Theory Weekly Highlights for December 2004

December 17, 2004

A Monte-Carlo code ORBIT-RF coupled with TORIC full wave solutions along the equatorial plane using a single dominant toroidal and poloidal wave number was applied to compare its results with TORIC calculations, and with DIII-D and C-Mod experimental measurements. TORIC assumes Maxwellian plasma distributions. For a previous DIII-D discharge with 4th harmonic damping on injected deuterium beam ions, the predicted ICRF power absorption from ORBIT-RF falls within a range of TORIC calculations. Experimentally measured enhanced neutron rate is reproduced to within 30 % from ORBIT-RF simulations. Preliminary C-Mod results of fundamental harmonic damping on minority H ions indicate that the computed RF absorbed power from ORBIT-RF is within 50% of the power absorption predicted from TORIC. Since ORBIT-RF treats the non-Maxwellian ion distribution self-consistently, the difference with TORIC results appears reasonable. Benchmarking the predicted tail energy spectrum from ORBIT-RF with C-Mod NPA spectrum is underway. For more quantitative comparisons, coupling of TORIC 2D full wave solutions as a function of (R,Z) to ORBIT-RF will be done.

December 10, 2004

For disruption mitigation using massive gas injection, the rate of delivery of neutrals into the plasma must be sufficiently fast to prevent avalanche runaway growth. The propagation of a neutral gas and cold temperature front into the plasma has been calculated assuming poloidally and toroidally symmetric injection and by solving for the cooling front propagation velocity as an eigenvalue problem. Although the neutral penetration distance into the plasma is small, the radiation cooling of the plasma is exceedingly fast for high Z dense gases. Thus, the plasma can very quickly cool down to a temperature where the neutrals become transparent to the plasma. For ITER-like parameters and with a gas density at the boundary about 100 times the plasma density, the calculations indicate that the neutral gas penetration time is similar in magnitude to the time required to initiate the runaway current, and the avalanche runaway may be avoided.

December 03, 2004

Previous simulations of nonlinear dynamics of peeling-ballooning modes exhibited explosive bursts of filamentary structures. However, the full energy and particle losses during the ELM crash could not be accounted for solely from loss of the filaments themselves. Two plausible mechanisms have been proposed to explain the increased losses, both of which may be important: 1) The filaments, which remain connected at the ends to the hot core plasma, act as conduits, transporting heat and particles along the filament which is then lost onto open field lines via fast diffusion and/or secondary instabilities. 2) The growth and propagation of the modes strongly damps the sheared rotation in the edge region, collapsing the edge barrier and leading to a temporary return to enhanced L-mode transport. Detailed predictions of these mechanisms are being investigated.

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