Theory Weekly Highlights for April 2018

April 27, 2018

A runaway electron post processing tool has been developed for the NIMROD 3D extended MHD code. This post processing tool traces relativistic drift kinetic trajectories of electrons calculated in NIMROD output fields. This diagnostic tool has been applied to the output fields from disruption mitigation simulations of shattered pellet injection in DIII-D H-mode plasmas. Further development of this tool will allow estimates of the runaway current and seed amplification. This tracer tool will also serve as a testbed for continued development of a more complete runaway electron coupling to NIMROD via the hybrid kinetic-MHD module initially developed to model energetic ions.

Phil Snyder presented plans and progress for the close coupling of theory and integrated simulation with the DIII-D program to the DIII-D Program Advisory Committee.

April 20, 2018

A predict-first workflow developed in conjunction with the DIII-D triangularity-scan experiment from last year was generalized to create a new OMFIT module, SEGWAY (Self-consistent Equilibrium Generation Workflow and Analysis). This workflow couples together EFIT, NEO, and the EPED model to generate equilibria with self-consistent pedestal structures as the shape and scalar plasma quantities are varied. The equilibria can then be analyzed as though they are reconstructions from an experiment (e.g., using linear M3D-C1 to compute the plasma response to 3D magnetic perturbations). The SEGWAY module will allow users to begin with any existing kinetic equilibrium reconstruction and perform numerical scans of shape, , q95, etc. to assist in the planning of upcoming experiments.

April 13, 2018

The newest version of OMFIT (v0.25.0) was publicly released. After only one month since v0.24.4, this release features over 1100 source code commits by 20 unique international contributors. In addition to many bug fixes, some of the most notable improvements include the ability to handle SSH interactive password requests for servers that do not allows private-public key authentication, and the added support for Docker that allows now OMFIT to run under Windows for the first time. More than half of the 92 physics modules that are available in OMFIT have been updated, some considerably (like OMFITprofiles, TRANSP, GPEC, and GYRO). Also, significant effort was invested in updating the online documentation, which for many of the most used modules [1] now features a detailed description that includes links to publications, external resources, and keeps a tally of the modules contributors and users.

April 06, 2018

A seamless fully relativistic formula was developed that corrects and improves the usual Coulomb log formulas for relativistic electrons by combining Moeller scattering for close collisions and simplified relativistic Born for distant collisions. In calculating the contribution to the stopping force and energy deposition of relativistic electrons due to free electrons in the target medium, the long-range nature of the coulomb force gives rise to the well-known logarithmic divergence in the scattering integrals. This is conventionally removed by introducing a small-angle cutoff roughly equal to the ratio of the electron de Broglie wavelength to the Debye screening length. Dirac’s equation gives an exact solution for the differential cross section for electron – electron (e-e) scattering in a Coulomb potential, namely Moeller scattering and includes the quantum mechanical exchange effect necessary for capturing close collisions. However, the heuristic cutoff must still be used to capture distant collisions since the stopping force depends on both. A Coulomb potential modified by a Debye screening term in the form of a Yukawa potential was used to calculate the differential cross section for e-e scattering using the Dirac form of the Born approximation to remove the small angle divergence.

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