Theory Weekly Highlights for September 2004

September 24, 2004

The new version of SAV (Signal Analysis and Visualization Code) is ready to be tested. In this version, the user interface is improved so that the user can input or change data parameters and computational parameters interactively, more signal arrays and test signal options are provided, and the visualization functionality is improved with better proper annotations and a resizable plot window. The action log and status message mechanism are also improved. With these and several other enhancements and corrections, this version is expected to be more robust than the previous version. By comparing against similar functionalities in the previous Heat Pulse Analysis code, the results of currently available computational functionalities were validated.

September 17, 2004

The ONETWO transport code was updated recently with several new NTCC modules. This includes the updated fast ion physics package NUBEAM and it's associated support codes, the PEDESTAL code and the inverse equilibrium code TOQ. Other NTCC modules previously included in the ONETWO code are TORAY, CURRAY, and GLF23. TOQ was recently reworked for submission as an NTCC module and was benchmarked within ONETWO by running a sample current evolution simulation case. The results are in general agreement with those using the inverse equilibrium solver and the direct solver present in ONETWO.

September 10, 2004

The model developed to explain the variety of tearing mode phenomena observed in DIII-D, in which the instability growth and the equilibrium time development are both taken into account, was shown to reproduce maximum and minimum heating rates for NTM onset. Similar qualitative behavior has been observed in DIII-D experiments; at slow beam power ramp rates NTMs do not appear, at extremely fast rates disruptions occur, and in between NTMs are triggered. Classically destabilized NTM evolution was analyzed in the presence of different heating rates. In the fast heating regime, the change in the linear stability as beta approaches the ideal limit dominates the evolution and the tearing mode grows faster than the rate of current relaxation. Above the maximum rate the integrated tearing mode growth time is longer than the heating time and the ideal mode becomes unstable before the NTM can grow. In the slow heating regime, the enhanced transport losses from the mode modify the current distribution and change the linear stability and this dominates the evolution; the tearing mode grows slowly enough to be affected by the current redistribution. Below the minimum heating rate, the seed island remains saturated and does not evolve into a large NTM. Between the two critical heating rates, classically destabilized NTMs are observed, and the predicted beta value reached as a function of island size agrees with experiment. Nonlinear initial value simulations are being used to test the simple model predictions in the presence of nonlinear mode coupling and so far confirm this picture.

September 03, 2004

A new, more accurate method for treating the finite Larmor radius (FLR) terms in the gyro-fluid equations has been derived. With this improvement the growth rate for the ITG mode remains accurate for very large temperature gradients, deep into the region of perpendicular wavelength shorter than an ion gyroradius. In previous models the FLR terms fell off too suddenly in this region, resulting in a growth rate that also decayed too rapidly. This improvement is expected to be important for the near separatrix region where the temperature and density gradients can be very steep.

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