We report synthesis and characterization of a batch of three cesium potassium antimonide photocathodes that have been grown on pure copper substrates via a ternary co-deposition method whose ...procedure is described herein. A deposition system that was designed for synthesis of two-element photocathodes has been utilized for synthesis of the aforementioned three-element photocathodes with slope of the in situ photocurrent as the driver for the growth process. A variation of substrate temperature and deposition parameters among the three photocathodes during synthesis has yielded a maximum quantum efficiency of 6% for 140°C substrate temperature. Lifetime studies performed in a 65-kV DC electron gun on two of the photocathodes, but under oxidized states, at tens of μA average currents (ampere-level peak currents) extracted utilizing a 532-nm wavelength, 1-kHz repetition rate laser, have resulted in charge-lifetimes of 6.13 C and 13.78 C, respectively. X-ray photoelectron spectroscopy analysis of the photocathode with the highest quantum efficiency has revealed a nearly impurity-free surface with stoichiometry Cs1.3K1.8Sb1.0 when pristine. Furthermore, it has been found that oxidation of the alkali surface atoms as well as carbon adsorption from hydrocarbons and minor fluorine uptake are the causes of quantum efficiency reduction during laser illumination in the utilized experimental set-up.
The use of superconducting radio frequency (rf) cavities in particle accelerators necessitates that copper (Cu) surfaces are coated by thin niobium (Nb) films, predominantly synthesized by magnetron ...sputtering. A key feature of the rf cavities is that they exhibit a complex three-dimensional geometry, such that during Nb film growth vapor is not deposited on a flat substrate. The latter, combined with the line-of-sight nature of the deposition flux in conventional magnetron sputtering methods (including direct current magnetron sputtering; DCMS) yields films with porous columnar morphologies on surfaces of the cavities that do not face the magnetron source. High-power impulse magnetron sputtering (HiPIMS) is a variant of sputtering that generates highly-ionized fluxes. Using electrical fields, such fluxes can be deflected to trajectories that are closer to the substrate normal and, thereby, dense and uniform layers can be deposited on all surfaces of the rf cavities. In the present work, we use classical molecular dynamics simulations to model Nb film growth on Cu substrates at conditions consistent with those prevailing during DCMS and HiPIMS. Our computational results are in qualitative agreement with experimental data (also generated in the present study), with respect to film morphology. Based on this agreement and by studying the evolution of the simulated systems, we suggest that the morphology of HiPIMS-grown films (as compared to their DCMS counterparts) is the result of the combined effects of deflection of ionized sputtered particles to trajectories parallel to the substrate normal, bombardment-induced interruption of crystal growth, and ballistic atomic rearrangement along with dynamic thermal annealing caused by energetic film-forming species. Moreover, the predictions of our model with respect to dynamic processes at the film-substrate interface and their effect on local epitaxial growth are discussed.
Abstract
All over Run 2, the LHC beam-induced heat load on the cryogenic system exhibited a wide scattering along the ring. Studies ascribed the heat source to electron cloud build-up, indicating an ...unexpected high Secondary Electron Yield (SEY) of the beam screen surface in some LHC regions. The inner copper surface of high and low heat load beam screens, extracted during the Long Shutdown 2, was analysed. On the low heat load ones, the surface was covered with the native Cu
2
O oxide, while on the high heat load ones CuO dominated at surface, and it exhibited a very low carbon coverage. Such chemical modifications increase the SEY and inhibit a proper conditioning of the affected surfaces. Following this characterisation, the mechanisms for CuO build-up in the LHC beam pipe were investigated on a newly commissioned cryogenic system allowing electron irradiation, surface chemical characterisation by X-ray Photoelectron Spectroscopy and SEY measurements on samples held below 15 K. In parallel, curative solutions against the presence of CuO in the LHC beam screens were explored, which could be implemented in-situ to recover a proper conditioning and lower the beam-induced heat load.
