Ag Mathewson

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 effect of bakeout temperature on the electron and ion induced gas desorption coefficients of some technological materials

The electron and ion induced neutral gas desorption coefficients at 1.4 keV have been measured for stainless steel, Inconel 600, Inconel 718, a titanium alloy, OFHC copper and an aluminium alloy after bakeout at temperatures ranging from 150 to 600°C. The surface composition was also determined by Auger spectroscopy after heating at various temperatures up to 1000°C. The similarity of the results from all the materials investigated was explained in terms of a porous surface oxide layer which provided a reservoir for desorbable gas.

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

Abstract 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.

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.

Hydrogen content and outgassing of air-baked and vacuum-fired stainless steel

Abstract The hydrogen content of stainless steel samples, treated by 400 °C heat treatment for 2, 4, 8, 16 and 24 h in ambient air, was determined by the 1H(15N, αγ)12C nuclear resonance reaction, and a standard extraction method. Two reference samples were also analysed, one virgin sample which was not heat treated at all, and one which had been vacuum fired (950 °C, 1 h). The results show that the heat treatment in ambient air results in an outgassing rate comparable to vacuum fired material although the amount of hydrogen in the samples was only lowered by approximately 50%, by the air baking, compared to a reduction to

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.

Vacuum problems in particle accelerators due to interaction of synchrotron radiation, electrons and ions with surfaces

Abstract In high energy particle accelerators where bunches of electrons, positrons and protons are accelerated to energies in the GeV range, and in the near future up to many TeV for protons in the LHC and SSC projects, the pressure requirements are typically in the 10−10 torr range. However, in operation, the walls of the vacuum system are subjected to bombardment by electrons, ions and/or photons which desorb tightly bound gas and results in large pressure increases detrimental to the operation of the accelerator. Another complication is that the vacuum chamber may be at a temperature of about 10 K. Cryopumping surfaces which are initially beneficial finally turn out to introduce several liabilities—for example, only a few monolayers of cryopumped H2 already has a vapour pressure at 10 K well in excess of 10−6 torr. These and other effects will be described in detail along with the methods used to deal with them.

Photon induced molecular desorption from condensed gases

Abstract With a view to predicting the behaviour of the cold vacuum system of the CERN Large Hadron Collider (LHC), measurements are presented of the synchrotron radiation induced desorption from thick layers of H 2 , CH 4 , CO and CO 2 condensed on stainless steel at 77K to 68K for CO 2 , 4.2 K for CO and CH 4 and to 3.3K for H 2 . The critical energy of the radiation was 284 eV and up to 10 monolayers of CO 2 or 20 monolayers of CO, CH 4 and H 2 were condensed. Photodesorption yields from condensed H 2 at 3.3 K were found to be about five molecules per photon at a few monolayers. The photodesorption yields from approximately the same thicknesses of condensed CH 4 and CO at 4.2 K and CO 2 at 68 K were found to be at least three orders of magnitude lower than that for H 2 and apparently, within the limits of the experiment, independent of the surface coverage. Sticking coefficients for CO 2 were measured at 77K and found to increase almost linearly with coverage, reaching a value of 0.2 at approximately 20 monolayers. An adsorption isotherm for CO 2 on stainless steel at 77K was also measured.

Langmuir probe studies and ion energies in gas discharges used for cleaning vacuum chambers

From 4 × 10 −3 torr to 5 × 10 −2 torr the electron temperature in ArAr/O 2 and H 2 gas discharges decreases with evidence of more than one electron population with different temperatures. Electron and ion densities of ~ 10 15 m −3 were measured. The plasma potentials lay within a few volts of the anode potential and remained almost constant to the dark space near the chamber wall. All ions exhibited peaks in their energy distribution at or close to that corresponding to anode potential. Due to collisions with gas molecules the O + distribution in the argon + 10% oxygen discharge exhibited a peak corresponding to exactly half the anode potential. In the hydrogen discharge, the product H 2 O + was observed resulting from interaction of H o or H + with the oxide of the wall.