Computation of turbulence in magnetically confined plasmas

Abstract

Plasmas are magnetically confined if the orbits of the charged particles around the magnetic field lines are small compared to the system size. Nevertheless, particles and energy are lost via transport resulting from the fluid-like drifts of these orbits across the magnetic field. Computation of this type of low frequency electromagnetic turbulence is introduced with emphasis on nonlinear character and energetics. The physical situation is low frequency dynamics with perpendicular forces in quasistatic balance and fast gyrofrequency leading to treating the orbit gyrocentres rather than the particles themselves. The broadband turbulence necessarily involves the ion gyroradius scale in any tokamak application. Hence the use of ‘gyrofluid’ and ‘gyrokinetic’ computational models. Computations must also treat the interplay between electromagnetic wave dynamics along the magnetic field and fluid-like turbulence across it, with a method independently checked against both of these sub-processes. Tokamak core and edge turbulence differ according to the ratios of the parallel (electron) and perpendicular (E-cross-B) transit frequencies. The turbulence is also in energetic contact with flows and currents associated with the equilibrium, so computations must be well behaved over very long run times and at least the perturbed equilibrium carried self-consistently. These considerations are illustrated by treating the case of ion temperature gradient instabilities giving rise to turbulence which is then suppressed by self generated E-cross-B flows. The methods of diagnosis of the physical processes are detailed. The situation of edge turbulence and interaction with the equilibrium is briefly addressed.