Production of energetic electrons during magnetic reconnection

The production of energetic electrons during magnetic reconnection is explored with full particle simulations and analytic analysis. Density cavities generated along separatrices bounding growing magnetic islands support parallel electric fields that act as plasma accelerators. Electrons because of their low mass are fast enough to make multiple passes through these acceleration cavities and are therefore capable of reaching relativistic energies.     

A second-order approximation to natural convection for large Rayleigh numbers and small Prandtl numbers

The problem under investigation is that of fluid flow within an enclosed rectangular cavity. It is assumed that one wall is maintained at a constant temperature T₁ (hot wall) and the other wall is maintained at a constant temperature T₀ (cold wall). At the remaining walls, two separate cases are studied. In the first, an adiabatic boundary condition is assumed. That is, the normal derivative of the temperature function is assumed to be 0. In the second, it is assumed the temperature varies linearly from T₀ to T₁. The purpose of this paper is the application of a second order numerical technique to the problem of fluid flow within a heated closed cavity. The method is a modification of a method developed by Shay¹ and applied to the driven cavity problem. In order to test the viability of this technique, it was decided to extend the technique to the problem of natural convection in a square. Jones² proposed that this problem is suitable for testing techniques that may be applied to a wide range of practical problems such as reactor insulation, cooling of radioactive waste containers, solar energy collection and others.³ The technique makes use of second-order finite difference approximations to all derivatives in the governing equations. Furthermore, second-order approximations are also used to determine boundary vorticities and, when the adiabatic boundary condition is used, for the boundary temperatures as well. In some works, where second-order approximations are used at interior points, second-order boundary approximations have been sacrificed in favour of a more stable, but first-order boundary approximation. The current approximations are generated by writing the unknown value of a function at a given interior node as a linear combination of unknown function values at all of the neighbouring nodes. Then the function values at these neighbouring nodes are expanded in a Taylor series about the given node. Through appropriate regrouping of terms and the use of the equations to the solved, constraints are imposed on the coefficients of the linear combination to yield a second-order approximation. As it turns out, there are more unknowns than constraints and, as a result, we are left with some freedom in choosing coefficients. In this work this freedom was used to choose coefficients in such a way as to maximize stability of the resulting system of equations. In other words, the approximations to the governing partial differential equation are individually determined at each point dependent on the direction of flow in order to generate the best possible stability. This idea is analogous to that used in the derivation of the upwind method. However, the current method is second-order accurate where the upwind method is only first-order accurate. Thus, what is generated is an easily implemented second-order method that yields a system of equations that has proved easy to solve. The system of equations is solved via the method of successive overrelaxation. The stability of the method is shown in the convergence for a wide range of Rayleigh numbers, Prandtl numbers and mesh sizes. Level curves of the stream, vorticity and temperature functions are provided for Rayleigh numbers (Ra) as large as 100,000, Prandtl numbers (Pr) as small as 0.0001, and mesh sizes as small as 0.0125. Values of the Nusselt number have also been calculated through the use of Simpson's rule, and a second order approximation to the normal derivative of the temperature along the cold wall. Comparisons are made with other current works to aid in the verification of this methods' accuracy and also with the first-order upwind method to demonstrate superiority over the first-order method.     

Ion-molecule reactions and collision-activated dissociation of C4H 4 +. isomers: A case study in the use of the MS3 capabilities of a pentaquadrupole mass spectrometer

