Electron-cloud effect

Abstract: A two-ring electron-positron collider with asymmetric energies—called the SuperKEKB—has been designed by the High Energy Accelerator Research Organization (KEK) as an upgrade of the KEKB B-factory (KEKB), which completed 12 years of operation in 2010. It is anticipated that the SuperKEKB will reach a luminosity of 8 × 1035 cm−2 s−1, which is approximately 40 times larger than that of the original KEKB. The upgrade of the vacuum system is a key factor that will allow the SuperKEKB to achieve unprecedented high performance. Most of the beam pipes, especially in the positron ring, are newly manufactured to manage the electron cloud effect, and to reduce beam impedance, which is essential to keep the low-emittance beam stable. Our design of the vacuum system implements recent technologies and draws on various experiences and studies during the operation of the original KEKB. The basic design is near completion, and manufacturing of beam pipes and the major vacuum components, such as bellows chambers, gate valves and supports, are in progress. The installation of these components will start in 2013 with the aim of commissioning the SuperKEKB in 2014.

Abstract: In order to upgrade the Large Hadron Collider (LHC) performance to be oriented towards higher energies and higher intensities in the future, a series of improvements of the existing LHC injectors is planned to take place over the next few years. Electron cloud effects are expected to be enhanced and play a central role in limiting the performance of the machines of the CERN complex. Electron cloud phenomena in beam pipes are based on electron multiplication and can be sufficiently suppressed if the Secondary Electron Yield (SEY) of the surface of the beam pipes is lower than unity. The goal of this work is to find and study a thin film coating with reliably low initial SEY, which does not require bake-out or conditioning in situ with photons, is robust against air exposure and can easily be applied in the beam pipes of accelerators. In this work, amorphous carbon (a-C) thin films have been prepared by DC magnetron sputtering for electron cloud mitigation and antimultipactor applications. In the first part of this thesis, the experimental set-ups for the a-C thin film coatings used in this work are described. In the second part, a summary of all the key experimental findings obtained on the a-C thin films in the laboratory, as well as the results of the measurements carried out with LHC type beams in the Super Proton Synchrotron (SPS), i.e. the last injector before the LHC are shown. The influence of different coating parameters, such as power, discharge gas pressure and substrate temperature on SEY has been investigated. The a-C thin films have been characterized by SEY measurements, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Nuclear Reaction Analysis (NRA), X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy. The SEY of the a-C samples after long termexposure to various atmospheres has been studied. Different surface treatments, such as conditioning by electron beam, annealing under vacuum and ion bombardment, on the sample surfaces have been investigated. Apart from the characterizations in laboratory, a-C thin films have also been applied to the liners in the electron cloud monitors and to vacuum chambers of three dipole magnets in the SPS. The electron cloud effect has been studied in the SPS with LHC type beams. In the third and final part, several other coating techniques as well as materials have been investigated for low SEY applications are presented. To gain a deep understanding of the properties of our amorphous carbon coating, the amorphous carbon thin film has been compared with other pure graphite samples. In conclusion, the present study has largely improved the knowledge of the electron cloud understanding with LHC type beams in the SPS. Amorphous carbon coatings are believed to be a potential solution for the electron cloud in the high-energy particle accelerators.

Abstract: A beam duct with an antechamber scheme for high-current accelerators was designed and the test chambers were studied experimentally. The duct consists of two channels, i.e., a beam channel where a beam circulates and a Synchrotron Radiation (SR) channel (antechamber) aside where the SR passes through. By using the antechamber scheme, the maximum power density of SR can be diluted at the side wall. The impedance is small owing to the pumping ports not being at the beam channel, but at the SR channel. Photoelectrons inside the beam channel are also expected to be reduced, which would be a big merit for a positron ring to suppress the electron cloud effect since the photoelectron is a major source of electrons composing the cloud. Two copper test chambers were manufactured with different methods, by pressing and by drawing. These chambers showed a good static vacuum property, i.e., gas desorption rates with less than 3.5 × 10 - 9 Pa m 3 s - 1 m - 2 after baking. After the installation to the positron ring of the KEK B-factory (KEKB), electron numbers in the beam channel, temperatures and pressures were measured during beam operation. The electrons in the beam channel were found to be reduced by a factor of 4 at 1.5 A compared to the case of the usual circular chamber. The reduction, however, was much larger, about 1/300, at a beam current of about 20 mA where the photoelectrons were dominant and the multiplication of electrons by the multipactoring was small. The temperatures were almost in agreement with the expectation. Vacuum scrubbing by photons proceeded almost smoothly, although pressure bursts were sometimes observed, especially for one test chamber, which was possibly due to discharges at the transverse joints in the beam chamber. Various instructive information had been obtained for future practical beam ducts for high-intensity accelerators.