Numerical modelling of H-mode plasmas on JET
Creators: name: Moulton, David James
Publisher: name: Imperial College London
Year: 2012

description: This thesis describes the numerical modelling of H-mode plasmas on the Joint European Torus (JET), both during and between edge localised modes (ELMs). The multi-fluid code EDGE2D-EIRENE has been benchmarked against experimental data from the inter-ELM phase of an ITER-relevant JET plasma. Despite matching all other reliable experimental measurements to within a factor ∼ 1.5, the simulated radiation was located primarily at the targets, whereas the experimental radiation was located primarily at the X-point. This discrepancy occurred because the majority of radiating carbon ions in the simulation were located near the targets. In addition, the inner target Dα measurement was underestimated in the simulation by a factor of 9. A wide parameter scan did not change these two discrepancies. The inclusion of a thermoelectric parallel current in the simulations was found to be consistent with experimental measurements and to give more realistic degrees of in/out power asymmetry. A fuelling scan was carried out from the benchmarked simulation and compared to experiment. For similar increases in upstream density, the simulated and experimental outer target profiles were seen to agree well. This lead to an experimentally verified scaling of [Equation appears here. To view, please open pdf attachment] for the maximum parallel energy flux density at the outer target as a function of the electron separatrix density at the outer mid-plane. Preliminary simulations with nitrogen seeding are also discussed. The ELM transport phase has also been modelled. By making a simple modification to the analytic ion free streaming equations (Fundamenski et al. 2006 PPCF, 48:109-156), the equations are shown to match the impulse responses for target particle and energy fluxes, as calculated by a 1d kinetic code, to a high degree of accuracy. The linearity of the force-free Vlasov equation means that a distributed ELM source in time and space can be modelled by convolving the impulse responses with the ELM source function. Although this introduced some error (due to the fact that, in reality, the potential evolves in time for a distributed ELM source), the agreement with even a complex 1d3v particle-in-cell code was remarkably good. A new code is presented that tracks portions of the ELM distribution function in three-dimensional space as they free stream to the vessel walls. The code is simple and fast and is shown to reproduce similar scalings for the maximum energy density as were found in experiment. It also reproduced an experimentally observed profile for the energy density at the outer target, and encouragingly the agreement was seen to improve when a more physically realistic distribution for the ELM influx was used in the poloidal direction. The simulated rise and fall times were too short compared to experiment when an impulse ELM influx was used and reasons for this discrepancy are discussed. Also demonstrated is the ability of the code to model the effect of stochasticity of filament size on the surface energy flux density at the target.