Theory Weekly Highlights for July 2014

July 25, 2014

A series of global, linear GYRO simulations of alpha particle-driven Alfvén eigenmodes (AEs) in a hybrid ITER scenario has revealed at least three distinct AE branches: two BAE or EPM-like modes competing below n=20 and a wide global TAE that dominates at higher n. GYRO identifies the n=22 global TAE as the most unstable mode, differing from GEM predictions that place the most unstable mode at n=16. With refinements in the equilibrium specification, the stability threshold is now identified at about 50% above the classically predicted alpha-particle density (neglecting the heating beam). By contrast, a steady-state scenario with negative central shear is dominated by a single global TAE across n values, also peaking near n=22 and unstable even at the classical alpha density. Modifications have been made to GYRO to consider most physics of an anisotropic heating beam alongside the isotropic alpha particles. Investigations of the steady-state ITER case with both effects are underway as part of a code comparison and verification effort between GYRO, GEM, GTC, M3D-K, TAEFL, and NOVA-K.

July 18, 2014

M3D-C1 was used to calculate the plasma response at the location of the center-post magnetic probes for several DIII-D discharges with generally good agreement between the measured and calculated response. These calculations have been made possible by the new resistive wall capability in M3D-C1 (see highlight from May 30 2014). The calculated response is found to be particularly sensitive to the resistivity in the open field-line region, which is treated as a low-temperature, low-density plasma in M3D-C1. When this region is treated as highly resistive, M3D-C1 results are in relatively good agreement with MARS-F, which treats that region as a vacuum. When the open field-line region is treated as more conductive, currents in that region screen the plasma response from the probes, resulting in a reduction in the predicted response. A low-temperature, high-resistivity model of the open field-line region is found to yield best agreement with the experimental data. Investigations into the importance of two-fluid effects and rotation on the predicted response are ongoing.

July 11, 2014

In work presented in an invited presentation at the 41st EPS conference on plasmas physics by Gary Staebler, the mean field toroidal and parallel momentum equations evolving the parallel and ExB toroidal velocities have been found to exhibit both one-step H-mode transitions and limit cycle oscillations (LCO). Using data from a DIII-D discharge to evaluate the momentum diffusivity and collisional poloidal damping rate, the frequency of the LCO, the amplitude of the ExB velocity, and the density fluctuation amplitude were well matched to the data using a simple model of the turbulent transport based on gyrokinetic properties. The fast timescale of the L/H transition poloidal velocity trigger is also well matched. This work established that the mean field momentum transport equations derived using standard ordering methods are able to simulate both LCO and H-mode transitions.

July 03, 2014

Rui Ding, a new post-doc for the PSI-SciDAC project arrived last week. A meeting was held at GA with Brian Wirth, who is the PI of the PSI-SciDAC project, to review the validation research plan. Ding will be a member of the DIII-D Boundary Physics Center and will participate in the planning of the upcoming helium plasma DiMES experiment. In preparation, he will use the ERO PMI code to analyze selected DiMES data from old experiments.

These highlights are reports of research work in progress and are accordingly subject to change or modification