Systematic study of niobium thermal treatments for superconducting radio frequency cavities employing x ray photoelectron spectroscopy

The structural and chemical composition of the surface layer 100 140 nm of niobium radiofrequency cavities operating at cryogenic temperature has enormous impact on their superconducting characteristics. During the last years, cavities treated with a new thermal processing recipe, so called nitrogen infusion, have demonstrated an increased efficiency and high accelerating gradients. The role and importance of nitrogen gas has been a topic of many debates. In the present work we employ variable energy synchrotron x ray photoelectron spectroscopy XPS , to study the niobium surface subjected to the following treatments vacuum annealing at 800 C, nitrogen infusion, and vacuum heat treatment as for the infusion process but without nitrogen supply. Careful analysis of XPS energy distribution curves revealed a slightly increased thickness of the native oxide Nb2O5 for the infused samples amp; 8764;3.8 nm as compared to the annealed one amp; 8764;3.5 nm which indicates insignificant oxygen incorporation into niobium during 120 C baking and no effect of nitrogen on the formation of oxides or other niobium phases. By conducting an additional in situ annealing experiment and analyzing the niobium after the failed infusion process, we conclude that the vacuum furnace hygiene particularly during the high temperature stage is the prerequisite for success of any treatment recip

Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens

The performance of time-resolved photoemission experiments at fs-pulsed photon sources is ultimately limited by the e–e Coulomb interaction, downgrading energy and momentum resolution. Here, we present an approach to effectively suppress space-charge artifacts in momentum microscopes and photoemission microscopes. A retarding electrostatic field generated by a special objective lens repels slow electrons, retaining the k-image of the fast photoelectrons. The suppression of space-charge effects scales with the ratio of the photoelectron velocities of fast and slow electrons. Fields in the range from −20 to −1100 V/mm for Ekin = 100 eV to 4 keV direct secondaries and pump-induced slow electrons back to the sample surface. Ray tracing simulations reveal that this happens within the first 40 to 3 μm above the sample surface for Ekin = 100 eV to 4 keV. An optimized front-lens design allows switching between the conventional accelerating and the new retarding mode. Time-resolved experiments at Ekin = 107 eV using fs extreme ultraviolet probe pulses from the free-electron laser FLASH reveal that the width of the Fermi edge increases by just 30 meV at an incident pump fluence of 22 mJ/cm2 (retarding field −21 V/mm). For an accelerating field of +2 kV/mm and a pump fluence of only 5 mJ/cm2, it increases by 0.5 eV (pump wavelength 1030 nm). At the given conditions, the suppression mode permits increasing the slow-electron yield by three to four orders of magnitude. The feasibility of the method at high energies is demonstrated without a pump beam at Ekin = 3830 eV using hard x rays from the storage ring PETRA III. The approach opens up a previously inaccessible regime of pump fluences for photoemission experiments

Features of designing hydraulic excavator in APM WinMachine

The urgency of the work is due to the need for design departments involved in the design of hydraulic excavators in techniques. Allowing to reduce the weight of excavators while providing at the same time sufficient reliability. The purpose of the work: development of a technique for application in the design of excavators of calculation modules based on the use of finite elements. Research methodology: modeling of working equipment. For a hydraulic excavator with a "direct" shovel working equipment, a mathematical model for calculating effort, an algorithm and a program in an algorithmic language have been developed, which allow to determine the working area of the excavator, possible digging forces, and efforts in the elements of the working equipment. To calculate stresses in the design of the working equipment, two modeling options are proposed: the models for the Strucrure 3D computational module are compiled separately for the bucket of the handle and the boom, the interaction of the models is carried out by efforts that are determined by the specified digging forces; a complete model of all the working equipment for the calculation module is compiled, without the need to calculate the loads between the elements, the calculation is carried out directly by the digging force. For the first variant formulas of calculation of efforts in elements of the working equipment are resulted. For the second variant, it is suggested to use a plate-rod model, and recommendations are given for the implementation of the relationships between the boom, the handle and the bucket. The results of stress calculations for the working equipment are presented. © The Authors, published by EDP Sciences, 2018

Study of the formation and properties of In–CuPcF4_4 nanocomposite materials in the mode of the millisecond recording of photoelectron spectra

International audienceA study of the formation processes and properties of nanocomposite materials consisting of indium nanoparticles in a thin film of the organic semiconductor copper tetrafluorophthalocyanine (CuPcF$_4$) is presented. The results are obtained by the new setup for dynamic photoelectron spectroscopy, which allows the recording of spectra in a millisecond interval using the ARGUS photoelectron spectrometer and synchrotron radiation (PETRA III/DESY, Germany). The evolution of the core level (CL) spectra: C1s, N1s, and In3d 5/2 recorded directly during the deposition of indium onto the CuPcF$_4$ surface under ultrahigh vacuum conditions is traced. The thickness of the indium coating during deposition increased from 0 to 5 nm. In this range of coatings, more than 150 spectra are recorded for each CL with a recording rate of 0.1 s/spectrum. The following is established: the significant diffusion of indium atoms deep into the organic matrix is observed; in fact, no chemical interaction of indium with carbon atoms is found; indium atoms are located in places close to pyrrole nitrogen of the CuPcF$_4$ molecule. Apparently, during the interaction of In atoms and pyrrole nitrogen atoms, a negative charge is transferred from indium to the CuPcF$_4$ molecule. Thus, data on the fast-flowing processes of the formation of organometallic In-CuPcF$_4$ interfaces are obtained

Systematic study of niobium thermal treatments for superconducting radio frequency cavities employing x-ray photoelectron spectroscopy

The structural and chemical composition of the surface layer (100–140 nm) of niobium radiofrequency cavities operating at cryogenic temperature has enormous impact on their superconducting characteristics. During the last years, cavities treated with a new thermal processing recipe, so-called nitrogen infusion, have demonstrated an increased efficiency and high accelerating gradients. The role and importance of nitrogen gas has been a topic of many debates. In the present work we employ variable-energy synchrotron x-ray photoelectron spectroscopy (XPS), to study the niobium surface subjected to the following treatments: vacuum annealing at 800 °C, nitrogen infusion, and vacuum heat treatment as for the infusion process but without nitrogen supply. Careful analysis of XPS energy-distribution curves revealed a slightly increased thickness of the native oxide Nb2O5 for the infused samples (∼3.8 nm) as compared to the annealed one (∼3.5 nm) which indicates insignificant oxygen incorporation into niobium during 120 °C baking and no effect of nitrogen on the formation of oxides or other niobium phases. By conducting an additional in-situ annealing experiment and analyzing the niobium after the failed infusion process, we conclude that the vacuum furnace hygiene particularly during the high-temperature stage is the prerequisite for success of any treatment recipe

Rotated domain network in graphene on cubic SiC 001

The atomic structure of the cubic SiC 001 surface during ultra high vacuum graphene synthesis has been studied using scanning tunneling microscopy STM and low energy electron diffraction. Atomically resolved STM studies prove the synthesis of a uniform, millimeter scale graphene overlayer consisting of nanodomains rotated by 13.5 relative to the amp; 9001;110 amp; 9002; directed boundaries. The preferential directions of the domain boundaries coincide with the directions of carbon atomic chains on the SiC 001 c 2 2 reconstruction, fabricated prior to graphene synthesis. The presented data show the correlation between the atomic structures of the SiC 001 c 2 2 surface and the graphene SiC 001 rotated domain network and pave the way for optimizing large area graphene synthesis on low cost cubic SiC 001 Si 001 wafer