Theory Weekly Highlights for July 2019

July 26, 2019

Gary Staebler and Federico Halpern attended the 46th EPS Conference on Plasma Physics in Milan, Italy. Staebler presented a poster entitled “A new system of gyro-fluid equations with Onsager symmetry”, while Halpern presented a poster on “Anti-symmetric plasma fluid models with exact discrete conservation”. Both authors emphasized the need to satisfy underlying physical symmetries in order to improve the accuracy and practicality of plasma transport and MHD solvers.

July 19, 2019

An analytical formula for the fuel source density nF() averaged over the magnetic flux surface psi following pellet injection has been derived. During pellet motion the ablated and ionized fuel will be deposited within some cross-sectional area projected onto the poloidal plane and localized near the pellet, before the material homogenizes within the magnetic flux surfaces. It is assumed that this deposition area is on the order of the cross-sectional area of the ionized ablation column sliced transverse to the magnetic field. Previous pellet deposition codes employed a point source deposition approximation, which overpredicts the surface-averaged density, particularly when the obliquity between the pellet trajectory and the flux surface normal becomes large. A key finding of this study is that as the obliquity approaches 90 degrees, grazing trajectory, the areal source model predicts a well behaved nF, and removes the unphysical divergence stemming from the point source model. To highlight the difference exhibited between areal and point source models for near grazing trajectories, we considered vertical pellet injection into a CFETR-like tokamak with a negative triangularity plasma cross section using a pellet ablation/deposition module with limited grad-B induced drift in a forth coming journal publication.

July 12, 2019

Michele Romanelli (UKAEA) and Jorge Ferreira (IPFN) visited GA for two weeks to work on the integration of the European Transport Solver (ETS) and OMFIT. The approach taken by the ETS is to couple physics components so that data is only exchanged among them via ITER IMAS data structures, while the transport simulation is orchestrated by the Kepler workflow manager. In this scheme, OMFIT enables loading the experimental data of different devices into IMAS, and provides ETS with a convenient user interface to facilitate setting up and executing the simulation within Kepler. Since all the data is cast in well-specified IMAS data structures independently of the experiment of origin, the group was able to seamlessly carry out JET and DIII-D transport simulations in ETS. The ability of OMFIT to interface with ITER IMAS databases has been greatly enabled by the OMAS numerical library. Such developments allow physicists to combine the convenience of the OMFIT environment with the large set of IMAS Python actors that have being developed by EUROFUSION and the ITER IO, as well demonstrated in the recent OMFIT-ETS integration effort.

July 05, 2019

The runaway electron (RE) distributions driven by a large toroidal electric field induced by a drop in the temperature profile due to disruption or pellets are comprehensively simulated using the 3D Fokker-Planck solver CQL3D, recently coupled to the Ampere-Faraday (AF) equations. The evolution of the toroidal current in a plasma occurs on a resistive time scale. The toroidal electric field rapidly increases due to an abrupt temperature drop. This is an example of Lenz’s law. For example, in simulations with KPRAD of neon pellet injection into a DIII-D discharge, the electron temperature Te drops from 2 keV to 10 eV in 0.1 msec and Zeff increases from 1 to 4, giving that the electric field increases 3500 times to 0.8 V/cm. A more comprehensive AF model was recently implemented in CQL3D, taking account of the time-development of the full-plasma-width toroidal electric field with an iterative technique for the toroidal field previously developed to maintain the implicit-in-time evolution of CQL3D. The degree of runaway current formation is reduced in AF augmented CQL3D, but the basic mechanism of “hot-tail runaways” remains a dominant contribution to the REs at early times after the Te drop in these simulations. On the other hand, a NIMROD simulation of shattered-pellet shutdown of DIII-D plasma gives a slower thermal quench; when the plasma profiles and electric field are coupled one-way to CQL3D, the “hot-tail” REs are much less, and growth of RE is dominated by the knock-on process. A manuscript summarizing this work has just been submitted to Nuclear Fusion.

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