1,900 research outputs found
Hollow cathodes with BaO impregnated, porous tungsten inserts and tips
The technology of impregnated materials is described and some inherently advantageous characteristics of impregnated cathodes are discussed. Thermionic emission measurements are presented for oxide coated and impregnated cathodes. Five cathode configurations with barium oxide impregnated porous tungsten inserts and/or tips have been fabricated and tested. Reliability, durability, and stability of operation are characterized. One of the cathodes has accumulated over 9000 operational hours, another has been cycled on and off more than 800 times
Mordell-Weil Torsion and the Global Structure of Gauge Groups in F-theory
We study the global structure of the gauge group of F-theory compactified
on an elliptic fibration . The global properties of are encoded in the
torsion subgroup of the Mordell-Weil group of rational sections of .
Generalising the Shioda map to torsional sections we construct a specific
integer divisor class on as a fractional linear combination of the
resolution divisors associated with the Cartan subalgebra of . This divisor
class can be interpreted as an element of the refined coweight lattice of the
gauge group. As a result, the spectrum of admissible matter representations is
strongly constrained and the gauge group is non-simply connected. We exemplify
our results by a detailed analysis of the general elliptic fibration with
Mordell-Weil group and as well as a further
specialization to . Our analysis exploits the
representation of these fibrations as hypersurfaces in toric geometry.Comment: 42 pages, 10 figures; v2: references adde
Inline self-diffraction dispersion-scan of over octave-spanning pulses in the single-cycle regime
We present an implementation of dispersion-scan based on self-diffraction (SD
d-scan) and apply it to the measurement of over octave-spanning sub-4-fs
pulses. The results are compared with second-harmonic generation (SHG) d-scan.
The efficiency of the SD process is derived theoretically and compared with the
spectral response retrieved by the d-scan algorithm. The new SD d-scan has a
robust inline setup and enables measuring pulses with over-octave spectra,
single-cycle durations and wavelength ranges beyond those of SHG crystals, such
as the ultraviolet and the deep-ultraviolet.Comment: 8 pages, 5 figure
Cascaded Nondegenerate Four-Wave Mixing Technique for High-Power Single-Cycle Pulse Synthesis in the Visible and Ultraviolet Ranges
We present a new technique to synthesize high-power single-cycle pulses in
the visible and ultraviolet ranges by coherent superposition of a multiband
octave-spanning spectrum obtained by highly-nondegenerate cascaded four-wave
mixing of femtosecond pulses in bulk isotropic nonresonant media. The
generation of coherent spectra spanning over two octaves in bandwidth is
experimentally demonstrated using a thin fused silica slide. Full
characterization of the intervening multicolored fields using
frequency-resolved optical gating, where multiple cascaded orders have been
measured simultaneously for the first time, supports the possibility of direct
synthesis of near-single-cycle 2.2 fs visible-UV pulses without recurring to
complex amplitude or phase control, which should enable many applications in
science and technology.Comment: 13 pages, 4 figures. Submitted to Physical Review
Optical properties of ion beam textured metals
Copper, silicon, aluminum, titanium and 316 stainless steel were textured by 1000 eV xenon ions from an 8 cm diameter electron bombardment ion source. Simultaneously sputter-deposited tantalum was used to facilitate the development of the surface microstructure. Scanning electron microscopy of the ion textured surfaces revealed two types of microstructure. Copper, silicon, and aluminum developed a cone structure with an average peak-to-peak distance ranging from 1 micron for silicon to 6 microns for aluminum. Titanium and 316 stainless steel developed a serpentine ridge structure. The average peak-to-peak distance for both of these materials was 0.5 micron. Spectral reflectance was measured using an integrating sphere and a holraum reflectometer. Total reflectance for air mass 0 and 2, solar absorptance and total emittance normalized for a 425 K black body were calculated from the reflectance measurements
ArgR is an essential local transcriptional regulator of the arcABC-operon in Streptococcus suis and crucial for biological fitness in acidic environment
Streptococcus suis is one of the most important pathogens in pigs and can also cause severe infections in humans. Despite its clinical relevance very little is known about the factors contributing to its virulence. Recently, we identified a new putative virulence factor in Streptococcus suis, the arginine deiminase system (ADS), an arginine catabolic enzyme system encoded by the arcABC-operon, which enables Streptococcus suis to survive in acidic environment. In this study, we focused on ArgR, an ADS associated regulator belonging to the ArgR/AhrC arginine repressor family. Using an argR knock-out strain we could show that ArgR is essential for arcABC-operon expression and necessary for the biological fitness of Streptococcus suis. By cDNA expression microarray analyses and quantitative real time RT-PCR we found that the arcABC-operon is the only gene cluster regulated by ArgR, which is in contrast to many other bacteria. Reporter gene analysis with gfp under the control of the arcABC promoter demonstrated that ArgR is able to activate the arcABC promoter. Electrophoretic mobility shift assays with fragments of the arcABC promoter and recombinant ArgR, and chromatin immunoprecipitation with antibodies directed against ArgR revealed that ArgR interacts with the arcABC promoter in vitro and in vivo by binding to a region from -147 to 72 bp upstream of the transcriptional start point. Overall our results show that in Streptococcus suis ArgR is an essential, system specific transcriptional regulator of the ADS directly interacting with the arcABC promoter in vivo
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