Theory Weekly Highlights for September 2008

September 26, 2008

An initial version of a visualization program written using C++ with OpenGL allows for real-time interactive 3D viewing of magnetic field lines produced from the TRIP3D code. Visualization from arbitrary viewing angles and camera positions, as well as a poloidal slice from an arbitrary toroidal angle, allow detailed inspection of potentially complicated perturbed field line geometry. Color gradients along field lines allow the user to rapidly visualize instances where many toroidal rotations of the same line are displayed. Magnetic perturbation coil locations with coil current information represented by color are also displayed.

September 19, 2008

The first test of the Trapped Gyro-Landua Fluid (TGLF) transport model with data from MAST has been completed in collaboration with Greg Colyer from Culham Laboratory. It was found that outside the sawtooth region (q > 1) TGLF predicted the ion and electron temperatures to within 17% and 26% respectively for eight discharges. The electron temperature prediction was on average 11% too high whereas the ion temperature was 4% too low. It was found that electron-ion collisions play an important role in improving the transport by reducing the trapped electron drive. The new electron-ion collision model in TGLF, which improves the fidelity to gyro-kinetic theory, also resulted in an improved prediction of the MAST temperatures. In addition, the shaped geometry modification to the Chang-Hinton formula proposed by Belli and Candy to give better agreement with the NEO code numerical neoclassical ion thermal diffusivity has a significant impact on the ion temperature prediction. The Belli-Candy modification yields much better agreement with the data. This motivates coupling the NEO code to the transport code in the future.

September 12, 2008

A new EFIT version with several improvements 09/04/2008 is now available on the GA Hydra and Linux computers. A new vertical position stabilization option similar to that used in Real-Time EFIT (rtEFIT) that has better convergence properties is now available. An EFIT input SNAP file with this option can be found in the area /link/efit/efit_snap.dat_jta_f, with both the F-coil currents and the reference poloidal flux included as fitted data, as is done in rtEFIT. The new EFIT version also includes two corrections related to toroidal magnetic field compensation of magnetic data and this version is linked to the previous version of the MSE shared library with 0.35 degree uncertainty in pitch angles. This new version is available from the usual /link/efit area with the command efitd6565d or efitd129d.

Vincent Chan, Ming Chu, and Lang Lao attended the Opening Meeting for the International and Scientific Advisory Committees for the Center for Magnetic Fusion Theory (CMFT), of the Chinese Academy of Science in Hefei, China on September 2-3, 2008. CMFT is a research unit set up to support theory and modeling for the EAST superconducting tokamak.

September 05, 2008

A detailed analysis of the stability calculations from the bean and oval sawtooth experiments shows that the key that determines the reconnection process and ultimate consequences for the crash is the differing poloidal displacement of the ideal quasi-interchange and internal kink modes. Earlier analysis of the normal component showed mixed results, with the mode in the oval case appearing to be a mixture of the quasi-interchange and the standard kink. However, the poloidal displacement for the oval clearly indicates a quasi-interchange like convective cell. For the bean shape on the other hand, except for a short time immediately after the crash and before becoming ideally stable in the mid sawtooth cycle (see April 6 2007 highlight at Theory Weekly Highlights for April 2007), the unstable ideal mode has the characteristics of the internal kink in which the poloidal motion is sharply localized at the q = 1 surface. The results are consistent with the Wesson picture of the quasi-interchange where the reconnection occurs slowly through the sawtooth ramp phase and for the standard internal kink, the mode reconnects rapidly during the crash. The implications for interpreting the experimental observations are being worked through; experimentally, the oval has large precursor oscillations and the bean has larger successor oscillations.

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