Theory Weekly Highlights for April 2006

April 28, 2006

The new Advanced Imagery Laboratory (AIL) is ready for use as a conference room and as a remote collaboration environment for meetings and experimental operations. This lab is intended to provide a convenient environment for the remote collaboration needs of the DIII-D staff. The lab is equipped with an IP-based H.323 videoconferencing system, an AG node and a VRVS node, as well as an echo-cancellation speakerphone, network connectivity, and cameras. An externally addressable camera is on order. There is also a three front project display system for H.323/AG/VRVS video, a portable computer display, and an additional computer display. The AIL also now houses the 8-tile rear projection system.

Jon Kinsey presented an invited talk on: “Parametric Dependencies of Transport Using Gyrokinetic Simulations Including Kinetic Electrons” at the combined April 2006 APS and Sherwood meetings in Dallas Texas on Sunday April 23.

April 21, 2006

The theoretical framework used in the ELITE edge stability code has now been extended to include higher order flow shear terms in addition to the terms that dominate for short wavelength modes. The strongly sheared toroidal flows found in the H-mode edge region of tokamaks can impact peeling-ballooning modes thought to be responsible for many types of edge localized modes (ELMs). ELITE can now accurately calculate growth rates with flow shear for mode numbers from 5 to several hundred, and has been successfully benchmarked against both the MARS and CASTOR codes for the long wavelength end of this range. The newly extended ELITE, in conjunction with MARS, is being used to quantify a new model of the Quiescent H-mode, as well as to further explore flow shear effects in ELMing plasmas.

April 14, 2006

Nonlinear stability analyses of ELMs using equilibrium reconstructions based on DIII-D discharges 113207 and 113317, in the NIMROD code, show well resolved filamentary structures near the plasma edge. Linearly unstable modes are in the range 5 < n < 25. The linear growth rates peak at approximately n=12 for 113317 and n=15 for 113207. Other modes are driven non-linearly, some by two-wave interaction and some by three-wave interaction. The differences in the linear spectrum affect which modes are nonlinearly driven; higher n peaking linear spectrum drives higher mode numbers nonlinearly, and ultimately a larger number of smaller filaments. In the nonlinear regime, the time evolution of the kinetic energy, using a range of toroidal modes (up to ~ 40), show that the fine structure of the filaments increases as the calculation progresses. The penetration depth of the nonlinear mode into the plasma is in reasonable agreement with experiment but the difference between the two cases is too small to make a distinction. In contrast, previous studies of model equilibria under the same computational conditions showed larger losses of temperature and density than expected from experimental data. Toroidal flow shears the structure of the nonlinear mode and limits the extension of the filaments into the vacuum region, while having a smaller effect on the penetration inward. Careful studies of families of equilibria representing the series of times leading to an ELM event are expected to shed more light on the differences between the two discharges and how the theoretical results compare with experiment. These results are contributing to the DOE theory milestone.

April 07, 2006

Corrections to δW to restabilize the Alfven continuum were implemented and shown to work extremely well in test cases. The Finite Hybrid Element method used in the GATO, ERATO, and KINX ideal MHD codes is well known to numerically destabilize the Alfven continuum slightly; localized continuum modes then converge from the unstable side to the marginal point quadratically with increasing mesh. The technique to restabilize these modes at a finite mesh, first used in the KINX code, was applied to the GATO representation. With the correction, the numerically destabilized continuum modes were shifted to just slightly on the stable side (~ 10-7), leaving the physically unstable modes hardly affected. With this new feature, the need for numerical convergence studies is greatly alleviated and the way is open to use GATO as a true δW code, which should make simple stability determinations much faster. It also is a key prerequisite for using GATO to study the plasma response to exte rnally imposed perturbations.

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