Theory Weekly Highlights for October 2006

October 27, 2006

A model for the zonal flow shear has been added to the saturated intensity model of the TGFL transport model. Zonal flows are the short wavelength radial electric field structures generated by the turbulence. The ExB flow shear due to the zonal flows is subtracted from the linear growth rate just like the equilibrium ExB velocity shear. This is a generalization of the “quench rule”. The zonal flow shear is driven by the linearly unstable modes and the amplitude of the zonal flow shear is determined by fitting to the database of non-linear GYRO runs. The GYRO database used for the fitting of the intensity model has been increased to 86 cases. The saturated intensity model now achieves fractional deviations of 16% for the ion energy flux, 15% for the electron energy flux, and 28% for the particle flux over the whole database. The zonal flow shear model strongly reduces the contribution to the fluxes from the lowest toroidal mode numbers. The new TGLF transport model will be presented in an invited talk at the APS-DPP meeting.

October 20, 2006

A stability study using ELITE was conducted on a series of DIII-D discharges from the NSTX similarity experiment. Despite very low current and magnetic field, these discharges behave precisely as expected from the peeling-ballooning model. The discharges are stable during the ELM-free period and then become unstable to ballooning or peeling-ballooning modes just before ELMs are observed.
Four GA theory oral papers were presented at the 21st IAEA Fusion Energy Conference in Chengdu, China Oct 16-21, 2006:

  1. “Mechanism for Maintaining the Quasi-Steady State Central Current Density Profile in Hybrid Discharges” by M. Chu,
  2. “Stability and Dynamics of the Edge Pedestal in the Low Collisionality Regime: Physics Mechanism for Steady-State ELM-Free Operation” by P. Snyder,
  3. “A Comprehensive Theory-Based Transport Model” by G. Staebler, and
  4. “Coupled ITG/TEM-ETG Gyro-Kinetic Simulations” by J. Candy.
October 13, 2006

The new collaborative Wiki-based fusion web site is scheduled for release on Tuesday, October 17 to replace the current site at . This new site takes advantage of MediaWiki so that web pages can be collaboratively created and maintained by members of the DIII-D National Team. It includes a more organized and secure structure, which is divided into two main parts: a general area for the public and an internal area that carries more detailed content for DIII-D scientists. From our initial testing, it is anticipated that the collaborative nature will allow the web site to remain more up to date as well as provide a useful vehicle for rapid information dissemination to the DIII-D team.

October 06, 2006

Calculations for the ideal internal kink mode in both DIII-D discharge and model circular cross section equilibria with q below one and slightly inverted shear have revealed a sequence of unstable n = 1 ideal modes that are global eigenmode manifestations of Mercier instability. They occur in only some situations and can now be seen clearly in the new version of the GATO code; previously they were usually masked by the numerically destabilized but now restabilized continuum. These modes exhibit the Sturmian behavior expected from spectral theory for the ideal MHD operator that the eigenvalues are ordered strictly with the number of zero crossings. Numerically, the eigenvalues extend into the stable continuum and full convergence studies are needed to confirm if they actually have an accumulation point at marginal stability. The first few are of the same order of magnitude growth rate as the ideal internal kink. This has implications for the plasma response to external perturbations since with no wall the modes also perturb the boundary and therefore contribute to the plasma response to an external perturbation.

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