The surface cleanliness of 316 L + N stainless steel studied by SIMS and AES

Some cleaning methods for 316 L + N stainless steel including solvent cleaning, high temperature treatment in vacuo and gas discharge cleaning have been studied by SIMS and AES with a view to providing a clean vacuum chamber surface with low gas desorption under ion bombardment. After solvent cleaning the main surface contaminant was found to be C and its associated compounds. Laboratory investigations on small samples of stainless steel showed that clean surfaces could be obtained by heating in vacuo to 800°C followed by exposure to air and by argon or argon/10% oxygen discharge cleaning. Due to a cross contamination within the vacuum system, the 800°C treated chamber gave positive desorption coefficients under ion bombardment. The pure argon discharge cleaned chambers proved stable giving negative desorption coefficients up to 2200 eV ion energy even after several weeks storage between discharge treatment and installation. No results have yet been obtained on the stability of an argon/10% oxygen discharge cleaned vacuum chamber.     

Vacuum stability and residual gas density estimation for the vacuum chamber upgrade of the ATLAS interaction region of the Large Hadron Collider

The CERN Large Hadron Collider (LHC) has 54 km of ultra-high vacuum (UHV) beam chambers out of which about 90% are at cryogenic temperature (1.9 K) and the rest at room temperature. During operation, the residual gas density in the beam pipes is dominated by beam induced effect such ion, electron and photon-stimulated gas desorption. Therefore, the computation of gas density profile is of great importance to confirm the vacuum stability, and to estimate the beam lifetime. Moreover, the gas density profiles are essential to determine the machine induced background in the experimental areas, and to define the pressure profile in the cryogenic sectors where there is no vacuum instrumentation available.In this paper, the vacuum stability is studied for a newly proposed upgrade of the vacuum chamber at the ATLAS interaction point, using the vacuum stability code called VASCO. The residual gas density profile along the ATLAS vacuum chambers and the effects of photon and electron flux hitting the vacuum chamber walls are presented and analysed.     

Gas dynamics modelling for particle accelerators

Design of an accelerator vacuum chamber requires an input from different scientific disciplines such as surface science, material science, gas dynamics, particle beam dynamics, and many others. Although vacuum scientists work on the boundary field between these disciplines, gas dynamics is the one that allows joining them to the vacuum science for particle accelerators. The vacuum requirements (usually UHV or XHV) in particle accelerators are defined by beam-gas interactions that should be negligible compared to other phenomena that limit the quality of the beam. At such low pressures the main source of gas in the vacuum chamber is a molecular desorption from materials used for the vacuum chamber and its components. The outgassing rates vary over a very wide range and depend on material, cleaning procedure, treatments, temperature, bombardment by particles and accumulated irradiation dose. The gas dynamics is used to design the research facilities to accurately measure and to study outgassing rates at different conditions. By applying these data to the accelerator vacuum design, one would have to consider that outgassing is often non-uniform and changes with time with different functions. The most time-efficient way of beam vacuum optimization is using a 1D diffusion model where all parameters are defined as a function of longitudinal coordinate (along the beam path). A full 3D modelling with TPMC codes provides much more accurate results, however, being time consuming work is not ideal for pumping and design optimization and is used for complex components and for finalized design.     

A mechanism of current noise reduction in systems of ion beam formation from vacuum arc plasma

The influence of a residual gas plasma on the ion beam transport in the drift chamber of an ion source based on the vacuum arc discharge was numerically simulated by the pellet injection method. The plasma formed as a result of ionization of the residual gas causes a change in the potential profile in the drift chamber. This leads to the appearance of an extended slow ion transport zone (occurring at a virtual anode potential) and to a change in the mechanism of suppression of the ion current noise as compared to the well-known mechanism operative in an equipotential vacuum gap. The results of model calculations show that the modified mechanism is related to a spatial redistribution of the density of ions (in the beam characterized by a certain scatter of velocities) in the slow ion transport zone.     

