Beam Vacuum Research Articles

Ultrabroadband terahertz source and beamline based on coherent transition radiation

Coherent transition radiation (CTR) in the THz regime is an important diagnostic tool for analyzing the temporal structure of the ultrashort electron bunches needed in ultraviolet and x-ray free-electron lasers. It is also a powerful source of such radiation, covering an exceptionally broad frequency range from about 200 GHz to 100 THz. At the soft x-ray free-electron laser FLASH we have installed a beam transport channel for transition radiation (TR) with the intention to guide a large fraction of the radiation to a laboratory outside the accelerator tunnel. The radiation is produced on a screen inside the ultrahigh vacuum beam pipe of the linac, coupled out through a diamond window and transported to the laboratory through an evacuated tube equipped with five focusing and four plane mirrors. The design of the beamline has been based on a thorough analysis of the generation of TR on metallic screens of limited size. The optical propagation of the radiation has been computed taking into account the effects of near-field (Fresnel) diffraction. The theoretical description of the TR source is presented in the first part of the paper, while the design principles and the technical layout of the beamline are described in the second part. First experimental results demonstrate that the CTR beamline covers the specified frequency range and preserves the narrow time structure of CTR pulses emitted by short electron bunches.

Applications of SU-8 in the development of a Single Ion Hit Facility

The Lund Nuclear Microprobe (LNM) has been adapted to be used as a Single Ion Hit Facility (SIHF) for proton cell irradiation experiments at low dose. In order to test the capabilities of the system, Ni dot arrays and artificial cells have been fabricated with the photopolymer SU-8 and common lithographic techniques. The primary purpose of the Ni dot arrays was to determine the targeting accuracy of the beam in vacuum and in air. Additionally, this sample was employed to evaluate the system performance during cell target irradiation experiments. The Ni dot arrays were also used for beam characterization. The artificial cells were originally fabricated to test the software for cell recognition and localization, developed and implemented at the LNM. However, this sample became very functional to make small adjustments at the irradiation chamber for cell irradiation experiments. A description of the samples, fabrication procedure and applications are presented in this paper.

An instrument for combining x-ray multiple diffraction and x-ray topographic imaging for examining crystal microcrystallography and perfection

Diffraction imaging using x-ray topography (XRT) and x-ray multiple diffraction (XRMD) provide valuable tools for examining the growth defects in crystals and the distributions from ideal lattice symmetry (microcrystallography). The topographic x-ray multiple diffraction microprobe (TMDM) combines the complementary aspects of both techniques enabling XRT and XRMD studies within the same instrument providing a useful resource for the structural characterization of materials that are not very stable in vacuum and electron beam environments. The design of the TMDM instrument is described together with data taken on GaAs (001) and potassium dihydrogen phosphate (001).

LHC World Largest Vacuum Systems Being Commissioned at CERN

The CERN Large Hadron Collider (LHC) with its 26.7 km of circumference and three different vacuum systems for the beams and insulation vacuum for magnets and liquid helium transfer lines, will have the world's largest vacuum system operating over a wide range of pressures and employing an impressive array of vacuum technologies. This system is composed by 54 km of UHV vacuum for the circulating beams and 50 km of insulation vacuum. Over the 54 km of UHV beam vacuum, 48 km of this are at cryogenic temperature (1.9 K). The remaining 6 km of beam vacuum containing the insertions for “cleaning” the proton beams, radiofrequency cavities for accelerating the protons as well as beam-monitoring equipment is at ambient temperature and uses non-evaporable getter (NEG) coatings. The noble gases and methane is pumped out by 780 ion pumps. Pressure readings are provided by 170 Bayard-Alpert gauges and 1084 gauges (Pirani and cold cathode Penning). The cryogenic insulation vacuums while technically less demanding, impress by their size (50 km) and volume (15000 m3). Once roughed using mechanical pumps, the vacuum relies on the cryopumping which allows reaching pressure in the 10-4 Pa range.

NSLS-II Project and Its Vacuum System Design

National Synchrotron Light Source II (NSLS-II) is a 3-GeV, 792-meter circumference, 3rd generation synchrotron radiation facility being constructed at Brookhaven National Laboratory. It will replace the existing NSLS, a three-decade old, 2nd-generation light source. To deliver photon beams with <1 nm spatial resolution and 0.1 meV energy resolution for the users, NSLS-II will have extremely high brightness, flux and stability; and ultra low emittance of ≤1 nm-rad. The design of the storage ring and the vacuum systems will be presented, with emphasis on beam vacuum chamber design, fabrication, ultrahigh vacuum pumping arrangement, photon beam tracking, absorber positioning and pressure profiles.

