Theory Weekly Highlights for March 2006

March 31, 2006

A new version, Version 4.0, of the ONETWO transport code was developed that features full integration of the NuBeam NTCC module, as well as the ability to run simpler and faster NFREYA-derived beam calculations for between-shot analysis in DIII-D. In addition, the coupling of ONETWO to TORAY was improved by supplying additional MHD-derived information to TORAY. The current drive calculations were updated to be compatible with the new interface and may result in some, typically less than five percent, changes in ECCD. The NTCC Multi-mode confinement module MM95 was added to ONETWO as well and comparisons with GLF23 are underway.

March 24, 2006

A general approach to determining the nonlinear plasma response to external nonaxisymmetric perturbations as a constrained 3D equilibrium was formulated. This approach was shown to be closely related to the Almost Ideal MHD (AIMHD) theory proposed previously by Torkil Jensen. However, the specific implementation of AIMHD originally proposed by Jensen ignored the possibility that the profiles may have different functional dependences within different simply connected regions (i.e. within islands). When this is accounted for, it appears that AIMHD can be formulated with reasonable physically valid constraints. New constraints need to be developed that describe the profiles in the new island flux regions. Some ideas on this can be obtained from transport modeling in addition to conservation laws.

A prototype graphical client code for automated input preparation of TRANSP was completed. This new client code, called AutoTRANSP, streamlines the process of preparing TRANSP runs for execution on FusionGrid.

March 17, 2006

Analysis of DIII-D Hybrid discharges just before onset of a 2/1 tearing mode found that the tearing stability at q = 2 is highly sensitive to qmin approaching unity, as a result of the ideal βN limit rapidly decreasing toward the experimental βN. In approximately a third of DIII-D Hybrid discharges, the high performance phase is terminated by the growth of these large 2/1 tearing modes. The analysis used the experimental equilibria with q0 constrained within the uncertainty of the reconstruction between 0.98 and 1.05. Varying q0 in this range resulted in little or no change in the equilibrium near q=2 or elsewhere. For qmin ~ 1.02 and above, the 2/1 tearing stability is only weakly dependent on βN. However, when qmin =1.01 and below, the 2/1 tearing mode (at q = 2) becomes strongly unstable as the equilibrium approaches the n=1 ideal limit. This suggests that the proximity to the q=1 resonance is critical to the 2/1 tearing mode stability, and is responsible for the onset of the mode. The 2/1 onset could therefore be prevented by a slight increase in qmin.

March 10, 2006

Electromagnetic fluctuations have been added to the new Trapped Gyro-Landau Fluid (TGLF) equations. Linear stability calculations show good agreement with the gyro-kinetic linear stability code GKS. Magnetic field fluctuations both parallel and perpendicular to the equilibrium field have been added. Except for parallel velocity shear, the TGLF equations now have the same comprehensive physics as the GKS code. TGLF will be made available as a much faster alternative to GKS for linear stability analysis of DIII-D data.

March 03, 2006

In re-examining the calculation of angular momentum flux in the Pfirsch-Schluter regime, (see July 01 2005 highlight at: Theory Weekly Highlights for July 2005) it was found that the expression for the flux differs substantially from the previous expression obtained by Hazeltine in 1974. The relevant distribution function was obtained by expansions in basis functions (Burnett functions) in lieu of a variationa l principle. The flux itself was calculated using the adjoint equation method recently employed for the same problem in the banana regime while retaining the poloidal variation of electrostatic potential. The new expression predicts a steady-state electric field at the edge pedestal region that exhibits a well structure that is observed in some experiments. The results are presently being written up for publication.

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