950 research outputs found
Nonlinear structures in interchange mode turbulence
Generation of linearly unstable streamers and nonlinearly saturated zonal flows have been investigated using two-dimensional simulations of interchange mode turbulence. It has been shown that nonlinear diamagnetic interactions tend to inhibit spectral cascades towards larger length scales and consequently suppress the nonlinear excitation of zonal flows. The latter has also been found to be in qualitative agreement with a theoretical analysis based on the reductive perturbation method
Zonal flow generation in ion temperature gradient mode turbulence
In the present work the zonal flow (ZF) growth rate in toroidal
ion-temperature-gradient (ITG) mode turbulence including the effects of
elongation is studied analytically. The scaling of the ZF growth with plasma
parameters is examined for typical tokamak parameter values. The physical model
used for the toroidal ITG driven mode is based on the ion continuity and ion
temperature equations whereas the ZF evolution is described by the vorticity
equation. The results indicate that a large ZF growth is found close to
marginal stability and for peaked density profiles and these effects may be
enhanced by elongation.Comment: 20 pages, 5 figure
Occupational asthma: a review.
Occupational asthma is the most common form of occupational lung disease in the developed world at the present time. In this review, the epidemiology, pathogenesis/mechanisms, clinical presentations, management, and prevention of occupational asthma are discussed. The population attributable risk of asthma due to occupational exposures is considerable. Current understanding of the mechanisms by which many agents cause occupational asthma is limited, especially for low-molecular-weight sensitizers and irritants. The diagnosis of occupational asthma is generally established on the basis of a suggestive history of a temporal association between exposure and the onset of symptoms and objective evidence that these symptoms are related to airflow limitation. Early diagnosis, elimination of exposure to the responsible agent, and early use of inhaled steroids may play important roles in the prevention of long-term persistence of asthma. Persistent occupational asthma is often associated with substantial disability and consequent impacts on income and quality of life. Prevention of new cases is the best approach to reducing the burden of asthma attributable to occupational exposures. Future research needs are identified
Zonal flow generation in collisionless trapped electron mode turbulence
In the present work the generation of zonal flows in collisionless trapped
electron mode (TEM) turbulence is studied analytically. A reduced model for TEM
turbulence is utilized based on an advanced fluid model for reactive drift
waves. An analytical expression for the zonal flow growth rate is derived and
compared with the linear TEM growth, and its scaling with plasma parameters is
examined for typical tokamak parameter values.Comment: 20 pages, 4 figure
Influence of the radio frequency ponderomotive force on anomalous impurity transport in tokamaks
Trace impurity transport in tokamaks is studied using an electrostatic, collisionless fluid model for ion-temperature-gradient and trapped-electron mode driven turbulence in the presence of radio frequency (rf) fields, and the results are compared with neoclassical predictions. It is shown that the inward impurity convective velocity (pinch) that is usually obtained can be reduced by the rf fields, in particular close to the wave resonance location where the rf ponderomotive force may be significant. However, the impurity diffusivity and convective velocity are usually similarly affected by the ponderomotive force, and hence the steady-state impurity density peaking factor -∇nz/nz is only moderately affected by the rf fields
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Predicted versus monitored performance of energy-efficiency measures in new commercial buildings from energy edge
Energy Edge is a research-oriented demonstration program involving 28 new commercial buildings in the Pacific Northwest. This paper discusses the energy savings and cost-effectiveness of energy-efficiency measures for the first 12 buildings evaluated using simulation models calibrated with measured end-use data. Average energy savings per building from the simulated code baseline building was 19%, less than the 30% target. The most important factor for the lower savings is that many of the installed measures differ from the measures specified in the design predictions. Only one of the first 12 buildings met the project objective of reducing energy use by more than 30% at a cost below the target of 56 mills/kWh (in 1991 dollars). Based on results from the first 12 calibrated simulation models, 29 of the 66 energy-efficiency measures, or 44%, met the levelized cost criterion. Despite the lower energy savings from individual measures, the energy-use intensities of the buildings are lower than other regional comparison data for new buildings. The authors review factors that contribute to the uncertainty regarding measured savings and suggest methods to improve future evaluations
Negative-energy perturbations in cylindrical equilibria with a radial electric field
The impact of an equilibrium radial electric field on negative-energy
perturbations (NEPs) (which are potentially dangerous because they can lead to
either linear or nonlinear explosive instabilities) in cylindrical equilibria
of magnetically confined plasmas is investigated within the framework of
Maxwell-drift kinetic theory. It turns out that for wave vectors with a
non-vanishing component parallel to the magnetic field the conditions for the
existence of NEPs in equilibria with E=0 [G. N. Throumoulopoulos and D.
Pfirsch, Phys. Rev. E 53, 2767 (1996)] remain valid, while the condition for
the existence of perpendicular NEPs, which are found to be the most important
perturbations, is modified. For ( is the
electrostatic potential) and ( is
the total plasma pressure), a case which is of operational interest in magnetic
confinement systems, the existence of perpendicular NEPs depends on ,
where is the charge of the particle species . In this case the
electric field can reduce the NEPs activity in the edge region of tokamaklike
and stellaratorlike equilibria with identical parabolic pressure profiles, the
reduction of electron NEPs being more pronounced than that of ion NEPs.Comment: 30 pages, late
Mechanisms for conduction via low-frequency noise measurements of High-Tc Thin-film microbridges
We have investigated possible mechanisms for conduction in high-T/sub c/ thin-film microbridges biased into the voltage state via the low-frequency noise properties. Measurements on thinned YBCO microbridges indicate that the voltage noise power spectral density S/sub V/(f) is proportional to the DC voltage.Peer Reviewe
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