Theory Weekly Highlights for July 2016

July 29, 2016

Studies of the benchmark L-mode edge shortfall discharge 128913 using extended MHD models in BOUT++ have revealed linear instability properties unlike other L-mode cases studied with BOUT++. In particular, inclusion of the electron pressure gradient drive in the vorticity equation, the minimum essential physics for elementary drift waves, brings a flute-like drift wave to the top of the linear instability spectrum. At relevant toroidal mode numbers n=50-200, this mode requires over an order of magnitude greater resolution to track than the peeling-ballooning or resistive-ballooning modes typically treated in BOUT++. For instance, in verification case discharge 119919, a colder edge leads to a resistive-ballooning mode dominating the spectrum even as additional physics is added. In addition to the greater resolution requirements, linear simulations tend to develop a boundary-driven numerical instability that thwarts clean characterization of the mode. To solve this problem, a new eigenvalue solver is being developed. The new solver will run as a stand alone code, uncoupled to BOUT++, allowing it to be spectral in the toroidal direction and consequently much more efficient for the eigenvalue problem. This developmental effort is well under way, with basic validation tests planned soon.

July 22, 2016

Eric Held (Utah State University) and Scott Kruger (Tech-X) visited GA to continue a collaboration on simulations of the giant sawteeth observed in DIII-D discharge #96047. A key challenge for these simulations, which employ the NIMROD code, is efficiently modeling the highly energetic fast ions from the fast wave antenna. This issue is now largely resolved and a path forward was agreed on. The first step will be to reproduce, in a complete simulation of the sawtooth cycle, the Porcelli predictions for the giant sawtooth onset published in Choi et al, Phys. Plasmas, 14 112517 (2007). The initial fast ion distribution for the NIMROD simulations can be obtained from the archived ORBIT-RF modeling done in previous work.

Nan Shi from the Institute of Plasma Physics in Hefei has arrived at GA to begin a two-month working visit. Nan will work on predictive modeling for the China Fusion Engineering Test Reactor (CFETR), and this visit also provides the impetus for improvements to TGYRO in connection with dynamic evolution of impurity ions in TGYRO.

July 15, 2016

The scalings of the resistive kink growth rates with respect to the resistivity at the q = 2 surface in the Limiter and Divertor low edge q discharges (see Highlight from October 23 2015 at Theory Weekly Highlights for October 2015) were found to overlay almost completely, indicating a possible universal scaling that is insensitive to the underlying equilibrium. The only significant difference between the two cases appears to be that for the Divertor case the ‘diffusive’ regime where the growth rate scales linearly with the resistivity at q=2 is missing. The full implications and generality of this are not yet clear but it does imply at least that the thresholds for the instability are quite different.

July 08, 2016

In collaboration with PPPL scientists, equilibrium analysis of plasmas in DIII-D using the EFIT code was upgraded to include the internal magnetic field determined from spectroscopic measurements of motional-Stark-effect line-splitting (MSE-LS). MSE-LS provides measurements of the magnitude of the internal magnetic field, rather than the pitch angle as provided by MSE line-polarization (MSE-LP) used in most tokamaks to date. EFIT MSE-LS reconstruction algorithms were enhanced to include MSE-LS and verified. The capability of MSE-LS to provide significant constraints on the equilibrium analysis was evaluated. Reconstruction results with both synthetic and experimental MSE-LS data from 10 DIII-D discharges run over a range of conditions show that MSE-LS measurements can contribute to the equilibrium reconstruction of pressure and safety factor profiles. Adequate MSE-LS measurement accuracy and number of spatial locations are necessary. The seven available experimental measurements provide useful additional constraints when used with other internal measurements. Using MSE-LS as the only internal measurement yields less current profile information.

July 01, 2016

Orso Meneghini participated in the EU integrated modeling code-camp in Prague. Over the course of two weeks OMFIT was interfaced with the European integrated modeling data system, so that data can be read seamlessly, manipulated, visualized and written to and from the EU platform. Furthermore, through OMFIT it is possible to execute the EU physics codes (commonly referred to as ‘actors’), by means of their Python interface. In fact, this has been the first time that the Python interface was tested and used for realistic physics applications. For example, an OMFIT module for the CHEASE equilibrium actor was developed, and the ability to conveniently perform parametric scans was showcased in a live demo during the meeting. The OMFIT framework is now installed on the EU integrated modeling server (‘Gateway server'), and the new functionalities have been tested by the camp attendees. With minimal effort, this work can be adapted to the ITER integrated modeling data structure, IMAS, which shares many similarities with the EU system.

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