Theory Weekly Highlights for January 2003

January 31 2003

In collaboration with Dr. Zhang Cheng, a visiting scientist from ASIPP, in China, an effective numerical scheme to reconstruct current-hole equilibria based on a virtual internal magnetic constraint approach was developed and implemented in the EFIT equilibrium reconstruction code. Initial reconstruction results for a DIII-D discharge with a current hole are very encouraging. Tightly converged equilibria with a large central current-hole [q(0) ~ 105] are efficiently reconstructed using MSE and magnetic data with 5 virtual constraints. Both the magnetic and the MSE data are well fitted. The pressure profile is flat inside the current-hole region.

January 24 2003

Three versions of the gyrofluid equations have been incorporated in a new version of the successful GLF23 transport model and compared with each other, as well as to the original GLF23 model using trial wavefunctions, in order to improve accuracy for negative magnetic shear conditions. All three new models, the original Waltz et al. four-moment model and the four and six moment models of Beer et al, were solved for the linear ITG mode instability using a Hermite basis function approach. All three give a more accurate fidelility to the exact gyro-kinetic ITG modes than the original GLF23 model. The six-moment model is the most accurate but is also the most expensive in cpu time. A compromise between accuracy and cpu time is possible with only four Hermite basis functions and the four moment Beer model. A final decision on which models to retain will be made after evaluating the trade-offs.

January 17 2003

A new time-stepping scheme based on the Implicit-Explicit (IMEX) Runge-Kutta algorithms has been implemented in the GYRO code that greatly relieves the “box” instability in which the longest wavelength n=0 mode remained stable only for unacceptably small time steps, and which previously hampered simulations for very large radial domain sizes. The IMEX algorithms were developed to solve various highly stiff problems such as reacting shock waves and hydrodynamics with strong relaxation. In GYRO, implicit stages are applied to the fast electron parallel term only, and can be done in a radially diagonal way that does not appreciably increase the computer time required for a single time step. However, preliminary testing with a second-order IMEX scheme shows a dramatic increase in the maximum allowable time step for linear electromagnetic modes. Further testing with nonlinear simulations and 3rd/4th order schemes is underway.

January 10 2003

The Data Analysis Applications Group (DAAG) recently presented detailed near-term and long-term plans of its support for DIII-D to the scientific staff. The plans received positive feedback. Several questions regarding the direction of the major data analysis tools and the potential hardware support requirements were also raised. To address these concerns, the DAAG will seek user input throughout the development period of the new codes, and especially at the design stage. Demonstrations to the scientific staff, illustrating new capabilities of the proposed system will be arranged and comparisons will be made with the existing environment. A detailed staging plan will be worked out and proposed together with the Computer Group and with input from users. The presentation of DAAG's support plan for DIII-D can be found at: DIII-D Data Analysis Applications Group Plans: 2003.

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