Transport Length Scales Research Articles

The quiescent double barrier regime in DIII-D

The quiescent double barrier (QDB) regime is a high performance regime recently identified in DIII-D and characterized by a double transport barrier structure (core and edge) that can be maintained for several seconds, often limited only by the pulse length capabilities of the DIII-D hardware. The QDB regime has been sustained for up to 25 τE with fusion performance of up to βNH89≈7. The edge barrier is ELM-free, but modulated by low frequency MHD activity that allows density control via an external cryopump. The core barrier is similar to those seen in previous internal transport barrier experiments, but is maintained without complete stabilization of turbulence. Instead, the turbulence correlation lengths become very short so as to minimize the transport length scales. The two barriers are separated by a region of high transport that is a consequence of a zero-crossing in the E×B shearing rate. These discharges typically possess highly peaked density profiles. This has several implications: narrow bootstrap current profile, reduced beta limit and increased impurity retention. We will report on studies of each of these issues.

Length-scale dependence of surface relief gratings in azobenzene side-chain polymers

Polarized irradiation of azobenzene side-chain polymers yields intriguing surface relief structures. Up to now, the physical mechanism of this process is unclear. We report on a study aimed at clarifying the intrinsic length scales of the substantial mass transport which evolves during laser irradiation. We vary the period of an incident intensity grating by more than an order of magnitude, and investigate the resulting relief amplitudes with atomic force microscopy.

Current methods and research strategies for modeling atmospheric mercury

The atmospheric pathway of the global mercury cycle is known to be the primary source of mercury contamination to most threatened aquatic ecosystems. Current efforts toward numerical modeling of atmospheric mercury are hindered by an incomplete understanding of emissions, atmospheric transformations, and deposition processes. While much effort has been made to quantify the total mass flux of mercury to the atmosphere from various natural and anthropogenic sources, discrimination of the chemical and physical forms of these emissions is just beginning in response to early modeling exercises showing this discrimination to be critical for accurate modeling estimates of the sources responsible for observed mercury deposition. A similar discrimination of ambient concentrations of mercury throughout the atmosphere is needed in order to develop a clear understanding of atmospheric transformation processes, both chemical and physical, which govern the length scale of atmospheric mercury transport and patterns of its deposition in both wet and dry processes. In this paper, current atmospheric mercury modeling techniques and the information obtained from them are described. A strategy for future field research and numerical model development is proposed which is intended to allow a confident identification of the sources of atmospheric mercury responsible for observed contamination of aquatic ecosystems.

Length scales of charge transport in organic photorefractive materials

The drift length of charge carriers has a significant influence on the dynamics of the space-charge field in organic photorefractive materials. This letter introduces a relatively simple method for the determination of the drift length, which takes into account that the charge carrier mobility depends on the sample thickness. By combining results of time-of-flight and holographic time-of-flight experiments using the stochastic transport model of Scher and Montroll, the effective drift length can be determined as 2.4 μm in the investigated photorefractive glass.

Resuspension of fall-out material following the Chernobyl accident

Air concentration and surface contamination data have been collected from a cross-section of sites across Europe where measurements have been made for 2 or 3 years after the Chernobyl accident. Data for over 20 European sites are considered; for U.K. sites data up to 1992 have been analysed. The results have shown that 137Cs derived from Chernobyl could be measured in air up to 6 years after deposition. Values of the resuspension factor ( K) have been calculated and show a reduction with time at all sites. The decrease in K could be described by an exponential function (time constant=0.02 to 0.12 month −1) or a power law (with an exponent about −0.5 to −1.0), with similar degrees of correlation. A systematic decrease in K at sites with increasing deposition was also observed. In the first year after the accident, the time integral of the deposition flux was a large fraction of the initial deposition. Depending upon the site, between 0.01 and 1.0 times the initial deposit was accumulated in deposition collectors. This represents a significant contamination risk well after the original deposition event. Evidence suggests, however, that the length scale for transport of most of the resuspended material is very limited.

