Joseph Thomas Parker | Searcharxiv

arXiv2017

Testing nonlocal models of electron thermal conduction for magnetic and inertial confinement fusion applications

Three models for nonlocal electron thermal transport are here compared against Vlasov-Fokker-Planck (VFP) codes to assess their accuracy in situations relevant to both inertial fusion hohlraums and tokamak scrape-off layers. The models tested are (i) a moment-based approach using an eigenvector integral closure (EIC) originally developed by Ji, Held and Sovinec; (ii) the non-Fourier Landau-fluid (NFLF) model of Dimits, Joseph and Umansky; and (iii) Schurtz, Nicolaï and Busquet's multigroup diffusion model (SNB). We find that while the EIC and NFLF models accurately predict the damping rate of a small-amplitude temperature perturbation (within 10% at moderate collisionalities), they overestimate the peak heat flow by as much as 35% and do not predict preheat in the more relevant case where there is a large temperature difference. The SNB model, however, agrees better with VFP results for the latter problem if care is taken with the definition of the mean free path. Additionally, we present for the first time a comparison of the SNB model against a VFP code for a hohlraum-relevant problem with inhomogeneous ionisation and show that the model overestimates the heat flow in the helium gas-fill by a factor of ~2 despite predicting the peak heat flux to within 16%.

physics.plasm-ph↗

arXiv2016

Gyrokinetic simulations of fusion plasmas using a spectral velocity space representation

Magnetic confinement fusion reactors suffer severely from heat and particle losses through turbulent transport, which has inspired the construction of ever larger and more expensive reactors. Numerical simulations are vital to their design and operation, but particle collisions are too infrequent for fluid descriptions to be valid. Instead, strongly magnetised fusion plasmas are described by the gyrokinetic equations, a nonlinear integro-differential system for evolving the particle distribution functions in a five-dimensional position and velocity space, and the consequent electromagnetic field. Due to the high dimensionality, simulations of small reactor sections require hundreds of thousands of CPU hours on High Performance Computing platforms. We develop a Hankel-Hermite spectral representation for velocity space that exploits structural features of the gyrokinetic system. The representation exactly conserves discrete free energy in the absence of explicit dissipation, while our Hermite hypercollision operator captures Landau damping with few variables. Calculation of the electromagnetic fields becomes purely local, eliminating inter-processor communication in, and vastly accelerating, searches for linear instabilities. We implement these ideas in SpectroGK, an efficient parallel code. Turbulent fusion plasmas may dissipate free energy through linear phase mixing to fine scales in velocity space, as in Landau damping, or through a nonlinear cascade to fine scales in physical space, as in hydrodynamic turbulence. Using SpectroGK to study saturated electrostatic drift-kinetic turbulence, we find that the nonlinear cascade suppresses linear phase mixing at energetically-dominant scales, so the turbulence is fluid-like. We use this observation to derive Fourier-Hermite spectra for the electrostatic potential and distribution function, and confirm these spectra with simulations.

physics.plasm-ph↗