Vacuum Chamber Materials Research Articles

Reduction of background scattered light in vacuum systems for cold atoms experiments

Recent advances in the understanding and control of cold atom systems have resulted in devices with extraordinary metrological performance. To further improve the performance in these systems, additional methods of noise reduction are needed. Here, we examine the noise reduction possible from vacuum compatible low reflection coatings in cold atom systems by characterizing a black coating and its compatibility in a Magneto-Optical Trap (MOT). We demonstrate that the commercially available PCO35® coating provides low-reflectivity surfaces that are ultra-high vacuum compatible. The reflective properties of the coating are compared to titanium, a common vacuum chamber material, and the reduction to scattered light is characterized over a range of angles and wavelengths. The outgassing properties of the coating are measured to be less than that of the vacuum system used to test the coating, which is limited to 3 × 10−8 mbar L cm−2 s−1. The coating is applied to a vacuum chamber housing a rubidium prism MOT, and its vacuum compatibility is assessed and compared to an identical non-coated system. Finally, the effect of scattered light reduction in a generalized system is explored theoretically. These results show promise for reducing background light in cold atom experiments via the use of low-reflectivity coatings.

Reflectivity and photoelectron yield from copper in accelerators

For the design of high-performance particle accelerators and vacuum systems for high-energy colliders, the choice of materials and of their surface treatment is an essential prerequisite. Physical and optical properties of such materials and their coatings are input parameters for programs to simulate their performance and to validate machine design. Materials behavior after exposure to synchrotron radiation (SR), produced by the circulating particles, need to be studied because of its detrimental consequences, such as photo induced desorption, heat load, vacuum and beam induced instabilities. Reflectivity, its angular distribution and photo yield, i.e., the number of photoelectrons produced per incident photon, are essential ingredients to simulation codes. Such parameters must be studied not only on materials as they are in accelerators, but also in conditions as close as possible to the operative ones. In this work, we present results of such an experimental campaign, carried out at the Optics Beamline of BESSY-II. This experimental setup, designed to investigate quasiperfect x-ray-optical elements by ``at-wavelength'' metrology (from 35 eV to 1850 eV), is also an ideal tool to perform reflectivity and photo yield studies of vacuum chamber materials. As will be discussed, different roughness and various nano-, micro- or macro- modifications of a Cu-surface significantly influence the parameters under study. Energy- and angle-integrated values for the total reflectivity and photo yield are derived. Such integrated values are representative for material behavior under ``white-light'' irradiation for the various accelerators discussed here.

Dynamic pressure monitoring in the front-end vacuum systems at the TPS facility

The Taiwan Photon Source Front End (TPS FE) vacuum systems are located between the storage ring and beam lines and include various high-heat-load (HHL) components. We investigate the change in dynamic pressure over time in the FE vacuum system, including measurements of HHL component temperatures, residual gas analyzer (RGA) data analysis, and the effect of changing the insertion device (ID) gap during beam current operation. Our results reveal that the exponential slopes of dynamic pressures in the FE versus accumulated beam dose is nearly −0.5 for all cases. With beam current at 400 mA and ID gap close to 7 mm, the dynamic pressure and HHL component temperature are 1.5 × 10−8 Pa and 45 °C, respectively. The RGA data show that the main dynamic pressure contributions are H2, H2O, CO and CO2. H2O, CO and CO2 come from the chamber surface, while H2 comes from the vacuum chamber materials. The dynamic pressure was ∼2.3 × 10−11 Pa/mA at 1000 Ah, and it continues to decrease with an increase in accumulated beam dose. These results indicate that the total dynamic pressure reaches to a value of 10−7 Pa at full operation current in the FE systems.

Field Quality Dependence on Vacuum Chamber Materials in Quadrupole Magnets

In accelerator magnet design, important criteria are the magnetic field distribution and the magnetic field quality. Accelerator magnets are usually designed for the ideal case in which no magnetic material is present in the field region. In practice, however, the presence of a vacuum chamber inside the magnet aperture can affect the magnetic field in case its material has nonnegligible relative permeability. We quantify this effect based on numerical simulations with a conventional quadrupole magnet model. The results show that there is a measurable effect of the configuration, the material choice, and the presence of welding seam in particular.

