Magnetic Reconnection in Extreme Astrophysical Environments

Magnetic reconnection is a basic plasma process of dramatic rearrangement of magnetic topology, often leading to a violent release of magnetic energy. It is important in magnetic fusion and in space and solar physics --- areas that have so far provided the context for most of reconnection research. Importantly, these environments consist just of electrons and ions and the dissipated energy always stays with the plasma. In contrast, in this paper I introduce a new direction of research, motivated by several important problems in high-energy astrophysics --- reconnection in high energy density (HED) radiative plasmas, where radiation pressure and radiative cooling become dominant factors in the pressure and energy balance. I identify the key processes distinguishing HED reconnection: special-relativistic effects; radiative effects (radiative cooling, radiation pressure, and Compton resistivity); and, at the most extreme end, QED effects, including pair creation. I then discuss the main astrophysical applications --- situations with magnetar-strength fields (exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares and magnetically-powered central engines and jets of GRBs. Here, magnetic energy density is so high that its dissipation heats the plasma to MeV temperatures. Electron-positron pairs are then copiously produced, making the reconnection layer highly collisional and dressing it in a thick pair coat that traps radiation. The pressure is dominated by radiation and pairs. Yet, radiation diffusion across the layer may be faster than the global Alfv\'en transit time; then, radiative cooling governs the thermodynamics and reconnection becomes a radiative transfer problem, greatly affected by the ultra-strong magnetic field. This overall picture is very different from our traditional picture of reconnection and thus represents a new frontier in reconnection research.Comment: Accepted to Space Science Reviews (special issue on magnetic reconnection). Article is based on an invited review talk at the Yosemite-2010 Workshop on Magnetic Reconnection (Yosemite NP, CA, USA; February 8-12, 2010). 30 pages, no figure

The Physics of Star Cluster Formation and Evolution

© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00689-4.Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and effectively channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.Peer reviewe

Growth of high T{sub c} superconducting fibers using a minaturized laser-heated float zone process. Annual progress report, January 1, 1993--December 31, 1993

This report covers the research done on {open_quotes}Growth of High Tc Superconducting Fibers using a Miniaturized Laser-Heated Float Zone Process{close_quotes} during the 12 months from Jan. 1, 1993 until Dec. 31, 1993. The effort during this period were directed into two areas; the influence of growth conditions on the properties of the superconducting fibers and the construction of the advanced fiber growth station. In the first area of emphasis, studies were done on constitutional super cooling effect, the influence of processing parameters on Tc, the correlation between Tc and growth parameters and the mechanical properties of 2212 fibers. These studies showed that there are two types of interfacial breakdowns; one type that involves low temperature inclusions caused by excessive solute buildup and another involving high temperature inclusions which require two conditions to be met. These condition are: (1) significant compositional gradients in the melt and (2) an interface melt temperature near the peritectic decomposition temperature. Analysis of the experimental data lead to the hypothesis that fibers with the highest crystallinity are grown from SrO-rich 2212 melts. Evaluation of the constitutional supercooling responsible for the high temperature inclusions suggested that growth under these conditions was most vulnerable to disruption by HT inclusions. Tc increased with growth temperature for as-grown fibers. The concentration of SrO in the fibers had a parabolic relationship with temperature. The same parabolic relationship was observed between composition and Tc. The thermal history of 2212 crystals has been shown to influence their oxygen content which played a significant role in determining their Tc`s. Fiber heat treatment and the ambient gaseous atmosphere were found to dominate the Tc variations measured in this study

Growth of high T{sub c} superconducting fibers using a miniaturized laser-heated float zone process. Annual progress report, January 1, 1992--December 31, 1992

This report covers the research done on {open_quotes}Growth of High Tc Superconducting Fibers using a Miniaturized Laser-Heated Float Zone Process{close_quotes} during the 12 months from Jan. 1, 1992 until Dec. 31, 1992. The major part of the work focused on phase relations and kinetics in the Bi{sub 2}O{sub 3}-SrO-CaO-CuO (BSCCO) system. By analyzing the crystal and melt composition, and the growth temperature of the float-zone samples, new data was obtained on the phase relationships. These results were shown to form a subset of solid solubility ranges reported by other investigators and was typical of the data available from other flux growth experiments. These experiments resulted in the development of a technique for the growth of long, single-phase 2212 samples. This was highly depended on starting material composition with Bi{sub 2.1}Sr{sub 1.8}Ca{sub 1.1}Cu{sub 2}O{sub y} being the most successful. Examination of the single phase 2212 growth interfaces was used to characterize the crystal/melt equilibrium conditions. These studies showed that 2212 crystal solidify from Bi{sub 2}O{sub 3}-rich and SrO-poor melts. Increasing melt concentrations of bismuth and cooper oxide increased the growth temperature. The sum of the bismuth and copper oxide in the crystals was invariant leading to the conclusion that the segregation of bismuth and copper oxide is interdependent. Work also proceeded on the new LHPG growth station

