Theory Weekly Highlights for July 2003

July 25 2003

An updated version of the NTCC supported fast and slow wave ion-cyclotron ray tracing code CURRAY was created and installed at GA. A common transport interface for CURRAY was introduced, which allows the code to be run in stand-alone mode or to be driven in a time dependant manner from a transport code. The ONETWO code was adapted to serve as an example of using the new interface. The CURRAY code was tested using L and H mode DIII-D cases. Previous results for small and large aspect ratio NSTX and ITER cases were verified. A report detailing the results was prepared with Dr. T.K. Mau of UCSD.

July 18 2003

The Monte-Carlo orbit guiding center code, ORBIT-RF, was upgraded to include several enhancements. The previous version, which was written in a mixture of F90 and F77 syntax, has now been completely rewritten in F90, allowing users to take full advantage of the dynamic allocation features of F90. Also, The parallelizing programs (MPI) of the code were rewritten and successfully tested on the LUNA (17 processors) and STELLA (43 processors) local clusters at GA. The result using 8000 test particles (with 42 processors on STELLA) showed a smoother energy spectrum for the excited fast ions in velocity space compared to that using only 1000 test particles, as was anticipated. When the net NB fast ion driven current density profile is calculated using a momentum balance equation between electrons and the injected NB fast ions, the profile using 8000 test particles also showed a slightly reduced peak compared to that using 1000 test particles; however, both results are still in a good agreement with the simulations results from ONETWO. Implementation of the high harmonic absorption of ICRF waves by NB fast ions is presently underway in order to better model DIII-D experiments.

July 11 2003

A novel and promising fueling technique was examined that utilizes a pre-cooled miniature Laval nozzle to produce a slender “needle thin” gas jet with a very high density. With this method, steady state central fueling in ITER would be possible if the jet density at the plasma boundary exceeds ~ 3×1016 cm-3, corresponding to a jet radius of less than ~ 1 cm. When the jet enters the plasma from the high field side, it quickly becomes heated and ionized, converting the gas jet into a high-energy-density plasma jet and causing it to E × B drift down the magnetic field gradient towards the plasma center, similar to the ionized material ablated from a solid pellet. In analogy with expanding laser plasmas undergoing isothermal cooling, a self-similar solution of the hydrodynamic equations governs the early time jet heating and expansion in the plasma, and the pressure of the ionized jet material can quickly reach a maximum value of 10 to 30 times the ambient plasma pressure by the time the free jet radial expansion is converted to flux tube flow and radial drift begins. If such high-density gas jets can produce the necessary high-pressure plasmoids at the plasma boundary in ITER, this method will avoid many of the technologically challenging aspects of continuous multiple pellet injection.

July 04 2003

In collaboration with the NIMROD team, nonlinear stability simulations of a sawtoothing DIII-D discharge, #86144, using the NIMROD code have demonstrated seeding of a linearly stable m/n = 3/2 tearing mode from a sawtooth crash. In the experiment, the 3/2 seed island is initiated and is then driven past the neoclassical threshold by the time the n=1 mode saturates and begins decaying. The seeding mechanism is qualitatively reproduced in the simulations, but so far the driven 3/2 island saturates and decays at a much smaller amplitude than is observed experimentally. The calculations are challenging since the heat flow dynamics within the small helical 3/2 island must be handled correctly and the calculations need to be performed at very high, experimentally realistic values of the Lundquist number S ~ 108. Work is continuing to provide a more quantitatively accurate simulation by improving the neoclassical closures needed to properly describe the island physics.

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