Theory Weekly Highlights for February 2006

February 24, 2006

A new similarity model to determine the conditions for spark ignition of a liner has been developed. To achieve high fusion energy gain, all inertial fusion concepts rely on igniting and burning a portion of the cold dense fuel or liner by heat deposition from alpha particles released from a centrally ignited hot-spot of relatively much lower mass. The key ingredient in the new model is that the stopping length of the alpha particles incident on the inner liner layer increases with liner temperature. The front velocity and temperature of the liner heating zone scale in time respectively as Vf ~ 1/t(2/5) and T ~ t2/5. In terms of the fuel burn-up fraction Fb and the product of the radius times the density, ρ*r, of the hot spot, the liner heating zone temperature scales as T ~ [Fb*ρ*r]2/5. As a result, the model predicts that magnetized target hot spots that achieve ignition at much lower ρ*r and Fb than ICF targets, will produce liner temperatures much too low to “fire-up” the liner fuel.

February 17, 2006

GYRO received an Innovative and Novel Computational Impact on Theory and Experiment (INCITE) award for 400,000 processor-hours on the ORNL National Center for Computational Sciences (NCCS) Cray X1E. The award will support simulations which couple ion-temperature-gradient plus trapped-electron-mode (ITG/TEM) turbulence to electron-temperature-gradient (ETG) turbulence. This award adds to the recent GYRO Users Group NCCS award of 440,000 processor-hours on the X1E.

Preliminary (and tentative) results on coupled ITG/TEM-ETG simulations using a reduced ion-to-electron mass ratio can be found on the GYRO website at (see publications).

February 10, 2006

A second mechanism, in addition to the novel KAW mechanism described earlier (see Oct. 28 highlight at Theory Weekly Highlights for October 2005 is being considered as a candidate for providing the negative central current drive that maintains the axis safety factor q0 > 1 in DIII-D hybrid discharges. The new scheme also relies on the development of a large co-rotating 2/2 sideband excited by the rotating 3/2 island. We propose that the magnetic perturbations due to the 3/2 and 2/2 modes scatter and redistribute the energetic particles. In ONETWO transport code simulations, it is found that the NB-induced current profile depends on the anomalous transport assumed for the energetic ions. The efficiency of the NB current drive depends on the anomalous transport much more strongly than the neutron production rate. This explanation is similar to a previous idea that toroidal Alfven eigenmodes (TAEs) excited by injected energetic beam ions result in a redistribution of the energetic particles at low density. However, in the higher density hybrid discharges considered here, the TAEs are absent but the 3/2 mode with 2/2 sideband plays an analogous role. This mechanism is expected to work in concert with the KAW to control the safety factor profile and maintain its central value above unity in the hybrid discharges.

February 03, 2006

A GYRO development workshop was held during the week of Jan 16-20 2006. Users from PPPL, ORNL and the University of Texas came to contribute to the development of new standards for experimental profile data, and to refine the TRANSP-GYRO and ONETWO-GYRO interfaces. Much of the machinery for handling experimental profiles in global simulations, and the mapping of experimental data to a given point for local simulations, was logically reorganized and improved. In addition, two days of the meeting were devoted to more general-interest GYRO lectures, which were attended by three graduate students from UCLA as well as more seasoned GYRO users. The final deliverable is the release of a much improved, polished and documented version 5.0.0 of GYRO. See for more details.

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