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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,441 records · Page 80
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Nanoscale insight into the p-n junction of alkali-incorporated Cu(In,Ga)Se 2 solar cells

The effects of alkali diffusion and post-deposition treatment in three-stage processed Cu(In,Ga)Se 2 solar cells are examined by using atom probe tomography and electrical property measurements. Cells, for which the substrate was treated at 650 °C to induce alkali diffusion from the substrate prior to absorber deposition, exhibited high open-circuit voltage (758 mV) and efficiency (18.2%) and also exhibited a 50 to 100-nm-thick ordered vacancy compound layer at the metallurgical junction. Surprisingly, these high-temperature samples exhibited higher concentrations of K at the junction (1.8 at.%) than post-deposition treatment samples (0.4 at.%). A model that uses Ga/(Ga + In) and Cu/(Ga + In) profiles to predict bandgaps (+/-17.9 meV) of 22 Cu(In,Ga)Se2 solar cells reported in literature was discussed and ultimately used to predict band properties at the nanoscale by using atom probe tomography data. The high-temperature samples exhibited a greater drop in the valence band maximum (200 meV) due to a lower Cu/(Ga + In) ratio than the post-deposition treatment samples. There was an anticorrelation of K concentrations and Cu/(Ga + In) ratios for all samples, regardless of processing conditions. In conclusion, changes in elemental profiles at the active junctions correlate well with the electrical behaviour of these devices.

14 SOLAR ENERGY↗
DOE OSTI2016

Simulation of fission gas diffusion in UO{sub 2} by vacancy clusters

The low solubility of fission gas in nuclear fuel leads to inter- and intra-granular bubbles. These bubbles affect performance of the fuel rod and increase the fuel temperature through the increase of porosity in the fuel pellets and decreased thermal conductivity across the pellet-clad gap as gas is released into the plenum. The nucleation, growth, and inter-connectivity of the fission gas bubbles ultimately depend on the diffusivity of gas atoms through the fuel matrix. Using mechanisms and data previously established from density functional theory (DFT) and empirical potential calculations, diffusion and reaction models have been derived for xenon atoms, uranium atoms, and vacancies in the substitutional and interstitial locations. Although similar to previous models, the diffusion and reaction rates have been re-derived in a more robust manner. Previous studies have under-predicted the experimentally observed diffusivity of xenon atoms in UO{sub 2}, however it has been previously shown that by including xenon atoms bound to two vacancies, the calculated diffusivity approaches the measured data. The derived model for species diffusion and inter-species reactions has been implemented in the MARMOT phase-field code in a modular structure so as to capture the enhanced diffusion of xenon due to bound gas + vacancy-cluster movement. The MARMOT code allows for the diffusion, reaction, and source rates to be solved simultaneously, resulting in steady state or transient concentrations of each species, and ultimately the diffusion behavior of xenon atoms. By including the diffusion of clusters of different combinations of xenon and vacancies, the calculated diffusivity more closely captures the measured diffusivity of xenon when using preliminary parameters. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

A FETI approach to domain decomposition for meshfree discretizations of nonlocal problems

We propose a domain decomposition method for the efficient simulation of nonlocal problems. Our approach is based on a multi-domain formulation of a nonlocal diffusion problem where the subdomains share “nonlocal” interfaces of the size of the nonlocal horizon. This system of nonlocal equations is first rewritten in terms of minimization of a nonlocal energy, then discretized with a meshfree approximation and finally solved via a Lagrange multiplier approach in a way that resembles the finite element tearing and interconnect method. Specifically, we propose a distributed projected gradient algorithm for the solution of the Lagrange multiplier system, whose unknowns determine the nonlocal interface conditions between subdomains. Several two-dimensional numerical tests on problems as large as 191 million unknowns illustrate the strong and the weak scalability of our algorithm, which outperforms the standard approach to the distributed numerical solution of the problem. Finally, this work is the first rigorous numerical study in a two-dimensional multi-domain setting for nonlocal operators with finite horizon and, as such, it is a fundamental step towards increasing the use of nonlocal models in large scale simulations.

