Book Description

One of the most challenging problems facing plasma physicists today involves the modeling of plasma turbulence and transport in magnetic confinement experiments. The most successful model to this end so far is the reduced gyrokinetic model. Such a model cannot be solved analytically, but can be used to simulate the plasma behavior and transport with the help of present-day supercomputers. This has lead to the development of many different codes which simulate the plasma using the gyrokinetic model in various ways. These models have achieved a large amount of success in describing the core of the plasma for conventional tokamak devices. However, numerous difficulties have been encountered when applying these models to more extreme parameter regimes, such as the edge and scrape-off layer of the tokamak, and high plasma devices, such as spherical tokamaks. The development and application of the gyrokinetic model (specifically with the gyrokinetic code, GENE) to these more extreme parameter ranges shall be the focus of this thesis. One of the main accomplishments during this thesis project is the development of a more advanced collision operator suitable for studying the low temperature plasma edge. The previous collision operator implemented in the code was found to artificially create free energy at high collisionality, leading to numerical instabilities when one attempted to model the plasma edge. This made such an analysis infeasible. The newly implemented collision operator conserves particles, momentum, and energy to machine precision, and is guaranteed to dissipate free energy, even in a nonisothermal scenario. Additional finite Larmor radius correction terms have also been implemented in the local code, and the global code version of the collision operator has been adapted for use with an advanced block-structured grid scheme, allowing for more affordable collisional simulations. The GENE code, along with the newly implemented collision operator developed in this thesis, has been applied to study plasma turbulence and transport in the edge (tor = 0:9) of an L-mode magnetic confinement discharge of ASDEX Upgrade. It has been found that the primary microinstabilities at that radial position are electron drift waves destabilized by collisions and electromagnetic effects. At low toroidal mode numbers, ion temperature gradient driven modes and microtearing modes also seem to exist. In nonlinear simulations with the nominal experimental parameters, the simulated electron heat flux was four times higher than the experimental reconstruction, and the simulated ion heat flux was twice as high. However, both the ion and electron simulated heat flux could be brought into agreement with experimental values by lowering the input logarithmic electron temperature gradient by 40%. It was also found that the cross-phases between the electrostatic potential and the moments agreed well for the part of the binormal spectrum where the dominant transport occurred, and was fairly poor at larger scales where minimal transport occurred. Finally, a new scheme for evaluating the electromagnetic fields has been developed to address the instabilities occurring in nonlinear local and global gyrokinetic simulations at high plasma . This new scheme is based on evaluating the electromagnetic induction explicitly, and constructing the gyrokinetic equation based on the original distribution, rather than the modified distribution which implicitly takes into account the induction. This new scheme removes the artificial instability occurring in global simulations, enabling the study of high scenarios with GENE. The new electromagnetic scheme can also be generalized to a full-f implementation, however, it would require updating the field matrix every time-step to avoid the cancellation problem. The new scheme (including the parallel nonlinearity) does not remove the local instability, suggesting that that instability (caused by magnetic field perturbations shorting out zonal flows) is part of the physics of the local model.