In a new analysis of pellet cloud drift dynamics, it was found that the effect of toroidicity becomes increasingly important as the cloudlet elongates with time. With toroidicity, different segments of the cloudlet start drifting on different flux tubes and the differential slippage causes the cloudlet to fragmentize in a rather orderly fashion; beginning with the outermost fluid cells, the cells are sequentially shed, one by one. The remaining cells, being fairly well aligned within a common magnetic flux tube at any given moment, continue to drift coherently in the large-R direction until the next end cell is shed. The mass shedding model was incorporated into our PRL code (Pressure Relaxation Lagrangian), which obtains the mass deposition profile in the plasma. This can then serve as the source profile in any tokamak transport code. A preliminary comparison was made to the experimental profile from DIII-D shot # 98796 and good agreement was found and can be seen at http://web.gat.com/theory/weekly_highlights/attachments/pellet_cloud.pdf .
NIMROD simulations of DIII-D discharges, completed in collaboration with SAIC, and which capture the nonlinear extended MHD dynamics of the NTM seeding by sawteeth and spontaneous NTM generation have been made publicly available via MDS+. These results are now being analyzed with advanced 3D visualization tools using the SCIRun package developed in collaboration with the University of Utah.
Full-physics simulations on the global gyrokinetic code GYRO have been speeded up enormously. Previously, full-physics runs (which are 20 times more expensive than state-of-the-art ITG runs) with trapped and passing electrons, finite-beta, real geometry, profile variation and ExB shear, etc., required five 24hr restarts on 128 processors of SEABORG at NERSC. This resulted in a formidable 7 to 10 day job turn-around. The recent computational advances enable scaling well beyond 128 processors so that these jobs can now be done in a single 24hr run on 512 processors. This is close to the same job turn-around time obtained for the reduced-physics ITG runs of a year ago. The new full physics GYRO runs can now be done in production mode but this will require a large increase in the MPP time available to us; otherwise, a full production schedule will exhaust our upcoming yearly NERSC allocation in as little as three months.
A complete set of transport equations for the plasma and the electromagnetic field have been obtained in the cylindrical limit that is appropriate for large aspect ratio flux surfaces of arbitrary shapes. Novel algorithms are proposed for solving the problem of plasma transport with concurrent change of flux surface geometry. Besides presenting a most comprehensive model of tokamak plasma transport, these results are also useful for studying how long-duration discharges are maintained by bootstrap current. Extension of the theory to include angular momentum transport and implementation of the algorithms will be attempted in future work.
These highlights are reports of research work in progress and are accordingly subject to change or modification