Theory Weekly Highlights for December 2010

December 17, 2010

It was recently shown (Kinsey, IAEA2010) that the TGLF model predicts an ITER Q, the ratio of fusion power to input power, which scales as βped2/Paux0.8 in the absence of an input torque, indicating a very stiff core. Here Paux is the input auxiliary power and βped is the H-mode pedestal β. From the definition of the Fusion performance Q and the scaling of the fusion reactivity with density (~n2) and temperature (~T2), if the core transport is perfectly stiff, then Q scales like βped2/Paux. Perfect core stiffness means that β 0/ βped, where β 0 is β on the magnetic axis, is independent of power throughput. Stiffness then depends on the operating point as well as the transport model. For example published work from the 2002 Snowmass review showed ITER-Q scaling like 1/Paux0.9 for the more stiff GLF23 and 1/Paux0.5 for the less stiff MM95 model at fixed βped. TGLF is validated on a large database of DIIID, Cmod, TFTR experiments compiled over the past decade. Those experiments used to validate TGLF have now been shown to operate with core stiffness comparable to the reported 2010 IAEA ITER predictions. By quantifying stiffness, S, as the ratio of the percent change in transported power to the percent change in local temperature gradient, the radial profiles of S from TGLF for DIII-D and JET H-modes used for the TGLF validation are comparable to the stiff ITER projection. A working group is planning new ITER relevant DIII-D experiments to more precisely quantify core stiffness, for example to quantify the weak increase of β 0/ βped with power against TGLF simulations of the same discharge, as well as any favorable increase of βped with power.

December 10, 2010

For various DIII-D H-mode discharges, NEO was used to systematically study finite-orbit-width (FOW) effects due to steep gradients and access the validity of local neoclassical transport in the near-edge region. Three test case discharges were studied: a typical pedestal width and height, a narrow pedestal, and a large pedestal width. For all three cases, no breakdown of local neoclassical theory was observed for quantities that vary on the electron scale such as the electron energy flux, Qe. Furthermore, local neoclassical theory appears to be valid for the bootstrap current in the near-edge. Only a moderate finite-orbit-width effect was found for the ion energy flux, Qi. Nonlinear full distribution function (Nonlinear-F) simulations may be required for Qi near the edge for ψN > 0.93, but other effects not included here, such as ion orbit loss, may be more important in this region. Regarding the geometry effects, up-down asymmetry flux-surface shaping effects on the non-local transport were found to be weak, but adequate numerical resolution is essential. Specifically, high theta-grid resolution, more than twice the standard resolution, is needed in the edge, due to the strong shaping effects, to prevent artificial enhancement of the FOW effect.

December 03, 2010

U.S. Patent #7,831,008, November 9, 2010, was awarded to Paul Parks and Francis Perkins for the invention “Microwave-Powered Pellet Accelerator”. This invention couples gyrotron power to a composite pusher-pellet module consisting of small conducting particles such as beryllium or lithium embedded homogeneously in a deuterium ice “pusher” located behind the pellet. The metallic particles absorb microwave power by means of eddy current dissipation vaporizing the ice into a high-pressure “propellant” gas, which continually absorbs wave energy and drives the pellet down a waveguide/launch tube. This new method represents a ten-fold increase in pellet velocity from ITER’s provisional fueling system, and will allow for much deeper penetration of the fusion fuel into ITER.

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