Theory Weekly Highlights for July 2004

July 30, 2004

The Friday Science Meeting has been made available for remote participation. The presentation video is captured with a high-resolution network camera and is broadcast via the camera's web-server. The meeting audio is broadcast using the ESNET Telephone Bridge. Remote DIII-D researchers and collaborators can now conveniently participate in the Friday Science Meetings with their office computers and telephones. The remote participation information is available on the web at and is included in the regular Friday Science Meeting announcements.

July 23, 2004

A detailed investigation of a spherically symmetric plasma liner from multiple high-velocity plasma jets imploding on a magnetized plasmoid, found that to obtain ignition conditions, extremely supersonic jets (Mjet ~ 60) are required in order to deliver the requisite ram pressure to the target. It had previously been assumed that the merged jets would form a homogeneous plasma liner with the same Mach number as the original N jets. Instead, a supersonic, radially inward flow is created along the contact surface at the edges of the intersecting jets, where the small change in direction at an angle θ ~ 1/√N ~ 2/N has to be accomplished through a standing shock. The flow is refracted across this shock into a narrow hexagonal boundary layer surrounding each jet and containing the hot shocked material. After the merging is completed, the shocked layer is no longer in stable pressure equilibrium and starts to break up, thus exchanging heat with the cool interior. In the strong shock limit pertaining here, (Mjet*θ)2 »1, the resultant energy equipartition reduces the liner Mach number to Mliner ~ 1/θ(3/2) ~ Nsup>(3/4)</sup> independent of Mjet. For N = 78, Mliner = 9.1. A more refined examination to confirm this result is underway.

July 16, 2004

In an experiment to investigate the effects of 3D error fields on magnetic surfaces in DIII-D, slowly rotating n = 1 traveling waves at 5 Hz and various amplitudes (~ 0.1 - 0.3 % of the poloidal equilibrium field) were applied to perturb the edge magnetic surfaces by pre-programming the I-Coil currents. At 0.1 % perturbation, the observed difference in the vertical separatrix location between magnetic reconstructions, which assume toroidal symmetry, and Thomson scattering measurements of electron temperature at a single toroidal location, responds in phase with the applied perturbed field with an oscillation amplitude ~ 2 cm. At 0.3 % perturbation, however, the amplitude of the difference in separatrix location grows in time, correlated with an early discharge termination, due to the appearance of a locked mode. The results are consistent with the conjecture that the observed separatrix location differences between magnetic and Thomson scattering measurements in some DIII-D discharges are due to the small toroidal asymmetry of the external shaping coil locations and that the plasma response is important in amplifying the error field and can enhance this difference. This will be investigated in future work.

July 09, 2004

Convergence tests for the latest version of the TWIST-R linear resistive MHD code have now found near-quadratic convergence in the matching data Δ' and Gamma' with radial mesh, over a range of Mercier index 0.5 < μ < 1.5 values for a Solov'ev equilibrium series. The study also showed the expected pole in the tearing mode matching data Δ'(μ) at μ = 1. In the latest version, numerical bugs were corrected, which resulted in physically reasonable solutions, and a more flexible and more consistent implementation of the poloidal fourier decomposition was provided to minimize the expansion truncation errors. Numerically computed, circular cross section toroidal equilibria were also tested and Δ' and Γ' were computed, with good convergence properties obtained for these cases as well. For all of these studies, a new tool to study and summarize the convergence of the matching data over a range of cases was also developed; this tool analyzes the convergence rate automatically and finds converged values as a best fit from the computed data. This tool should be extremely useful in future studies.

July 02, 2004

Lang Lao and Vincent Chan participated in the 2nd US-PRC Magnetic Confinement Workshop in Kunming China, June 23-25, and presented two talks on recent DIII-D results and validation of simulations against experiments. The workshop was hosted by the Southwest Institute of Physics and attended by about 30 scientists, including six US institutions. The purpose of the workshop was to discuss collaboration ideas for the next two years.

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