LHCb VErtex LOcator (VELO): Module production and performance

LHCb is the dedicated b-hadron experiment at the LHC. At its center is the VErtex LOcator (VELO). Located in a secondary vacuum system within the LHC primary beam vacuum, the VELO's primary purpose is to identify and trigger on bottom hadron decays through the precise reconstruction of the primary and secondary vertices. The VELO detector system is presented with special focus on the module specifications, design and fabrication. The production of the 42 VELO modules, which finished in February 2007, is described. The thermal, electrical, and radiation-hard performance of the finished modules is detailed.

Acceleration of electrons by a Bessel-Gaussian beam in vacuum

The acceleration of electrons by using a Bessel-Gaussian (BG) beam in vacuum is studied. It is shown that the axial electric field of a linearly or circularly polarized BG beam of order n = 1 can be used to accelerate electrons. The general features of the acceleration of electrons by using a linearly or circularly polarized BG beam, such as the transversal and axial electric-field components, phase velocity, slippage distance, accelerating potential, and energy gain etc., are analyzed.

Quantum interaction among intense laser beams in vacuum

Quantum electrodynamics (QED) predicts that electromagnetic fields interact among each other also in vacuum. We study the possibility of experimentally revealing this interaction by using soon available laser fields with intensities of order of $10^{23}\text{--}10^{26}\,\text{W/cm$^2$}$ . First, a few processes are first reviewed where vacuum polarization effects can be detected, like laser-assisted photon-photon scattering and the light diffraction by a strong standing wave. The possibility of enhancing these effects by using a plasma is also mentioned. Finally, the process of photon splitting in a laser field is discussed in detail together with its possible experimental observation.

Direct Acceleration of Ions With Variable-Frequency Lasers

In this paper, a method is proposed for producing monoergetic high-quality ion beams in vacuum, via direct acceleration by the electromagnetic (EM) field of two counterpropagating variable-frequency lasers. Ions are trapped and accelerated by a beat-wave structure with variable phase velocity, allowing for fine control over the energy and the charge of the beam via tuning of the frequency variation. The physical mechanism is described with a 1-D theory, providing the general conditions for trapping and scaling laws for the relevant features of the ion beam. Two-dimensional EM particle-in-cell simulations, in which hydrogen gas is considered as an ion source, confirm the validity and the robustness of the method.

Commissioning of the Superconducting Ring Cyclotron for the RIKEN RI Beam Factory

Since 1997, RIKEN Nishina Center has been constructing the Radioactive Isotope Beam Factory (RIBF) and succeeded in beam commissioning of its accelerator complex at the end of 2006. The world's first superconducting ring cyclotron (SRC) is the final booster in the RIBF accelerator complex which is able to accelerate all-element heavy ions to a speed of about 70% of the velocity of light. Assembling of the superconducting sector magnets was completed in August 2005. The superconducting coils were successfully cooled down and excited for tests on many operational aspects: magnetic force, coil protection and quality of magnetic field, showing that they work as designed. After a series of tests the other components were installed and tested under stray fields from the sector magnets. Local magnetic shields were put to the components which could not work under the stray fields. After setups of beam vacuum and radio frequency, beam commissioning started. The first beam was extracted at the end of 2006 and the first uranium beam was extracted in March 2007.

A ceramic radial insulation structure for a relativistic electron beam vacuum diode

For one kind of a high current diode composed of a small disk-type alumina ceramic insulator water/vacuum interface, the insulation structure was designed and experimentally investigated. According to the theories of vacuum flashover and the rules for radial insulators, a "cone-column" anode outline and the cathode shielding rings were adopted. The electrostatic field along the insulator surface was obtained by finite element analysis simulating. By adjusting the outline of the anode and reshaping the shielding rings, the electric fields were well distributed and the field around the cathode triple junction was effectively controlled. Area weighted statistical method was applied to estimate the surface breakdown field. In addition, the operating process of an accelerator based on a spiral pulse forming line (PFL) was simulated through the PSPICE software to get the waveform of charging and diode voltage. The high voltage test was carried out on a water dielectric spiral PFL accelerator with long pulse duration, and results show that the diode can work stably in 420 kV, 200 ns conditions. The experimental results agree with the theoretical and simulated results.