Isomeric C₄H ₄ ^+. radical cations vinylacetylene (a), butatriene (b), methylene cyclopropene (c), and the nonaromatic cyclobutadiene (d), generated, respectively, from the neutral precursors 3-butyn-1-ol (1), 1,4-dichloro-2-butyne (2), benzene (3), and 7,8-benzotricyclo [4.2.2.0^2,5]deca-3,7,9-triene (4), undergo diagnostically different ion-molecule reactions with allene, isoprene, furan, and thiophene. It is speculated that adducts are generated by [2 + 2] cycloadditions with the first reagent and [4 + 2] Dials-Alder cycloadditions with isoprene, furan, and thiophene. The initially formed cycloaddition adducts fragment rapidly, isomerize, or undergo further addition of neutral reagent to yield a complex set of products. With a pentaquadrupole mass spectrometer, MS³ experiments that employ three stages of ion mass analysis are used to help elucidate the ion-molecule reactions and to distinguish the isomeric C⁴H ₄ ^+. ions. Among these experiments, the reaction intermediate spectrum reveals the nature of the intermediates connecting the reactant to a selected product while the sequential product spectrum provides mechanistic and structural information on the adducts and other ion-molecule products. The unique combination of ion-molecule reactions with collision-activated dissociation employed here provides valuable information on the chemistry of ionized cyclobutadiene, including its proclivity to undergo [2 + 2] and [4 + 2] cyc1oadditions.     

An extraction-based assay for neutral anionophores: the measurement of high binding constants to steroidal receptors in a nonpolar solvent

The extraction-based protocol for measuring binding constants, developed by Cram and co-workers, has been extended for use with anionic substrates. The method is especially useful for high-affinity receptors, allowing very high binding constants to be measured in nonpolar solvents. Distribution constants K(d) between chloroform and water have been obtained for tetraethylammonium chloride and bromide, thus calibrating the method for these two substrates. Application to steroidal podands 5-9 has confirmed the ability of electron-withdrawing groups to enhance hydrogen-bond donor capabilities. Binding constants of approximately 3 x 10(7) M(-1) have been measured for the most powerful receptor 7. An X-ray crystal structure of 15, the methyl ester analogue of 7, reveals a well-defined binding site preorganised for anion recognition.     

Evidence for an elongated (>60 ion skin depths) electron diffusion region during fast magnetic reconnection

Observations of an extremely elongated electron diffusion region occurring during fast reconnection are presented. Cluster spacecraft in situ observations of an expanding reconnection exhaust reveal a broad current layer ( approximately 10 ion skin depths thick) supporting the reversal of the reconnecting magnetic field together with an intense current embedded at the center that is due to a super-Alfvénic electron outflow jet with transverse scale of approximately 9 electron skin depths. The electron jet extends at least 60 ion skin depths downstream from the X-line.     

Usage of turbulence for superresolved imaging

We have used the turbulent flow of hot air to improve the diffraction limits of resolution in an imaging system. A time-multiplexing approach was applied and used the turbulent flow near the object to encode its spatially high frequencies. The turbulent perturbations were extracted and numerically implemented to decode the high-resolution image. Experimental verification demonstrated the proposed concept.     

The Las-Vegas Processor Identity Problem (How and When to Be Unique)

We study the classical problem of assigning unique identifiers to identical concurrent processes. In this paper, we consider the asynchronous shared memory model, and the correctness requirement is that upon termination of the algorithm, the processes must have unique IDs always. Our results include tight characterization of the problem in several respects. We call a protocol solving this task Las Vegas if it has finite expected termination time. Our main positive result is the first Las-Vegas protocol that solves the problem. The protocol terminates in O(log n) expected asychronous rounds, using O(n) shared memory space, where n is the number of participating processes. The new protocol improves on all previous solutions simultaneously in running time (exponentially), probability of termination (to 1), and space requirement. The protocol works under the assumption that the asynchronous schedule is oblivious, i.e., independent of the actual unfolding execution. On the negative side, we show that there is no finite-state Las-Vegas protocol for the problem if the schedule may depend on the history of the shared memory (an adaptive schedule). We also show that any Las-Vegas protocol must know n in advance (which implies that crash faults cannot be tolerated) and that the running time is Ω(log n). For the case of an arbitrary (nonoblivious) adversarial schedule, we present a Las-Vegas protocol that uses O(n) unbounded registers. For the read-modify-write model, we present a constant-space deterministic algorithm.     

