Abstract
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.
Highlights
The Large Hadron Collider (LHC) is a 27-km circumference accelerator where two proton beams circulate in opposite directions [1,2]
The comparison between different surfaces shows that amorphous carbon coating reduces drastically the electron cloud (EC) buildup, thanks to its low secondary electron yield (SEY) and photoelectron yield (PY), while copper and ex situ nonevaporable getter (NEG) coated surfaces suffer of EC multipacting, even after several months of operation
The goal was to compare the behavior of ex situ NEG and amorphous-carbon coating with well-known unbaked copper samples
Summary
The Large Hadron Collider (LHC) is a 27-km circumference accelerator where two proton beams circulate in opposite directions [1,2]. Batches of a maximum of 72 bunches are injected in the LHC from the Super Proton Synchrotron (SPS) at an energy of 450 GeV. Once the LHC is filled, the beam energy is ramped up to 6.5 TeV for proton collisions in the so-called stable beam phase [3,4]. LHC beams generate electron clouds (EC) as predicted by simulations [5,6,7,8]. This phenomenon affects beam performance, gas density, and heat load in the cryogenic systems
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