Electron flux and pressure dynamic in the LHC vacuum pilot sector as a function of beam parameters and beam pipe properties

The Large Hadron Collider (LHC) is affected by the electron cloud (EC) phenomenon that can provoke beam instabilities, detrimental heat loads and pressure increases in the vacuum system. An innovative dedicated system called vacuum pilot sector (VPS) provides a continuous monitoring of the electron flux and of the pressure signals thanks to electron pickup and vacuum gauges. The VPS system is installed in a room temperature, field-free part of the LHC storage ring. Several technical surfaces, such as ex situ nonevaporable getter (NEG), amorphous carbon coating and copper, are simultaneously tested. The main outcomes of this study show that the EC signals have: (1) a linear dependence upon the number of bunches and upon the bunch population in the multipacting regime, (2) a multipacting threshold at a given bunch population, (3) a reduction under beam conditioning, (4) a strong dependence on the filling pattern and beam energy. The comparison between different surfaces shows that amorphous carbon coating reduces drastically the EC buildup, thanks to its low secondary electron yield (SEY) and photoelectron yield (PY), while copper and ex situ NEG coated surfaces suffer of EC multipacting, even after several months of operation. The multipacting rate coefficients are higher for copper than for ex situ NEG, as predicted from the SEY estimation. Other detailed experimental observations are discussed in this paper.

Electron cloud studies for the LHC and future proton colliders

César Octavio Domínguez Sánchez De La Blanca

The Large Hadron Collider (LHC) is the world’s largest and most powerful particle collider. Its main objectives are to explore the validity of the standard model of particle physics and to look for new physics beyond it, at unprecedented collision energies and rates. A good luminosity performance is imperative to attain these goals. In the last stage of the LHC commissioning (2011-2012), the limiting factor to achieving the design bunch spacing of 25 ns has been the electron cloud effects. The electron cloud is also expected to be the most important luminosity limitation after the first Long Shut-Down of the LHC (LS1), when the machine should be operated at higher energy and with 25-ns spacing, as well as for the planned luminosity upgrade (HL-LHC) and future high energy proton colliders (HE-LHC and VHE-LHC). This thesis contributes to the understanding of the electron cloud observations during the first run of the LHC (2010-2012), presents the first beam dynamics analysis for the next generation of high energy hadron colliders, and assists in the prediction of how electron clouds will impact the performance of the future high-luminosity and high-energy machines. In particular, the thesis discusses a method to benchmark pressure measurements at the LHC against electron cloud build-up simulations for identifying the most relevant surface parameters. This method allowed monitoring the effectiveness of LHC “scrubbing runs”, revealing that in the warm regions, the maximum Secondary Electron Yield, δmax, decreased from an initial value of about 1.9 down to about 1.2 (with a low-energy electron reflectivity R ≈ 0.3), thanks to surface conditioning. In addition, the “map formalism”, a good approximation to quickly explain and predict electron cloud effects, has been further developed and applied, for the first time, to optimize the scrubbing process at the LHC. For the HL-LHC, several novel filling schemes have been analyzed in terms of luminosity performance and electron cloud activity. Only a few of them are compatible with an electron cloud activity lower than for the baseline scenario. We highlight a promising option which could be a good fallback scenario in case the electron cloud effects prevent the injection of the baseline beam. This option could also be considered for the nominal LHC after the LS1 if electron cloud turns out to be a serious obstacle. Regarding the future high-energy proton colliders (HE- and VHE-LHC), in the frame of this thesis a performance model was developed to predict the luminosity as a function of time and to optimize the beam parameters, carrying out the first ever performance analysis for these machines. Several scenarios have been considered, including round and flat beams as well as different bunch spacings. The parameters presented in this thesis have been submitted as an input for the most recent update of the European Strategy for Particle Physics. Finally, we also report the electron cloud studies performed for both high energy machines. The large amount of primary photoelectrons generated by synchrotron radiation at these high energies motivates the consideration of high efficiency photon stops as well as other mitigation techniques (e.g. a-C coatings and clearing electrodes). Although for both machines (HE- and VHE-LHC) a tentative bunch spacing of 25 ns has been considered as baseline assumption, the results of this thesis suggest the possibility of going down to 5 ns, since such a beam would present several advantages.

EPFL2013

Electron Cloud and Synchrotron Radiation characterization of technical surfaces with the Large Hadron Collider Vacuum Pilot Sector

Elena Buratin

This PhD thesis presents the experimental study on electron cloud (EC) and synchrotron radiation (SR) phenomena affecting the LHC storage ring performance. These phenomena are one of the major issues of the LHC storage ring, generating beam instabilities, pressure rise and heat loads on the beam pipes. This experimental study is carried out with an innovative system installed in a room temperature and field free section of the Large Hadron Collider (LHC), the Vacuum Pilot Sector (VPS). ln this system, several surfaces are simultaneously tested, in particular standard copper, amorphous carbon coating, NEG. Thanks to a wide spectrum of detectors, the EC and SR signals were studied as a function of beam parameters and beam pipe properties. The EC behaviour increases linearly with the number of bunches and bunch population above the multipacting threshold. This phenomenon increases linearly also with the bunch length for the Long Straight Sections of LHC (LSS). As expected, the bunch spacing has a crucial importance on the formation of EC dynamic: the 50 ns filling scheme avoids the multipacting effect at 450 GeV. At 6.5 TeV the photoelectron contribution is visible, with no multipacting. This scheme is not sufficient for the LHC collision studies and cannot be used as standard filling scheme. Instead, the 25 ns bunch spacing scheme suffers of EC; in this case this phenomenon can be drastically reduced only by bombarding the surface with electrons during dedicated scrubbing runs, or thanks to the installation of beam pipe surface with a low SEY, such as the amorphous-carbon coating. The measurements performed with pick-ups show the evolution of the EC dynamics for the first time in the LHC. The energy spectrum detectors and the gas analysers were also installed for the first time in the LHC history: the electron cloud spectrum and the gas composition could be measured during the machine operation. The electron energy spectrum evidences a peak at around 100 eV, corresponding to the accelerated electrons responsible for multipacting. The gas analysers measured the main gasses present in the LHC ultra-high vacuum system, such as methane, carbon dioxide and monoxide, water, hydrogen and traces of 2-Propynenitrile 3-fluoro (C3FN), probably linked to the effect of radiation into the kapton cables. It has been possible to estimate the Electron Stimulated Desorption (ESD) parameter of a copper surface as a function of the accumulated electron dose. The heat loads due to impedance, EC and SR were measured and, finally, the operation of HL-LHC machine has been estimated in terms of electron current, pressure and power deposition thanks to the results of this study. Doubling of the bunch population will increase the electron current and the pressure of one order of magnitude, while the deposited power due to EC will be smaller than 1 W/mchamber for a beam pipe SEY lower than 1.8.

EPFL2020

Electron cloud studies for the LHC and future proton colliders

César Octavio Domínguez Sánchez De La Blanca

EPFL2013

Electron Cloud and Synchrotron Radiation characterization of technical surfaces with the Large Hadron Collider Vacuum Pilot Sector

Elena Buratin

EPFL2020