Delay effects of the interactions between neutrons emitted during plasma experiments performed on the DPF-1000 U facility and construction materials

The Institute of Plasma Physics and Laser Microfusion operates the biggest plasma focus device built so far in the world. It is identified as DPF-1000 U (Dense Plasma Focus Upgrade). The plasma produced by the described device constitutes a pulse of highly effective neutron source with the neutron yield ranging up to 1012 n per impulse. The precise composition of the stainless steel from which the vacuum chamber of the plasma focus device is made, was determined by neutron activation analysis. It was found that nuclear reactions that occur inside the stainless steel are mainly (n, γ), (n, p) and (n, α) reactions. Taking into consideration the neutron energy spectrum and the material composition, 63 nuclear reactions leading to vacuum chamber material activation were identified in total. It was observed that in the first hour after shut-down, the main activity comes from 59Fe and 59Ni isotopes. One year after the shut-down, the main contribution to the observed radioactivity of the experimental chamber material was related to the presence of 54Mn isotope, while after 10 years the only significant contribution to the activity will be made by molybdenum isotopes such as 93mMo and 99Mo.

Analysis of High Vacuum System Based on the Applications of Vacuum Materials

In this study, the outgassing effects of selected vacuum materials on the vacuum characteristics were simulated by the VacSim Multi simulation tool. This investigation examined the feasibility of reliably simulating the outgassing characteristics of common vacuum chamber materials (aluminum, copper, stainless steel, nickel plated steel, Viton A). The optimum design factors for these vacuum systems were suggested based on the simulation results. The baking- out effects of the modeled systems and materials on the performance of the vacuum system were also analyzed. The simulation predicted that the overall outgassing effect was more significant in the turbomolecular pump system than in the diffusion pump system and that the utilization of a booster pump has a greater effect on the evacuation time than on the ultimate pressure.

Vacuum chamber surface electronic properties influencing electron cloud phenomena

In the vacuum science community, it is now commonly accepted that, for the present and next generation of accelerators, the surface electronic properties of the vacuum chamber material have to be studied in detail. Moreover, such studies are of valuable help to define the cleaning procedures of the chosen materials and to identify the most efficient vacuum commissioning. In the case of the large hadron collider (LHC) the proton beam stability, in the presence of an electron cloud, is analysed using beam induced electron multipacting (BIEM) simulations requiring a number of surface related properties, such as photon reflectivity, electron and photon induced electron emission, heat load, etc. and their modification during machine commissioning and operation. Such simulation codes base their validity on the completeness and reliability of the aforementioned input data. In this work we describe how a surface science approach has been applied to measure, total electron yield (SEY) as well as energy distribution curves excited by a very low-energy electron beam (0–320 eV), from the industrially prepared Cu co-laminated material, the adopted LHC beam-screen material, held at cryogenic temperatures (about 9 K). The data show that the SEY converges to unity at zero primary electron energy and that the ratio of reflected to secondary electrons increases for decreasing energy below about 70 eV, and becomes dominant below electron energies of about 20 eV. These observations lead to the notion of long-lived low-energy electrons in the accelerator vacuum chamber, which could be an issue for the LHC, damping rings and future accelerators.

VUV photoemission studies of candidate Large Hadron Collider vacuum chamber materials

In the context of future accelerators and, in particular, the beam vacuum of the Large Hadron Collider (LHC), a 27 km circumference proton collider to be built at CERN, VUV synchrotron radiation (SR) has been used to study both qualitatively and quantitatively candidate vacuum chamber materials. Emphasis is given to show that angle and energy resolved photoemission is an extremely powerful tool to address important issues relevant to the LHC, such as the emission of electrons that contributes to the creation of an electron cloud which may cause serious beam instabilities and unmanageable heat loads on the cryogenic system. Here we present not only the measured photoelectron yields from the proposed materials, prepared on an industrial scale, but also the energy and in some cases the angular dependence of the emitted electrons when excited with either a white light (WL) spectrum, simulating that in the arcs of the LHC, or monochromatic light in the photon energy range of interest. The effects on the materials examined of WL irradiation and /or ion sputtering, simulating the SR and ion bombardment expected in the LHC, were investigated. The studied samples exhibited significant modifications, in terms of electron emission, when exposed to the WL spectrum from the BESSY Toroidal Grating Monochromator beam line. Moreover, annealing and ion bombardment also induce substantial changes to the surface thereby indicating that such surfaces would not have a constant electron emission during machine operation. Such characteristics may be an important issue to define the surface properties of the LHC vacuum chamber material and are presented in detail for the various samples analyzed. It should be noted that all the measurements presented here were recorded at room temperature, whereas the majority of the LHC vacuum system will be maintained at temperatures below 20 K. The results cannot therefore be directly applied to these sections of the machine until measurements at cryogenic temperatures, i.e., in the presence of cryosorbed gas layers, are obtained. However, these results are directly relevant to all the warm regions of the LHC vacuum system, such as the experimental vacuum chambers and warm element vacuum chambers in the insertion regions.