42 ENGINEERING↗
DOE OSTI2019

Ultrafast flow chemistry for the acid-catalyzed conversion of fructose

Biomass conversion to chemicals and fuels has been epitomized by batch processing and long reaction times that limit the economic viability of future biorefineries and prevent modular manufacturing near the source-production site that is necessary due to the biomass’ large water content. Herein we introduce a continuous flow microreactor to enable process intensification and one thousand-fold device miniaturization for remote chemical processing. We demonstrate the approach in HCl-catalyzed fructose dehydration in water. We characterize and quantify the mixing using laser induced fluorescence (LIF) of fluorescent dyes and particle image velocimetry (PIV) of fluorescent microspheres. The estimated mixing times ranged from 0.03 to 4.8 s for residence times from 1 to 120 s, respectively. Moreover, a curved channel geometry induces secondary (Dean) vortices that produce a three-fold increase in mixing by diffusion through advection along the cross-sectional direction of the channel, creating dissipative chaotic mixing under laminar flow. We then obtain for the first time isothermal kinetics of fructose dehydration at short contact times and high temperatures and compare to a previously published, hybrid ab initio and data-driven kinetic model. Rigorous model optimization is carried out and an optimal HMF yield of 54% is attained in a single aqueous phase. High temperatures and low pH improve the reactor throughput by increasing the HMF space-time yield. We demonstrate the highest ever productivity (HMF yield per unit time). Importantly, this can be achieved at ~100% fructose conversion, eliminating the need for recycling to improve economics. Furthermore, optimization of the reactor dimensions and the specific energy dissipation rate results in ~60% pressure-drop related energy savings. We describe a methodology to scale-up microreactors to millimeter size and increase productivity 16-fold under optimal conditions. The energy efficiency and cost analysis for HMF production from corn stover of a typical corn farm indicate an optimal strategy for numbering-up millimeter size reactors and, importantly, they underscore the potential feasibility of producing bioproducts in a distributed manner.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

MODULATION OF GALACTIC ELECTRONS IN THE HELIOSPHERE DURING THE UNUSUAL SOLAR MINIMUM OF 2006–2009: A MODELING APPROACH

The last solar minimum activity period, and the consequent minimum modulation conditions for cosmic rays, was unusual. The highest levels of Galactic protons were recorded at Earth in late 2009 in contrast to expectations. A comprehensive model was used to study the proton modulation for the period from 2006 to 2009 in order to determine what basic processes were responsible for solar modulation during this period and why it differs from proton modulation during previous solar minimum modulation periods. This established model is now applied to studying the solar modulation of electron spectra as observed for 80 MeV–30 GeV by the PAMELA space detector from mid-2006 to the end of 2009. Over this period the heliospheric magnetic field had decreased significantly until the end of 2009 while the waviness of the heliospheric current sheet decreased moderately and the observed electron spectra increased by a factor of ∼1.5 at 1.0 GeV to ∼3.5 at 100 MeV. In order to reproduce the modulation evident from seven consecutive semesters, the diffusion coefficients had to increase moderately while maintaining the basic rigidity dependence. It is confirmed that the main diffusion coefficients are independent of rigidity below ∼0.5 GV, while the drift coefficient had to be reduced below this value. The 2006–2009 solar minimum epoch indeed was different than previously observed minima, at least since the beginning of the space exploration era. This period could be called “diffusion-dominated” as was also found for the modulation of protons.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

TIME-DEPENDENT ELECTRON ACCELERATION IN BLAZAR TRANSIENTS: X-RAY TIME LAGS AND SPECTRAL FORMATION

Electromagnetic radiation from blazar jets often displays strong variability, extending from radio to γ -ray frequencies. In a few cases, this variability has been characterized using Fourier time lags, such as those detected in the X-rays from Mrk 421 using Beppo SAX. The lack of a theoretical framework to interpret the data has motivated us to develop a new model for the formation of the X-ray spectrum and the time lags in blazar jets based on a transport equation including terms describing stochastic Fermi acceleration, synchrotron losses, shock acceleration, adiabatic expansion, and spatial diffusion. We derive the exact solution for the Fourier transform of the electron distribution and use it to compute the Fourier transform of the synchrotron radiation spectrum and the associated X-ray time lags. The same theoretical framework is also used to compute the peak flare X-ray spectrum, assuming that a steady-state electron distribution is achieved during the peak of the flare. The model parameters are constrained by comparing the theoretical predictions with the observational data for Mrk 421. The resulting integrated model yields, for the first time, a complete first-principles physical explanation for both the formation of the observed time lags and the shape of the peak flare X-ray spectrum. It also yields direct estimates of the strength of the shock and the stochastic magnetohydrodynamical wave acceleration components in the Mrk 421 jet.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

