Theory Weekly Highlights for March 2015

March 27, 2015

In collaboration with scientists from ASIPP-Hefei and other institutes, detailed transport analysis of several high performance discharges with internal transport barriers have been performed for the DIII-D high poloidal-beta EAST-demonstration experiments using the OMFIT/TGYRO transport package with the TGLF turbulent and the NEO neoclassical transport models. The results show that in these high poloidal-beta discharges, ion energy transport is very much neoclassical and the TGLF+NEO predicted ion temperature profiles agree closely with the experimental measured profiles using a static experimental electron-ion energy exchange. For these same discharges, TGLF predicts a very small electron turbulent energy flux in the outer plasma region that leads to very high electron temperature profiles significantly exceeding the experimental measured values. The electron transport results are largely insensitive to reductions in the rotational shear or Shafranov shift stabilization. In addition the ETG mode is robustly stable, even at very high wave number. Ideal-ballooning analysis of these discharges indicate that in the outer region the plasmas are in the second ballooning stable zone, and this may play a role in the low predicted electron turbulent energy flux. Detailed analyses are being performed to investigate this discrepancy in the electron energy flux.

March 20, 2015

In a previous highlight (see highlight from November 7 2014 at Theory Weekly Highlights for November 2014) it was reported that with a modest enhancement of the resistivity in the region around q = 2 above Spitzer, fast growing resistive kink modes strongly peaked at the edge were found that can explain the m/n = 2/1 disruption observed in the diverted L-mode DIII-Discharge #150513 when q95 reached below 2.0. The Spitzer resistivity profile was obtained from the measured Te profile. New calculations found that the enhanced resistivity corresponds to a relatively modest modification in the Te profile shape in the region between q = 2 and the edge, approximately of the same order as the uncertainty in the measured profile. This implies that any modification of the Spitzer model needed to explain the observed instability as a resistive kink only needs to be small. A number of plausible reasons for a breakdown in the model, sufficient to provide the required resistivity enhancement can be identified; for example, finite orbit effects, or possibly stochastization of a thin edge layer. Additionally, small uncertainties from the equilibrium reconstruction in the position of q = 2 near where the resistivity profile becomes very steep could easily account for the needed enhancement.

Nate Ferraro, Emily Belli, Brendan Lyons and Alan Turnbull from the GA Theory Group, along with Todd Evans and Dmitry Orlov from DIII-D attended the 2015 Sherwood Theory Meeting at the Courant Institute in New York.

March 13, 2015

Linear two-fluid M3D-C1 simulations of recent n=2 Resonant Magnetic Perturbation (RMP) ELM suppression experiments show that the tearing drive at the q=4 surface is strongly enhanced during the period of ELM suppression. This enhancement is apparently due to the reduction in the magnitude of the perpendicular electron rotation at that surface, which in turn is due to the change in the radial electric field in the pedestal observed during ELM suppression. Motivated by recent observations of a strong n=1 response to applied n=2 (and n=3) fields, both the n=1 and n=2 responses were calculated and both n=1 and n=2 tearing drive were found to be enhanced during ELM suppression. In the cases considered, the q=4 surface is at the top of the pedestal (normalized poloidal flux ~ 94%), and therefore tearing at this surface may provide a mechanism for limiting the growth of the pedestal.

March 06, 2015

As part of the Advanced Tokamak Modeling (AToM) project, the EPED solver was implemented as an IPS component, executed via OMFIT. The mission of AToM is to enhance and extend the predictive modeling capabilities that currently exist within the US magnetic fusion program. An initial application is to evaluate the importance of self-consistently solving the coupled core transport, pedestal structure, and equilibrium problems. Peeling-ballooning (PB) and kinetic ballooning modes (KBM) impose constraints on the pedestal structure, which in turn strongly affects core temperature and pressure in H-mode plasmas. Furthermore, PB stability is in turn impacted by the Shafranov shift caused by the core pressure, leading to a feedback cycle between core and pedestal. The EPED calculations are typically completed in a few minutes running at NERSC with over 700 CPUs. Work is under way to validate the aforementioned workflow against a set of DIII-D experimental discharges.

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