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The \textNJst method was proposed by Liu and Yu to infer a species tree topology from unrooted topological gene trees. While its statistical consistency under the multispecies coalescent model was established only for a four-taxon tree, simulations demonstrated its good performance on gene trees inferred from sequences for many taxa. Here, we prove the statistical consistency of the method for an arbitrarily large species tree. Our approach connects \textNJst to a generalization of the STAR method of Liu, Pearl, and Edwards, and a previous theoretical analysis of it. We further show \textNJst utilizes only the distribution of splits in the gene trees, and not their individual topologies. Finally, we discuss how multiple samples per taxon per gene should be handled for statistical consistency. [ABSTRACT FROM PUBLISHER]

The field-reversed configuration heating experiment (FRCHX) was designed to form closed-field-line magnetized target plasmas for magnetoinertial fusion and other high energy density plasma research. These plasmas are in a field-reversed configuration and are formed via a reversed-field theta pinch on an already magnetized background plasma. To extend the duration and temporal uniformity of the pinch, the capacitor bank driving the reversed-field discharge is crowbarred near the current peak. Four parallel rail-gap switches are used on the FRCHX for this application to ensure a low-inductance crowbar discharge path and to accommodate the large magnitude of the discharge current (often greater than 1 MA). Historically, parallel operation of spark gap switches in a crowbarring arrangement has often proved to be difficult due to the very low voltage present on the bank and across the switches at the time of peak current. In a low-inductance design, triggering can be further complicated by the rapid collapse of what little voltage there is across the switches as soon as the first spark gap begins conduction. This paper reports on the efforts that were made to develop a low-inductance crowbar switch for the FRCHX and to ultimately enable successful triggering and operation of the four parallel rail-gap switches used in the crowbar. The design of the low-inductance parallel switch assembly is presented first, followed by a description of the triggering scheme employed to ensure conduction of all four switches. [ABSTRACT FROM AUTHOR]

In 1991, Turchi et al. reported evidence for a 2000 km/s aluminum plasma that originated from the upstream boundary of a wire array armature in a plasma flow switch (PFS). The 2008 article by Turchi et al. posits that if such high $Z$ plasma could instead be composed of deuterium or a deuterium–tritium mixture, then the resultant multi-keV plasma would make an effective target for magnetized plasma compression to fusion conditions. This report documents several experiments executed in an effort to achieve an ultrahigh-speed flow in a deuterium plasma. The first phase of this research concentrated on extension of the earlier work to a lower current system that would emulate the PFS used in series with an imploding liner load. The apparatus was also modified to permit pulsed injection of deuterium gas along the insulated coaxial electrodes between the PFS armature and the vacuum power feed. The experiments met with limited success, exhibiting evidence of a 550 km/s plasma flow which convected a small fraction of the total magnetic field. Two subsequent tests were conducted using foam armatures. In both cases, current prematurely shunted upstream in the vacuum feed. Several possible causes were explored for the shunting of the current. Among the modifications implemented, the gas injection system was altered to increase both the quantity of gas adjacent to the armature while facilitating an increased pressure gradient between the armature and the current feed. A series of low-energy shots were conducted to examine the impact of several proposed design modifications on current delivery to the armature. These experiments demonstrated that the hardware assembled for this investigation was unlikely to forestall breakdown in the injected gas as required by Turchi et al. Nevertheless, two experiments were conducted to evaluate performance with foam armatures. Both experiments exhibited good current delivery to the armature, behaving initially like the low-energy experiments. The magnetic flux convected downstream was greater than in any of the prior experiments, though significant work remains to demonstrate the ultrahigh-speed plasma flow concept. [ABSTRACT FROM PUBLISHER]

