Author: Zach R. Williams
Requested Type: Poster
Submitted: 2016-02-11 17:31:36
Co-authors: M.J. Pueschel, P.W. Terry
University of Wisconsin-Madison
1150 University Ave
Madison, WI 53706
Gyrokinetic simulations of Pulsed Poloidal Current Drive discharges in the Madison Symmetric Torus exhibit unusually strong zonal flows. In order to more accurately describe the discharge, an external, constant-in-time magnetic perturbation was introduced into nonlinear flux-tube simulations to model the effect of residual tearing mode fluctuations. Such perturbations lead to a radial current that weakens zonal flows via electron streaming from rational surfaces. With weakened zonal flows, heat and particle fluxes increase to the order of experimentally observed values, resulting in a degradation of confinement. The perturbation exhibits rapid growth, increasing by orders of magnitude on timescales much faster than those of the characteristic turbulence. While this reinforcement appears to be at variance with island-healing scenarios common in tokamaks, it occurs in simulations with no ExB shear flow. An analytic calculation is presented to describe this behavior. Unlike the standard approach based on a kinetically modified MHD energy principle, this response is calculated directly from a fully gyrokinetic model. Selective modification of simulation parameters shows the reinforcement to be driven by parallel electron streaming and curvature. The island growth saturates after moderately short time, providing a convenient comparison point between the analytic calculation and simulation. Specifically, the reinforcement is seen, both in simulation and the calculation, to scale with beta and initial imposed island width.