Overcoming spatial and temporal stiffness in MHD simulations for fusion applications

By: Dr. Ravi Samtaney


Magnetohydrodynamics (MHD) is arguably the most popular mathematical model for the macroscopic simulations of fusion plasmas. In this talk we will focus on the resistive single-fluid MHD equations, the solutions of which can exhibit near-singular layers (or even discontinuities in the absence of diffusion terms). We rely on locally adaptive structured mesh refinement (AMR) methods to mitigate the range of spatial scales in MHD. We will present results from AMR simulations of MHD applications relevant to the fusion program. These will include pellet injection - a proven method to refuel tokamaks; magnetic reconnection - a canonical problem in plasma physics involving thin current sheets; and an example in MHD shock refraction where five or more discontinuities meet at a single point.

Explicit time-stepping methods to simulate MHD for fusion applications become prohibitively expensive due to the CFL constraint on the time-step. To overcome the temporal stiffness associated with the fast compressive and Alfven waves in MHD, we have developed a nonlinearly implicit time stepping method using a Jacobian-Free Newton-Krylov approach (JFNK). At the heart of our JFNK method is a PDE-operator based preconditioner (exact for a 1D system of hyperbolic PDEs), to effectively solve the resulting large ill-conditioned linear system.

We will conclude with our future plans on combining the AMR and JFNK approaches under the auspices of the DOE SciDAC Program.

Acknowledgment: This work is a collaboration between the APDEC, CEMM and TOPS SciDAC Centers, and is supported under the DOE SciDAC program (US-DOE Contract no. DE-AC020-76-CH03073).

Last modified: Fri Oct 12 13:09:28 EDT 2007