Theory Weekly Highlights for September 2016

September 30, 2016

A systematic scan of the rotation zero-crossing in single-fluid plasma response calculations using the M3D-C1 extended magnetohydrodynamics code has led to the discovery of new, physics effects that may be important for understanding ELM-suppression by 3D magnetic perturbations. Depending on the rotation shear at the zero-crossing, non-resonant current can be driven there. The quasilinear electromagnetic torque density induced by this non-resonant plasma response acts to drive the zero-crossing point toward dynamically stable points located near rational surfaces. This allows for increased resonant response. We hypothesize that these effects play an important role in bifurcations into ELM suppression. Finally, analysis of these plasma responses revealed that the changes to the resonant field should be measurable by magnetic sensors on the high-field side of the tokamak. The low-field side magnetic signal and the divertor footprint structure, however, are largely insensitive to these changes in the plasma response.

September 23, 2016

The neural-network models reported previously (see Highlight from May 09 2014) have now been directly integrated within the TGYRO transport solver. The models are designed to reproduce the TGLF prediction of the turbulent transport fluxes in the core of the plasma (TGLF-NN model) and the EPED1 predictions of the pedestal structure (EPED1-NN model). The self-consistent core-pedestal coupled solution that was previously obtained by running the original models within OMFIT can now be achieved with a computational speedup of thousands. An additional advantage of using neural networks is that their output is a smooth function of the inputs, which significantly improves the convergence properties of TGYRO. The predicted TGYRO solution with evolving electron and ion temperature, density, rotation profiles and dynamic pedestal has been validated with a database of 200 DIII-D experimental discharges. Statistically, the global normalized pressure predicted by the coupled TGLF-NN/EPED1 NN core-pedestal model is found to be on average 90% of the experimental value for this set of DIII-D discharges.

September 16, 2016

Lang Lao attended and chaired the 8th ITER Integrated Modeling Expert Group (IMEG) Meeting at ITER Headquarters, Cadarache, France from September 12-14, 2016. The IMEG consists of two representatives from each of the seven ITER partners. The main goal of this years meeting was to review progress since the last IMEG meeting and advise ITER on its Integrated Modeling (IM) Program. The IM program aims to develop an Integrated Modeling Analysis Suite (IMAS) and an infrastructure with a workflow management system and a data model to support ITER plasma operations and research. Discussions centered on progress since the last IMEG meeting, including each domestic members’ progress and plans on local installation, testing, and adaptation of IMAS.

September 09, 2016

Brendan Lyons attended the Joint Varenna-Lausanne International Workshop on the Theory of Fusion Plasmas in Varenna, Italy. He presented an invited talk entitled “Extended magnetohydrodynamic insights into edge-localized mode suppression/mitigation by three-dimensional magnetic perturbations,“ with particular focus on recent work documenting the role of the single-fluid rotation zero-crossing on the plasma response to 3D magnetic perturbations.

Phil Snyder is attending the EU TTF meeting in Leysin, Switzerland.

September 02, 2016

An analysis of the effect of error fields on the magnetic topology near the proposed Small Angle Slot (SAS) divertor in DIII-D has determined how uncertainties in the magnetic probe signals, used by EFIT in 2D equilibrium reconstruction, translate into uncertainties in the determination of the strike point and angle on the divertor. Preliminary statistical analysis on the data collected from the study found a non-linear dependence of the strike point and angle uncertainties on the initial error field parameter and has yielded quantified estimates of the uncertainties. Further analyses incorporating proposed updates of the magnetic diagnostic setup near the SAS divertor have also been initiated, along with preparations for a more complete 3D study.

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