A unified framework for polycrystal plasticity with grain boundary evolution

Plastic deformation in polycrystals is governed by the interplay between intra-granular slip and grain boundary-mediated plasticity. However, while the role played by bulk dislocations is relatively well-understood, the contribution of grain boundaries (GBs) has only recently begun to be studied. GB plasticity is known to play a key role along with bulk plasticity under a wide range of conditions, such as dynamic recovery, superplasticity, severe plastic deformation, etc., and developing models capable of simultaneously capturing GB and bulk plasticity has become a topic of high relevance. In this paper we develop a thermodynamically-consistent polycrystal plasticity model capable of simulating a variety of grain boundary-mediated plastic processes in conjunction with bulk dislocation slip. The model starts from the description of a single crystal and creates lattice strain-free polycrystalline configurations by using a specially-designed multiplicative decomposition developed by the authors. This leads to the introduction of a particular class of geometrically necessary dislocations (GND) that define fundamental GB features such as misorientation and inclination. The evolution of the system is based on an energy functional that uses a non-standard function of the GND tensor to account for the grain boundary energy, as well as for the standard elastic energy. Our implementation builds on smooth descriptions of GBs inspired on diffuse-interface models of grain evolution for numerical convenience. Here we demonstrate the generality and potential of the methodology by simulating a wide variety of phenomena such as shear-induced GB sliding, coupled GB motion, curvature-induced grain rotation and shrinkage, and polygonization via dislocation sub-grain formation.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Spatial variation in microbial processes controlling carbon mineralization within soils and sediments

Soils have a defining role in global carbon cycling, having one of the largest dynamic stocks of C on earth—3300 Pg of C are stored in soils, which is three-times the amount stored in the atmosphere and more than the terrestrial land plants. An important control on soil organic matter (SOM) quantities is the mineralization rate. It is well recognized that the rate and extent of SOM mineralization is affected by climatic factors and mineral-organic matter associations. What remained elusive is to what extent constraints on microbial metabolism induced by the respiratory pathway, and specifically the electron acceptor in respiration, control overall rates of carbon mineralization in soils. Therefore, physical factors limiting oxygen diffusion such as soil texture and aggregate size (soil structure) may therefore be central controls on C mineralization rates. The goal of our research was therefore to determine if variations in microbial metabolic rates induced by anaerobic microsites in soils are a major control on SOM mineralization rates and thus storage. We performed a combination of laboratory experiments and field investigations will be performed to fulfill our research objectives. We used laboratory studies to examine fundamental factors of respiratory constraints (i.e., electron acceptor) on organic matter mineralization rates. We ground our laboratory studies with both manipulation of field samples and in-field measurements. Selection of the field sites is guided by variation in soil texture and structure while having (other environmental/soil factors constant. Our laboratory studies defined redox gradients and variations in microbial metabolism operating at the aggregate-scale (cm-scale) within soils using a novel constructed diffusion reactor. We further examined micro-scale variation in terminal electron accepting processes and resulting C mineralization rates within re-packed soils. A major outcome of our research is the ability to quantitatively place the importance of aggregate-based heterogeneity in microbial redox processes and the resulting lack of oxygen on the rate of carbon mineralization. Collectively, our research shows that anaerobic microsites are prevalent in soils and are important regulators of soil carbon persistence, shifting microbial metabolism to less efficient anaerobic respiration and selectively protecting otherwise bioavailable, reduced organic compounds such as lipids and waxes from decomposition. Further, shifting from anaerobic to aerobic conditions leads to a 10-fold increase in volume-specific mineralization rate, illustrating the sensitivity of anaerobically protected carbon to disturbance. Vulnerability of anaerobically protected carbon to future climate or land use change thus constitutes a yet unrecognized soil carbon-climate feedback that should be incorporated into terrestrial ecosystem models.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2012

