Theory Weekly Highlights for February 2004

February 27 2004

Calculations using the MARS-F code have revealed a synergistic effect between active feedback and rotational stabilization of the resistive wall mode (RWM) with the feedback coil located outside the plasma chamber. This synergistic stabilization is most effective when the growth rate of the RWM, γ, has been slowed by rotation to a sufficiently low value of γτw < 3, where τw is the flux diffusion time constant of the resistive wall. At that time the feedback gain required for stabilization is reduced considerably. This synergistic effect may be utilized advantageously in future devices such as ITER where plasma rotation by itself may not be sufficient to stabilize the RWM.

February 20 2004

The theory developed for the onset and early evolution of tearing modes in high beta sawtoothing discharges in DIII-D appears to be valid in Alcator C-mod as well. Extensive nonlinear simulations of the seeding mechanism for DIII-D previously established the crucial role of evolving the linear stability in addition to modeling the nonlinear drive in the neoclassical island evolution. Preliminary analyses of equilibrium reconstructions from a similar Alcator C-mod experiment are consistent with the importance of both the classical linear and the nonlinear coupling drive to the tearing mode onset and early evolution, even though the sawtooth physics and neoclassical drive are expected to be different from that in DIII-D. The theory predicts that the details of the sawtooth physics and neoclassical drive are not important in determining the relative amplitude of the nonlinear drive to the early island evolution. This relative amplitude appears to be consistent with the theory in both experiments.

February 13 2004

Previous theories of transport barrier formation have put forward the idea that a radial region that contains a minimum-q (zero shear) point is favorable to the formation of an ion transport barrier. However, studies of reversed shear plasmas using the GYRO code show, on the contrary, that there is no transport suppression in such a minimum-q region. These simulations indicate that the appearance of non-resonant ITG modes, which are neglected in the barrier formation theories, ensure that transport remains smooth across the point where shear vanishes. This result implies that theories of ITB formation that invoke special gyrokinetic effects at zero shear can be discounted; one needs to look at non-gyrokinetic explanations.

February 06 2004

MDSplus data from 2003 was migrated from the old RAID array to the new Atlas server. The long-term plan is to replace the old RAID array with a new mass storage system, at which time all MDSplus data will be served from the new Atlas server the new Atlas server serves data faster than the old system. Summary data from DIII-D discharge 70200 to the present were loaded into the DIII-D Relational Database (D3DRDB). These data values include scalars that summarize the characteristics of each discharge. A list of quantities in the summaries table is available at

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