O.B. Malyshev

Investigation of synchrotron radiation‐induced photodesorption in cryosorbing quasiclosed geometry

The results of 4.2‐K photodesorption experiments in two quasiclosed geometries are reported, (1) a simple tube and (2) a tube with a coaxial perforated liner. These experiments were designed to measure separately the desorption coefficients of tightly bound and physisorbed molecules. The results are important for the beam tube vacuum of the next generation of superconducting proton colliders that have been contemplated—the 20‐TeV Superconducting Super Collider in the United States and the 7.3‐TeV Large Hadron Collider at CERN.

Low secondary electron yield engineered surface for electron cloud mitigation

Secondary electron yield (SEY or δ) limits the performance of a number of devices. Particularly, in high-energy charged particle accelerators, the beam-induced electron multipacting is one of the main sources of electron cloud (e-cloud) build up on the beam path; in radio frequency wave guides, the electron multipacting limits their lifetime and causes power loss; and in detectors, the secondary electrons define the signal background and reduce the sensitivity. The best solution would be a material with a low SEY coating and for many applications δ < 1 would be sufficient. We report on an alternative surface preparation to the ones that are currently advocated. Three commonly used materials in accelerator vacuum chambers (stainless steel, copper, and aluminium) were laser processed to create a highly regular surface topography. It is shown that this treatment reduces the SEY of the copper, aluminium, and stainless steel from δmax of 1.90, 2.55, and 2.25 to 1.12, 1.45, and 1.12, respectively. The δmax furt...

Development of a Superconducting Helical Undulator for a Polarised Positron Source

A method of producing a polarised positron beam from e+e-pair production in a target by circularly polarised γ-radiation is being investigated. Polarised photons are to be generated by the passage of a high energy electron beam (250 GeV as anticipated in the International Linear Collider - ILC) through a helical undulator. For production of 20 MeV photons, an undulator with a period of 14 mm, a bore of approximately 4 mm and magnetic field on axis of 0.8 T is required. First prototypes have been constructed using both superconducting and permanent magnet technologies which are capable of producing the necessary magnetic field configuration in the undulator. This paper details the design, construction techniques and field measurement results of the first superconducting prototype and compares the results with simulation.

Synchrotron radiation induced gas desorption from a Prototype Large Hadron Collider beam screen at cryogenic temperatures

The performance of the vacuum system of the Large Hadron Collider will depend critically on the synchrotron radiation induced gas desorption and on the readsorption of molecules on the cold surfaces. The present design of the system is based on a so‐called beam screen inserted in the 1.9 K cold bore of the magnets. Gas molecules desorbed will therefore readsorb on the beam screen which is held at a temperature between 5 and 20 K. Pumping slots in the beam screen enable some of the desorbed gas to be pumped onto the 1.9 K surface of the cold bore.

In situ ultrahigh vacuum residual gas analyzer “calibration”

Knowing the residual gas spectrum is essential for many applications and research in ultrahigh vacuum (UHV). Residual gas analyzers (RGAs) are used for both qualitative and quantitative gas analyses, where the quadrupole mass analyzers are now the most popular. It was found that RGAs supplied by different manufacturers are not necessarily well calibrated for quantitative gas analysis. A procedure applied for in situ RGA “calibration” against a calibrated UHV total pressure gauge is described in this article. It was found that special attention should be paid to H2 calibration, as RGAs are usually much more sensitive to H2 than ionization gauges. The calibration coefficients are quite reproducible in Faraday cup mode, however, using the secondary electron multiplier requires frequent checks of the calibration coefficients. The coefficients obtained for the RGA allow the use of the RGA as an accurate device for gas spectrum analysis.

Influence of deposition pressure and pulsed dc sputtering on pumping properties of Ti–Zr–V nonevaporable getter films

The performance of a UHV vessel can be improved with a new CERN technology nonevaporable getter (NEG) coating, which is already widely used for accelerator vacuum chambers. Better understanding of the processes involved in NEG film deposition, activation, and poisoning should allow optimization and engineering of the film properties, which are necessary for a particular application. Ti–Zr–V NEG films were created by magnetron sputtering from a single Ti–Zr–V target, and the NEG performance and morphology dependence on deposition pressure, sputtering conditions, and substrate surface roughness have been investigated. It was found that the average grain size of the Ti–Zr–V film was 5–6 nm and was broadly independent of the substrate material and deposition conditions. However, film topography and density were shown to depend very much on the substrate surface roughness and deposition conditions. Rough substrates, high working pressures, and the absence of ion bombardment produced open columnar structures, w...

