Double-discharge copper-vapor laser

Power supply for discharge pulses consists of two capacitors that are made to discharge synchronously with adjustable time intervals. First pulse is switched with hydrogen thyratron, and second by spark gap. Lasing action peaks for appropriate combination of these two parameters

Hydrogen film cooling of a small hydrogen-oxygen thrust chamber and its effect on erosion rates of various ablative materials

An experimental investigation was conducted to determine what arrangement of film-coolant-injection orifices should be used to decrease the erosion rates of small, high temperature, high pressure ablative thrust chambers without incurring a large penalty in combustion performance. All of the film cooling was supplied through holes in a ring between the outer row of injector elements and the chamber wall. The best arrangement, which had twice the number of holes as there were outer row injection elements, was also the simplest. The performance penalties, presented as a reduction in characteristic exhaust velocity efficiency, were 0.8 and 2.8 percentage points for the 10 and 20 percent cooling flows, respectively, The best film-coolant injector was then used to obtain erosion rates for 19 ablative materials. The throat erosion rate was reduced by a factor of 2.5 with a 10 percent coolant flow. Only the more expensive silica phenolic materials had low enough erosion rates to be considered for use in the nozzle throat. However, some of the cheaper materials might qualify for use in other areas of small nozzles with large throat diameters where the higher erosion rates are more acceptable

Nonlinear optics in Xe-filled hollow-core PCF in high pressure and supercritical regimes

Supercritical Xe at 293 K offers a Kerr nonlinearity that can exceed that of fused silica while being free of Raman scattering. It also has a much higher optical damage threshold and a transparency window that extends from the UV to the infrared. We report the observation of nonlinear phenomena, such as self-phase modulation, in hollow-core photonic crystal fiber filled with supercritical Xe. In the subcritical regime, intermodal four-wave-mixing resulted in the generation of UV light in the HE12 mode. The normal dispersion of the fiber at high pressures means that spectral broadening can clearly obtained without influence from soliton effects or material damage

Accuracy of self-evaluation in a peer-learning environment: An analysis of a group learning model

There is a global competitive demand for graduates with soft skills, and higher education institutions are tasked to reduce the employee skill gap. Thus, we investigated the students' perceptions of peer assessment in facilitating engagement in soft-skill development through group work activities. Using group work to measure the effectiveness of students' feedback on their assessment, we posit that students perceive self-assessment in group work as a tool that represents fairness. By focusing on learning in a peer-assisted learning environment, the study is a two-period different observation on the effectiveness and validity of peer assessment practice. We applied a group learning model over two academic sessions to investigate if students can self-evaluate accurately in a peer-learning environment. The employed methods included both qualitative and quantitative analysis. The findings of the study differ from previous findings that students cannot self-assess accurately. Empirically, there was no significant difference between the peer marks and tutor marks. The study also found that peer learning improves students' quality of assessment as they reflect on their work better

Preliminary Results from Recent Measurements of the Antiprotonic Helium Hyperfine Structure

We report on preliminary results from a systematic study of the hyperfine (HF) structure of antiprotonic helium. This precise measurement which was commenced in 2006, has now been completed. Our initial analysis shows no apparent density or power dependence and therefore the results can be averaged. The statistical error of the observable M1 transitions is a factor of 60 smaller than that of three body quantum electrodynamic (QED) calculations, while their difference has been resolved to a precision comparable to theory (a factor of 10 better than our first measurement). This difference is sensitive to the antiproton magnetic moment and agreement between theory and experiment would lead to an increased precision of this parameter, thus providing a test of CPT invariance.Comment: 6 pages, 4 figure

Control of porous structure in flexible silicone aerogels produced from methyltrimethoxysilane (MTMS): the effect of precursor concentration in sol–gel solutions

Controllable nanoporous structure in MTMS-based silicone aerogels is required to improve their thermal conductivity. Silicone aerogels were formed in a two-step acid–base catalysed sol–gel process combined with supercritical drying. The influence of MTMS concentration, specifically the molar ratio of methanol:MTMS and water:MTMS in the sol–gel process was studied in relation to the porous structure of resultant silicone aerogels. Samples were characterised to determine the dimensions of micro, meso and macro-pore structure by means of both nitrogen gas adsorption–desorption for detection of pores 300 nm. Porosity, pore volume distribution and Brunau er–Emmitt–Teller surface area in the silicone aerogels were all found to be influenced by adjustment of the molar ratio of methanol:MTMS and the molar ratio of water:MTMS during sol–gel processing

Geodynamic and metabolic cycles in the Hadean

International audienceHigh-degree melting of hot dry Hadean mantle at ocean ridges and plumes resulted in a crust about 30km thick, overlain in places by extensive and thick mafic volcanic plateaus. Continental crust, by contrast, was relatively thin and mostly submarine. At constructive and destructive plate boundaries, and above the many mantle plumes, acidic hydrothermal springs at ~400°C contributed Fe and other transition elements as well as P and H2 to the deep ocean made acidulous by dissolved CO2 and minor HCl derived from volcanoes. Away from ocean ridges, submarine hydrothermal fluids were cool (=100°C), alkaline (pH ~10), highly reduced and also H2-rich. Reaction of solvents in this fluid with those in ocean water was catalyzed in a hydrothermal mound, a natural self-restoring flow reactor and fractionation column made up of carbonates and freshly precipitated Fe-Ni sulfide and greenrust pores and bubbles, developed above the alkaline spring. Acetate and the amino acetate glycine were the main products, much of which was eluted to the ocean. Other organic byproducts were retained, concentrated and reacted within the compartments. These compartments comprising the natural hydrothermal reactor consisted partly of greigite (Fe5NiS8). It was from reactions between organic modules confined within these inorganic compartments that the first prokaryotic organism evolved. These acetogenic precursors to the Bacteria diversified and migrated down the mound and into the ocean floor to inaugurate the "deep biosphere". Once there the Bacteria, and the recently differentiated Archaea, were protected from cataclysmic heating events caused by large bolide impacts. Geodynamic forces led to the eventual obduction of the deep biosphere into the photic zone where, initially protected by a thin veneer of sediment, the use of solar energy was mastered and photosynthesis emerged. The further evolution to oxygenic photosynthesis was effected as catalytic [CaMn4+] bearing molecules that otherwise would have been interred in the mineral ranciéite in the shallow marine manganiferous sediments, were sequestered and invaginated within the cyanobacterial precursor where, energized by light, they could oxidize water with greater efficiency. Thus, chemical sediments were required both for the emergence of chemosynthesis and of oxygenic photosynthesis, the two innovations that did most to change the nature of our planet

Evaluation of hidden Markov models robustness in uncovering focus of visual attention from noisy eye-tracker data

A robust way to unconver the focus of visual attention from (simulated) noisy eye tracking data provided by the hidden Markov model was discussed. It was found that a hidden simi-Markov model (HSMM) with explicit state duration PDF representing task-constrained visual attention was more stable and accurate to represent visual attention duration. HSMM used an additional Gaussian component to the observation distribution PDF with larger standard deviation to ensure less differentiation between eye movement positions for away from the object. Analysis shows that HMM and HSMM performed better in terms of accuracy and instability than the baseline non-HMM method