The objective of the field-reversed configuration heating experiment (FRCHX) is to obtain a better understanding of the fundamental scientific issues associated with high-energy density laboratory plasmas (HEDLPs) in strong, closed-field-line magnetic fields. These issues have relevance to such topics as magneto-inertial fusion, laboratory astrophysical research, and intense radiation sources, among others. To create HEDLP conditions, a field-reversed configuration (FRC) plasma of moderate density is first formed via reversed-field theta pinch. It is then translated into a cylindrical aluminum flux conserver (solid liner), where it is trapped between two magnetic mirrors and then compressed by the magnetically driven implosion of the solid liner. A requirement is that, once the FRC is stopped within the solid liner, the trapped flux inside the FRC must persist while the compression process is completed. With the present liner dimensions and implosion drive bank parameters, the total time required for implosion is \sim25~\mus . Lifetime measurements of recent FRCHX FRCs indicate that trapped lifetimes following capture are now approaching \sim14~\mus (and therefore, total lifetimes after formation are now approaching \sim19~\mus ). By separating the mirror and translation coil banks into two so that the mirror fields can be set lower initially, the liner compression can now be initiated 7–9 \mus before the FRC is formed. A discussion of FRC lifetime-limiting mechanisms and various experimental approaches to extending the FRC lifetime will be presented. [ABSTRACT FROM PUBLISHER]

Ranked gene trees, which consider both the gene tree topology and the sequence in which gene lineages separate, can potentially provide a new source of information for use in modeling genealogies and performing inference of species trees. Recently, we have calculated the probability distribution of ranked gene trees under the standard multispecies coalescent model for the evolution of gene lineages along the branches of a fixed species tree, demonstrating the existence of anomalous ranked gene trees (ARGTs), in which a ranked gene tree that does not match the ranked species tree can have greater probability under the model than the matching ranked gene tree. Here, we fully characterize the set of unranked species tree topologies that give rise to ARGTs, showing that this set contains all species tree topologies with five or more taxa, with the exceptions of caterpillars and pseudocaterpillars. The results have implications for the use of ranked gene trees in phylogenetic inference. [ABSTRACT FROM PUBLISHER]

Plasma guns offer opportunities to generate and direct plasma flows at high energy density. Typically, such guns comprise coaxial electrodes that are connected to high-current sources (e.g., capacitor banks, pulse lines, inductive stores, or magnetic-flux-compression generators). The basic interactions include ionization of materials such as injected gas or preinstalled wires/foils, acceleration of these materials by the Lorentz force, and expulsion of the resulting plasma flows. We review the use of a particular arrangement in the form of a plasma flow switch that acts as a multimegampere commutator, but it can also provide a magnetized-plasma target for compression by an imploding liner. In a quite separate concept, a plurality of quasi-steady plasma guns in a spherical array provides converging, collimated jets to compress plasma with stand-off from the plasma generators and chamber walls. Such stand-off in a repetitively pulsed system can be crucial for the development of fusion power reactors at megagauss energy densities. [ABSTRACT FROM AUTHOR]

Magnetized target fusion (MTF) is a means to com- press plasmas to fusion conditions that uses magnetic fields to greatly reduce electron thermal conduction, thereby greatly reducing compression power density requirements. The compression is achieved by imploding the boundary, a metal shell. This effort pursues formation of the field-reversed configuration (FRC) type of magnetized plasma, and implosion of the metal shell by means of magnetic pressure from a high current flowing through the shell. We reported previously on experiments demonstrating that we can use magnetic pressure from high current capacitor discharges to implode long cylindrical metal shells (liners) with size, symmetry, implosion velocity, and overall performance suitable for compression of FRCs. We also presented considerations of using deformable liner-electrode contacts of Z-pinch geometry liners or theta pinch-driven liners, in order to have axial access to inject FRCs and to have axial diagnostic access. Since then, we have experimentally implemented the Z-pinch discharge driven deformable liner-electrode contact, obtained full axial coverage radiography of such a liner implosion, and obtained 2 ½ dimensional MHD simulations for a variety of profiled thickness long cylindrical liners. The radiographic results indicate that at least 16 times radial compression of the inner surface of a 0.11-cm-thick Al liner was achieved, with a symmetric implosion, free of instability growth in the plane of the symmetry axis. We have also made progress in combining 2 ½-D MHD simulations of FRC formation with imploding liner compression of FRCs. These indicate that capture of the injected FRC by the imploding liner can be achieved with suitable relative timing of the FRC formation and liner implosion discharges. [ABSTRACT FROM AUTHOR]