The structure of electrospray beams in vacuum

Electrospray atomization of liquids in the cone-jet mode generates narrow droplet distributions with average diameters as small as a few nanometres. This ability is important for technologies such as colloid thrusters, nanoparticle generation and ion beam processes, and the optimization of these applications requires an understanding of the physics and structure of the associated beams. This paper presents a detailed experimental characterization of electrosprays in vacuum and formulates an analytical model of the beam. A key feature of our model is the use of a simplified expression for the electric field induced by the space charge. This simplification leads to a time-independent Eulerian formulation compatible with an analytical solution, in contrast to the direct simulation of a multitude of droplets which must be simultaneously tracked to account for Coulombic interactions. We find that the beams open up in an initial region relatively insensitive to the external electrodes, a process dominated by the electric repulsion between droplets and the initial droplet inertia. Although the external electric field modifies the trajectories of the droplets downstream of this initial region, the effect is moderate in our typical electrospray source and the analytical solution in the space charge region explains well the far-field beam structure observed experimentally. We also describe a numerical scheme that implements the full effect of the external electric field and provides a more accurate solution.

Investigation of an expanding copper plasma across a transverse magnetic field generated by strip-type electron-beam

The electron beam assisted thermal processes are well-established technique for various applications such melting, evaporation, isotope purification, ion-implantation etc. The processes are time average, wherein process is carried out over a larger duration so that real time dependent variations gets averaged out. During electron beam evaporation and atomic beam generation, ions also get generated inherently due to interaction of primary and back-scattered electrons with the atomic vapour along with other routes like Saha ionization, secondary electron ionization etc. The presence of ions has a bearing in the quality of deposited films and influence the ion implantation process. It also has strong effect on dilution factor in isotope purification. Thus the knowledge of ion content, its distribution and evolution in the freely expanding atomic beam in vacuum is important for any process optimisation in electron beam evaporation applications. The thermal ion content in atomic vapour for a given element as a function of aspect ratio (height/length) and source Knudsen number can be used as most inexpensive technique to characterized the evaporation on time average as well as teal time processes. With this intension, a series of experiments were carried out in a 100 kW strip beam evaporator with 130mm strip electron gun with copper as target material. A wedge shaped copper atomic beam is generated from a two dimensional evaporating source (120mm × 6mm) by impingement of the electron beam up to 90kW power. Disk type Langmuir probe is used for measuring I-V characteristic plots for deriving the thermal ion flux. The measurements are carried out as a function source Knudsen number (depends on incident electron beam power), and the probe distance from source that defines the aspect ratio. Along with probe data, the atom flux is also simultaneously measured using quartz crystal thickness monitor. The evolution of ions and atoms with distance from source is compared.

Large high-vacuum systems for CERN accelerators

CERN operated over the more than 50 years of its existence particle accelerators and storage rings ranging from a few tens of metre to 27 km, the size of its latest project, the Large Hadron Collider (LHC) which is under construction and will be started in 2008. The challenges began with the Intersection Storage Rings (ISR) in the seventies. With a beam pipe length of 2 × 1 km, this accelerator required innovative solutions like bake-out and glow discharge to achieve the required static vacuum level, fight against beam-induced pressure increases and cancel beam neutralisation by trapped electrons. The vacuum system of the Large Electron Positron (LEP) storage ring (in operation between 1989 and 2001) of a total length of 27 km had to cope with very high levels of synchrotron power. The beam vacuum system of LHC (2 × 27 km) integrates some parts at 1.9 K and others at room temperature and will also have to cope with dynamic effects. In addition to the beam vacuum system, LHC requires insulation vacuum for the superconducting magnets and the helium distribution line. Whereas the required pressure is not very low, the leak detection and localisation is significantly more demanding for the insulation vacuum than for the beam vacuum because of the large volumes and the thermal insulation.When the size of an accelerator grows, the difficulties are not only to get a clean and leak tight vacuum system, but also to be able to measure reliably pressure or gas composition over long distances. Furthermore, in the case of LHC the integration of the beam vacuum system was particularly difficult because of the complexity induced by a superconducting magnet scheme and the reduced space available for the beam pipes. Planning and logistics aspects during installation, including the usage of mobile pumping and diagnostic means, were much more difficult to manage in LHC than in previous projects.

