…Jenkins and Smithe [13][14][15] propose the following timedomain sheath boundary condition. Ignoring loss terms, terms related to dielectric coatings, and assuming known constant sheath width δ sh :…”

Resonance in radio frequency sheath admittance and enhanced impurity emission near the ion cyclotron frequency

“…The problem presented by RF sheaths is that they are associated with enhanced impurity production. Increased erosion from the Faraday screen and surrounding limiters during ICRH operation is well-characterized [4][5][6][7]. This impurity production is attributed to the nonlinear voltage-current trend which produces an additional DC so-called rectified potential.…”

Hybrid iterative approach for simulation of radio-frequency fields in plasma

“…Nonlinear hybrid models with fully kinetic ions and drift kinetic electrons are starting to be used in fusion RF modeling. 23 Modeling of time evolution of RF fields in nonlinear fluid plasma models finds a number of applications in fusion plasmas, including modeling of an antenna near and far fields 24,25 in an ion cyclotron frequency range and modeling of propagation of a RF beam near the plasma edge in an ECR frequency range. 26 It seems that the main limitation of using the time evolution approach for solving wave equations in a frequency domain is due to accumulation of numerical errors during the time integration.…”