Theory Weekly Highlights for August 2019

August 30, 2019

Alfven eigenmode (AE) driven energetic particle (EP) losses are typically very intermittent in time. Studies with a simplified time dependent quasilinear critical gradient model (QLCGM) find that intermittency in the EP losses results from turbulent noise in the AE plasma damping rate, which determines the critical gradient. The work builds on the success of the time independent critical gradient model which provides the time average radial profile of EP transport losses, and has been validated by DIII-D experiments and used for ITER predictions. The predicted intermittency increases with the EP loss rate, in agreement with DIII-D observations.

August 23, 2019

Yueqiang Liu visited CCFE during August 3-22, working on MAST-U International collaboration project. He discussed the progress of the 3D response project with CCFE colleagues and carried out new MARS-Q simulations on the plasma flow profile control using the n=2 RMP fields produced by the ELM control coils designed for MAST-U.

August 16, 2019

CGYRO has been used to study the influence of kinetic electrons on reversing the gyroBohm isotope scaling of gyrokinetic turbulent energy flux ( In the ion-dominated regime, the turbulent ion energy flux increases as the ion mass increases, in agreement with simple gyroBohm scaling arguments. However, in Trapped Electron Mode (TEM) dominated regimes driven by the density gradient, where the electron transport is comparable to the ion transport, a strong reversal from the gyroBohm scaling is observed, with the ion energy flux decreasing as the ion mass increases. Mixing length estimates are not able to capture this effect. The origin of the reversal was found to be due to the nonadiabatic correction, beyond lowest-order bounce averaging, of the electron parallel motion, and scales as the square root of the electron-to-ion mass ratio. The nonadiabatic electron response acts to further destabilize the TEM turbulence for light ions, thereby giving rise to the gyroBohm reversal for hydrogen compared to deuterium.

August 09, 2019

During the summer, a SULI student has been exploring a new, more systematic approach for the flux-surface parameterization used throughout GACODE. The new method more accurately represents the flux-surface geometry in the plasma edge, where the shaping is strongly non-circular and close to singular. In particular, the addition of a delta-sequence has enabled good fitting very close to the X-point. Work is underway to incorporate this new parameterization into GACODE as a generalization to the existing Miller-type model. Importantly, the new method is consistent with, and reduces to, the Miller model when the added shape parameters are set to zero. Thus, there is no difficulty recovering results obtained with the conventional parameterization in terms of elongation, triangularity, and squareness.

August 02, 2019

The OMFIT GATO module (see Highlight from June 29 2018 at was replaced with a new version that is more robust and includes a number of additional features. In addition to greatly improved robustness, the new module resurrected the ability to run over a sequence of different equilibria. Several additional features were added, including options to plot various quantities, including from scans, a GUI interface for packing the mesh, and options to add further namelist variables not present in the public module. The ultimate goal is to develop a ‘Stability Manager’ where stability can be calculated from the same input, using any stability code of choice, and with the output in a uniform format that can be accessed by other external tools (diagnostic, graphics, synthetic diagnostics, etc.) irrespective of the origin the data. In this regard, the TOQ equilibrium code module was also upgraded to permit more flexible stand-alone work flows and interface with the Stability Manager.

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