Non-Ambipolarity of Microturbulent Transport

Abstract

When exact magnetic surfaces are assumed to exist, the gyrokinetic theory of microturbulence gives the same radial transport for ions and electrons. But, exact magnetic surfaces do not exist in the presence of what is called electrostatic microturbulence. When the plasma pressure is non-zero, a turbulent electric potential is accompanied by a turbulent magnetic field, which splits the rational magnetic surfaces with which it resonates. If the magnetic field is assumed to have an ideal topology-conserving evolution, delta function current densities arise on resonant surfaces. The singularity of the current density allows islands to open quickly, but there is no singularity that allows a rapid closure. Islands remain and do not flutter into and out of existence. A relative rotation of the electron fluid in neighboring island chains produces a non-dissipative force that can lock the islands together and produce a non-ambipolar transport. At sufficient plasma pressure, the islands associated with different resonant rational surfaces can overlap. When this occurs some magnetic field lines will cross the entire radial region occupied by overlapping islands. The effect on the electron fluid is to create a viscosity-like force, which is dissipative and tends to remove gradients in the electron rotation. This also produces a non-ambipolar transport. Under many assumptions, the island locking force is larger than the viscosity-like force.