### Theory Weekly Highlights for September 2017

##### September 29, 2017

An OMFIT integrated-modeling workflow has been developed to generate predictive kinetic equilibria for DIII-D experiments. This workflow uses the EFIT code to modify aspects of existing, experimental equilibrium reconstructions (e.g., shape, current, plasma β) subject to constraints that ensure self-consistent pedestal structure. In particular, the EPED1-NN model (neural nets trained on results from the EPED1 model) is used to constrain the pedestal pressure, and the bootstrap current calculated by the NEO drift-kinetic equation solver is used to constrain the pedestal current density. This workflow was used to perform predict-first simulations for a DIII-D experiment exploring the effect of triangularity on the plasma response to 3D magnetic perturbations. Results from this predict-first analysis were used to guide the planning and execution of the experiment, facilitating the development of optimal target plasmas. Post-experiment validation revealed good agreement between the shape and pedestal structure of the predicted and reconstructed equilibria. This work and additional validation using M3D-C1 plasma response analysis will be the subject in an invited presentation by Brendan Lyons at the upcoming APS-DPP meeting, and entitled “Predict-first experimental analysis using automated and integrated MHD modeling.”

##### September 22, 2017

Initial TRIP3D analyses investigating a possible 3D misalignment of the toroidal field magnetic axis suggest that the 3D effects could enhance the excursions of the field lines comprising the divertor footprint. The error fields were assumed to consist of a simple tilt and shift in the toroidal magnetic field with respect to the central symmetry axis. Magnetic field-line tracing of the vacuum error field to assess its effects on typical SAS equilibria indicates the corresponding divertor footprint is wider than that typically produced by the bus-work of the toroidal coils. Future analyses will incorporate magnetic plasma response effects into the analysis.

##### September 15, 2017

The MARS-F MHD code has been applied to compute plasma response for various ITER scenarios, ranging from the 5 MA/1.8 T hydrogen H-mode plasma from the pre-fusion power operation (PFPO) phase to the 15 MA/5.3 T, Q=10 plasma from the fusion power operation phase. Generally, similar plasma response is computed for different ITER scenarios with similar safety factor q (i.e. by scaling both plasma current and toroidal field) and sufficiently fast edge/pedestal flow. Response however can be different at slow edge/pedestal flow even with the same q. The response is certainly different with different q. For the 5 MA/1.8 T hydrogen plasma during the PFPO-1 phase, with limited power supply capabilities, the ELM control with two off-middle rows may be a better choice than the 3-row configuration, according to the edge-peeling response criterion. Part of the work has been presented at the 16th International H-mode Workshop in Saint-Petersburg, Russia (Sept 13-15, 2017), by ITER collaborator A. Loarte.

##### September 08, 2017

The 3D initial-value extended MHD code NIMROD has recently been updated to include a Particle-in-Cell (PiC) based Shattered Pellet Injection (SPI) model that builds upon the Massive Gas Injection (MGI) work done by V. Izzo. SPI will be the primary disruption mitigation system in ITER and critical to its mission. SPI propels a low-Z cryo-pellet along a flight tube into a sharp bend that shatters the pellet into a plume of shards that is injected at high velocity to quench the plasma. The SPI-PiC model uses a small number of macro-particles to represent the plume of shards. The shards are modeled as uniform spheres following a straight line trajectory. At each time step, an ablation formula derived by P.B. Parks is then used to ablate the shards and reduce their sizes accordingly. NIMROD simulations of SPI disruption mitigation will provide a qualitative and quantitative assessment of the thermal load mitigation efficiency, radiation distribution and resulting heat loads. An animation of a NIMROD SPI simulation of ITER with a mixed deuterium-neon pellet showing a complete thermal quench of a 15MA 350MJ plasma can be found at https://youtu.be/y9LF5zMWirI . This animation shows the strong interplay of MHD activity in the shard ablation and quench dynamics. Initially, the edge of the plasma is shed via low toroidal and poloidal number edge modes causing a steepening edge gradient that ultimately drives the core collapse and thermal quench.

##### September 01, 2017

Dr. Zhao Deng returns to the China Engineering Physics Laboratory after a two-year post-doctoral appointment in the GA Theory and Computational Group. He spent 18 months of his Peking University graduate student years in the Theory Group working with Dr. Ron Waltz on nonlinear 6D cyclokinetics resulting in three publications. In the past two years, Dr. Zhao continued to work with Dr. Waltz on applying the Gaussian Radial Basis Function (GRBF) methods to 6D Vlasov-Poisson kinetics and applying GRBF to an extended fluid moments approach. Dr. Zhao also worked with Dr. Lang Lao and the University of Science and Technology of China (USTC) China Fusion Engineering Test Reactor (CFETR) Physics Team on developing integrated modeling workflows and applying them to simulate CFETR operation scenarios.

**Disclaimer**

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