Polar [4+2+] diels-alder cycloaddition to nitrilium and immonium ions in the gas phase: Applications of multiple stage mass spectrometry in a pentaquadrupole instrument

Multiple stage MS² and MS³ mass spectrometric experiments, performed using a pentaquadrupole instrument, are employed to explore the gas-phase ion-molecule chemistry of several nitrilium [R-C≡N⁺-H (1), R-C≡N⁺-CH₃ (2), and H-C≡N⁺-C₂H₅ (3)] as well as immonium ions RR¹C=N⁺R²R³ (4) with the neutral diene isoprene. Polar [4+2⁺] Diels-Alder cycloaddition is observed for nitrilium ions when the energy gap between the lowest unoccupied molecular orbital (LUMO) of the ion and the highest occupied molecular orbital (HOMO) of the isoprene is small and the competing proton transfer reaction is endothermic. Thus, C-protonated methyl isonitrile H-C≡N⁺-CH₃ (2a) and its higher homolog H-C≡N⁺-C₂H₅ (3a) form abundant [4+2⁺] cycloadducts with isoprene, but several protonated nitriles 1 do not; instead they show exothermic proton transfer as the main ion-molecule reaction. Replacement of the methyne hydrogen in 2a by a methyl, ethyl, or phenyl group (2b–d) raises the LUMO-HOMO gap, which greatly decreases the total yield of ion-molecule products and precludes cycloaddition. On the other hand, the electron-withdrawing acetyl and bromine substituents in 2e and 2f substantially lower the LUMO energy of the ions and cycloaddition reaction occurs readily. The simplest member of the immonium ion series, CH₂=NH ₂ ⁺ (4a), reacts readily by cycloaddition, whereas alkyl substitution on either the carbon or nitrogen (4b–f) dramatically lowers the overall reactivity, which substantially decreases or even precludes cycloaddition. In strong contrast, the N-phenyl (4g) and N-acetyl (4h) ions and the N-vinyl-substituted immonium ion, N-protonated 2-aza-butadiene (4i), react extensively with isoprene, mainly by [4+2⁺] cycloaddition. However, the isomeric C-vinyl-substituted ion (4j) displays only modest reactivity in both the proton-transfer and the cycloaddition channels. Collision-induced dissociation (CID) of the cycloadducts performed by on-line MS³ experiments demonstrates that they are covalently bound and supports their assignments as cycloaddition products. Retro Diels-Alder fragmentation is a major process for cycloadducts of both the immonium and the nitrilium ions, but other fragmentation processes also are observed. The cycloadduct of 4a with butadiene displays CID fragmentation identical to that of the authentic ion produced by protonation of 1,2,3,6-tetrahydropyridine, which thus strengthens the [4+2⁺] cycloaddition proposal. AM1 calculations also support the formation of the [4+2⁺] cycloadducts, which are shown in several cases to be much more stable than the products of simple addition, that is, the ring-open isomers.     

Strain-gage stability measurements for three years at 150°C in air

The zero shifts of nickel-chromium foil strain gages (modified Karma) were measured over a period of three years at a constant temperature of 150°C in air. Three gage lengths were included—1.585 mm (1/16 in.), 3.170 mm (1/8 in.), and 6.340 mm (1/4 in.). The strain gages were bonded to constant-stress cantilever beams which were subjected to nomial mechanical strain levels of 0, ±780 μm/m, and ± 1350 μm/m. Each strian gage was connected by threewire leads to a Wheatstone-bridge circuit for the test duration. The data support two general observations: (1) short gage lengths suffer larger zero shifts than longer gage lengths, and (2) gages in compression suffer large zero shifts than gages in tension. On the assumption that the major cause of the zero shifts is a combination of corrosion of the foil and creep of the gage/epoxy/beam system, the author suggests a possible way to correct for the zero shift by experimentally determining the combined corrosion/creep effect and substracting it from the strain-gage readings. Some of the data appear to be consistent with this assumption, but some of the data do not.