Numerical study of neoclassical plasma pedestal in a tokamak geometry

Abstract

The fundamental properties of steep neoclassical plasma pedestals in a quiescent tokamak plasma have been investigated with a new guiding center particle code XGC: an X-point included Guiding Center code. It is shown that the width of the steepest neoclassical pedestals is similar to an experimentally observed edge pedestal width, and that a steep pedestal must be accompanied by a self-consistent negative radial electric field well. It is also shown that a steep neoclassical pedestal can form naturally at a quiescent diverted edge as the particle source from the neutral penetration (and heat flux from the core plasma) is balanced by the sharply increasing convective ion loss toward the separatrix. The steep neoclassical pedestal and the strong radial electric field well are suppressed by an anomalous diffusion coefficient of a strength appropriate to an L-mode state; nonetheless, the E×B shearing rate increases rapidly with pedestal temperature. Additionally, the present study shows that a steep pedestal at the diverted edge acts as a cocurrent parallel momentum source.

Neoclassical physics in full distribution function gyrokinetics

“…III and IV complement the existing studies of neoclassical physics in the plasma edge [64][65][66] whilst providing a successful cross-comparison between this Monte-Carlo scheme and the previously detailed Eulerian-based operators implemented in GYSELA.…”

Compressed ion temperature gradient turbulence in diverted tokamak edge

“…Monte Carlo neutral particles can also be simulated together in the XGC family codes. 10 In the present work, however, the neutral particle routine is not used.…”

“…3,4,10,11 In the present study, we use full-f marker particles for neoclassical/turbulent ions and neoclassical electrons. Adiabatic electrons are used for the ITG turbulence response.…”

Numerical calculation of neoclassical distribution functions and current profiles in low collisionality, axisymmetric plasmas

“…While sophisticated, three-dimensional MHD time evolution codes exist (e.g., NIMROD 5 and M3D-C1 6 ), it is more difficult to find kinetic neoclassical codes that can be used in such a study. The XGC0 7 and DKES 8 codes do solve for the bootstrap current in 3D toroidal geometries. XGC0, however, is a PIC code that requires enormous computing power.…”