Vacuum system for superconducting LINAC at TIFR

The superconducting heavy ion LINAC booster at the TIFR-BARC facility has been operational. Seven super conducting cryostats containing 4 quarter wave resonators each along with beam lines, bending magnets, switching magnet, diagnostics and vacuum system have been commissioned. The heart of the cryogenic system for the heavy ion superconducting LINAC booster is a custom-built liquid helium refrigerator made by Linde Kryotechnik, Switzerland. The Refrigerator is rated for 300 Watts at 4.5 K with a dual JT (Joule-Thomson valve) at the final cooling stage, which allows simultaneous connections to the cryogenic loads (the LINAC module cryostats) and to a liquid helium storage dewar (1000 litres). The two-phase helium at 4.5 K produced at the JT stage in the refrigerator is delivered to the cryostats through a cryogenic distribution system. The cryogenic distribution system for the LINAC is designed to deliver both liquid helium and liquid nitrogen to the cryostats. The details of UHV system, indigenously developed beam line components, pumps and module cryostats will be presented.

Accelerator vacuum systems at DESY

The research center DESY in Germany is one of the leading accelerator centers worldwide. At the facilities located in Hamburg more than 3000 scientists from all over the world do research with photons and in the field of particle physics. Since 1992 the 6-km-long HERA storage ring was used for high energy collisions of electrons/positrons and protons. It consists of two rings. The one for the 27.5 GeV electrons/positrons uses normal conducting magnets. The other one bends the up to 920 GeV protons with superconducting magnets. Data taking at HERA has been terminated only recently. Synchrotron light for 36 experimental stations is generated by the 4.5 GeV storage ring DORIS. The 2.3 km long storage ring PETRA is presently rebuilt into PETRA III, one of the most brilliant X-ray sources worldwide, to start user operation in 2009. Therefore the vacuum system is completely replaced. The construction of the European X-ray Free Electron Laser XFEL, a new international research facility, just has started next to DESY. Its extremely intense X-ray laser flashes with tunable wavelengths down to 0.1 nm will open up completely new experimental possibilities for nearly all fields of natural sciences. The basic process adopted to generate the X-ray pulses is SASE (Self-Amplified Spontaneous Emission). Therefore electron bunches are brought to high energy of about 20 GeV through a superconducting linear accelerator, and conveyed to up to 250 m long undulators where the X-rays are generated. The beam vacuum system of this 3.4 km long straight facility contains sections operated at room temperature as well as at 2 K in the areas of the superconducting accelerating structures. In addition to standard UHV requirements the vacuum system needs to preserve the particle cleanliness of the superconducting cavity surfaces. Further challenges are the undulator vacuum chambers filling more than 750 m with extreme requirements to the surface quality. Unique research opportunities worldwide are offered already now by the 250 m long free-electron laser FLASH, the prototype for the XFEL, which is under operation since several years as a user facility. The vacuum systems of the various DESY accelerator facilities will be discussed in this paper in more detail.

Post-treatment of thermal spray coatings on magnesium

Magnesium alloys have a beneficial combination of high strength to weight ratio, good machinability and high recycling potential. Despite this, the application of magnesium still is behind that of other constructive materials mainly due to low wear and corrosion resistance. For more demanding applications, a large amount of surface treatment methods are developed to overcome this problem. Thermal spraying is an efficient and flexible method of coating deposition and is widely used for protection of different materials against corrosion and wear. Nevertheless, the bonding of thermal spray coatings on magnesium alloys is not sufficient, so the following post-treatment processes are needed. One of such possibilities is high energy beam treatment of thermally sprayed coatings. During the heat treatment of magnesium substrates with coating the remelting of coating and a thin surface layer of substrate occurs. Depending on the combination of applied coating system and treatment method, different processes can be realised in modified layers: the alloying of magnesium substrate with other elements to improve corrosion properties, redistribution of hard particles from composite coating and new phases formation during the processing to improve the wear resistance of magnesium alloys. In the present work some examples concerning the laser and electron beam treatment of aluminium based composite coatings as well as infra red irradiation of zinc based coatings are described. Coatings are deposited on magnesium substrates (AM20, AZ31, AZ91) by arc spraying with Zn, ZnAl4 and ZnAl15 solid wires and cored wires in aluminium core with powder filling containing different hard particles, such as boron, silicon and tungsten carbide or titanium oxide. Remelting of thermal spray coatings is carried out by means of continuous irradiation of СО 2-laser in nitrogen or argon atmosphere, electron beam in vacuum and focused tungsten halogen lamp line heater in atmosphere. Microstructure of sprayed coatings as well as that of modified surface layers is investigated by metallographic methods. Corrosion properties are estimated by electrochemical measurements. Abrasion wear resistance of the modified layers is determined by scratch test, corundum grinding disk test and Rubber wheel test. It is shown that all methods applied for processing of thermal spray coatings lead to formation of modified surface layers in magnesium substrate with improved wear and corrosion properties. Different mechanisms of microstructure formation such as redistribution of chemical composition of composite coating components, partial remelting of hard phase particles, and new phases formation are discussed. Electrochemical behaviour of modified surface layers is mostly improved due to alloying, homogenization of element distribution and strong decrease of as-sprayed coating porosity. Abrasion wear resistance of processed magnesium substrates strongly depends on the microstructure and usually is 5 to 20 times higher compared with base material.

