Theory Weekly Highlights for July 2009

July 31, 2009

In simulations using the NEO code to analyze the impurity poloidal flow in the H-mode edge for a typical C-Mod plasma, large differences in the magnitude of the computed and measured poloidal flows were found in the core, even accounting for the relatively large error in the Charge Exchange Recombination System measurements. The simulations were performed with three species: deuterium ions, electrons, and boron impurities. In the pedestal, the results are closer, suggesting that the flows are, in fact, essentially neoclassically-driven there, although NEO predicts a more narrow pedestal width than is observed. By solving the higher-order drift-kinetic equations, the finite-orbit-width effects were found to be negligible even in the steep-gradient region, as was previously found also for typical DIII-D H-mode edge plasmas, so cannot account for the differences. Other effects, such as orbit loss, are being explored.

July 24, 2009

Ongoing work under the ITER contract “ITER CODAC: Operation Request Gatekeeper” is examining the computer science issues associated with secure remote instrumentation control for magnetic fusion experiments. The project will define a vision for a gatekeeper software system intended to be the only channel for interaction with incoming requests to the secured area of the ITER Plant Operations Zone (POZ). The Gatekeeper’s decision on whether to pass a request inside the POZ is based on security verification (authentication and authorization), grammar validation (is the request well formed), and validity checks (is a request within a specified range of values). If all these tests are passed, the request is queued for entry inside the POZ where a detailed validation check is made before execution. The prototype system is to be implemented on the DIII-D Plasma Control System, and tested during the remote support of KSTAR and EAST operations.

Jeff Candy and Emily Belli returned from a ten-day visit to IPP-Greifswald working with Per Helander on neoclassical transport.

July 17, 2009

A recently developed analytical model for the plasma cooling, forced by the injection of pellets, was applied to the Low-Z shell pellet experiment done on DIII-D using small (mm sized) gas-filled polystyrene (PS) pellets. The low sublimation energy per electron ablated for PS justified the use of the standard ablation model for cryogenic pellets. Additionally, the ablation constant was reduced by a factor of 0.61 in order to get agreement with the measured shell burnout distance at r/a = 0.45. This slowing is probably explained by the heat sink involved in “cracking” the C8H8 monomer vapor shield into smaller molecules, and atoms. The measured and predicted temperature profiles just prior to shell burnout at 0.98 msec were found to be similar but the magnitude of the temperature depression from the pellet depends on the field line connection length, which varies depending on the rational and irrational surface locations. An unexplained result is that significant “pre-cooling” takes place for r/a < 0.45 before any current perturbation can occur, which is indicative of a large heat diffusivity of ~ 10 m2/s. It is speculated that this is caused by drift-ballooning modes activated by the altered pressure profile.

Dr. Guoqiang Li returned to the Institute of Plasma Physics in China after a six-month visit to GA, where he collaborated on the development of the IMFIT Integrated Modeling and Fitting tool.

July 10, 2009

The DIII-D web-portal system has been requested by collaborators at the National Fusion Research Institute (NFRI) in Korea for the KSTAR experiment. This system ( has been deployed at DIII-D since the beginning of 2009 to support experimental activities of researchers worldwide and provides multiple online services, such as real-time experiment status monitoring, diagnostic data access, data visualization as well as interactive collaborations. The system offers a customizable interface with personalized page layouts and list of services for users to select. Each individual user can create a unique working environment to fit their own needs and interests.

July 03, 2009

An upgrade has been completed to include the effects of drift orbits in the runaway electron tracing model in NIMROD. Using the model to study runaway electron confinement on stochastic magnetic fields, produced during a disruption by integrating single particle orbits as the MHD fields are evolved, it was found that the curvature drift, which is proportional to the relativistic factor gamma can become macroscopic for highly relativistic electrons. The electrons average over magnetic perturbations during a poloidal transit. Thus electrons in stochastic regions can appear well confined. This was verified by comparing a small number of electron orbits computed for a static magnetic configuration having good flux surfaces, islands, and stochastic regions with and without drift terms. More extensive comparisons with many electrons show that the drift terms can also degrade confinement in some regions, such as when individual field lines approach the edge but do not escape. The model will used to study whether disruption induced MHD can degrade runaway electron confinement sufficiently to avoid runaway avalanching.

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