A benchmark stability calculation between the ELITE and GATO codes, carried out for intermediate n modes in an NSTX equilibrium found remarkably good overall agreement in the mode growth rates and mode structures for n > 8. This is a particularly challenging case for both codes due to the small aspect ratio resulting in strong edge gradients and high edge q. Extremely high radial and poloidal resolution is required to resolve the rational surfaces and the poloidal mode oscillations. Nevertheless, the growth rates for the most unstable edge mode were in agreement to within 10% for 8 < n < 12 and a detailed comparison of the mode structure for the n = 10 case showed remarkable agreement (see mode structure overlay). For this case, internal modes are present which for many n dominate the edge mode. For the ELITE code, the radius of convergence to the most unstable mode is small for these cases and an accurate initial guess is required. A second case with increased edge current density is underway in an attempt to confirm the encouraging but preliminary agreement obtained in this first case, and to simultaneously minimize the convergence difficulties.

In an attempt to modernize the approach to collaborative software development in the fusion group, the code base for GYRO, NEO, TGLF and TGYRO has been moved out of CVS and into GIT. GIT is a modern distributed revision control generally considered to be superior to traditional centralized systems such as CVS and SVN. The source code is no longer hosted at GA but off-site at GitHub (https://github.com). GitHub is secure and cost-effective, and offers numerous management tools, including automated generation of tar and zipballs, an issue-tracker, and web-based navigation of the entire source tree including diffs and tags. This significant rethinking of the development workflow is a prototype for the various other existing fusion codes in production use. GitHub repositories for ELITE and GATO have also been created and work is underway to move the respective code bases there. Codes that are moved to GitHub will be required to develop and include a regression suite to insure code correctness after modifications. These codes will also be integrated into a unified build system for GA fusion codes in order to address the issue of portability and may offer a prototype for large scale fusion-wide simulation projects such as FSP.

In collaboration with Varun Tangri and Paul Terry (U. Wisc.), the tokamak large aspect ratio circular “s-α” equilibrium model option in GYRO has been generalized to a “Toroidal Bessel Function Model” (TBFM) suitable for gyrokinetic simulation of micro-turbulence in RFPs. TBFM spans the tokamak ordering, where Bp/Bt ~ r/(Rq) is small to the RFP where it is large, with the toroidal field Bt small and possibly reversed in sign between the core and edge. GYRO linear stability checks on ITG modes show the TBFM agrees well with the more exact Miller finite aspect ratio shaped geometry option. In principle, RFP experimental profile data, normally known on the poloidal flux grid, could be mapped directly onto the midplane minor radius grid in Miller geometry used in GYRO. Note that tokamak transport profile data (and transport codes) is stored on the toroidal flux grid, which is not convenient for an RFP. This new GYRO facility is intended for simulation of RFP turbulence when the RFP dynamo tearing modes are suppressed by external current drive. A paper has been recently submitted to Physics of Plasmas.

Dr. C.S. Chang (NYU and PPPL) visited GA from January 5 through 7 for a highly productive exchange focused on improving our understanding of the density pumpout, non-axisymmetric field penetration, and ELM suppression from the internal I-coils in DIII-D Resonant Magnetic Field (RMP) experiments. Dr. Chang gave a well-attended seminar on calculations from his XGC0 code, which predicts some observed features in the RMP experiments. A full day was also spent in wide ranging discussions of the physics of the pumpout, field penetration, and ELM suppression, culminating in a prioritized list of further code work and experiments intended to elucidate the physical mechanisms responsible.