DISCOVERY OF A HALO AROUND THE HELIX NEBULA NGC 7293 IN THE WISE ALL-SKY SURVEY

We report the discovery of an extended halo ({approx}40' in diameter) around the planetary nebula NGC 7293 (the Helix Nebula) observed in the 12 {mu}m band from the Wide-field Infrared Survey Explorer all-sky survey. The mid-infrared halo has an axisymmetric structure with a sharp boundary to the northeast and a more diffuse boundary to the southwest, suggesting an interaction between the stellar wind and the interstellar medium (ISM). The symmetry axis of the halo is well aligned with that of a northeast arc, suggesting that the two structures are physically associated. We have attempted to fit the observed geometry with a model of a moving steady-state stellar wind interacting with the ISM. Possible combinations of the ISM density and the stellar velocity are derived from these fittings. The discrepancies between the model and the observations suggest that the stellar mass loss has a more complicated history, including possible time and angle dependences.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

The Cosmic Ultraviolet Baryon Survey (CUBS) – III. Physical properties and elemental abundances of Lyman-limit systems at z < 1

ABSTRACT We present a systematic investigation of physical conditions and elemental abundances in four optically thick Lyman-limit systems (LLSs) at z = 0.36–0.6 discovered within the Cosmic Ultraviolet Baryon Survey (CUBS). Because intervening LLSs at z < 1 suppress far-UV (ultraviolet) light from background QSOs, an unbiased search of these absorbers requires a near-UV-selected QSO sample, as achieved by CUBS. CUBS LLSs exhibit multicomponent kinematic structure and a complex mix of multiphase gas, with associated metal transitions from multiple ionization states such as C ii, C iii, N iii, Mg ii, Si ii, Si iii, O ii, O iii, O vi, and Fe ii absorption that span several hundred km s−1 in line-of-sight velocity. Specifically, higher column density components (log N(H i)/cm−2≳ 16) in all four absorbers comprise dynamically cool gas with $\langle T \rangle =(2\pm 1) \times 10^4\,$K and modest non-thermal broadening of $\langle b_\mathrm{nt} \rangle =5\pm 3\,$km s−1. The high quality of the QSO absorption spectra allows us to infer the physical conditions of the gas, using a detailed ionization modelling that takes into account the resolved component structures of H i and metal transitions. The range of inferred gas densities indicates that these absorbers consist of spatially compact clouds with a median line-of-sight thickness of $160^{+140}_{-50}$ pc. While obtaining robust metallicity constraints for the low density, highly ionized phase remains challenging due to the uncertain $N\mathrm{(H\, {\small I})}$, we demonstrate that the cool-phase gas in LLSs has a median metallicity of $\mathrm{[\alpha /H]_{1/2}}=-0.7^{+0.1}_{-0.2}$, with a 16–84 percentile range of [α/H] = (−1.3, −0.1). Furthermore, the wide range of inferred elemental abundance ratios ([C/α], [N/α], and [Fe/α]) indicate a diversity of chemical enrichment histories. Combining the absorption data with deep galaxy survey data characterizing the galaxy environment of these absorbers, we discuss the physical connection between star-forming regions in galaxies and diffuse gas associated with optically thick absorption systems in the z < 1 circumgalactic medium.

Zahedy, Fakhri S. (ORCID:0000000178692551)↗
DOE OSTI2014

Initial RattleSnake Calculations of the Hot Zero Power BEAVRS

The validation of the Idaho National Laboratory's next generation of reactor physics analysis codes is an essential and ongoing task. The validation process requires a large undertaking and includes detailed, realistic models that can accurately predict the behavior of an operational nuclear reactor. Over the past few years the INL has developed the RattleSnake application and supporting tools on the MOOSE framework to perform these reactor physics calculations. RattleSnake solves the linearized Boltzmann transport equation with a variety of solution meth­ ods. Various traditional reactor physics benchmarks have already been performed, but a more realistic light water reactor comparison was needed to solidify the status of the code and deter­ mine its fidelity. The INL team decided to use the Benchmark for Evaluation and Validation of Reactor Simulations, which was made available in early 2013. This benchmark is a one­ of-a-kind document assembled by the Massachusetts Institute of Technology, which includes two cycles of detailed, measured PWR operational data. The results from this initial study of the hot zero power conditions show the current INL analysis procedure with DRAGON4 cross section preparation and using the low order diffusion solver in RattleSnake for the whole core calculations yield very encouraging results for PWR analysis. The radial assembly power distributions, radial detector measurements and control rod worths were computed with good accuracy. The computation of the isothermal temperature coefficients of reactivity require further study.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2016

