Mesoscale plasma dynamics, transport barriers and zonal flows: simulations and paradigms

Abstract Tokamaks are magnetic confinement fusion devices which seek to produce power from fusion reactions between the isotopes of hydrogen (deuterium and tritium). The understanding of turbulent transport processes which govern the energy, momentum and current distributions in tokamak plasmas is important to optimising the economically viable design of future power plants based on the tokamak concept. With the advent of powerful modern computers it has become possible to model the plasma dynamics on the so-called “mesoscale” which consists of electromagnetic turbulence with wavelengths intermediate to the ion gyro radius and the system size of typical tokamaks. This paper attempts to describe one such approach which evolves the two-fluid model of a tokamak plasma globally (i.e., both on the macroscale and the mesoscale), using a nonlinear, electromagnetic, three-dimensional code CUTIE. Recent researches, both theoretical and experimental, on tokamaks indicate the spontaneous (or, externally induced) generation of so-called “zonal flows”, which, under well-defined conditions, can lead to substantial reduction of turbulent transport in localized regions known as transport barriers. This type of confinement enhancement is of great importance in the design and construction of practical fusion power plants and has been the subject of intensive study. In addition to the computational approach based on CUTIE simulations, we also describe some simpler paradigmatic models which are designed to illustrate the genesis of zonal flows by characteristic drift wave fluctuations and the effects of such highly sheared advective flows on the system dynamics. These models help one to understand in a much clearer fashion the rather complex processes simulated by CUTIE.