Oswald Grobner

Studies of photon induced gas desorption using synchrotron radiation

Abstract 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 H 2 , CO 2 , CO and CH 4 with H 2 O 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 o scaling. The implications of the results in terms of the expected beam-gas lifetime in LEP are discussed.

The Large Hadron Collider (LHC) in the LEP tunnel

After the remarkable start-up of LEP, the installation of a Large Hadron Collider, LHC, in the LEP tunnel will open a new era for the High Energy Physics. This report summarizes the main LHC parameters and subsytems and describes the more recent studies and developments.

Synchrotron radiation induced neutral gas desorption from samples of vacuum chambers

Abstract 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.

Beam Induced Gas Desorption in the CERN Intersecting Storage Rings

The maximum beam intensity achieved in the ISR has been limited up to now by the beam induced pressure rise which builds up with the stacked proton current. This pressure increase can be explained by ion induced gas desorption from the vacuum chamber. The pressure P, as a function of the stacked proton current I, can be described in good approximation by P = Po/[1-(k?/S)I], where Po is the pressure without beam, S the pumping speed, ? the net gas desorption coefficient in molecules per incident gas ion and ? is a constant. This concept shows the existence of a critical current at which the pressure goes to infinity. The desorption coefficient depends on the primary ion energy, the type of ions and above all, on the surface conditions. Surface treatments yielding low and even negative values of ? are discussed together with experimental results obtained.

The antiproton decelerator: AD

A simplified scheme for the provision of antiprotons at 100 MeV/c based on fast extraction is described. The scheme uses the existing p~ production target area and the modified Antiproton Collector Ring in their current location. The physics programme is largely based on capturing and storing antiprotons in Penning traps for the production and spectroscopy of antihydrogen. The machine modifications necessary to deliver batches of 1/spl times/10/sup 7/ p~/min at 100 MeV/c are described. Details of the machine layout and the experimental area in the existing AAC Hall are given.

Neutral gas desorption and photoelectric emission from aluminum alloy vacuum chambers exposed to synchrotron radiation

In an aluminum alloy vacuum chamber exposed to synchrotron radiation, the photoelectron currents produced were measured with the photons incident at low angles on the side wall and compared with normal incidence. The calculated photocurrents for normal incidence, using published values of the photoyield for oxidized Al, agree to within 15% with the measured values. Differences in the photocurrent dependence on photon spectrum at normal and glancing incidence were attributed to low‐energy photons being totally reflected and hence producing no photoelectrons. It was established that, at glancing angles of incidence down to 11 mrad, a substantial—more than 20%—fraction of the synchrotron radiation is scattered around the vacuum chamber from the initial point of impact. During exposure to synchrotron radiation, the gases desorbed were H2, CO, CO2, and CH4. The similar shapes of the dependence of the gas desorption and the photoelectron currents on the photon spectrum suggested that it is mainly the photoelect...

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.

Comparison of photodesorption yields using synchrotron radiation of low critical energies for stainless steel, copper, and electrodeposited copper surfaces

Photon‐stimulated gas desorption yields from metal surfaces, typically used in large vacuum systems, such as stainless steel, copper, and electrodeposited copper, have been measured using synchrotron radiation with critical energies from 12.4 to 284 eV, thus allowing to obtain initial yields applicable to future large hadron colliders. The main gases desorbed from all surfaces are: H2, CO2, CO, and CH4. Water is also desorbed and greatly increases with exposure until a saturation value is reached. In situ bakeout reduces the amount of water desorbed, but only slightly reduces the yield of the other gases. The dependance of the yields on critical energy is almost linear, except for H2O. Photoelectron yields have also been measured, showing a good linear relation with the pressure increase, thus suggesting a process where a photon produces an electron that subsequently desorbs a gas molecule.

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.

VACUUM FOR PARTICLE ACCELERATORS AND STORAGE RINGS

Abstract Accelerator vacuum systems must be measured in kilometres rather than metres and operate down to or below 10−8 Pa. Vacuum requirements and design constraints are reviewed for different machines using, as examples, the intersecting storage rings (ISR) for protons, the large electron positron (LEP) machine for electrons and the ‘cold-bore’ large hadron collider (LHC) again for protons. Various dynamic interactions between the particle beam and the vacuum system which are specific to accelerators are described. This is followed by a general discussion on pumping, especially distributed NEG pumping, cold-bore design and certain operational aspects of large systems.