Gyrokinetic simulation of micro-turbulence in magnetically confined plasmas

Abstract

Small scale turbulence in toroidal magnetic fusion experiments (Tokamaks) causes the rapid loss of heat from the plasma. This is of great importance since it limits the fusion performance of proposed reactor concepts. Electron temperature gradient driven (ETG) modes have been proposed as a source of anomalous electron thermal loses in tokamaks. It is widely acknowledged that the electrostatic potential in ETG turbulence can develop into radially elongated structures known as streamers. Understanding the conditions that permit streamers to produce experimentally significant transport is a topic of great interest. Analysis of the ETG mode at long wavelengths where both the ions and electrons are adiabatic (have a Boltzmann response) show that the ETG mode is inherently electromagnetic. Mixing length estimates of the thermal transport coefficient in this regime peak at collisionless skin-depth scales, providing a possible beta (the ratio of plasma pressure to magnetic pressure) dependence of the resulting transport. Preliminary nonlinear flux-tube simulations of the electromagnetic ETG mode produce large transport from the magnetic nonlinearity, while streamers in the electrostatic potential are still formed.