Turbulent transport of impurities and their effect on energy confinement

By presenting linear and nonlinear gyrokinetic studies, based on a balanced neutral beam injection deuterium discharge from the DIII-D tokamak, we demonstrate that impurities alter the scaling of the transport on the charge and mass of the main species, and even more importantly, they can dramatically change the energy transport even in relatively small quantities. A poloidally varying equilibrium electrostatic potential can lead to a strong reduction or sign change of the impurity peaking factor due to the combined effect of the in-out impurity density asymmetry and the EXB drift of impurities. We present an approximate expression for the impurity peaking factor and demonstrate that impurity peaking is not significantly affected by impurity self-collisions.Comment: Accepted for publication in Plasma Physics and Controlled Fusio

Characteristics of drug and dietary supplement inquiries by college athletes.

BackgroundIn the United States, the National Center for Drug Free Sport manages the drug-testing programs for athletes of the National Collegiate Athletic Association (NCAA). Through its Resource Exchange Center (REC), Drug Free Sport supports athletic staff and athletes with information regarding drugs and dietary supplements.PurposeTo characterize the types of drug-related and dietary supplement-related inquiries submitted to Drug Free Sport through the REC.Study designCross-sectional study.MethodsAll inquiries submitted to the REC for the period of September 1, 2005, through June 30, 2006, were reviewed. The data were categorized by the method of inquiry submission; the name of the substance in question; the sex, sport, and NCAA division of the athlete involved; the nature of the inquiry; and the response provided by the REC regarding the NCAA's status of the substance in question.ResultsPseudoephedrine, acetaminophen/hydrocodone, and albuterol were the most commonly self-searched medications; stimulants accounted for the majority of banned medications. Dietary supplements accounted for 80% of all inquiries submitted to the REC via the Banned Drug Inquiry Form. Among all dietary supplements, creatine was the most commonly inquired. Banned substances accounted for 29% of all inquiries.ConclusionsThere were more than 10 000 inquiries regarding the status of medications, dietary supplements, and other substances for NCAA athletes during the 2005-2006 academic year. It is helpful for athletes to have resources that help them navigate banned-substance lists and so avoid the inadvertent use of banned substances.Clinical relevanceEducating athletes regarding the stimulant content of various dietary supplements and addressing the lack of clinical trials to support stated claims and safety appear critical

Asymmetries in Mars' Exosphere: Implications for X-ray and ENA Imaging

Observations and simulations show that Mars' atmosphere has large seasonal variations. Total atmospheric density can have an order of magnitude latitudinal variation at exobase heights. By numerical simulations we show that these latitude variations in exobase parameters induce asymmetries in the hydrogen exosphere that propagate to large distances from the planet. We show that these asymmetries in the exosphere produce asymmetries in the fluxes of energetic neutral atoms (ENAs) and soft X-rays produced by charge exchange between the solar wind and exospheric hydrogen. This could be an explanation for asymmetries that have been observed in ENA and X-ray fluxes at Mars.Comment: Submitted to Space Science Review. v2: Minor changes in text and figure

Competing Alignments of Nematic Liquid Crystals on Square Patterned Substrates

A theoretical analysis is presented of a nematic liquid crystal confined between substrates pat- terned with squares that promote vertical and planar alignment. Two approaches are used to eluci- date the behavior across a wide range of length scales: Monte Carlo simulation of hard particles and Frank-Oseen continuum theory. Both approaches predict bistable degenerate azimuthal alignment in the bulk along the edges of the squares; the continuum calculation additionally reveals the possi- bility of an anchoring transition to diagonal alignment if the polar anchoring energy associated with the pattern is sufficiently weak. Unlike the striped systems previously analyzed, the Monte Carlo simulations suggest that there is no "bridging" transition for sufficiently thin cells. The extent to which these geometrically patterned systems resemble topographically patterned substrates, such as square wells, is also discussed.Comment: 11 pages, 12 figure

Fuel injection assembly for gas turbine engine combustor

A fuel injection assembly for a gas turbine engine combustor, including at least one fuel stem, a plurality of concentrically disposed tubes positioned within each fuel stem, wherein a cooling supply flow passage, a cooling return flow passage, and a tip fuel flow passage are defined thereby, and at least one fuel tip assembly connected to each fuel stem so as to be in flow communication with the flow passages, wherein an active cooling circuit for each fuel stem and fuel tip assembly is maintained by providing all active fuel through the cooling supply flow passage and the cooling return flow passage during each stage of combustor operation. The fuel flowing through the active cooling circuit is then collected so that a predetermined portion thereof is provided to the tip fuel flow passage for injection by the fuel tip assembly

