Comparisons of the nonlinear and the quasilinear model for the bump-on-tail instability with phase decorrelation

Joint Varenna-Lausanne International Workshop 2014

Tholerus, S.^1 ; Hellsten, T.^1 ; Johnson, T.^1

KTH Royal Institute of Technology, School of Electrical Engineering, Department of Fusion Plasma Physics, Stockholm

SE-100 44, Sweden^1

关键词: Characteristic time;    Diffusive process;    Energetic particles;    Parameter studies;    Phase space structures;    Quasi-linear model;    Theoretical values;    Wave-particle interactions;   

Others  :  https://iopscience.iop.org/article/10.1088/1742-6596/561/1/012019/pdf
DOI  :  10.1088/1742-6596/561/1/012019

来源: IOP

【 摘 要 】

The dynamics of discrete global modes in a toroidal plasma interacting with an energetic particle distribution is studied, and in particular when the dynamics of the system using the nonlinear and quasilinear descriptions are macroscopically similar. The dynamics can be described with a nonlinear bump-on-tail model in a two-dimensional phase space of particles. A Monte Carlo framework is developed for this model with an included decorrelation of the wave- particle phase, which is used to model extrinsic stochastisation of the wave-particle interactions. From this description, a quasilinear version of the model is also developed, which is described by a diffusive process in energy space due to the added phase decorrelation. Due to the reduced dimensionality of phase space, the quasilinear description is typically less computationally demanding than the nonlinear description. The purpose of the studies is to find conditions when a quasilinear model sufficiently describes the same phenomena of the wave-plasma interactions as a nonlinear model does. Via numerical and theoretical parameter studies, regimes where the two models overlap macroscopically are found. These regimes exist above a given threshold of the strength of the decorrelation, where coherent phase space structures are destroyed on time scales shorter than characteristic time scales of nonlinear particle motion in phase space close to the wave-particle resonance. Specifically for the quasilinear model, a theoretical value of the time scale of quasilinear flattening is derived and numerically verified.