Abstract
In accelerator beam chambers and RF waveguides, electron cloud and multipacting can be mitigated effectively by reducing the secondary electron yield (SEY). In recent years, it has been ...established that laser-engineered surface structuring is a very efficient method to create a copper surface with a SEY maximum close to or even below unity. Different laser pulse durations, from nanoseconds to picoseconds, can be used to change surface morphology. Conversely, the characteristics that minimise the SEY, such as the moderately deep grooves and the redeposited nanoparticles, might have unfavourable consequences, including increased RF surface resistance. In this study, we describe the techniques used to measure the surface resistance of laser-treated copper samples using an enhanced dielectric resonator with 12 cm diameter sample sizes operating in the GHz range. The quantification basis lies in a non-contact measurement of the high-frequency losses, focusing on understanding the variation of surface resistance levels depending on the specifics of the treatment and possible post-treatment cleaning procedures.
Adsorption of nano‐scaled titanium(IV) oxide particles on electroplated zinc is performed by a simple dip‐coating technique in an aqueous titanium(IV) oxide suspension prepared with a stirred media ...mill. X‐ray photoelectron spectroscopy, scanning electron microscopy and X‐ray fluorescence spectroscopy are carried out to investigate the composition of the zinc surface and the thickness and porosity of the adsorbed titania films. The zinc surface formed during the electrodeposition process is of oxyhydroxide nature and the thickness of the adsorbed titania particle layer is controlled by the pH value and the solid concentration of the suspension. In the range of 10 wt.%–30 wt.% titanium(IV) oxide, a linear dependence between the titania film thickness and the solid content of titania particles in the suspension is found. Highest film thicknesses are obtained in alkaline media (pH≥9). At 13.5 wt.% titania particles and pH values below pH = 2.4, the titania particle film is not closely packed and the zinc layer underneath is still visible in electron microscopy, which is a prerequisite for imbedding these particles by a thin second zinc layer for formation of a robust chromium(VI)‐free passivation layer containing the titania particles.
Translation
Die Adsorption von nanoskaligen Titandioxidpartikeln auf galvanischen Zinkoberflächen wird mit der einfachen Tauchbeschichtungstechnik in wässrigen Titandioxidsuspensionen, die mit einer Rührwerkskugelmühle hergestellt wurden, durchgeführt. Mittels Röntgenphotoelektronenspektroskopie, Rasterelektronenmikroskopie und Röntgenfluoreszenzanalysen werden die Zusammensetzung der Zinkoberfläche, die Dicke sowie die Porosität der adsorbierten Titandioxidschichten analysiert. Die durch den galvanischen Abscheidungsprozess entstehende Zinkoberfläche besteht aus Oxid‐ und Hydroxidverbindungen und die Dicke der adsorbierten Schicht ist mit dem pH‐Wert und der Feststoffkonzentration der Suspension kontrollierbar. Im Konzentrationsbereich von 10 wt.%–30 wt.% ergibt sich ein linearer Zusammenhang. Die größten Schichtdicken entstehen im alkalischen Bereich (pH≥9). Bei einer Feststoffkonzentration von 13,5 wt.% und einem pH‐Wert von 2,4 entstehen nicht vollkommen dicht gepackte Titandioxidschichten, sodass die darunterliegende Zinkoberfläche bei Mikroskopaufnahmen sichtbar bleibt. Dies ist Voraussetzung, dass die adsorbierten Partikel in einer zweiten Zinkschicht eingebettet werden können und nachfolgend in eine robuste Chrom(IV)‐freie Passivschicht, die diese Partikel enthält, umgewandelt werden kann.