Study of the space charge potential of ion beams

The energy distribution of slow residual gas ions created by an ion beam in collisions with the residual gas molecules inside an ion beam transport system has been measured with an electrostatic spectrometer. For this investigations Ar⁺ ion beams (dc) were used, with energies between 15 and 45 keV. The beam current has been varied from 0.01 up to 1.6 mA. Further parameters were the residual gas pressure, the gas composition, the beam diameter and the potentials of several clearing electrodes in the beam transport system. The experimental results are compared with the calculated values of the space charge potentials of the ion beam. In operating the spectrometer in a gated mode the time could be measured which is necessary to build up the space charge compensation in a pulsed ion beam. Additional residual gas ions produced at different radial distances from the ion beam axis by an electron beam allowed to determine the radial potential distribution.     

Design and performance of an instrument for soft landing of biomolecular ions on surfaces

A new ion deposition apparatus was designed and constructed in our laboratory. Our research objectives were to investigate interactions of biomolecules with hydrophilic and hydrophobic surfaces and to carry out exploratory experiments aimed at highly selective deposition of spatially defined and uniquely selected biological molecules on surfaces. The apparatus includes a high-transmission electrospray ion source, a quadrupole mass filter, a bending quadrupole that deflects the ion beam and prevents neutral molecules originating in the ion source from impacting the surface, an ultrahigh vacuum (UHV) chamber for ion deposition by soft landing, and a vacuum lock system for introducing surfaces into the UHV chamber without breaking vacuum. Ex situ analysis of surfaces following soft landing of mass-selected peptide ions was performed using 15 keV Ga+ time-of-flight secondary ion mass spectrometry and grazing incidence infrared reflection-absorption spectroscopy. It is shown that these two techniques are highly complementary methods for characterization of surfaces prepared with a range of doses of mass-selected biomolecular ions. We also demonstrated that soft landing of peptide ions on surfaces can be utilized for controlled preparation of peptide films of known coverage for fundamental studies of matrix effects in SIMS.     

NSRL电子储存环超高真空系统残余气体分析

在同步辐射电子储存环中,由于光电解吸的作用,将解吸大量的气体,而储存的束流电子与气体分子的碰撞将引起电子的用射,导致束流寿命的减小,因此残余气体分成为衡量一个电子储存环真空系统性能的重要的指标,本文将介绍有关ＮＳＲＬ储存环真空系统残气分析的部分工作。     

Vacuum sealing using surface activation bonding of Si wafer

Bonding technology of Si wafer for vacuum seal is important in MEMS. We have tried the vacuum seal using surface activation bonding without any binder. It is the ultimate bonding technique and gives the precise dimension due to the direct contact. The technique is, however, not easy. We have investigated the surface conditions in order to achieve the bonding. The surfaces cleaned by Ar ion beam bombardment were measured by XPS and AFM. The natural oxide on the Si surface was removed by Ar ion bombardment. The surface roughness depended on the condition and the irradiation time of the Ar ion beam. The surface bonding at room temperature was achieved for the clean surface of the surface roughness less than Ra = 1 nm, but it was not done with the rough surfaces more than Ra > 2 nm. The vacuum sealing was checked using the cavities made in the Si wafer. The cavity part sealed in vacuum was depressed in the atmosphere, which was measured using a needle-contact profiler and a 3D laser profiler. The gas in the cavity was measured with a mass spectrometer by clashing the seal in vacuum. Any other gas except Ar gas closed in the cavity was not detected. We concluded that the vacuum sealing using surface activation bonding of Si wafer was achieved. The sealing condition has not changed even after one year.     

An in situ glow discharge cleaning method for the LEP vacuum system

The performance of the vacuum systems of present day electron storage rings is limited by synchrotron radiation induced gas desorption. A method to reduce the electron induced desorption yield (thought to be a measure of the radiation induced desorption) by glow discharge cleaning has been developed. Initial beam lifetimes for standard LEP operation (Large Electron Position Storage Ring) have been calculated based on measured electron stimulated desorption yields and two different proposed pumping systems. (One with distributed ion pumps, the other with distributed getter pumps). After bakeout, both systems would provide an initial beam lifetime of ～ 4 min. After a helium glow discharge for 3 h at 150°C (at 1.5 torr and with a discharge current of 0.7 A m^?1), a lifetime of ～6.7 h would be obtained in the ion pumped system as compared to ～1.7 h in the getter pumped version. Results obtained with Argon discharge proved to be inferior.     

