Bhavin Patel from University of York, UK completed a 3 week visit to the GA theory group. He built a quasilinear transport code that uses GYRO for the linear eigenmodes and the TGLF SAT1 model for the saturated potential. This model extends quasilinear theory to plasma conditions where the TGLF model is not accurate, such as high pressure spherical tokamaks. For his PhD thesis, Bhavin will use this model to predict plasma profiles using TGYRO for the UK's STEP spherical torus fusion reactor design study.

It has been found that Shafranov shift stabilization of turbulence creates a bifurcation in transport, leading to distinct confinement states in the high poloidal beta scenario on DIII-D: An H-mode confinement state with a high edge pedestal, versus an enhanced confinement state with a low pedestal and an internal transport barrier (ITB). The bifurcation is observed experimentally in the ion energy transport with respect to mid-radius pressure gradient. Simultaneously, the electron transport exhibits a similar but less dramatic behavior with respect to pressure gradient. The Shafranov shift is found to increase at the same time as the transition to enhanced confinement, and quasilinear gyro-Landau fluid modeling shows a reduction of predicted energy flux consistent on-set of the ITB. Transient perturbations such as ELMs are likely a trigger for the transition between states by lowering the edge pressure and increasing mid-radius pressure gradient. A paper describing this entitled “Shafranov shift bifurcation of turbulent transport in the high poloidal beta scenario on DIII-D” has been accepted for publication in Nuclear Fusion Letters.

The NIMROD impurity radiation model has been upgraded to allow for the inclusion of multiple impurity species in a single simulation, required for modeling of dispersive shell pellet (DSP) injection with distinct shell and payload materials. The new capability has been successfully tested for the case of a carbon shell and beryllium payload material, which have different radiation characteristics particularly at low temperature. Although Be may be of interest as a payload material for ITER, in order to reproduce the DIII-D DSP experiments, in which a carbon shell with a boron payload is used, new atomic data for boron will need to be added to the code.