Theory Weekly Highlights for January 2015

January 30, 2015

The extended-MHD code M3D-C1 has been modified to read ideal-MHD equilibria from VMEC. This capability allows nonlinear calculations to be initialized in M3D-C1 with non-axisymmetric ideal-MHD equilibria, in order to model the resistive evolution of these plasmas. A test case using a VMEC equilibrium for a n=3 perturbed DIII-D discharge has been carried out. The nonlinear M3D-C1 calculation shows an initial Alfven-timescale ringing that results from the interpolated VMEC equilibrium quickly relaxing to an ideal-MHD equilibrium on the M3D-C1 grid, and then islands opening on > 100 microsecond timescales. In addition to providing a method of calculating islands from VMEC equilibria complementary to PIES and SIESTA, this will allow M3D-C1 to calculate nonlinear 3D response starting from a fully screened state. Previously, nonlinear M3D-C1 calculations were initialized with the vacuum (penetrated) fields, and it is expected that the final state in these calculations can depend on the initial conditions.

January 23, 2015

A new module for the OMFIT framework has been developed to enable integrated simulations with the NIMROD code. The module allows the user to set up, remotely execute, and analyze NIMROD simulations using a convenient graphical user interface. Remote execution on the Edison and Hopper machines at NERSC has been demonstrated for both interactive and batch runs. NIMROD regression cases can be loaded into the OMFIT module, to be used as templates to setup new simulations, and the functionality of the module has been tested for all of these regression cases. Initial application of the new module was to the execution of massive gas injection simulations on DIII-D (see Theory Weekly Highlights for January 2015 for January 09). The ability to run integrated simulations was also demonstrated by coupling the NIMROD and EFIT modules in OMFIT in order to scan linear stability over a series of time steps in one shot. It is anticipated that the NIMROD module can be used in integrated workflows designed to assess proximity to stability bounds by generating ensembles of nearby equilibria, as well as in benchmarking exercises with other codes. This effort is in support of both the AToM integrated modeling SciDAC project, and separate UCSD and GA research initiatives.

January 16, 2015

The 3D NEO code has been used to explore the effects of toroidal non-axisymmetries on neoclassical transport at low collisionality, nu. A novel hyper-viscosity model in pitch angle was found to improve convergence since the distribution function is nearly discontinuous across the trapped-passing particle boundary. For regularization of the divergent 1/nu transport at low collisionality, various higher-order effects have been explored. With addition of the ExB drift velocity (which is of the order of the gyro-radius squared) to the drift-kinetic equation, the ion 1/nu regime disappears as E_r increases, transitioning to a nu-scaled regime at very low electron collisionality. The results in the highly collisional regimes are, however, unaffected. Regularization of the transport to the super-banana plateau and super-banana regimes requires addition of a different higher-order term, namely the toroidal drift. The theory for the transport in this region is somewhat ambiguous, however, since the ExB drift velocity and toroidal drift velocity are higher-order terms in the drift-kinetic theory. A reformulation of the drift-kinetic theory to more rigorously treat the 1/nu regime is an open problem.

January 09, 2015

NIMROD modeling results were used to evaluate possible configurations for radiation diagnostic upgrades on DIII-D. Presently, two fast-radiated power measurements are used to diagnose radiation asymmetry during MGI on DIII-D, which NIMROD predicts to be inadequate to accurately diagnose the toroidal peaking factor (TPF). Possible diagnostic configurations with the number of detectors ranging from 3-6 were analyzed for a set of six radiated energy curves from NIMROD simulations during the pre-TQ and TQ for each of three cases. For every diagnostic configuration considered, the synthetic diagnostic TPF for all six curves was compared to the real TPF from the simulation. Slit-detectors measuring radiation from a narrow region in toroidal angle, as well as uncollimated detectors viewing a large volume of the plasma were also compared. In every case the individual synthetic measurements were fitted to a periodic test function (n=0+n=1 for 3 or 4 detectors, or n=0+n=1+n=2 for 5 or 6 detectors), and the best-fit curve was used to determine the “measured TPF”. In general, the biggest improvement in measured TPF occurred when increasing from 2 (present configuration) to 3 detectors, with modest improvements seen when further increasing the number of detectors. In the simulation with two simultaneous gas jets, ability to resolve the n=2 component (at least 5 detectors) was especially beneficial. Inclusion of some uncollimated detectors provided equally good TPF results, and may be beneficial for other metrics, such as total radiated energy.

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