SOLPS modelling of ELMing H-mode

Numerical simulation of the tokamak scrape-off layer (SOL) is an essential tool for the prediction of the conditions to be expected in future fusion reactors such as the ITER project, now under construction in Southern France. One particularly important issue regards the estimation of the expected transient power loads on plasma-facing components (PFC) due to magnetohydrodynamic plasma relaxations, known as Edge Localised Modes (ELMs). These loads are a major cause of concern for ITER owing to the very severe restrictions on PFC lifetime (especially the divertor targets) that they will impose if their amplitude is not maintained below a given size. Even though SOL plasma modelling has reached a comparatively high level of sophistication (the ITER divertor is being designed in part with complex edge plasma codes), the majority of simulations are performed for steady state conditions, necessarily excluding the description of transient events. This thesis explores the utility and validity of the fluid plasma, Monte-Carlo neutrals approach to SOL and divertor modelling in the presence of time dependent ELM phenomena. It aims to test the most complex tool of this type currently available, the fluid (B2.5)-neutral Monte-Carlo (EIRENE) code package SOLPS5, against a variety of ELM sizes in two very different tokamaks, TCV and JET. Although the SOLPS package has been the modelling tool of choice for ITER design, it has not yet been systematically used for the study of ELM transients. A key element throughout is rigorous benchmarking – seeking the best possible agreement between both experiment and simulation and between different codes for the same experiment, using as many different measurements as possible to constrain the model. Such benchmarking attempts are still, unfortunately, comparatively rare on today's machines. Fully time-dependent simulations (2-D plasma, 3-D neutrals) have been performed of four H-mode plasmas, two each on TCV and JET, covering Type III and Type I ELMs over a range of pedestal collisionality and energy expelled per ELM from ΔWELM ∼ 0.005 → 0.7 MJ. The high end of this limit corresponds to the current maximum ΔWELM which is thought to be tolerable on ITER for acceptable divertor target lifetime. The two tokamaks differ radically in size, input power and divertor geometry, but share carbon as the main PFC material. The SOLPS5 simulations have thus been performed with all carbon charge states included but do not feature activated poloidal drift terms. The approach is first to seek the closest match to experimental upstream, pedestal/SOL and downstream target profiles during the inter-ELM phase. This is achieved through the specification of radially varying perpendicular particle and heat diffusivities and/or convective radial velocity in order to account for the different transport levels in the edge and SOL regions. Poloidal variation of these transport coefficients is also applied to distinguish between main chamber SOL and