The secondary electron yield of TiZr and TiZrV non evaporable getter thin film coatings

The secondary electron yield (SEY) of two different non evaporable getter (NEG) samples has been measured 'as received' and after thermal treatment. The investigated NEGs are TiZr and TiZrV thin film coatings of 1 mm thickness, which are sputter deposited onto copper substrates. The maximum SEY dmax of the air exposed TiZr and TiZrV coating decreases from above 2.0 to below 1.1 during a 2 hour heat treatment at 250 °C and 200 °C, respectively. Saturating an activated TiZrV surface under vacuum with the gases typically present in ultra high vacuum systems increases dmax by about 0.1. Changes in elemental surface composition during the applied heat treatments were monitored by Auger electron spectroscopy (AES). After activation carbon, oxygen and chlorine were detected on the NEG surfaces. The potential of AES for detecting the surface modifications which cause the reduction of SE emission during the applied heat treatments is critically discussed

A Summary of Main Experimental Results Concerning the Secondary Electron Emission of Copper

The secondary electron emission of surfaces exposed to the impact of energetic electrons contributes significantly to the electron cloud build-up. For the prediction of the consequences of this effect the measurements of the secondary electron yield carried out at CERN are an important source of information. New experimental results concerning the total secondary electron yield for very low primary electron energy (between 5 eV and 50 eV) will be also given in the case of as received copper. Furthermore the energy distribution of the re-emitted electrons is drastically influenced by the primary electron energy. The ratio of the number of reflected electrons to the total number of re-emitted electrons has been measured and its variation with the primary electron energy will be shown. As a consequence of these new experimental data, a numerical approximation to express the secondary electron yield as a function of the primary electron energy will be given for the low incident electron energy region (E < 50 eV). It has been shown that the decrease of the secondary electron yield due to the electron bombardment could reduce sufficiently the consequences electron cloud effect. To understand further the origin of this decrease, the results of experiments showing the variation of the electron induced desorption yield with the incident electron dose will be compared to the concomitant reduction of the secondary electron yield

The variation of the secondary electron yield and of the desorption yield of copper under electron bombardment: Origin and impact on the conditioning of the LHC

The operation of most radio-frequency components in accelerators rely on the conditioning obtained usually by gradually increasing the power fed into these devices. This effect is related to the reduction of the desorption yields and of the secondary electron yields of surfaces exposed to electron bombardment. In LHC, similar decreases will also limit the detrimental consequences of the electron cloud phenomenon on the beam stability, the power deposited in the cryogenic system and the gas density. The parallel evolution of the desorption yield and of the secondary electron yield will be discussed and compared to the changes to the surface composition as observed by surface analytical techniques. The possible origin of the secondary electron yield decrease will also be discussed at the light of these new results

The Secondary Electron Yield of Technical Materials and its Variation with Surface Treatments

Secondary electron emission of surfaces exposed to oscillating electromagnetic field is at the origin of the multipacting effect that could severely perturb the operation of particle accelerators. This contribution tries to illustrate by measurement results, the origin of the secondary electron emission as well as the main reasons for the discrepancies between technical materials and pure metals. The variation of the secondary electron yield with the incident electron energy will be discussed for various types of technical surfaces. The influence of a gas condensation on these surfaces will also be addressed in the context of the LHC accelerator. Various treatments aiming at a permanent reduction of the secondary electron yield will be presented. A special attention will be paid to the decrease of the secondary electron yield under electron or photon impact and to its possible beneficial consequences for the processing of devices prone to multipacting

Measurement of the electron cloud properties by means of a multi-strip detector in the CERN SPS

Electron cloud effects presently limit the performances of the CERN SPS with LHC type beams [1] and are of concern for the LHC itself [2]. Electron multipacting in the SPS produces dramatic dynamic pressure increases and strong transverse instabilities [3]. In the LHC the electron cloud is expected to significantly increase the heat load in the cryogenics system. Estimates of these effects are based on computer simulations of the electron cloud build-up and of its spatial distribution in field free regions and in strong magnetic fields. The accuracy of such simulations is therefore a key issue for component design and for the definition of the operating strategies for the LHC. In 2001 a multi-strip detector has been installed in the SPS to study the electron cloud and to provide experimental data to validate the models and to better constrain their input parameters. After a description of the monitor characteristics and of its associated electronics an overview of its performance and of the results of the measurements conducted with different proton beam parameters are presented. The measurements are compared with simulation results. Possible monitor upgrades are also discussed

Electron Cloud and Beam Scrubbing in the LHC

An adequate dose of photoelectrons, accelerated by low-intensity proton bunches and hitting the LHC beam screen wall, will substantially reduce secondary emission and avoid the fast build-up of an electron cloud for the nominal LHC beam. The conditioning period of the liner surface can be considerably shortened thanks to secondary electrons, provided heat load and beam stability can be kept under control; for example this may be possible using a special proton beam, including satellite bunches with an intensity of 15-20% of the nominal bunch intensity and a spacing of one or two RF wavelengths. Based on recent measurements of secondary electron emission, on multipacting tests and simulation results, we discuss possible "beam scrubbing" scenarios in the LHC and present an updat

Electron Cloud Effects in the CERN SPS and LHC

Electron cloud effects have been recently observed in the CERN SPS in the presence of LHC type proton beams with 25 ns bunch spacing. Above a threshold intensity of about 4 X 10^12 protons in 81 consecutive bunches, corresponding to half of the nomina

Electron conditioning of technical surfaces at cryogenic and room temperature in the 0–1 keV energy range

In the superconducting magnets of the Large Hadron Collider (LHC) at CERN, most of the beam-induced heat load is intercepted by a beam-screen (BS) cryogenically cooled to 5–20 K. When circulating the bunched proton beam, an electron cloud (EC) can form and bombard the BS copper surface with high doses of predominantly low-energy electrons, which desorb gas and consequently increase the pressure. The beam-induced pressure rise decreases during operation as the electron irradiation diminishes the secondary electron yield (SEY) and the electron-stimulated desorption (ESD) yield, a phenomenon referred to as ‘beam conditioning’. Low ESD and SEY values achieved rapidly are requisite to mitigate EC and maintain UHV in storage rings. We report data on ESD and SEY electron conditioning completed at cryogenic temperature with 0–1 keV electrons up to an electron dose of 5.10−3 C mm−2. Our results show that SEY conditioning depends on the primary electron energy and also that ESD yield significantly decreases with temperature. At 15 K, the amorphous-carbon coating and laser-treated copper present SEY below 1.1 and have initial ESD yields 3–6 times lower than OFE copper. Our results conform to the SEY and ESD's general understanding and extend it towards cryogenic temperatures. •Technical copper has 4–80x lower electrodesorption yields at 15 K than at 260 K.•Electrodesorption energy threshold and conditioning rate remain unchanged at 15 K.•Low-energy e− have a limited conditioning effect on the secondary electron yield.•Carbon-coated and laser-treated copper retain low SEY and ESD at 15 K