Preparation of the target plasma represents a critical issue in liner compression techniques to achieve fusion conditions. We consider the use of an ultrahigh speed plasma flow from a special coaxial-gun arrangement known as the plasma flow switch. Experiments have demonstrated that this arrangement can provide plasma flows with speeds in excess of 2000 km/s. Stagnation of such a plasma flow results in fully stripped aluminum plasma with electron temperatures of 30 keV. Substitution of deuterium or a deuterium-tritium mixture could provide target plasma at kilovolt temperatures within an imploding liner. Such temperatures suggest that, even if substantial heat loss occurred during liner compression, fusion-level temperatures would be possible. The concatenation of events to generate the ultrahigh speed flow, to direct it into the implosion chamber, and to arrange liner dynamics for effective compression demands numerical simulation, which is based on initial analytical estimates. Both types of calculation for exploring this concept are discussed. [ABSTRACT FROM AUTHOR]

Field reversed theta pinch technology is employed with programmed cusp fields at the theta coil ends to form high-density field-reversed configuration (FRC) plasmas. The well-formed FRC plasmas have volume-averaged density of 2 – 4 × 1022 m-3, total temperature (Te + Ti) of 300–500 eV, and plasma lifetime between 10–20 μs in 50–70 mtorr of deuterium static gas fill. The achieved FRC parameters are very close to the desired target plasma requirements for magnetized target fusion. [ABSTRACT FROM AUTHOR]

We describe a program to demonstrate the scientific basis of magnetized target fusion (MTF). MTF is a "potentially low-cost path to fusion which is intermediate in plasma regime between magnetic (MFE) and inertial fusion energy (IFE). MTF involves the Compression of a magnetized target plasma and pressure times volume (PdV) heating to fusion relevant conditions inside a converging flux conserving boundary. We have chosen to demonstrate MTF by using a field-reversed configuration (FRC) as our magnetized target plasma and an imploding metal liner for compression. These choices take advantage of significant past scientific and technical accomplishments in MFE and defense programs research and should yield substantial plasma performance (nr > 1013 s-cm³ T > 5 key) using an available pulsed-power implosion facility at modest cost. We have recently shown the density, temperature, and lifetime of this FRC to be within a factor of 2-3 of that required for use as a suitable target plasma for MIT compression for a fusion demonstration. [ABSTRACT FROM AUTHOR]

Presents a study which dealt with the development of a high-current low-inductance crowbar switch for the formation of long-lifetime field-reversed configurations. Overview of hardware design; Field analysis of cable headers; Results of test on crowbar switch currents.

Presents a study which discussed the design, fabrication and operation of a linear implosion system at peak currents of 16 megamperes. Basic design of the pulsed power insulators; Liner fabrication of glide planes; Installation of lower glide-plane.

Presents a study which discussed the design and demonstration of an imploding solid liner for compressing a field reversed configuration. Indication of multiple diagnostics; Application of liner implosion technique in liner-on-plasma experiments.

Presents information on a study which illustrated the theoretical analysis and circuit simulations of high voltage pulses using explosive formed fuses. Analytic considerations; Circuit modeling of the circuit parameters.

The article discusses various topics reported within the issue including papers related to multimegagauss magnetic fields discussed at the 12th International Conference on Megagauss Magnetic Field Generation and Related Topics held on July 13-8, 2008 in Novosibirsk, Russia, an article on magnetic flux compression by A. Orlov and S. Sorokin, and another article on EMIR high-current explosive generator by V. Selemir.