Theory Weekly Highlights for October 2007

October 26, 2007

A formula derived for the poloidal rotation velocity of impurity ions in a plasma with two ion species resolved the discrepancy between predictions from NCLASS and DIII-D measurements. The derivation was based on neoclassical theory in the banana regime and large aspect ratio circular flux surfaces, in which the toroidal flow is allowed to approach the impurity ion thermal speed. Preliminary application to measured profiles of carbon ion velocity in DIII-D reported in the literature [W.M. Solomon et al, Phys. Plasmas 13 056116 (2006)] shows promising agreement. The direction is correctly predicted by the formula, in contrast to NCLASS, which predicts rotation in the opposite direction. The magnitude predicted by the formula is also in reasonable agreement.

October 19, 2007

A new study of the pedestal height constraint in ITER using the ELITE stability code and an extensive set of model ITER equilibria makes quantitative pedestal predictions for ITER. The new study includes a model of diamagnetic stabilization and explores the impact of variations of magnetic field and current, as well as Shafranov shift. The pedestal height constraint scales as the product of magnetic field and current and with the pedestal width to the 3/4 power, but varies only weakly with the Shafranov shift at the design density, although a larger effect is found at reduced density. For the reference ITER shape and density, at power levels sufficient for the Type I ELM regime, the pedestal height constraint is given within 20% uncertainty as βNped = 0.8(Δψ/0.05)3/4, where Δψ is the pedestal width in normalized poloidal flux.

October 12, 2007

Disruption mitigation simulations using the NIMROD code coupled with the KPRAD radiation code suggest that adding a small fraction of boron to a dense helium gas jet can increase the total radiated power during the initial jet penetration and reduce the thermal quench onset time. C-Mod plasmas contain approximately 1% boron, which is a considerably better radiator than helium below 10eV. Even at that small a fraction, the boron can contribute as much to the total radiated power as the dense helium gas jet. Simulations in which boron radiation has been added to a helium jet, predict an increase in both the total radiated power during the initial jet penetration and a reduction in the quench onset time. The results strongly motivate the addition of carbon radiation to DIII-D simulations with a helium gas jet.

October 05, 2007

Local gyrokinetic simulations of a DIII-D L-mode discharge, using GYRO, are able to simultaneously reproduce experimentally measured ion and electron heat diffusivities as well as the root-mean-square (RMS) density fluctuation levels measured by beam emission spectroscopy (BES) at mid-radius (ρ = 0.5). Synthetic BES signals, which account for the finite wavenumber sensitivity of the volume-sampling measurement, are generated from GYRO to perform quantitative comparisons with the experimental signals. In contrast, local simulations of the outer core (ρ = 0.75) are found to under predict the ion and electron heat fluxes by a factor 4. The synthetic RMS density fluctuations at this location are less than half the level measured by BES. Additional comparisons of frequency spectra and correlation lengths, as well as to Te fluctuations obtained by the new correlation electron cyclotron emission (CECE) diagnostic are underway, and will be presented at the 2007 APS-DPP meeting.

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