A study of the photodesorption process for cryosorbed layers of H2, CH4, CO or CO2 at various temperatures between 3 and 68 K

The vacuum performance of the large hadron collider (LHC) at CERN will depend critically on the photodesorption of gas by synchrotron radiation and the re-adsorption of these molecules back onto the cold surface. The results of photon-induced molecular desorption by synchrotron radiation with a critical photon energy of 284 eV for H 2 ,C H 4 ,C O or CO 2 cryosorbed on a stainless steel surface are presented. Most measurements have been carried out in a temperature range from about 3 to 20 K. Measurements for CO 2 were also performed at 68 K. The specific method used for this study has been to pre-deposit a known quantity of gas onto a cold surface, to irradiate the surface with a known photon dose and to measure the quantity of gas remaining on the cold surface by recording the pressure during warm-up. The average photodesorption yields of all gas species were found to increase with increasing surface coverage and to reach a saturation value. For H 2 this value is approximately 0.5 for a coverage exceeding 1017 molecules/cm2, for CH 4 and CO 2 saturation occurs at about 0.5 molecules/photon at &1019 molecules/cm2, while the corresponding final value for CO is about 0.04 molecules/photon at this coverage. ( 1999 Elsevier Science Ltd. All rights reserved.

Electron stimulated desorption from bare and nonevaporable getter coated stainless steels

An installation for investigation of the electron stimulated desorption (ESD) from both pumping and nonpumping tubular samples was designed and built. This installation allows studying ESD and sample sticking probability as a function of electron dose up to about 1023 e−/m2, electron energy in the range 10 eV–6.5 keV, and sampling temperature in the range 0–80 °C. Two samples were investigated: bare and Ti–Zr–V coated stainless steels. The ESD yields were measured as a function of electron accumulated dose, electron energy, and different NEG coating activation temperatures. The effect of electron stimulated pumping of CO saturated NEG coating was demonstrated for the first time and is in a good agreement with the effect of photon stimulated NEG activation measured earlier.

Activation and measurement of nonevaporable getter films

An experimental setup for studying the pumping and capacity properties of nonevaporable getter (NEG)-coated films has been designed and built in the ASTeC Vacuum Science Laboratory at Daresbury Laboratory. The measurement system is based on the dynamic expansion method with a gas injection system that is capable of injecting the residual gases present in a typical UHV system, such as H2, CO, CO2, and CH4. The test particle Monte Carlo model was used for accurate evaluation of NEG film sticking probability from the pressure reading during gas injection. The experimental measurements with NEG-coated samples have shown that the NEG film can be poisoned during the NEG film activation (and/or regeneration) by molecules of CO and CO2 desorbed from uncoated parts of the vacuum chamber. The effect of NEG film poisoning was reduced by lowering the temperature of the uncoated parts of the vacuum chamber during the NEG film activation and, as a result, a new activation procedure was developed for vacuum systems cont...

Method and setup for photodesorption measurements for a nonevaporable-getter-coated vacuum chamber

This article describes a method for measurement of the photodesorption yield and sticking probability for a vacuum chamber coated with nonevaporable getter (NEG). This method is based on a three-gauge method of measurement which was developed and used on a specially designed and assembled setup for the photodesorption yield measurements of test chambers. It has been modified for photodesorption measurements of a vacuum chamber with sorbing walls. Although a short description of the installation and the results of the photodesorption measurements with the NEG-coated vacuum chamber have been published [V. V. Anashin, I. R. Collins, R. V. Dostovalov, N. V. Fedorov, A. A. Krasnov, O. B. Malyshev, and V. L. Ruzinov, The 8th European Particle Accelerator Conference, Paris, France, 2002, p. 2550; Vacuum 75, 155 (2004)], the details of the method, the analysis, and the experimental setup are also described in this article. Special attention is paid to define the sensitivity limit of the method and the setup and to reduce electron- and photon-stimulated desorption in the measurement port.This article describes a method for measurement of the photodesorption yield and sticking probability for a vacuum chamber coated with nonevaporable getter (NEG). This method is based on a three-gauge method of measurement which was developed and used on a specially designed and assembled setup for the photodesorption yield measurements of test chambers. It has been modified for photodesorption measurements of a vacuum chamber with sorbing walls. Although a short description of the installation and the results of the photodesorption measurements with the NEG-coated vacuum chamber have been published [V. V. Anashin, I. R. Collins, R. V. Dostovalov, N. V. Fedorov, A. A. Krasnov, O. B. Malyshev, and V. L. Ruzinov, The 8th European Particle Accelerator Conference, Paris, France, 2002, p. 2550; Vacuum 75, 155 (2004)], the details of the method, the analysis, and the experimental setup are also described in this article. Special attention is paid to define the sensitivity limit of the method and the setup and t...