Studies on charge production from Cs2Te photocathodes in the PITZ L-band normal conducting radio frequency photo injector

This paper discusses the behavior of electron bunch charge produced in an L-band normal conducting radio frequency cavity (RF gun) from Cs2Te photocathodes illuminated with ps-long UV laser pulses when the laser transverse distribution consists of a flat-top core with Gaussian-like decaying halo. The produced charge shows a linear dependence at low laser pulse energies as expected in the quantum efficiency limited emission regime, while its dependence on laser pulse energy is observed to be much weaker for higher values, due to space charge limited emission. However, direct plug-in of experimental parameters into the space charge tracking code ASTRA yields lower output charge in the space charge limited regime compared to measured values. The rate of increase of the produced charge at high laser pulse energies close to the space charge limited emission regime seems to be proportional to the amount of halo present in the radial laser profile since the charge from the core has saturated already. By utilizing core + halo particle distributions based on measured radial laser profiles, ASTRA simulations and semi-analytical emission models reproduce the behavior of the measured charge for a wide range of RF gun and laser operational parameters within the measurement uncertainties.Comment: 15 pages, 16 figures, 2 table

Report on Gun Conditioning Activities at PITZ in 2013

Recently three RF guns were prepared at the Photo Injector Test Facility at DESY, location Zeuthen PITZ for their subsequent operation at FLASH and the European XFEL. The gun 3.1 is a previous cavity design and is currently installed and operated at FLASH, the other two guns 4.3 and 4.4 were of the current cavity design and are dedicated to serve for the start up of the European XFEL photo injector. All three cavities had been dry ice cleaned prior their conditioning and hence showed low dark current levels. The lowest dark current level as low as 60 amp; 956;A at 65MV m field amplitude has been observed for the gun 3.1. This paper reports in details about the conditioning process of the most recent gun 4.4. It informs about experience gained at PITZ during establishing of the RF conditioning procedure and provides a comparison with the other gun cavities in terms of the dark currents. It also summarizes the major setup upgrades, which have affected the conditioning processes of the cavitie

The construction of identity through visual intertextuality in a Bohemian early modern travelogue

Cultural historians have long been concerned with visual sources. Their research has centred on ways of reading images and how they can most accurately be interpreted. This article focuses on an alternative aspect of these visual sources: on how images were made and used. It analyses how the identity of a Bohemian Catholic, Bed?ich z Donín, is constructed by his use of images in a travelogue based on his pilgrimage in the early 17th century. Highlighting the process of ‘visual intertextuality’, it claims that the ways in which Donín adopts and adapts visual images reveals his association with various affinity groups. The distinction between ‘actual’ and ‘habitual’ intertextuality is applied to the analysis of this historical source and shows how competing voices are present in the images. This article is an example of how historians can use the methodologies of semioticians to benefit their research

Evaluation of the laser melting process of different materials for the front-side metallisation of silicon solar cells

This work evaluates the laser-assisted metallization process based on the melting/sintering of metallic powders for the formation of front-side electrical contacts for silicon solar cells, as a part of a two-step metallization concept without masks or photo-lithographical definition. A metallic seed layer is formed by a laser beam and then thickened by lightinduced silver plating [1]. The deposition of a homogeneous powder coating was found to be the crucial requirement for a reproducible process

Proof of damage-free selective removal of thin dielectric coatings on silicon wafers by irradiation with femtosecond laser pulses

The microstructural impact of selective femtosecond laser ablation of thin dielectric layers from monocrystalline silicon wafers was investigated. Various spots opened by 280 fs laser pulses at lambda = 1.03 µm wavelength and 50 fs pulses at 800 nm, respectively, were analyzed in detail using Raman and transmission electron microscopy. The results show clearly that the thin dielectric films can be removed without any detectable modification of the Si crystal structure in the opened area. In contrast, in adjacent regions corresponding to laser fluence slightly below the breaking threshold, a thin layer of amorphous silicon with a maximum thickness of about 50 nm is found at the Si/SiO2 interface after laser irradiation. More than one pulse on the same position, however, causes structural modification of the silicon after thin film ablation in any case

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Diversity of aerobic methanotrophic bacteria in a permafrost active layer soil of the Lena Delta, Siberia

With this study, we present first data on the diversity of aerobic methanotrophic bacteria (MOB) in an Arctic permafrost active layer soil of the Lena Delta, Siberia. Applying denaturing gradient gel electrophoresis and cloning of 16S ribosomal ribonucleic acid (rRNA) and pmoA gene fragments of active layer samples, we found a general restriction of the methanotrophic diversity to sequences closely related to the genera Methylobacter and Methylosarcina, both type I MOB. In contrast, we revealed a distinct species-level diversity. Based on phylogenetic analysis of the 16S rRNA gene, two new clusters of MOB specific for the permafrost active layer soil of this study were found. In total, 8 out of 13 operational taxonomic units detected belong to these clusters. Members of these clusters were closely related to Methylobacter psychrophilus and Methylobacter tundripaludum, both isolated from Arctic environments. A dominance of MOB closely related to M. psychrophilus and M. tundripaludum was confirmed by an additional pmoA gene analysis. We used diversity indices such as the Shannon diversity index or the Chao1 richness estimator in order to compare the MOB community near the surface and near the permafrost table. We determined a similar diversity of the MOB community in both depths and suggest that it is not influenced by the extreme physical and geochemical gradients in the active layer