Abstract
Electron cloud effects, which include heat load in the cryogenic system, pressure rise, and beam instabilities, are among the main intensity limitations for the LHC operation with 25 ns spaced bunches. A new observation tool was proposed and developed to monitor the e-cloud activity and it has already been used successfully during the LHC run 1 (2010-2012) and it is being intensively used in operation during the start of the LHC run 2 (2015-2018). It is based on the fact that the power loss of each bunch due to e-cloud can be estimated using bunch-by-bunch measurement of the synchronous phase. The measurements were done using the existing beam phase module of the low-level rf control system. In order to achieve the very high accuracy required, corrections for reflection in the cables and for systematic errors need to be applied followed by a post-processing of the measurements. Results clearly show the e-cloud buildup along the bunch trains and its time evolution during each LHC fill as well as from fill to fill. Measurements during the 2012 LHC scrubbing run reveal a progressive reduction in the e-cloud activity and therefore a decrease in the secondary electron yield. The total beam power loss can be computed as a sum of the contributions from all bunches and compared with the heat load deposited in the cryogenic system.
Highlights
At the beginning of the CERN Large Hadron Collider (LHC) run 1 (2009–2013), electron cloud (e-cloud) effects were limiting the LHC operation, leading to an excessive heat load in the cryogenic system, a degradation of the vacuum, transverse instabilities, emittance growth, and particle losses [1,2]
In this paper we present a new method for e-cloud observation that uses synchronous phase measurements
Bunch positions measured by the beam quality monitor (BQM) [11] were considered to extract the synchronous phase shift [12], but this method could not be used because the phase shift due to transient beam loading is included and it can be larger than the phase shift due to e-cloud
Summary
At the beginning of the CERN Large Hadron Collider (LHC) run 1 (2009–2013), electron cloud (e-cloud) effects were limiting the LHC operation, leading to an excessive heat load in the cryogenic system, a degradation of the vacuum, transverse instabilities, emittance growth, and particle losses [1,2]. Scrubbing with beams was proven to be an effective method for reducing the secondary electron yield (SEY) below the e-cloud buildup threshold for 50 ns beams, a long time was. Beams with 25 ns bunch spacing were injected later in 2011, but the strong e-cloud effects were limiting the beam intensity circulating in the ring and quickly degrading the beam quality. The number of bunches injected into the LHC is being increased since that time, but the excessive heat load in the cryogenic. Future LHC operation relies on efficient scrubbing of the beam pipe surface to further reduce its SEY and observation tools are required for optimization of the scrubbing strategy and time [4]. By comparing the synchronous phase measurements with macroparticle simulations the SEY can be estimated [5,6]
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