Theory Weekly Highlights for March 2010

March 26, 2010

Improvements have been made to the MATLAB code NC to calculate neoclassical ion thermal conductivity and parallel flow using the full Fokker Planck operator. The code now employs normalized velocity space basis functions to allow more terms in the expansion. It also uses the Knorr model to better define aspect ratio dependence and block matrix notations to separate velocity space and poloidal angle dependences, allowing easy extension to general magnetic geometry. The number of terms in the expansion increases as collisionality is reduced. Convergence to one percent accuracy has been demonstrated for the case with inverse aspect ratio of 0.1 and the customary collisionality parameter nustar of 1 using 40 each of Legendre polynomials, generalized Laguerre polynomials, and trigonometric functions.

March 19, 2010

GYRO simulations of energetic particle (EP) driven very long wavelength local TAE/EPM turbulence embedded in shorter wavelength ITG/TEM turbulence have shown that fixed gradient nonlinearly-saturated transport states can exist in a narrow range of EP pressure gradients. EP transport is enhanced over the ITG/TEM driven levels but the background plasma (BGP) transport is unchanged. The narrow range is bounded by the TAE/EPM linear threshold gradient and a critical gradient where runaway in time and unbounded EP transport onsets. However, ExB shear from the runaway long wavelength TAE/EPM modes stabilized the ITG/TEM turbulence and the BGP transport decreases. Since EP transport is limited by the EP source rate, the EP pressure gradients will profile relax to intermittent EP transport at the critical gradient. The critical gradient collapses to the linear threshold gradient if the BGP gradients are not large enough. The results suggest an important conclusion: The onset of local high n TAE/EPM modes does not degrade the background plasma confinement.

March 12, 2010

NIMROD simulations of DIII-D rapid shutdown by Ar-pellet injection showed that the application of RMP fields with n=1,2, and 3 symmetry reduces the magnitude of the runaway “prompt loss” that occurs at the start of the current quench. In these simulations, orbits for 1800 fast electrons were integrated during the rapid shutdown to examine confinement of supra-thermal and runaway electrons. The prompt loss reduction relative to the case with no RMPs occurred in every simulation. However, the n=1 fields show the most promise for sustaining a continuous runaway electron loss rate that exceeds the theoretical avalanche growth rate. In future simulations, the n=1 poloidal spectrum will be tailored to incorporate stronger resonant components. A toroidal resolution study has also found runaway electron confinement to be converged with 11 toroidal modes (n=0,10); the change in confined runaway electrons when the number of modes is doubled is 1%.

March 05, 2010

The MDSplus data system has recently been expanded to include a new server which doubles the overall usable disk storage to 13.5Tb. This expansion was critical, as the estimated disk usage would have reached over 95% by the end of this years run period. Data migration has been completed in order to spread the usage out between the different data servers.

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