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Author: Jugal Chowdhury
Requested Type: Pre-Selected Invited
Submitted: 2016-02-15 19:17:48

Co-authors: Yang Chen,Weigang Wan,Scott E. Parker

Contact Info:
University of Colorado
2000 Colorado Ave.
Boulder, CO   80302

Abstract Text:
Direct gyrokinetic simulation of tearing modes in tokamaks has been a great computational challenge. First, tearing modes have very fine mode structure at the tearing layer, which requires fine radial resolution. Second, low-n tearing modes have a very small growth rate. Here we present direct gyrokinetic simulation of both microtearing modes and low-n tearing modes. Ions are gyrokinetic and electrons are drift-kinetic. For the microtearing modes we present nonlinear simulation results for the edge of the NSTX spherical tokamak using the GEM code in the flux-tube limit [1]. For the high-n microtearing turbulence, fine radial resolution is also required, making the nonlinear simulations computationally challenging. We begin with an introduction to the electromagnetic gyrokinetic delta-f particle-simulation model and algorithm, including recent developments for studying tearing modes. We will present a detailed study of the microtearing mode at the pedestal top of NSTX plasmas. The dependence of the microtearing mode on various equilibrium quantities in the edge regions is presented. We also investigate microtearing modes in the core of NSTX. Core microtearing modes are found to depend upon electron-ion collisions, but in the edge, the mode is weakly dependent on collisions. Electrostatic physics plays a significant role in both cases. While microtearing is partially stabilized by the electrostatic potential in the core, it has a substantial destabilizing effect in the edge. We will also highlight recent success simulating global low-n tearing modes [2,3]. For low-n tearing modes a novel outer region boundary condition has been developed to allow high radial resolution in the current layer [2]. Nonlinear results for a single (m=2,n=1) tearing mode will be reported.
[1] J. Chowdhury, et al. Phys. Plasmas 23, 012513 (2016).
[2] Y. Chen, et al. Phys. Plasmas 22, 042111 (2015).
[3] Y. Chen, et al, Phys. Plasmas, to appear (2016).