Synergistic cross-scale coupling of turbulence in a tokamak plasma

For the first time, nonlinear gyrokinetic simulations spanning both the ion and electron spatio-temporal scales have been performed with realistic electron mass ratio ((m[subscript D] [over m [subscript e])[superscript 1 over 2] = 60.0), realistic geometry, and all experimental inputs, demonstrating the coexistence and synergy of ion (k[subscript θρs] ~O(1.0)) and electron-scale (k[subscript θρe] ~O(1.0)) turbulence in the core of a tokamak plasma. All multi-scale simulations utilized the GYRO code [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] to study the coupling of ion and electron-scale turbulence in the core (r/a = 0.6) of an Alcator C-Mod L-mode discharge shown previously to exhibit an under-prediction of the electron heat flux when using simulations only including ion-scale turbulence. Electron-scale turbulence is found to play a dominant role in setting the electron heat flux level and radially elongated (k[subscript r] ≪ k[subscript θ]) “streamers” are found to coexist with ion-scale eddies in experimental plasma conditions. Inclusion of electron-scale turbulence in these simulations is found to increase both ion and electron heat flux levels by enhancing the transport at the ion-scale while also driving electron heat flux at sub-ρ[subscript i] scales. The combined increases in the low and high-k driven electron heat flux may explain previously observed discrepancies between simulated and experimental electron heat fluxes and indicates a complex interaction of short and long wavelength turbulence.United States. Dept. of Energy. Office of Science (Contract DE-AC02-05CH11231)United States. Dept. of Energy (Contract DE-FC02-99ER54512-CMOD)United States. Dept. of Energy. Fusion Energy Postdoctoral Research Program (Oak Ridge Institute for Science and Education

An Enhanced Nonlinear Critical Gradient for Electron Turbulent Transport due to Reversed Magnetic Shear

The first nonlinear gyrokinetic simulations of electron internal transport barriers (e-ITBs) in the National Spherical Torus Experiment show that reversed magnetic shear can suppress thermal transport by increasing the nonlinear critical gradient for electron-temperature-gradient-driven turbulence to three times its linear critical value. An interesting feature of this turbulence is nonlinearly driven off-midplane radial streamers. This work reinforces the experimental observation that magnetic shear is likely an effective way of triggering and sustaining e-ITBs in magnetic fusion devices.Comment: 4 pages, 5 figure

An Enhanced Nonlinear Critical Gradient for Electron Turbulent Transport due to Reversed Magnetic Shear

The first nonlinear gyrokinetic simulations of electron internal transport barriers (e-ITBs) in the National Spherical Torus Experiment show that reversed magnetic shear can suppress thermal transport by increasing the nonlinear critical gradient for electron-temperature-gradient-driven turbulence to three times its linear critical value. An interesting feature of this turbulence is nonlinearly driven off-midplane radial streamers. This work reinforces the experimental observation that magnetic shear is likely an effective way of triggering and sustaining e-ITBs in magnetic fusion devices.Comment: 4 pages, 5 figure

Higher Order Force Gradient Symplectic Algorithms

We show that a recently discovered fourth order symplectic algorithm, which requires one evaluation of force gradient in addition to three evaluations of the force, when iterated to higher order, yielded algorithms that are far superior to similarly iterated higher order algorithms based on the standard Forest-Ruth algorithm. We gauge the accuracy of each algorithm by comparing the step-size independent error functions associated with energy conservation and the rotation of the Laplace-Runge-Lenz vector when solving a highly eccentric Kepler problem. For orders 6, 8, 10 and 12, the new algorithms are approximately a factor of $10^3$, $10^4$, $10^4$ and $10^5$ better.Comment: 23 pages, 10 figure

Multi-scale gyrokinetic simulations: Comparison with experiment and implications for predicting turbulence and transport

To better understand the role of cross-scale coupling in experimental conditions, a series of multi-scale gyrokinetic simulations were performed on Alcator C-Mod, L-mode plasmas. These simulations, performed using all experimental inputs and realistic ion to electron mass ratio ((mi/me)1∕2 = 60.0), simultaneously capture turbulence at the ion (kθρs∼(1.0)) and electron-scales (kθρe∼(1.0)). Direct comparison with experimental heat fluxes and electron profile stiffness indicates that Electron Temperature Gradient (ETG) streamers and strong cross-scale turbulence coupling likely exist in both of the experimental conditions studied. The coupling between ion and electron-scales exists in the form of energy cascades, modification of zonal flow dynamics, and the effective shearing of ETG turbulence by long wavelength, Ion Temperature Gradient (ITG) turbulence. The tightly coupled nature of ITG and ETG turbulence in these realistic plasma conditions is shown to have significant implications for the interpretation of experimental transport and fluctuations. Initial attempts are made to develop a “rule of thumb” based on linear physics, to help predict when cross-scale coupling plays an important role and to inform future modeling of experimental discharges. The details of the simulations, comparisons with experimental measurements, and implications for both modeling and experimental interpretation are discussed.United States. Department of Energy (DE-AC02-05CH11231)United States. Department of Energy (DE-FC02-99ER54512-CMOD)United States. Department of Energy (DE-SC0006957)United States. Department of Energy (DE-FG02-06ER54871