Thomas Jenkins

Calculating electron cyclotron current drive stabilization of resistive tearing modes in a nonlinear magnetohydrodynamic model

A model which incorporates the effects of electron cyclotron current drive (ECCD) into the magnetohydrodynamic equations is implemented in the NIMROD code [C. R. Sovinec et al., J. Comput. Phys. 195, 355 (2004)] and used to investigate the effect of ECCD injection on the stability, growth, and dynamical behavior of magnetic islands associated with resistive tearing modes. In addition to qualitatively and quantitatively agreeing with numerical results obtained from the inclusion of localized ECCD deposition in static equilibrium solvers [A. Pletzer and F. W. Perkins, Phys. Plasmas 6, 1589 (1999)], predictions from the model further elaborate the role which rational surface motion plays in these results. The complete suppression of the (2,1) resistive tearing mode by ECCD is demonstrated and the relevant stabilization mechanism is determined. Consequences of the shifting of the mode rational surface in response to the injected current are explored, and the characteristic short-time responses of resistive te...

Modeling plasma chemistry, sputtering, and RF sheath effects in low-temperature and fusion plasmas

Summary form only given. We present an overview of ongoing enhancements to VSim, a plasma modeling code capable of both PIC/MCC and fluid finite-difference time-domain representations. A new sub-grid kinetic sheath boundary condition1 enables the physical effects of DC and RF sheath physics to be included in macroscopic-scale plasma simulations that need not explicitly resolve sheath scale lengths. Evolution of local sheath potentials on complex plasma-facing components, together with the associated particle fluxes and sputtering effects, can be simulated using such techniques; we will demonstrate a model of impurity sputtering from the ICRF antenna in the Alcator C-Mod fusion device. Test particle ion populations, inserted in the simulation near the antenna and evolving in response to the electromagnetic RF fields which it produces, are used to predict particle and heat fluxes to localized antenna components. These ions gain energy as they pass through the sheath to strike the surface, and the self-consistent evolution of local sheath potential can be used to compute this energy gain and more accurately predict local sputtering yields. Augmentations of the sheath boundary condition to include more general magnetic field and plasma configurations (e.g. resistive sheath dissipation) will also be presented. Complex chemistry scenarios arising in low-temperature plasmas can be modeled in VSim via an interface with MUNCHKIN, a standalone python/C++/SQL code that identifies reaction paths for a given set of input species, solves 1D rate equations to analyze the time-dependent chemical evolution of the system, and generates corresponding VSim input blocks with appropriate crosssections or reaction rates. These features, together with others such as calculation of reaction rates from userspecified distribution functions, principal path analysis (for reducing the number of simulated chemical reactions while retaining accuracy), and parallelization for high-performance analysis, will also be discussed.