Sustained Acceleration of Over-dense Plasmas by Colliding Laser Pulses

We review recent PIC simulation results which show that double-sided irradiaton of a thin overdense plasma slab by ultra-intense laser pulses from both sides can lead to sustained comoving acceleration of surface electrons to energies much higher than the conventional ponderomotive limit. The acceleration stops only when the electrons drift transversely out of the laser beam. We show results of parameter studies based on this concept and discuss future laser experiments that can be used to test these computer results.Comment: 9 pages 6 figures. AIP Conference Proceedings for 2005 Varenna Conf. on Superstrong Fields in Plasmas (AIP, NY 2006

Emission model of gamma-ray bursts

The emission mechanisms of cosmic gamma-ray bursts are reviewed. In particular, the thermal synchrotron model is discussed as the most viable mechanism for the majority of the continuum emission. Within this framework various information about the source region can be extracted. The picture that emerges is that of a hot (kT = .2 - 1.0 sq mc), thin sheet of dense pair-dominated plasma emitting via cyclo-synchrotron radiation in a strong magnetic field (B approximately one-hundred billion to one trillion gauss). Speculations on the origin and structure of this sheet are attempted. The problem of high-energy photons above pair production threshold escaping from the source is also considered

Pseudoscalar or vector meson production in non-leptonic decays of heavy hadrons

We have addressed the study of non-leptonic weak decays of heavy hadrons ($\Lambda_b, \Lambda_c, B$ and $D$), with external and internal emission to give two final hadrons, taking into account the spin-angular momentum structure of the mesons and baryons produced. A detailed angular momentum formulation is developed which leads to easy final formulas. By means of them we have made predictions for a large amount of reactions, up to a global factor, common to many of them, that we take from some particular data. Comparing the theoretical predictions with the experimental data, the agreement found is quite good in general and the discrepancies should give valuable information on intrinsic form factors, independent of the spin structure studied here. The formulas obtained are also useful in order to evaluate meson-meson or meson-baryon loops, for instance of $B$ decays, in which one has PP, PV, VP or VV intermediate states, with P for pseudoscalar mesons and V for vector meson and lay the grounds for studies of decays into three final particles.Comment: 54 pages, 7 figures, 13 tables; v2: 60 pages, 9 figures, 14 tables, discussion added, references added, version to appear in Eur.Phys.J.

Testing the Invariance of Cooling Rate in Gamma-Ray Burst Pulses

Recent studies have found that the spectral evolution of pulses within gamma-ray bursts (GRBs) is consistent with simple radiative cooling. Perhaps more interesting was a report that some bursts may have a single cooling rate for the multiple pulses that occur within it. We determine the probability that the observed "cooling rate invariance" is purely coincidental by sampling values from the observed distribution of cooling rates. We find a 0.1-26% probability that we would randomly observe a similar degree of invariance based on a variety of pulse selection methods and pulse comparison statistics. This probability is sufficiently high to warrant skepticism of any intrinsic invariance in the cooling rate.Comment: 4 pages, 1 figure, to appear in Proceedings of the Fourth Huntsville Symposium on Gamma-Ray Burst

Determination of water in organic solvents by flow-injection analysis with Karl Fischer reagent and a biamperometric detection system

A flow-injection system with a biamperometric flow-through detector provided with two platinum plate electrodes was tested for the determination of water with a two-component pyridine-free Karl Fischer reagent. The response was shown to be linear in the concentration range 0.03–0.11% water in methanol, ethanol or 2-propanol, with methanol as the carrier solvent. The maximum sampling frequency was about 150 samples per hr. It appeared to be possible to introduce a membrane separation step, thus allowing for the determination of water in fouled process streams. To avoid direct contact between the Karl Fischer solution and the pumping tubes, and thus extend the lifetime of the tubes, an indirect delivery system, based on replacement of the solution by pumped silicone oil, was also applied

Thermal Modeling and Experimental Validation in the LENS™ Process

Several aspects of the thermal behavior of deposited stainless steel 410 (SS410) during the Laser Engineered Net Shaping (LENSTM) process were investigated experimentally and numerically. Thermal images in the molten pool and surrounding area were recorded using a two-wavelength imaging pyrometer system, and analyzed using ThermaVizTM software to obtain the temperature distribution. The molten pool size, temperature gradient, and cooling rate were obtained from the recorded history of temperature profiles. The dynamic shape of the molten pool, including the pool size in both travel direction and depth direction, was investigated and the effect of different process parameters was illustrated. The thermal experiments were performed in a LENSTM 850 machine with a 3kW IPG laser for different process parameters. A three-dimensional finite element model was developed to calculate the temperature distribution in the LENS process as a function of time and process parameters. The modeling results showed good agreement with the experimental data.Mechanical Engineerin