Theory Weekly Highlights for June 2008

June 27, 2008

Low-n global eigenmode instabilities in equilibria with full shaped magnetic geometry reconstructed from experimental measurements in DIII-D have been demonstrated using the GYRO code. As part of our work on how to identify TAE/EPM (energetic particle) modes, these instabilities were driven by an assumed density gradient profile of energetic particles (with the background plasma gradients reset to zero). The characteristics of the frequency and growth rates are similar to those reported earlier (see March 28, 2008 Highlight) from local high-n flux tubes simulations in GYRO.

John Finn from LANL is visiting this week to collaborate with Lang Lao and a National Undergraduate Fellowship student Adam Jacob on a new method for equilibrium reconstructions taking account of correlations in the equilibrium data.

Dylan Brennan is visiting from the University of Tulsa to collaborate with DIII-D and GA theorists on the theory of coupling of tearing modes and turbulence and modeling of the interaction between a resistive wall and resistive MHD plasma modes.

June 20, 2008

Jeff Candy visited Chalmers University in Sweden for three weeks as part of his appointment as 2008 Jubileum Professor. He worked with Hans Nordman, Tunde Fulop and Per Strand on topics in drift-wave and neoclassical transport in tokamaks.

Ron Waltz attended the European Physical Society Meeting in Hersonissos, Crete and presented a paper “Gyrokinetic Theory and Simulation of Angular Momentum Transport”.

Alan Turnbull attended and presented an invited talk at the High Performance Computing Methods Symposium on June 16 and 17 in Lausanne, Switzerland. The symposium was in honor of Dr. Ralf Gruber who was the original developer of the ERATO 2D ideal MHD stability code on which GATO is essentially based, and of the 3D ideal MHD TERPSICHORE code. The presentation by Alan Turnbull was on “Ideal MHD Spectrum Calculations Using The TERPSICHORE Code For The ARIES-CS Configuration”.

June 13, 2008

The NIMROD code has been used to simulate the thermal quench of a DIII-D plasma after dilution cooling by massive deuterium injection. The simulation is initiated by increasing the density by 100 times everywhere, with a fixed pressure profile, so the initial plasma temperature is 40eV. This cool plasma can be easily simulated with NIMROD with no artificial resistivity enhancement. An in situ carbon fraction of 1% (of the equilibrium density) is assumed. Preliminary analysis of the simulations indicates that the carbon radiates most strongly at the plasma edge, producing an inward propagating cooling front, similar to gas jet injection. After 0.5 ms, the current and pressure gradients are reduced by the destruction of the flux surfaces due to MHD. Current is redistributed to the core, eventually leading to strong peaking on axis. Despite the large electric fields that result, the 100 fold density increase is chosen to exceed the Rosenbluth criterion to prevent runaway avalanching. The value of the perpendicular thermal conductivity in this temperature range is the most significant variable in accurately modeling the shutdown. Larger values will produce smaller gradients and could lead to an MHD-free thermal quench.

June 06, 2008

The stand-alone version of the ORBIT-RF code has now been uploaded as a public version into the GA computer group CVS directory. ORBIT-RF is a 5-D Monte-Carlo Hamiltonian ion guiding center code that has been used to interpret ICRF wave-plasma heating experiments in Alcator C-Mod, DIII-D, and other tokamaks. It is also proposed to be included as a new Fokker-Planck component in the SWIM and IMFIT (Integrated Modeling and Fitting) projects. The previous version required multiple steps to prepare input data, such as the equilibrium and global full-wave solutions, on different platforms. This complex set-up process is now simplified to be one-step on one platform - the GA Linux cluster Drop - by running new script files in the user's home directory. This will also provide much easier future coupling of ORBIT-RF to the SWIM and IMFIT code packages. A user guide and reference cases from the extensive benchmarking activities from collaboration between GA and the RF SciDAC and SWIM projects have also been uploaded into CVS.

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