VUV synchrotron radiation studies of candidate LHC vacuum chamber materials

For the design of future accelerators, in particular, the beam vacuum of the large hadron collider (LHC), a 27 km circumference proton collider to be built at CERN, ‘white light’ (WL) and monochromatic VUV synchrotron radiation (SR) have been used to measure, both qualitatively and quantitatively, electron emission from candidate vacuum chamber materials. Emphasis has been placed on measuring the photoelectron yields (PY) and the energy distribution of the emitted electrons. These parameters are relevant to gas desorption, the LHC cryogenic cooling capacities and the possible creation of an electron cloud which may cause beam instabilities. Proposed materials, prepared on an industrial scale, such as 50 μm OFE Cu co-laminated onto a high Mn-content stainless steel, exhibit significant modifications when exposed to the WL spectrum from the BESSY TGM7 beamline. Surface cleaning treatments, such as sample annealing and ion bombardment, induce substantial changes to the electron emission which therefore indicate that such surfaces would not be constant, in terms of electron emission, during machine operation. Surfaces which are considered to be constant electron emitters, such as annealed TiZr alloys and commercial non-evaporable getters, were also investigated. These results and their implications for the choice of the material to be used for the LHC are discussed.

Synchrotron light source measurements of photon stimulated desorption from vacuum chamber Materials

Synchrotron light source measurements of photon stimulated desorption from vacuum chamber Materials

Stainless steels and aluminum alloys as vacuum chamber materials

Stainless steels and aluminum alloys as vacuum chamber materials

Vacuum system of the Dubna heavy ion storage ring facility K4-K10

The complex K4-K10 of two storage rings is intended to provide the beams of exotic neutron deficient and neutron excess nuclei ranging from He to Ni. High momentum resolution of cooled exotic beams ( δp/ p = 10 −5 −10 −6) and maximum energy of 150–200 MeV amu −1 are the purpose of this project. Interactions of heavy ions with the molecules of the important constituents of the residual gas are analysed. For a successful operation of the ring complex, the pressure in the vacuum chambers must not be less than 1 × 10 −8 Pa. The gas load sources are analysed and the vacuum technology of the initial preparation of the vacuum chamber materials and the procedure of reaching the desirable vacuum are defined. The structures of the vacuum systems and their different parts are described. An ultra high vacuum test bench for modelling works and examination of individual parts of the vacuum system is created.

Synchrotron radiation induced neutral gas desorption from samples of vacuum chambers

Synchrotron radiation induced neutral gas desorption from samples of different vacuum chamber materials has been measured at the DCI storage ring at LURE, Orsay. In order to study the effect of a surface oxide layer on the desorption rate and its decrease during continued photon exposure (frequently called beam cleaning), a comparison has been made between lightly and heavily oxidized samples of aluminium and steel (Nimonic). The dependence of the desorption rate for hydrogen on the photon dose and on time for the different types of samples supports a model of gas diffusion from near surface layers and from the bulk to the surface. The experimental set-up also permitted the measurement of the photoelectron current from the different samples and hence comparison with electron stimulated neutral gas desorption measurements in the laboratory on identically prepared samples.

Surface study of Type 6063 aluminium alloys for vacuum chamber materials

The surface characterization of Type 6063 aluminum alloy was performed with the use of various surface analysis techniques, such as Auger electron spectroscopy, scanning electron microscopy, optical microscopy, surface roughness factor and thermal desorption spectroscopy. The samples prepared by two different surface treatments were chosen for utilization as a uhv chamber material; namely, the surfaces prepared by extrusion (Sample A) and finished with diamond lathe (Sample B). The surface roughness factor of Sample A measured by xenon adsorption was 12.6 which was 2.6 times larger than that of Sample B. Carbon was the main impurity on the surface of Sample A, and oxygen on Sample B. Magnesium was found to segregate almost entirely on the surface of Sample B, while it segregated partially on Sample A after heating at 300°C. H 2O and CO were the main outgassing components below the practical baking temperature (150°C) for the aluminum alloys. The amount of gas released from Sample A was much larger than Sample B. This could be interpreted by the differences in the surface roughness factor and the thickness of the oxide layer.