Plasma turbulence response to topology changes in the tokamak edge

点击翻译按钮获取中文摘要

The performance of nuclear fusion experiments depends sensitively on the ionized gas (=plasma) next to the wall of the device. This region is characterised by a change in the topology of the magnetic field lines. In the hot plasma region where fusion reactions happen, known as core, magnetic-field lines cover surfaces ergodically and never come into contact with the wall, whereas in the region near the wall, known as Scrape-Off-Layer (SOL), magnetic-field lines encounter the wall after spanning a finite length. This change in the topology of the magnetic-field lines has important effects on the response of the plasma to perturbations and hence on the turbulence that transports energy away from the hot plasma region. As a first step, the student will develop a kinetic model to describe the plasma near the wall. The density and the temperature of the plasma will be treated as normalization variables, and they will be evolved in time independently from the rest of the distribution function. Thanks to this separation between density, temperature and the rest of the distribution function, the model will connect smoothly with the fluid treatments used for the SOL and with kinetic treatments used for the core, where turbulence is a small perturbation to a mostly quiescent plasma.Once the model is developed, the student will derive fluid equations with hot ions. Even though ions are usually hotter than electrons, many fluid models for the SOL assume that ions are very cold to simplify the equations. The new fluid equations will be appropriate for both core and SOL, and they will be compared to existing SOL models. These equations will be implemented numerically to study the evolution of plasma quantities as the plasma flows from the core to the wall. The student will focus on the electric field because it is determined very differently in the SOL and in the core. In the SOL, the electric field is mostly determined by the potential at the wall, whereas in the core the electric field is determined by turbulent momentum transport.