Plasma dynamics in the tokamak edge

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Transport in the tokamak edge and scrape-off layer (SOL) plays a crucial role in determining the performance of a magnetic confinement fusion reactor. Turbulence in the edge likely sets the height of the pedestal that forms in high-confinement-mode discharges, and the associated transport sets the heat load to the vessel walls. Plasma dynamics in the edge and SOL are particularly challenging to model and to simulate due to, e.g., the presence of open field lines, neutral particles, extreme changes in magnetic geometry and large-amplitude fluctuations. As part of the ongoing ExCALIBUR/Neptune project, a group of researchers at Oxford - in collaboration with UKAEA colleagues - have created a novel, moment-based drift kinetic code to study the dynamics in the edge and SOL. The fundamental aim of the code is to solve efficiently a self-consistent version of the drift- and/or gyro-kinetic equation, while enabling a straightforward connection to fluid codes in the far SOL and delta-f gyrokinetic codes in the core. To our knowledge, all current codes are either prohibitively expensive for routine physics studies, use a simplified fluid treatment (possibly with a reduction in the number of dimensions considered) or use an inconsistent kinetic treatment. The moment-kinetic approach that we are pursuing should allow for a self-consistent kinetic treatment where required without undue numerical expense. At the moment the code solves only for either parallel dynamics (with or without the moment kinetic approach) or for a homogeneous, helical magnetic field (with only standard drift kinetics). Both versions of the code assume a simplified response for electrons that corresponds to a Boltzmann statistical equilibrium.Our proposal is for a PhD student based at Oxford to carry forward the code development; to continue with more advanced verification, validation and performance benchmarks; and to ultimately use the code to do fundamental physics studies. Key aspects of code development that could be explored include an extension to treat kinetic electrons, addition of collisions amongst charged particles, extension to 3D (non-helical) geometry, treatment of the separatrix region that ties together open and closed field line regions, addition of more sophisticated wall boundary conditions and a scheme for transitioning from drift kinetic to gyrokinetic or fluid models within a single simulation. There are a lot of avenues to explore, and which ones are addressed by the student will depend on the state of the code once the student gets up to speed, what will be seen as the most novel and timely contribution, which provides the most direct path to physics results and the relevance of the targeted physics to reactor design studies.This project falls within the EPSRC "Plasma and Lasers" and "UK Magnetic Fusion Research Programme" research areas.