Molecular Simulation Models of Carbon Dioxide Intercalation in Hydrated Sodium Montmorillonite

In this study, classical molecular dynamics simulations and density functional theory (DFT)-based molecular dynamics are used to elucidate the process of CO 2 intercalation into hydrated Na-montmorillonite at P-T conditions relevant to geological formations suitable for CO 2 storage. Of particular interest are the structural and transport properties of interlayer species after CO 2 intercalation. The conducted simulations allowed the research team to quantify expansion/contraction of smectite as a function of CO 2 and H 2 O compositions. The resulting swelling curves can be used to gauge the amount of stored CO 2 , compare it to the experiment, and estimate changes in geomechanical properties of the storage formation. The obtained results showed that the infrared signal of the asymmetric stretch vibration of CO 2 molecule is extremely sensitive to the solvent environment. The extent of the frequency shift relative to the gas-phase value can be used to probe hydration level in the interlayer with intercalated CO 2 . Interaction of supercritical CO 2 with brine in deep geological formations promotes an increase of hydrophobicity of clay surfaces. As a result of wettability alteration, estimated diffusion constants of CO 2 and H 2 O increase with the increased CO 2 load; this can contribute to faster migration of CO 2 throughout the formation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Mineralogical and transport controls on the evolution of porous media texture using direct numerical simulation

The evolution of porous media due to mineral dissolution and precipitation can change the bulk properties of subsurface materials. The pore-scale structure of the media, including its physical and mineralogical heterogeneity, exerts controls on porous media evolution via transport limitations to reactive surfaces and mineral accessibility. Here in this paper we explore how these controls affect the evolution of the texture in porous media at the pore scale. For this purpose, a pore-scale flow and reactive transport model is developed that explicitly tracks mineral surfaces as they evolve using a direct numerical simulation approach. Simulations of dissolution in single-mineral domains provide insights into the transport controls at the pore scale, while the simulation of a fracture surface composed of bands of faster-dissolving calcite and slower-dissolving dolomite provides insights into the mineralogical controls on evolution. Transport-limited conditions at the grain-pack scale may result in unstable evolution, a situation in which dissolution is focused in a fast-flowing, fast-dissolving path. Due to increasing velocities, the evolution in these regions is like that observed under conditions closer to strict surface control at the pore scale. That is, grains evolve to have oblong shapes with their long dimensions aligning with the local flow directions. Another example of an evolving reactive transport regime that affects local rates is seen in the evolution of the fracture surface. As calcite dissolves, the diffusive length between the fracture flow path and the receding calcite surfaces increases. Thus, the calcite dissolution reaction becomes increasingly limited by diffusion.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Reliability-informed end-of-use decision making for product sustainability using two-stage stochastic optimization

The concept of circular economy has been diffused in recent decades to promote economic growth that does not add to the burden on natural resource extraction. Re-X options (e.g., reuse, repair, refurbish, remanufacture, recycle) have been gradually adopted in the product development process and optimized to reduce or eliminate waste and pollution. Although manufacturing incorporating Re-X options can be more environmentally friendly, it involves more sources of uncertainty than traditional manufacturing since the end-of-use products can be collected from multiple origins with various quantities and qualities, and the market demand for both new and remanufactured products cannot be forecasted perfectly. Thus, there is a need to optimize the Re-X policy to alleviate the negative impacts of the higher uncertainty. One option is using the reliability information of new products to estimate the end-of-use conditions and applying multi-stage stochastic optimization to capture multiple demand scenarios. This paper develops a two-stage stochastic optimization model to optimize the quality thresholds for reuse, recycling, and remanufacturing options. Our objective is to minimize the total cost, energy consumption, and environmental impact of producing and providing warranty service for a product family. The model employs reliability information of product components to estimate the warranty service cost and the end-of-use conditions of the returned resources. A case study on a general product family is implemented to illustrate the efficacy of the optimization model. Finally, results show that the two-stage optimization can achieve cost and environmental impact reduction for a hybrid manufacturing and remanufacturing process.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2013