Nonequilibrium plankton community structures in an ecohydrodynamic model system

Abstract. Due to the local and global impacts of algae blooms and patchiness on water quality, carbon cycling and climate, models of plankton dynamics are of current interest. In this paper, the temporal and spatial patterns in natural plankton communities are interpreted as transient and stationary nonequilibrium solutions of dynamical nonlinear interaction-diffusion-advection systems. A simple model of phytoplankton-zooplankton dynamics (Scheffer, 1991) is presented in space and time. After summarizing the local properties as multiple stability and oscillations, the emergence of spatial and spatio- temporal patterns is considered, accounting also for diffusion and weak advection. In order to study the emergence and stability of these structures under hydrodynamic forcing, the interaction- diffusion-advection model is coupled to the hydrodynamic equations. It is shown, that the formation of nonequilibrium spatio-temporal density patterns due to the interplay of the deterministic nonlinear biological interactions and physical processes is a rare occurrence in rapidly flowing waters. The two-timing perturbation technique is applied to problems with very rapid single-directed steady flows. A channel under tidal forcing serves as and example for a system with a relatively high detention time of matter. Generally, due to the different time and length scales of planktic interactions, diffusion and transport, initial nonequilibrium plankton patches are simply moved through the system unless the strong hydrodynamic forces do not destroy them before.

Microscale isotopic zonation in graphite crystals: Evidence for channelled CO influx in granulites

Coarse graphite in association with cordierite and quartz within a vein in a metapelite locality at Chittikara (CHT) in southern India illustrates precipitation of graphite from CO 2-rich fluids. Carbon isotope traverses within individual graphite crystals using a microsampling technique reveal strong isotopic zonations in graphite crystal from the vein, with δ 13C values oscillating between −13‰ in the core and −8.5‰ in the rim. CO 2 trapped within fluid inclusions in the cordierite shows δ 13C= −5‰, suggesting isotopic equilbrium with the core composition of the graphite. A graphite crystal from the metapelite immediately adjacent to the vein is also zoned, with a smooth rimward increase in 13C. Disseminated graphite flakes in the host metapelites ca. 1 m from the vein are, however, unzoned, and possess strongly negative δ 13C values (−34.4‰). The rimward increase in 13C, the absence of other carbon-bearing phases and the sluggish exchange rates for carbon in graphites preclude diffusive re-equilibration, and suggest that the different zoning patterns in the coarse graphite flakes within and adjacent to the vein assemblage at CHT have resulted from precipitation in different parts of a highly channelled fluid regime, where fluids are non-wetting and move episodically by a hydraulic fracture mechanism. Our study provides the first unambiguous insight into graphite precipitation mechanisms, and has potential implications for the nature of fluid interconnectivity and CO 2 transport and equilibration length scales within the crust.

Effects of material mixing on planetary gamma ray spectroscopy

In many types of compositional remote sensing, a key problem involves separating the signals of two or more compositionally distinct materials that are mixed on a planet's surface. We investigate mixing effects in planetary gamma ray spectroscopy. Two important sources of planetary gamma ray emissions are inelastic scatter reactions induced by energetic neutrons and prompt capture reactions induced by thermal neutrons. The nature of the neutron energy spectrum at a planet's surface therefore exerts an important influence on the nature of the gamma radiation emitted and on the interpretation of compositional information from that radiation. The most important factors controlling the neutron energy spectrum are the hydrogen content xH and the macroscopic thermal neutron absorption cross section ∑ of the surface material. If a planetary surface is composed of several mixed materials with differering values of xH and/or ∑, then the emitted gamma ray spectrum can depend sensitively on the physical nature of the mixing. An important example may be Mars, where one expects to find rocks with low xH intermixed with fine‐grained soil that may have a significantly higher xH due to inclusion of water of hydration. If rocky materials are mixed through the soil at a size scale that is finer than the characteristic length scale for neutron transport (tens of centimeters), then they will be immersed in the thermal neutrons produced by scattering in the H‐rich soil. Such rock may be expected to generate strong prompt capture lines. However, rock segregated in bodies larger than this length scale will be shielded from the thermal neutrons, so their emission will be dominated by inelastic scatter lines. We quantify these effects by modeling gamma ray emissions from the Martian surface. Our model includes production of neutrons by cosmic ray interactions, neutron scattering, gamma ray production by inelastic scatter, thermal neutron capture, and natural radionuclides, attenuation of the gamma ray signal by passage through surface materials and the Martian atmosphere, production of the gamma ray continuum background, and the physics and statistics of gamma ray detection from an orbiting spacecraft. In order to investigate the kinds of effects that can occur, we consider mixing of “Viking soil” with rocks of “mafic” and “felsic” composition, in three geometries: intimate mixing, in which the mixing length scale is small compared to neutron transport length scales; checkerboard mixing, in which it is large; and layered mixing, in which a thin layer of soil overlies a semi‐infinite layer of rock. We also consider mixing of soil and ice in these geometries. We show that mixing geometry can have a major impact on observed spectra and hence on geochemical interpretations. We also show that significant steps can be made toward characterizing the mixing geometry and disentangling the soil and rock signals if adequate independent knowledge of the composition and concentration of the soil component is available.