Growth of high T{sub c} superconducting fibers using a miniaturized laser-heated float zone process. Annual progress report, October 15, 1989--November 5, 1990

This report covers the research accomplished on the program entitled {open_quotes}Growth of High Tc Superconducting Fibers using a Miniaturized Laser-Heated Float Zone Process{close_quotes} during the 12.5 month period from Oct. 15, 1989 to Nov. 5, 1990. Research was done in four areas: phase relationships, preparation of starting materials, growth studies and the advanced fiber growth apparatus. The phase relationship studies built on the work published by Ono. Comparison studies with the well known compound Ca{sub 3}Al{sub 2}O{sub 6} confirmed that the Bi{sub 2+x}(Sr,Ca){sub 3-x}Cu{sub 2}O{sub y} is incongruently melting and that cuprous oxide, calcium oxide and (strontium, calcium) cuprate are the higher melting compounds which coexist with the melt and the superconducting phase. The preparation of the starting materials is crucial to the stable growth of the fibers. Non-uniform distribution of second phase particles, gaseous inclusions or porosity can lead to instabilities. A process was developed to ensure uniform starting materials. `Ibis process involves grinding the individual starting materials to a uniform size (44 {mu}m). The resulting powders are mixed and calcined three times with regrinding between each calcining step. The calcined powder is then cold pressed and sintered, reground, re-pressed and sintered. Ibis final material is then cut into bars for feed material for fiber growth. Growth rate studies showed a relationship between the growth rate and the regions of stability for single and multiphase fibers. This was traced to changes in the Bi and Cu levels in the melt related to changes in the growth rate. It was also shown that fluctuation in laser power lead to CaO inclusions in the fibers. The necessary components for the Advanced Fiber Growth Apparatus have been determined. Some of the components have been ordered and others are being designed

Growth of high T{sub c} superconducting fibers using a miniaturized laser-heated float zone process. Final technical report, January 15, 1989--December 31, 1994

This report summarizes a four year program on the use of the laser-heated pedestral growth (LHPG) process for the preparation of long, flexible fibers of the high T{sub c} copper-oxide ceramic superconductors having wire-like morphology. The major question addressed was whether the LHPG method could produce high T{sub c} fibers of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} (2212) in lengths long enough for use as superconducting wires. Cold-pressing and sintering methods were developed to prepare uniform, single phase ceramic feedstock. Phase equilibrium studies revealed the relationship between thermal gradients, interface shape and phases produced by the LHPG process during incongruent solidification. The highest critical current densities over measured in bulk samples of Bi-2212 material, 60,000 A/cm{sup 2} at 68K, were achieved in single crystal and/or highly grain-oriented fibers. The first ever flexible, multi-cm fibers ({le}100 {mu}m in diameter) were prepared. Fibers diameters were ultimately reduced to 25 {mu}m (1 cm in length), and we were able to grow them up to 14 cm in length (100 {mu}m diameter). These fibers could be bent in radii less than 5 cm, but max. growth rates of {approximately}10 mm/hr did not permit them to be grown long enough for prototype motor windings. Superconducting Bi-2212 grain-aligned ribbons were grown for the first time by the LHPG method using platinum guide wires

Electronic transport properties of hot-pressed B6Si

Measurements of the electrical conductivity, Seebeck coefficient and Hall mobility from -300K to -1300K have been carried out on multiphase hotpressed samples of the nominal composition B6Si. In all samples the conductivity and the p-type Seebeck coefficient both increase smoothly with increasing temperature. By themselves, these facts suggest small-polaronic hopping between inequivalent sites. The measured Hall mobilities are always low, but vary in sign. A possible explanation is offered for this anomalous behavior