Pierre M Strubin

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.

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.

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...

Controls for the vacuum system of LEP

The very large size (27‐km circumference) of  LEP (CERN Large Electron Positron storage ring) makes it mandatory to provide extensive remote control for installation, commissioning, and operation. The control facilities will be available both at fixed locations and in the form of mobile consoles which will follow the installation teams. All operations must be accessible to vacuum technicians, who may have little or no computing knowledge. Therefore, menu driven consoles, using a screen and light pen, will be made available to the operators. The large number of pumps, gauges, and valves makes it necessary to find solutions to reduce the cabling costs. An original way of powering the sputter ion pumps uses a single power supply and cable for up to eight pumps, yet allows individual current monitoring for each pump. An ionization gauge power supply for eight heads is economical and saves rack space by multiplexing the ion currents while powering and regulating continuously all filaments. All power supplies must be integrated in the general control system of the LEP machine. This is done by using so‐called intelligent equipment controllers, which exchange messages with the main control room by a network of multidrop buses. A sophisticated data base management system is required in order to keep track of the numerous items connected to the vacuum system.

Assembly and quality control of the lhc cryostats at CERN: Motivations, means, results and lessons learnt

In 2001, the project management decided to perform at CERN the final assembly of the LHC superconducting magnets with cryostat parts and cold masses produced by European Industry in large series. This industrial-like production has required a very significant investment in tooling, production facilities, engineering and quality control efforts, in contractual partnership with a consortium of firms. This unusual endeavour of a limited lifetime represented more than 850,000 working hours spanning over five years, the work being done on a result-oriented basis by the contractor. This paper presents the reasons for having conducted this project at CERN, summarizes the work breakdown structure, the production means and methods, the infrastructure specially developed, the tooling, logistics and quality control aspects of the work performed and the results achieved, in analytical form. Finally, the lessons learnt are outlined.

Control and operational models for vacuum equipment

Operational models which describe the behaviour and the physical values associated with the vacuum equipment as seen by an operator have been studied for some time at CERN. Recently, they have been completed by control models, which define in a formal way the data structures required to access the physical values described in the operational model. The control models also define the operations that an application program has to send to the vacuum equipment to modify its state. Object modelling techniques (OMT) have been used to formalise the description of the models. In order to test the validity of the concepts, we have made a working prototype in the LEP accelerator.

Study of a new method to control precisely the evaporation rate of titanium sublimation pumps

Conventional ways of activating a sublimation pump, consisting of feeding a constant current or voltage to a titanium filament, do not give a control of the sublimation rate over the lifetime of the filament. The new method proposed here relies on the fact that a hot filament emits electrons. The temperature of the filament, and hence the sublimation rate, can be stabilized by regulating the electron emission, e.g., as is done for ionization gauges. Well controlled temperature and sublimation rates lead to an increase of the lifetime of the filament. This report presents the results of calculated temperatures and sublimation rates with and without emission stabilization. Experimental values are compared with the calculated ones.

Comparison of the synchrotron radiation induced gas desorption in aluminum vacuum chambers after chemical and argon glow discharge cleaning

In a dedicated synchrotron radiation beam line at the DCI storage ring at Orsay, France a series of measurements were made to evaluate the effect of argon glow discharge cleaning either as a pretreatment in the laboratory, or in situ, following the installation and pumpdown of the vacuum system in the LEP (Large Electron Positron storage ring) machine. On exposure to synchrotron radiation the gases desorbed were H2, CH4, CO, and CO2 with Ar in the glow discharge cleaned chambers. With continued exposure the gas desorption decreased with time except that the Ar showed a definite reluctance to clean up. When applied to LEP conditions, chemical cleaning only gave the required beam‐gas lifetime after the shortest exposure. The worst overall situation results from the in situ Ar discharge cleaning due to the low pumping speed for Ar in the LEP machine.In a dedicated synchrotron radiation beam line at the DCI storage ring at Orsay, France a series of measurements were made to evaluate the effect of argon glow discharge cleaning either as a pretreatment in the laboratory, or in situ, following the installation and pumpdown of the vacuum system in the LEP (Large Electron Positron storage ring) machine. On exposure to synchrotron radiation the gases desorbed were H2, CH4, CO, and CO2 with Ar in the glow discharge cleaned chambers. With continued exposure the gas desorption decreased with time except that the Ar showed a definite reluctance to clean up. When applied to LEP conditions, chemical cleaning only gave the required beam‐gas lifetime after the shortest exposure. The worst overall situation results from the in situ Ar discharge cleaning due to the low pumping speed for Ar in the LEP machine.

First experience with control and operational models for vacuum equipment in the AD decelerator

Control and Operational models for Vacuum Equipment have been studied at CERN for several years. A prototype implementation was tried out on ion gauges in LEP followed by a full-scale implementation for all vacuum equipment to be controlled in the newly built AD ring. In order to meet the tight time schedule, the existing hardware and software infrastructure of the PS complex has been used. The model server was built on top of this infrastructure. This has introduced some restrictions with respect to a full implementation of the models, but made the server available for all vacuum equipment already installed in the various accelerators which are connected to this control system. In order to test the server, a simplified man-machine interface has been created. This interface presents the available acquisition and control values in a very homogeneous way to the operator, making the advantage of the chosen model approach evident. It also makes additional diagnostic information, previously unavailable, accessible to the vacuum operators.

Gas desorption and surface conditioning of a synchrotron radiation source

Synchrotron radiation induced neutral gas desorption from baked Al alloy vacuum chambers cleaned using alkaline detergents has been measured in a test beam line at the DCI storage ring at LURE, Orsay, France.The gases desorbed were H2, CH4, CO and CO2 and with time the H2 and CO desorption decreased as D−1/2 where D is the radiation beam dose in mA hours. Similar results for electron induced neutral gas desorption from small samples showed a similar behavior and led to the proposal of a model of gas diffusion from near surface layers and from the bulk to the surface which determined the desorption characteristics. For H2 the model described well the behavior of the H2 desorption.Measurements of the photoelectron currents produced at 11 mrad glancing angle of incidence and also at normal incidence, when compared with calculation, showed differences which could be explained if the photons with energies up to between 1.2 and 1.5 keV were reflected at 11 mrad with reflectivities approaching 1. Using published...