Study of Air Ingress Across the Duct During the Accident Conditions

The goal of this project is to study the fundamental physical phenomena associated with air ingress in very high temperature reactors (VHTRs). Air ingress may occur due to a rupture of primary piping and a subsequent breach in the primary pressure boundary in helium-cooled and graphite-moderated VHTRs. Significant air ingress is a concern because it introduces potential to expose the fuel, graphite support rods, and core to a risk of severe graphite oxidation. Two of the most probable air ingress scenarios involve rupture of a control rod or fuel access standpipe, and rupture in the main coolant pipe on the lower part of the reactor pressure vessel. Therefore, establishing a fundamental understanding of air ingress phenomena is critical in order to rationally evaluate safety of existing VHTRs and develop new designs that minimize these risks. But despite this importance, progress toward development these predictive capabilities has been slowed by the complex nature of the underlying phenomena. The combination of inter-diffusion among multiple species, molecular diffusion, natural convection, and complex geometries, as well as the multiple chemical reactions involved, impose significant roadblocks to both modeling and experiment design. The project team will employ a coordinated experimental and computational effort that will help gain a deeper understanding of multiphased air ingress phenomena. This project will enhance advanced modeling and simulation methods, enabling calculation of nuclear power plant transients and accident scenarios with a high degree of confidence. The following are the project tasks: Perform particle image velocimetry measurement of multiphase air ingresses; and, Perform computational fluid dynamics analysis of air ingress phenomena.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2019

Interaction of magnesia-carbon refractory with metallic iron under flash ironmaking conditions

The interaction of a magnesia-carbon refractory with iron under flash ironmaking conditions has been investigated in this work. Oxidation of carbon from the magnesia-carbon refractory took place and the formation of magnesiowustite (Mg x Fe 1-x O) solid solution occurred as a result of the interaction between magnesia (MgO) in the refractory with iron (Fe). A kinetics model for the growth of magnesiowustite (Mg x Fe 1-x O) was developed based on the counterdiffusion of Fe 2+ and Mg 2+ cations through it. Experiments were conducted with iron powder and magnesia-carbon refractory in the temperature range 1200–1400 °C under gas atmospheres relevant to flash ironmaking and analyses of reacted samples using SEM-EDX and EPMA techniques confirmed the formation of magnesiowustite (Mg x Fe 1-x O). Using the kinetics model and the composition profiles determined with EPMA, the values of interdiffusion coefficient ( D ¯ F e - M g ), averaged over the composition range, were calculated. The activation energy ( E a ) for the solid-state diffusion process was determined to be 377 kJ/mol.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Gyro-induced acceleration of magnetic reconnection

The linear and nonlinear evolution of magnetic reconnection in collisionless high-temperature plasmas with a strong guide field is analyzed on the basis of a two-dimensional gyrofluid model. The linear growth rate of the reconnecting instability is compared to analytical calculations over the whole spectrum of linearly unstable wave numbers. In the strongly unstable regime (large Δ′), the nonlinear evolution of the reconnecting instability is found to undergo two distinctive acceleration phases separated by a stall phase in which the instantaneous growth rate decreases. The first acceleration phase is caused by the formation of strong electric fields close to the X-point due to ion gyration, while the second acceleration phase is driven by the development of an open Petschek-like configuration due to both ion and electron temperature effects. Furthermore, the maximum instantaneous growth rate is found to increase dramatically over its linear value for decreasing diffusion layers. This is a consequence of the fact that the peak instantaneous growth rate becomes weakly dependent on the microscopic plasma parameters if the diffusion region thickness is sufficiently smaller than the equilibrium magnetic field scale length. When this condition is satisfied, the peak reconnection rate asymptotes to a constant value.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