Theory Weekly Highlights for April 2004

April 30, 2004

The latest version TORAY-GA1.7 is ready for public release. Dimensions of both equilibrium and radial grid point arrays are now allocated automatically and the different common block files previously needed to run with different resolution are now replaced by a single file. TORAY-GA1.7 was validated to test its robustness and reproducibility across platforms. The test cases include comparisons of the absorbed power and driven current profiles for different launch locations, harmonic, X-mode or O-mode waves, different damping and current drive modules, and varying equilibrium and profile resolution. Within reasonable accuracy, the code gives the same results on both Linux (gemini) and HP (hydra) systems and interacts correctly with other codes, ONETWO, CQL3D, and GAFIT for each test case. This validation work is summarized in a file “Doc_toray1.7.doc” in the CVS directory.

April 23, 2004

The Linux cluster that performs between-shot EFIT, CER analysis and profile fitting analysis has been upgraded. It now has 12 dual-Xeon 2.66 MHz processors. The cluster can now complete a typical magnetic EFIT calculation in 30 seconds, including data retrieval, which is about half of the time that the previous cluster took. Whereas the old cluster was running at full capacity during operations, with the new cluster, significantly more analyses can be added between shots, such as map_to_rho and spectrometry analysis. Kinetic EFIT and power balance analysis an also potentially be included.

April 16 2004

Nonlinear extended MHD simulations of ELMs with NIMROD have shown an energy distribution in the mode structure peaked at low and high mode numbers. Low n modes initially have a lower linear growth rate compared to higher n. Assuming equipartition of energy in the toroidal modes as an initial condition, preliminary results indicate that the higher n modes grow linearly to large amplitude and their beating nonlinearly drives the lower n modes to large amplitude. Intermediate n = 5-10 modes are robustly linearly unstable but are not strongly driven by the higher n modes in the early nonlinear phase. The coupled modes form complex structures in flow velocity and temperature; high temperature areas are seen flowing out with vortices clearly evident, in agreement with other models. The temperature and flow structures are antisymmetric above and below the midplane. The simulations cannot yet be continued late into the nonlinear phase due to the need for higher toroidal mode resolution; presently up to n = 21 is included. Although inclusion of higher modes will almost certainly change some results significantly, the bipolar energy distribution is expected to remain in the early nonlinear phase, since it is driven by the nearest-neighbor beating process.

See also for details.

April 09 2004

A new, web-based, video conferencing software for audio and video broadcasting, VRVS, has been set up to make DIII-D operations widely available for remote participation. Both DIII-D operations and the 8:05AM pre-operation meeting are being broadcasted via dedicated “DIII-D Virtual Room”. Currently, there are three video streams from the control room and one video stream from the building 34 conference room available. The audio is the combination of physics operator audio, session leader audio, and pre-operation meeting room audio. A controllable, web-based, high-resolution camera is also installed in the DIII-D control room. As a result, DIII-D researchers and collaborators can now participate remotely in DIII-D experiments and the pre-operation meetings using their office desktop computer.

April 02 2004

Investigations into the ideal MHD stability of tokamaks with a current hole region, lead to the conclusion that their stability can be treated as a plasma with two 'vacuum like' regions. For perturbations with finite toroidal mode number, the current hole region behaves exactly like a vacuum region. This is consistent with standard tokamak external kink analyses where the stability of a vacuum and a current free and pressure gradient free plasma are identical when there are no rational surfaces in the region. For axisymmetric perturbations, however, the analysis is more subtle. In that case, the minimizing perturbation consists of the superposition of three different kinds of independent displacements: a vertical displacement, which does not contribute to the stabilization of the plasma, and two other displacements that compress the plasma and the toroidal magnetic field respectively. These are non-negative definite so cannot provide any destabilization. However, we have not yet been able to prove that these contributions can be made to vanish for the minimizing perturbation.

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