Measurement of Electron Clouds in Large Accelerators by Microwave Dispersion

Clouds of low energy electrons in the vacuum beam pipes of accelerators of positively charged particle beams present a serious limitation for operation at high currents. Furthermore, it is difficult to probe their density over substantial lengths of the beam pipe. We have developed a novel technique to directly measure the electron cloud density via the phase shift induced in a TE wave transmitted over a section of the accelerator and used it to measure the average electron cloud density over a 50 m section in the positron ring of the PEP-II collider at the Stanford Linear Accelerator Center.

A study of very thin DLC foils as a gas barrier

Measurements of vacuum tightness and mechanical strength of diamond-like carbon (DLC) foils in the thickness range of 1–7 μg cm −2 have been performed with a purpose to evaluate suitability of foils as a gas barrier. Hydrogen and argon at pressures from 10 −2 Pa to 20 kPa were used as test gases. The permeation rate specified as conductance density was found for the best sample of self-supporting foil to be around 1.5×10 −3 l and 3.3×10 −4 l s −1 cm −2 for H 2 and Ar, respectively. Conductance density of the same foils mounted on the frames with a mesh along the apertures as support was about twice higher than that for the self-supporting ones, likely due to the mechanical imperfections of the foil assemblies of the first ones. On the other hand, mesh-supported foils as thin as 3 μg cm −2 and of 5 mm in diameter were withstanding the pressure of up to 18 kPa, while self-supporting foils of the same thickness ruptured at around 1.2 kPa. There was no observed relation between thickness of the foil and its mechanical properties and permeation rate. This suggests that rather tears and pinholes present in foils are the limiting factors of the foil–vacuum tightness and strength. Results obtained in the studies, presented in this work, demonstrate the ability of very thin DLC to isolate a high vacuum beam line from a gas cell in a variety of applications and ability to withstand the gas pressure relevant, in particular, to some gas-filled ionization chambers.

The vacuum system of the European X-ray free electron laser XFEL

The European X-ray Free Electron Laser XFEL, a new international research facility, will be built at DESY/Hamburg. The XFEL will generate extremely brilliant and ultra short pulses of spatially coherent X-rays with tuneable wavelengths down to 0.1 nm, and exploit them for revolutionary scientific experiments at various disciplines. The basic process adopted to produce the X-ray pulses is SASE (Self-Amplified Spontaneous Emission). Therefore electron bunches are produced in a high-brightness gun, brought to high energy of about 20 GeV through a superconducting linear accelerator, and transported to up to 250 m long undulators, where the X-rays are generated. The beam vacuum system of the accelerator contains sections operated at room temperature as well as at 2 K in the areas of the superconducting accelerating structures, thus requiring an insulating vacuum system. In addition to standard UHV requirements, the vacuum system for this facility needs to preserve the cleanliness of the superconducting cavity surfaces. Therefore the preparation of all vacuum components for the 1.6 km long main linac includes cleaning of the components in a clean room to remove particles, installation into the accelerator in local clean rooms, and special procedures for pump down and venting. Further challenges are the undulator vacuum chambers filling more than 700 m, where a high surface quality with respect to surface roughness and thickness of oxide layers is mandatory to reduce wake field effects, and the vacuum systems for the various beam dumps, where exit windows acting as vacuum barriers of sufficient reliability need to be developed. In addition, a large amount of about 1.7 km of transport beam lines is required. The layout of the various vacuum sections as well as experience with prototype components will be described.