Asymmetric Electrostatic Dodecapole: Compact Bandpass Filter with Low Aberrations for Momentum Microscopy

Imaging energy filters in photoelectron microscopes and momentum microscopes employ spherical fields with deflection angles of 90{\deg}, 180{\deg} and even 2 x 180{\deg}. These instruments are optimized for high energy resolution, yet they come along with image aberrations when they are operated in high transmission mode with medium energy resolution. Here we present a new approach for bandpass-filtered imaging in real or reciprocal space, using an asymmetric electrostatic dodecapole. This multipole enables energy-dispersive beam deflection and correction of image aberrations up to the 3rd order. Owing to a deflection angle of only 4{\deg}, the total beam displacement in the filter is just ~10 mm. Hence, the entire instrument is compact and just requires a straight vacuum tube. The multipole is framed by transfer lenses in the entrance and exit branch. Two sets of 16 entrance and exit apertures with different sizes on piezomotor-driven holders allow selecting the desired resolution. The combination of apertures and dodecapole acts as a bandpass pre-selector in a high-energy time-of-flight momentum microscope at the hard X-ray beamline P22 at PETRA-III (DESY, Hamburg). At pass energies between 400 and 600 eV it transmits electrons with kinetic energies in the range of 20-40 eV and thus effectively eliminates unwanted intensity from higher-energy electrons in the ToF analyzer. At low pass energies, the instrument allows energy-filtered imaging without subsequent ToF analysis. In a laboratory experiment the 4{\deg} prototype reached < 500 meV resolution, which is sufficient for fast survey studies in the X-ray range.Comment: 16 pages, 6 figures, 26 reference

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

Multi-Mode Front Lens for Momentum Microscopy: Part II Experiments

We have experimentally demonstrated different operating modes for the front lenses of the momentum microscopes described in Part I. Measurements at energies from vacuum UV at a high-harmonic generation (HHG)-based source to the soft and hard X-ray range at a synchrotron facility validated the results of theoretical ray-tracing calculations. The key element is a ring electrode concentric with the extractor electrode, which can tailor the field in the gap. First, the gap-lens-assisted extractor mode reduces the field strength at the sample while mitigating image aberrations. This mode gave good results in all spectral ranges. Secondly, by compensating the field at the sample surface with a negative voltage at the ring electrode we can operate in zero-field mode, which is beneficial for operando experiments. Finally, higher negative voltages establish the repeller mode, which removes all slow electrons below a certain kinetic energy to eliminate the primary contribution to the space-charge interaction in pump-probe experiments. The switch from extractor to repeller mode is associated with a reduction in the k-field-of-view (10-20 % at hard-X-ray energies, increasing to ~50% at low energies). Real-space imaging also benefits from the new lens modes as confirmed by ToF-XPEEM imaging with 650 nm resolution.Comment: 22 pages, 9 figures, 56 reference

Time of flight photoelectron momentum microscopy with 80 500 MHz photon sources electron optical pulse picker or bandpass pre filter

The small time gaps of synchrotron radiation in conventional multi bunch mode 100 500 MHz or laser based sources with high pulse rate 80 MHz are prohibitive for time of flight ToF based photoelectron spectroscopy. Detectors with time resolution in the 100 ps range yield only 20 100 resolved time slices within the small time gap. Here we present two techniques of implementing efficient ToF recording at sources with high repetition rate. A fast electron optical beam blanking unit with GHz bandwidth, integrated in a photoelectron momentum microscope, allows electron optical pulse picking with any desired repetition period. Aberration free momentum distributions have been recorded at reduced pulse periods of 5 MHz at MAX II and 1.25 MHz at BESSY II . The approach is compared with two alternative solutions a bandpass pre filter here a hemispherical analyzer or a parasitic four bunch island orbit pulse train, coexisting with the multi bunch pattern on the main orbit. Chopping in the time domain or bandpass pre selection in the energy domain can both enable efficient ToF spectroscopy and photoelectron momentum microscopy at 100 500 MHz synchrotrons, highly repetitive lasers or cavity enhanced high harmonic sources. The high photon flux of a UV laser 80 MHz, lt;1 meV bandwidth facilitates momentum microscopy with an energy resolution of 4.2 meV and an analyzed region of interest ROI down to lt;800 nm. In this novel approach to sub m ARPES the ROI is defined by a small field aperture in an intermediate Gaussian image, regardless of the size of the photon spo

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