Reduced Modeling of Tokamak Scrape-Off-Layer Plasmas for Core-Edge Coupled Simulations

Abstract

The dynamic interplay between the core and the edge plasma has important consequences in the confinement and heating of fusion plasma. However, SOL models can be time consuming and require extensive human intervention to produce results. Therefore, a fast model that retains the important physics elements would be a valuable tool in support of experimental planning in the control room. For this purpose, even a reduced model that can be mapped to a 2D grid can make it possible to address core-edge physics problems, such as edge neutral sourcing, propagation and dissipation of RF waves, fast ion confinement, etc.. Adding this piece will enable better studies of plasma transport and confinement in a broad range of topics, including RF heating and current drive, fast ion confinement and losses, pedestal formation, and PFC heat loads. In this dissertation, a simple analytical theory for SOL plasma transport is developed and tested against experimental observations and high fidelity interpretive simulations. A fast and flexible SOL grid generator has been developed that can robustly accommodate transient phases, such as ramp-up/down or sawteeth oscillations. As a demonstration of the core-edge coupling, the reduced SOL model is mapped onto 2D using the grid, and then passed to the kinetic neutral transport code DEGAS2. The ionization sources and neutral densities produced are then compared with high fidelity simulations. Fast ion orbit tracing is also extended into the SOL region with modern and efficient algorithms. In particular, a novel numerical algorithm for integrating the stochastic differential equation for pitch angle scattering is presented, which conserves particle energy exactly. The new capabilities are tested and demonstrated in NSTX / NSTX-U discharges, with future possibilities discussed.