Length scales of convection-dispersion approaches to flow and tranport in porous media

Two length scales of one-dimensional transient flow and transport in porous media are introduced: The minimum mixing length, l s, also referred to as Lagrangean length scale, and the width of hydrodynamics spread, l s. The macroscopic path length, l c, in the medium causing hydrodynamic dispersion has to exceed markedly both l m and l s if the parameters derived from the application of convection-dispersion equations should be of predictive value. The length scales are derived and discussed. Particular attention is given to the hydrodynamic dispersion of soil moisture and its potential energy.

Chronology and mechanism of depletion in Lewisian granulites

The pattern of Th, U and Rb depletion has been investigated in a suite of contrasting lithologies from the classic Lewisian granulite terrain at Scourie, N.W. Scotland. The study is based on lithologies ranging from ultramafic to tonalitic gneisses, together with calcsilicate and pelitic metasediments, sampled on the metre scale. Because of their close proximity, all of these lithologies have experienced identical P-T-t histories since depletion; however, their expected dehydration and melting characteristics are markedly different. Pb isotope analyses of the samples define the latest time of U/Pb fractionation as 2.665±0.026 Ga, and indicate substantial U and Th depletion relative to Pb in tonalitic and mafic lithologies, and in some metasediments. In contrast, ultramafic samples show no U and Th depletion. Sm-Nd isotope analyses of the mafic and ultramafic samples define an age of 2.707±0.052 Ga, which is interpreted as the time of igneous differentiation of the mafic-ultramafic bodies. This time is indistinguishable from the latest time of Th−Pb and U−Pb fractionation. Overall there is a straightforward relationship between the magnitude of depletion, lithology and the stability of mineral phases. Tonalitic and mafic gneisses are depleted in Th and U relative to Pb, whereas ultramafic gneisses which retain amphibole show no evidence for depletion. The observations are consistent with loss of U, Th and Rb in fluids produced by metamorphic dehydration and melting. A role for externally derived CO2 in the depletion process is not excluded by the data but there is no evidence for it having existed. Limits are placed on the length scale of transport for several elements. U, Th and Rb are depleted regionally from mafic and tonalitic lithologies; transport distances must have been on the kilometre scale. In contrast, movements of Sm and Nd have been more restricted.

UNA PRIMERA ESTIMACION DEL TRANSPORTE DE S02 SOBRE LA CIUDAD DE MEXICO

Correlation coefficients for SO2, CO and suspended particles between four monitoring stations located in extreme NW, NE, SW and SE corners of Mexico City urban area are found to be significantly high. The air quality standard for SO2 (0.10 ppm) was exceeded more than 10% of the time in January 1976.
 SO2 concentrations as well as wind intensity at the same four stations were found to be log-normally distributed for the January data. Space-time correlations between the different stations and at different time-intervals were also studied to establish the transport length scale of SO2 over the city. A distinct peak of correlation at two-hour interval over the center west of city (Chapultepec Park) indicates a southward flow of contaminants along the foot hills of mountains to the west.

A space-time correlation study of oxidant and carbon monoxide in the Los Angeles basin

Abstract Autocorrelations of oxidant and carbon-monoxide between different sites and at different times and time intervals (within ± 5 h) in the Los Angeles Basin were studied to establish the transport length scale of the pollutants. Consideration of several ground-based station pairs suggests that oxidant has a transport scale on the order of 55 km along the San Gabriel Valley, east of Los Angeles, in the afternoon, when wind transport is most effective. However, the transport scale appears to be short to the north because of the mountain range, and to the south because of high local NO sources and the Palos Verdes convergence. The transport scale for CO on the other hand is short since local emissions can easily dominate the transported portion. In the afternoon, the lack of surface wind dependence in the oxidant correlations between Los Angeles and Pasadena, and between Los Angeles and Azusa may indicate the importance of high altitude transport.