Jim Callen visited GA this week with postdoctoral fellow Matt Beidler and graduate student Tyler Cote from U. Wisconsin, Madison. Several discussion sessions were held to discuss areas of mutual interest in the effects of 3D external fields on stability of locked modes and ballooning modes in tokamaks and issues of seeding of neoclassical tearing modes.

Temperature profile prediction with the TGLF turbulent transport model and the NEO neoclassical transport code, and linear gyrokinetic analysis with the CGYRO gyrokinetic code, has shown that the kinetic ballooning mode (KBM) stability controls the region between the internal and H-mode edge transport barriers. Two self-organized states are possible depending on the feedback between the KBM stability and the bootstrap current peaks of the transport barriers. The high bootstrap fraction of the high poloidal beta regime provides the strong coupling of the pressure profile to the magnetic shear, both of which control the KBM stability. An invited talk at the APS-DPP meeting was given by Gary Staebler on this research.

The influence of sonic toroidal rotation on main ion and heavy impurity gyrokinetic transport has been studied with the CGYRO gyrokinetic code. Nonlinear turbulent fluxes for zero tungsten gradient at finite Mach number show that, while the deuterium ions and electrons are negligibly affected by the centrifugal force, the tungsten particle flux is strongly affected by the centrifugal dynamics. Including only weak rotation terms (Coriolis drift, ExB shear, toroidal rotation shear drive), which is typical in gyrokinetics, gives a large error. With full rotation, the tungsten particle flux is radially inward, with its magnitude increasing strongly with increasing rotation shear. This indicates a strong inward pinch, which can lead to detrimental core accumulation in a reactor. Furthermore, at large collision rate, the neoclassical tungsten particle fluxes, computed with the NEO neoclassical transport code, become competitive with the turbulent fluxes, enhancing the pinch effect.

Brendan Lyons visited the Princeton Plasma Physics Laboratory from January 22 to 26 to collaborate with Dr. Stephen Jardin and Dr. Nathaniel Ferraro on two projects for the new SciDAC Center for Tokamak Transients Simulations (CTTS). They began testing of a new impurity model within the M3D-C1 extended-magnetohdyrodynamics (MHD) code, with the eventual goal of performing simulations of pellet-mitigated disruptions. In addition, they reviewed progress and developed plans for a new drift-kinetic equation solver intended for coupling to linear MHD codes to provide kinetic corrections in stability calculations (e.g., for the modeling of resistive-wall modes).