The influence of microgeometries on the Secondary Electron Yield (SEY) of surfaces is investigated. Laser written structures of different aspect ratio (height to width) on a copper surface tuned the ...SEY of the surface and reduced its value to less than unity. The aspect ratio of microstructures was methodically controlled by varying the laser parameters. The results obtained corroborate a recent theoretical model of SEY reduction as a function of the aspect ratio of microstructures. Nanostructures - which are formed inside the microstructures during the interaction with the laser beam - provided further reduction in SEY comparable to that obtained in the simulation of structures which were coated with an absorptive layer suppressing secondary electron emission.
We have demonstrated out-of-focus ultrafast pulsed laser processing of copper with a variable working distance, without the need for mechanical movement. This was achieved by employing a diffractive ...optical element. The method has been demonstrated in a practical application to reduce the secondary electron yield (SEY) of copper to below 1.3. We show that using an extended focus element not only increases the consistency of processing across a range of working distances, but also changes the topography of the produced structures, reducing the SEY. This presented approach shows promise in facilitating the Large Hadron Collider’s (LHC’s) upcoming high luminosity upgrade by preventing electron clouds.
Abstract
To achieve the vacuum quality required for the operation of particle accelerators, the surface of the vacuum vessels must be clean from hydrocarbons. This is usually achieved by wet ...chemistry processes, e.g., degreasing chemical baths that, in case of radioactive vessels, must be disposed accordingly. An alternative way exploits the oxygen plasma produced by a downstream RF plasma source. This technique offers the possibility of operating in-situ, which is an advantageous option to avoid the handling of voluminous and/or fragile components and a more sustainable alternative to large volume disposable baths. In this work, we test a commercial plasma source in dedicated vacuum systems equipped with quartz crystal microbalances (QCMs). The evolution of the etching rates of amorphous carbon (a-C) thin films deposited on the QCMs to mimic contamination are studied as function of operating parameters. We present the results of the plasma cleaning process applied to the real case of a hydrocarbons-contaminated large vacuum vessel. The studies are complemented by transport simulations and surface contamination monitoring by X-ray photoelectron spectroscopy (XPS) analysis. The evaluation of the vessel cleanliness, which is performed via residual gas analysis (RGA) measurements, is based on CERN’s outgassing acceptance criteria and agrees with both simulations and XPS results.
•Temporal evolution of Fuchs-Kliewer phonon spectra of pristine GaN(1-100) surfaces.•Monotonous decrease of multi-phonon scattering probabilities is not applicable.•Dissociative water adsorption is ...likely to change surface dielectric constants.
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Inelastic electron scattering is used to study surface phonon polaritons on H-covered and clean GaN(11¯00) surfaces. The Fuchs–Kliewer phonon of GaN(11¯00)–H gives rise to characteristic signatures of its single and multiple excitation in specular electron energy loss spectra. The loss intensities for multi-phonon scattering processes decrease according to a Poisson distribution. Vibrational spectra of this surface are invariant on the time scale of days reflecting its chemical passivation by the H layer. In contrast, vibrational spectra of pristine GaN(11¯00) are subject to a pronounced temporal evolution where spectroscopic weight is gradually shifted towards the multiple excitation of the Fuchs–Kliewer phonon. As a consequence, the monotonous decrease of the cross section for multiple quantum excitation as observed for the H-covered surface is not applicable. This remarkable effect is particularly strong in spectra acquired at low primary energies of incident electrons, which hints at processes occurring in the very surface region. Scenarios that may contribute to these observations are discussed.
Commercial titanium was treated in ambient atmosphere using pulsed Nd:YAG (λ=1064nm) laser irradiation. Repeated laser treatments induce a removal of surface contaminants as well as the formation of ...a nanostructured top layer exhibiting a large effective surface and nanometer roughness. The laser induced oxidation leads to the presence of a surface layer with strongly improved, hydrothermally stable adhesion when joined to a one-component, hot-curing epoxy-based adhesive. Changes in the material properties have been characterized with respect to the topography, the chemical composition and the crystal structure using SEM, cross-beam FIB, XPS and XRD analyses in order to correlate the adhesion behavior with the structural and chemical characteristics of the surface.
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