The characterization of magnesium oxide surfaces prepared by electron beam cleavage

The surface of freshly electron beam cleaved MgO was analyzed by in situ gold decoration experiments. The depositions were performed under controlled vacuum, temperature and supersaturation conditions. Particular attention was directed to the influence of ion and electron irradiation of the substrate surface prior to the depositions. Several significant results were obtained. The decoration behavior on electron beam cleaved (001)MgO is substantially different from published data obtained on mechanically cleaved (001)MgO surfaces. The differences are believed to be due to the extreme temperature conditions during the electron beam cleavage process. No such unusual behavior was found for the (111)-oriented MgO surfaces which are also generated during the electron beam cleavage process. Furthermore, the MgO surface is noticeably susceptible to electron irradiation damage under normal transmission electron microscope imaging conditions, and very small doses of noble gas ion bombardment induce strong structural surface damage which annihilates the differences in nucleation behavior between (100) and (111) substrate areas.     

Gas release of the dielectric coating of an anode and its effect on the characteristics of an ion diode with electron flux insulation by a radial magnetic field

Generation of an ion beam by an ion diode is accompanied by the release of gas from the surface of the dielectric coating of the anode. The magnitude of the pulse pressure of gas in the vacuum chamber depends on the type of the dielectric used and can reach 6 × 10^–4–2 × 10^–3 mm Hg. The magnitude of the limit vacuum chamber pressure at which ion beam parameters remain constant has been found. Characteristics of the ion beam current in the vacuum chamber pressure range of 2 × 10^–4–1.3 × 10^–3 mm Hg are presented.     

Hydrogen bombardment of the oxide layer on beryllium

Beryllium is being considered as a plasma-side material of low atomic number Z for use in fusion reactors. Untreated beryllium surfaces are usually covered by a thin oxide layer which forms during air exposure or from residual gas adsorption in vacuum. Secondary ion mass spectrometry measurements have been made to investigate the durability of this oxide layer during hydrogen ion bombardment at an energy of 1 keV, which is similar to that expected at the plasma edge in a fusion reactor. The surface condition of machined beryllium samples undergoing hydrogen bombardment was monitored by observing secondary ion emission of O^-, OH^- and BeO^-. On the basis of sputtering yield data for BeO, the oxide layer appears to be up to 10² monolayers in thickness. At the metal interface it was possible to determine the cross section for the removal of adsorbed oxygen. The measured value is 1.5 × 10^-17 cm², which is in agreement with bulk oxide sputtering data. This suggests that adsorbed oxygen forms a surface oxide with a binding energy that is similar to that of the bulk oxide. These results are used to predict the conditions in which an oxide layer will exist on exposed beryllium surfaces during fusion reactor operation.     

Mass and charge measurement of trapped dust in the CESR storage ring

The trapping of dust particles with an attendant reduction in beam lifetime and/or increased radiation rates is aperiodically observed in the Cornell Electron/positron Storage Ring CESR. A measurement of the stability of the trapped dust as a function of bunch current with a single bunch of electrons confirmed directly that the dust particles are heavier than any normal constituent of the residual gas in the vacuum chamber. The removal of individual dust particles from the beam via image charge forces were observed and the change in beam lifetime together with the current at which the dust left the beam permitted calculation of the dust mass and charge. The surface electric field of the dust particle as calculated from the experimental data agrees well with a theoretical calculation based upon an equilibrium between charging and discharging rates.     

Apparatus for the study of low energy ion trapping and gas release in solid targets

Experiment apparatus has been constructed to study the phenomena associated with the penetration and entrapment of rare gas atoms in solid targets following bombardment with low energy (50 eV-3 keV) monoenergetic ion beams. In this apparatus, ions strike a solid target and a fraction become entrapped. The fraction trapped is subsequently evaluated by heating the target in a well-defined manner, the evolved gas being recorded with a mass spectrometer. Additionally, using this thermal desorption technique, the rate of release of gas as a function of temperature was continuously monitored in order to investigate the details of the mechanisms of ion entrapment and subsequent thermal motion. In order to provide facilities for the study of a wide range of target materials, a programmable, electron bombardment heating system was incorporated into the apparatus.     

