Theory Weekly Highlights for May 2005

May 27, 2005

Application of the GLF23 transport model to ITER predicts that the toroidal rotation generated in ITER by negative ion neutral beams injected in the direction of the plasma current can significantly increase the fusion power produced compared to heating without torque. The increase in fusion power with beam power is predicted to be steeper for co-injected than for balanced beams. Beam voltages in the range of present technology (300-400keV) produced the highest rotation and fusion power enhancement for fixed beam power. The pedestal pressure needed to reach a target fusion power is also lower for co-injection.

May 20, 2005

New numerical investigations of edge stability using the NIMROD code have resolved an earlier apparent discrepancy between the NIMROD results and ideal linear stability codes DCON and ELITE. Kinetic equilibrium reconstructions from DIII-D discharges with edge localized modes including accurate edge reconstructions, as well as similar model equilibria, were typically found to be unstable at the edge with ELITE and DCON but unstable to a resistive interchange like mode located at the top of the pedestal with NIMROD. The NIMROD formulation includes resistivity and viscosity. Simulations of edge-localized modes in other cases had generally shown agreement between all three codes in mode structure and, in the case of ELITE and NIMROD, in growth rate. In the new analysis, when the resistivity and viscosity are significantly reduced in the edge region, and the ideal mode is fully converged, the linear eigenfunction transitions to an edge mode with growth rates in agreement with the ideal codes. The interchange-like mode in NIMROD is being investigated for a possible role in the large radial transport observed just inside the pedestal region in the experiments.

May 13, 2005

An analytic formula for the “effective” number of electrons Zeff to be used in calculating the penetration range of plasma electrons in target gases was derived for the case of target gases with many electron atoms, (e.g. argon). The electron penetration range is an important ingredient in the construction of gas jet penetration models in the low temperature edge of tokamaks (Te < 1 keV) and during the post thermal quench phase. In the many electron Thomas-Fermi model of the atom, the orbital electrons are considered as a completely Fermi-degenerate gas and the Fermi energy Ef and the local charge density distribution of the electron cloud, n ~ Ef3/2, are uniquely related to the radial coordinate. The incident electron can only lose energy on cloud electrons with Fermi energy less than the incident electron energy. This criterion, due to Sugiyama (1985), yields an analytic formula for the “effective” number of electrons Zeff to be used in the Bethe-like stopping power formula providing an important low-energy correction below ~1 keV. For a 400 eV electron incident on argon (atomic number Z = 18), the Zeff = 9.67. This correction increases the penetration range by a factor of almost two from that predicted by the standard Bethe-like formula.

May 06, 2005

Between-shot TRANSP simulations using the FusionGrid resource were successfully tested during DIII-D operation. The requests were triggered by the between-shot data analysis event system and submitted to the FusionGrid computer at PPPL. With advanced CPU reservation, the request preempted other processes on the computer to guarantee 100% CPU to the between-shot TRANSP run. The process was monitored by the Data Analysis Monitor (DAM) and the results were written back to the MDSplus server at DIII-D.

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