140
E. Metral et al.
involved, simple considerations have usually turned out to be insufficient to predict
the build-up thresholds and the change in electron cloud caused by some specific
parameter change. It was a gratifying confirmation of the predictive power of the
early simulations that the expected electron clouds were indeed observed at both
B factories (PEP-II and KEKB), at the Large Hadron Collider injectors PS and
SPS when operated with LHC-type beams, and finally in the LHC itself when
operation with trains of closely spaced bunches started. In these rings, the electron
cloud was seen to cause tune shift and emittance growth along the bunches of a
train, both coupled and single-bunch instabilities and a degradation of certain beam
diagnostics signals [126–128]. In the SPS and PS, significant beam loss could be
observed at the end of a train if no countermeasures were put in place. As the
electron cloud is potentially one of the main bottlenecks of the SPS after the upgrade
of the LHC injector chain in the framework of the LHC Injectors Upgrade (LIU)
project [129], dedicated electron diagnostics devices are installed in the machine to
measure the electron flux and spatial electron distribution on surfaces with different
coatings, as well as its variation with the time of exposure, i.e. what we call “beaminduced machine scrubbing” [129]. Since 2011, the electron cloud has been also
observed routinely in the LHC, causing beam instability and emittance growth at
the end of the multi-bunch trains but also additional heat load on the cold beam
screens of the arcs as well as of the matching and final focusing quadrupoles [130].
While the effects linked to electron cloud have quickly disappeared for beams with
50 ns bunch spacing thanks to a relatively rapid beam induced machine scrubbing,
running with 25 ns beams has proved to be rather challenging in this machine.
With this type of beams, even after extensive machine scrubbing, the undesired
effects of the electron cloud have remained visible on the beam and the machine
equipment. In particular, the 25 ns beam needs to be stabilised with high values of
chromaticity in both planes and large octupole settings. Besides, the large heat load
on the cold beam screens still remains very close to the capacity of the cryogenic
system in nominal operating conditions [131, 132]. While these effects have not
prevented running LHC close to the nominal conditions from 2015 to 2018, they
could still be a significant showstopper for future operation with double beam
current in the High Luminosity LHC era [133]. Concerning lepton machines, the
electron cloud is typically associated with a reduction of specific luminosity in ep colliders and is expected to be one of the main limiting factors for the damping
rings of future linear collider projects. The Cornell Electron Storage Ring (CESR)
was reconfigured in 2008 as a Test Accelerator (CesrTA) for a program of electron
cloud research with lepton beams. With its new local diagnostics for measurement
of cloud density and improved instrumentation for the characterization of the beam
dynamics of high intensity bunch trains interacting with the cloud, this test facility
provided for many years both a benchmark case for the existing simulation codes
and testing the effectiveness of several types of countermeasures [134]. Since the
processes of secondary electron emission and elastic reflection at the walls play a
fundamental role in causing beam induced multipacting, we now shortly describe
their key parameters. The true secondary yield for perpendicular incidence, δ, can
E. Metral et al.
involved, simple considerations have usually turned out to be insufficient to predict
the build-up thresholds and the change in electron cloud caused by some specific
parameter change. It was a gratifying confirmation of the predictive power of the
early simulations that the expected electron clouds were indeed observed at both
B factories (PEP-II and KEKB), at the Large Hadron Collider injectors PS and
SPS when operated with LHC-type beams, and finally in the LHC itself when
operation with trains of closely spaced bunches started. In these rings, the electron
cloud was seen to cause tune shift and emittance growth along the bunches of a
train, both coupled and single-bunch instabilities and a degradation of certain beam
diagnostics signals [126–128]. In the SPS and PS, significant beam loss could be
observed at the end of a train if no countermeasures were put in place. As the
electron cloud is potentially one of the main bottlenecks of the SPS after the upgrade
of the LHC injector chain in the framework of the LHC Injectors Upgrade (LIU)
project [129], dedicated electron diagnostics devices are installed in the machine to
measure the electron flux and spatial electron distribution on surfaces with different
coatings, as well as its variation with the time of exposure, i.e. what we call “beaminduced machine scrubbing” [129]. Since 2011, the electron cloud has been also
observed routinely in the LHC, causing beam instability and emittance growth at
the end of the multi-bunch trains but also additional heat load on the cold beam
screens of the arcs as well as of the matching and final focusing quadrupoles [130].
While the effects linked to electron cloud have quickly disappeared for beams with
50 ns bunch spacing thanks to a relatively rapid beam induced machine scrubbing,
running with 25 ns beams has proved to be rather challenging in this machine.
With this type of beams, even after extensive machine scrubbing, the undesired
effects of the electron cloud have remained visible on the beam and the machine
equipment. In particular, the 25 ns beam needs to be stabilised with high values of
chromaticity in both planes and large octupole settings. Besides, the large heat load
on the cold beam screens still remains very close to the capacity of the cryogenic
system in nominal operating conditions [131, 132]. While these effects have not
prevented running LHC close to the nominal conditions from 2015 to 2018, they
could still be a significant showstopper for future operation with double beam
current in the High Luminosity LHC era [133]. Concerning lepton machines, the
electron cloud is typically associated with a reduction of specific luminosity in ep colliders and is expected to be one of the main limiting factors for the damping
rings of future linear collider projects. The Cornell Electron Storage Ring (CESR)
was reconfigured in 2008 as a Test Accelerator (CesrTA) for a program of electron
cloud research with lepton beams. With its new local diagnostics for measurement
of cloud density and improved instrumentation for the characterization of the beam
dynamics of high intensity bunch trains interacting with the cloud, this test facility
provided for many years both a benchmark case for the existing simulation codes
and testing the effectiveness of several types of countermeasures [134]. Since the
processes of secondary electron emission and elastic reflection at the walls play a
fundamental role in causing beam induced multipacting, we now shortly describe
their key parameters. The true secondary yield for perpendicular incidence, δ, can