Studies of photon induced gas desorption using synchrotron radiation

In view of finalizing the design of the vacuum system of the Large Electron and Positron Storage Ring (LEP) we have studied synchrotron radiation induced neutral gas desorption. A 3 m section of an aluminum vacuum chamber has been exposed to the photon beam emerging from the electron storage ring DCI in Orsay, under conditions closely simulating the environment in a particle acceletor. In order of importance the gases desorbed were H₂, CO₂, CO and CH₄ with H₂O practically absent. Under the experimental conditions of an unbaked chamber and 11 mrad glancing incidence of the photons, the initial molecular desorption yields for these gases were typically 0.5, 8 × 10^?2, 2 × 10^?2 and 8 × 10^?3 molecules per photon respectively. These values could be reduced by about 1 to 2 orders of magnitude during continued photon exposure and most cases without evidence that this ‘beam cleaning action’ would be limited. After exposure to air and pumpdown of the previously cleaned chamber, we observe a significant memory effect. The dependence of the photon desorption on the angle of incidence has been studied down to a glancing angle of 11 mrad showing a definite deviation from the previously assumed 1/sin ø scaling. The implications of the results in terms of the expected beam-gas lifetime in LEP are discussed.     

Ion bombardment in electron microscopy investigations

Transmission electron microscopy, transmission high energy electron diffraction, reflection high energy electron diffraction and scanning electron microscopy were employed to evaluate the possibilities of ion bombardment used in the preparation of electron microscope specimens. The experimental results show that the uncontrolled use of ion bombardment for thinning of metal foils, cleaning layers or crystal surfaces etc. may often lead to incorrect interpretation of the electron microscopy investigations. In contrast, it is stressed that controlled ion bombardment directly in the column of the electron microscope is a very useful method for the observation and study of some very interesting phenomena which occur to the structure of the specimens during ion bombardment.     

Low-energy ion beam bombardment of human cancer cells in vacuum to induce DNA transfection

A process of low-energy ion-beam-induced gene transfection in human cancer cells was developed for non-viral DNA transfection. The whole process included maintenance of the cells in vacuum, ion beam bombardment of the cells with optimized ion beam condition leading to a medium cell viability, and DNA transfer. A method of using a specially designed liquid-nitrogen cooled sample holder to house the cells and frozen medium to cover and protect the cells was adopted to maintain the cells viable in the vacuum condition during ion beam bombardment. Nitrogen ion beams with energy in the range 7–28 keV were used to bombard human cancer cells (HEp-2, the human laryngeal epitheloid cancer cell line) to fluence orders of 10¹⁵–10¹⁶ ions/cm². After ion beam bombardment, the cell viability was studied. The ion beam condition which resulted in a relatively medium viability of the cells was chosen to operate ion bombardment to induce gene transfection. To the viable cells, DNA transfection using plasmid DNA pEGFPN2 was carried out. The experiment demonstrated that under appropriately controlled ion beam conditions and freezing environment, the human cells could survive and be transfected with exogenous DNA. This technique could be used as an alternative of non-viral gene transfections with a promising efficiency.     

The desorption of condensed noble gases and gas mixtures from cryogenic surfaces

In accelerators, operating at liquid-helium temperature, cold surfaces are exposed to intense synchrotron radiation and bombardment by energetic electrons and ions. Molecular desorption yield and secondary electron yield can strongly influence the performance of the accelerator. In order to predict the gas density during the operation, the knowledge of electron-induced desorption yields of condensed gases and of its variation with the gas coverage is necessary. Desorption yields under electron impact of various noble gases and gas mixtures condensed on a copper surface cooled at 4.2 K have been measured.     

The operation of the ISR facility in relation to the UHV environment

At CERN Geneva, two kilometers of UHV at 10⁻¹⁰ torr is in daily use at the Intersecting Storage Ring (ISR) facility. An appreciable fraction of the stainless steel vacuum chamber, some 200 m in fact, is continuously at 10⁻¹¹ torr.     

Vacuum performance of a beam screen with charcoal for the LHC long straight sections

The vacuum chamber inside the cryogenic magnets in the LHC Long Straight Sections will have a beam screen at a temperature between 5 and 20 K to protect the cold bore against the synchrotron radiation, electron and ion exposure. The desorbed molecules of H₂ will leave the inner part of the beam screen through the pumping slots on the beam screen and eventually condense on the cryosorber, which is mounted on the shadowed (outer) part of the beam screen for magnets operating at 4.5 K. The design of the experimental set-up, the results of the adsorption capacity measurements for charcoal, the pumping speed and the capture factor of the beam screen with charcoal for a proposed LHC vacuum chamber configuration are described in this paper.