Dr. Deng Zhou from the Hefei Institiute of Plasma Physics completed his one-year tenure at General Atomics and returned to China this week. During his stay Dr. Zhou contributed to the development of transport analyses by coupling the TOQ inverse equilibrium solver to the ONETWO transport code and initiating coupled transport and stability calculations. He subsequently went on to develop discharge scenarios for the EAST Experimental Advanced Superconducting Tokamak using the tools he developed at GA. He will continue his work on various EAST simulations using ONETWO after he returns to China.

Simulations of nonlinear dynamics of peeling-ballooning modes utilizing the 3D, two-fluid electromagnetic BOUT code show an explosive burst of one or many filamentary structures occurs, transporting heat and particles radially outward into the open field line region. Peeling-ballooning modes are believed to be responsible for edge localized modes (ELMs) in tokamaks. In the early phase of the simulations, the modes exhibit behavior consistent with linear peeling-ballooning studies. In the nonlinear phase, the explosive behavior is qualitatively similar to expectations from nonlinear ballooning theory, though significantly more complex dynamics is seen in the simulations, including a characteristic lull as perturbations approach equilibrium amplitude. Results have been compared to experimental observations of ELMs on DIII-D and MAST, and numerous similarities have been noted, including the poloidal extent and filamentary structure of the modes. This work was highlighted in the invited talk by P. Snyder at the 2004 APS Meeting in Savannah.

Preliminary nonlinear simulations of Edge Localized Modes using the extended MHD NIMROD code (http://nimrodteam.org) show an interesting distribution in the growth rates with toroidal mode number n. When the higher n modes are most unstable linearly, they grow to energies large enough for nonlinear coupling to become important and then nonlinearly drive low n modes. These are driven at much larger growth rates than the high n linear growth rates. This group of low n modes thus forms a “shelf” with increased instantaneous growth rate. The distribution is reversed when the lowest n modes are most unstable linearly and grow to large amplitude first. In this case groups of higher n modes are driven until each group reaches the nonlinear coupling energy and in turn begins driving the next higher group of modes. This causes multiple “shelves” at successively larger growth rate with increasing n. The physical meaning of these “shelves” and how they relate to the final nonlinear state of the system is being investigated.

The GATO ideal MHD stability code is now available as a worldwide computational service on FusionGrid. This Grid computing deployment offers a very easy and convenient facility for running GATO in a secure environment. Presently, the service runs on a 2-CPU server. Data preparation and the invoking of a run are done through an IDL-based PreGATO utility. Both input and output data are written into MDSplus in a tree structure designed to be appropriate for any stability code. Future plans for the GATO service include adding a batch run feature, adding a run queue for efficient use of resources, expanding the server to include many more compute nodes, creating a non-commercial software based PreGATO, and building a set of general MDSplus tools to process and visualize the results. This service is expected to be a prototype for future stability codes. For more information on the National Fusion Collaboratory Project (FusionGrid), see http://www.fusiongrid.org/