Theory Weekly Highlights for September 2013

September 27, 2013

The new RMPtran code calculates the quasilinear transport from external Resonant Magnetic Perturbations (RMPs). RMPtran takes the linear Braginskii two fluid response from the M3DC1 code to calculate the instantaneous quasilinear particle and toroidal angular momentum (TAM) transport. These flows as well the energy transport flows are proportional to the square of the screened external magnetic perturbations [(delta-B)2]. The focus of RMPtran will be on understanding the DIII-D RMP density pump out effect: when the RMP coils are switched on, why does most of the TAM leave the plasma by particle convection rather than by slowing the toroidal rotation? First results indicate a very large RMP induced ExB particle flux at the edge which may explain the density pump-out. TAM transport is broken into the Reynolds stress on the plasma (proportional to delta-Vx delta-Vphi) and Maxwell stress on the plasma and field (proportional to delta-Bx delta-Bphi) which results from the RMP applied JxB torque density. Most of the JxB torque is from the displacement radial current (time rate of change in Er) leaving little Maxwell stress on the plasma. Over the inner core (r/a < 0.8) much of this Maxwell stress on the plasma appears to be cancelled by the Reynolds stress. To understand the pump-out effect, we must compare and contrast the core and edge instantaneous flows just after RMP turn-on to those well after turn-on.

Professor Howard Wilson from the University of York is visiting to work with Phil Snyder and Amelia Dowsett on enhancements of the ELITE stability code.

September 20, 2013

The neoclassical kinetic code NEO has been used to analyze the accuracy and limitations of the Sauter model for the bootstrap current for multi-ion plasmas. The Sauter model is strictly valid only for pure plasmas. While the NEO results show that the Sauter model is able to accurately capture the electron-impurity interaction through the use of Zeff in ν*e, it does not accurately model the ion-impurity collisional interaction. However, the latter effect, which enters as a term proportional to the ion temperature gradient, is often sub-dominant compared with the pressure gradient and electron temperature gradient contributions to the bootstrap current terms that represent the electron-impurity interaction. Analysis of the predictions of the Sauter model for energetic impurities, which have not previously been studied, shows that it largely overestimates the collisional effect of the energetic species on the ion and electron dynamics. Direct calculations with NEO allow this effect to be treated accurately, enabling modeling of reactors like ITER with a significant concentration of alpha particles.

September 13, 2013

Orso Meneghini, Phil Snyder, Eric Bass, Chris Holland and Olivier Izacard attended the BOUT++ Workshop at LLNL. Dr. Meneghini presented a talk on the OMFIT integrated modeling suite, highlighting its capabilities to do equilibrium reconstruction and edge stability calculations, and potentially to couple directly to BOUT++. Dr. Snyder gave a talk on the physics of Edge Localized Modes and the EPED pedestal model, and future opportunities for edge physics studies with BOUT++. Dr. Holland presented a talk on synthetic diagnostics and experimental validation of simulation results. Future plans include closer coupling of BOUT++ to existing GA theory codes such as OMFIT and GACODE, code validation, and physics studies of edge turbulence, ELMs and tearing modes.

Amelia Dowsett, a graduate student at the University of York in the UK, is visiting GA theory for 6 weeks, working on further enhancements to the ELITE edge stability code.

September 06, 2013

The OMFIT integrated modeling framework has been extended to enable analysis of DIII-D kinetic profiles in presence of ELMs. This was achieved by binning and organizing the experimental data as a function of the ELM cycle. A GUI guides users throughout this process. The resulting electron density and temperature profiles can be used in any of the OMFIT modules. For example, ELM-aware kinetic EFITs have been used to assess the edge stability of DIII-D discharges. The experimental pressure and current profiles at the edge of the experimentally reconstructed equilibrium is perturbed via the new VARYPED module in OMFIT. The stability of the resulting set of equilibria is then tested with the ELITE code (also a new OMFIT module). Results compare well with previous findings and show that the steep experimental profiles occurring before an ELM crash are indeed at the boundary of the stability limit.

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