Theory Weekly Highlights for October 2005
October 28, 2005
Kinetic analyses of the m/n = 3/2 tearing mode observed in DIII-D hybrid discharges suggest that counter current driven by the kinetic Alfven wave (KAW) induced by the 2/2 sideband displacement may provide a novel mechanism to regulate the q profile and prevent the sawtooth crash in these discharges. In a previous highlight (see highlight for July 22 2005 at Theory Weekly Highlights for July 2005) the ideal 2/2 sideband was suggested as a mechanism to drive the needed current. However, further calculations indicated the driven current is too small to maintain q0 against the diffusion of current density into the core. In contrast, the new KAW scenario does predict sufficient current drive. When the central q value evolves to a value close to 1, the electromagnetic 2/2 sideband induced by the rotating 3/2 tearing mode can be converted into propagating KAWs. These waves drive current counter to the direction of rotation of the 3/2 island more efficiently than the Alfven wave proposed earlier.
October 21, 2005
A new theoretical study of the stability of the edge region in Quiescent H-Mode (QH mode) employing the ELITE, DCON and GATO codes, has found that QH discharges operate in the vicinity of the low-n kink/peeling stability limit. Previously, the mechanism that drives the “edge harmonic oscillation” (EHO) and allows steady state QH operation has been a puzzle, and this provides a possible candidate for the EHO. The combination of low density, which leads to high bootstrap current in the edge, and strong rotation shear, causes the limiting instability in the edge to be a rotationally-destabilized low-n kink/peeling mode, rather than the intermediate to high-n peeling/ballooning modes which generally limit the pedestal in ELMing discharges. The stability calculations find a QH density limit that increases with stronger discharge shaping, and agrees with observations on DIII-D. Calculations also suggest that the ITER pedestal should be in the allowed QH range for low pedestal densities. These results will be highlighted by P.B. Snyder at the APS/DPP meeting in Denver.
A new Quad Xeon server was purchased and installed to replace Zephyr, a very old Tru64 machine, and the old Atlas RAID array. The new server will be configured to serve MDSplus data. The new server has double the storage capacity of the old RAID array and orders of magnitude more server load capacity than Zephyr.
October 14, 2005
A prototype FusionGrid service along with a beta-version client PreONETWO for launching the ONETWO code is now available for testing. This limited-functionality prototype can be used to reserve a run ID and enter other metadata about the ONETWO run and upload inputs to the database. The facility can then launch a ONETWO run on FusionGrid and enables the user to view the output when the run is done. Access requires only a grid ID and authorization through ROAM, which can be provided on request, and a small number of environment variables set up correctly. The service can then be accessed by typing “preonetwo” on the command line from any of the Linux hosts in the LSF cluster. More information is available at: http://web.gat.com/comp/analysis/grid/onetwo.html
Several instructional demos are also provided at the following URLs:
October 07, 2005
The TORAY-GA ray tracing code for modeling electron cyclotron heating and current drive has been improved to calculate more accurately the driven parallel current density and the toroidal current density for ONETWO. The new version, TORAY-GA 1.8, runs only with ONETWO versions 3.91 and later. Previously, the driven toroidal current density in TORAY-GA was calculated using approximations to the flux-surface average of the magnetic field and the major radius. This has been corrected in the new version. Self-consistent calculations using TORAY-GA V 1.8 and ONETWO 3.91 are expected to result in corrections to the driven current density of approximately 5-10% in the outer part of the plasma for most DIII-D and ITER cases.
Disclaimer
These highlights are reports of research work in progress and are accordingly subject to change or modification