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E. Metral et al.
of multipacting by lowering the SEY via surface treatment; (iii) alleviating the
effect on the beam or on the diagnostics. In most cases a combined approach
is desirable, that’s why most machines affected by electron cloud problems have
usually chosen to implement more than one of these mitigating techniques. The
primary production of photoelectrons needs to be reduced, because it may be so
high that the electron cloud could reach saturation within a few bunch passages
even without any multipacting. This is the case at KEKB, the photon factory, if no
countermeasures were taken. The obvious solution is an antechamber to intercept
most of the synchrotron radiation, or also photon absorbers (as those designed for
the CLIC Damping Rings). For dipole fields, a saw-tooth pattern impressed on
the chamber wall, as was implemented for the LHC (actually on the beam screen
that forms the inner part of the chamber and serves to protect the cold bore of the
magnets from synchrotron radiation), is used for effectively reducing the photon
reflectivity thanks to the perpendicular impact. Weak solenoids of the order of 50
G are a possibility in field-free regions, which was successfully implemented in
the straight sections of KEKB and in RHIC. The solenoids do not really affect
the photoemission process, but they keep the photoelectrons close to the wall
and, thus, strongly mitigate the subsequent beam–electron interaction. Since the
gas ionization rate is linearly proportional to the vacuum pressure in the beam
chamber, the number of electrons created by gas ionization can only be reduced by
significant factors improving the vacuum. If field ionization is important, however,
a possible cure would be lengthening the bunches, though this will mainly be a
concern for future projects such as linear colliders or X-ray FELs operating with
positrons. Electrons generated by beam loss can be controlled if the localization of
the losses is known with good precision. For example, electrons produced by the
beam losses at a collimator can be controlled by solenoids or clearing electrodes.
A large number of electrons is also generated at the injection stripping foils, for
accelerators employing charge-exchange injection. At the SNS, a 10-kV clearing
voltage is applied to channel the electrons liberated at the stripping foil onto a
collector plate that is monitored by a TV camera, while solenoids are used along
the collimator straights.
The reduction of the SEY of the inner wall of the beam chamber can be achieved
in different manners. First of all, a serendipitous feature of the electron cloud build
up in an accelerator’s chamber is that, while the electrons hit the beam chamber with
high energy and multiply, they also ‘scrub’ the surface by first removing layers of
impurities responsible for high SEY values and eventually graphitising the surface
with a further reduction of the SEY from that of the pure metal [151]. This means
that, if a method is found to run an accelerator with electron cloud and stable beam,
e.g. by stabilising the beam against the electron cloud through appropriate machine
settings, it will be the electron cloud itself to gradually lower the SEY of the inner
wall of the chamber and eventually turn itself off. This obviously relies on that the
final SEY reachable through scrubbing is below the value that sets off the electron
cloud build up in the operational configuration. Besides, it may take a significant
amount of time to reach this condition, because the electron flux is decreasing while
scrubbing and the electron doses required to perform further SEY reduction steps are
E. Metral et al.
of multipacting by lowering the SEY via surface treatment; (iii) alleviating the
effect on the beam or on the diagnostics. In most cases a combined approach
is desirable, that’s why most machines affected by electron cloud problems have
usually chosen to implement more than one of these mitigating techniques. The
primary production of photoelectrons needs to be reduced, because it may be so
high that the electron cloud could reach saturation within a few bunch passages
even without any multipacting. This is the case at KEKB, the photon factory, if no
countermeasures were taken. The obvious solution is an antechamber to intercept
most of the synchrotron radiation, or also photon absorbers (as those designed for
the CLIC Damping Rings). For dipole fields, a saw-tooth pattern impressed on
the chamber wall, as was implemented for the LHC (actually on the beam screen
that forms the inner part of the chamber and serves to protect the cold bore of the
magnets from synchrotron radiation), is used for effectively reducing the photon
reflectivity thanks to the perpendicular impact. Weak solenoids of the order of 50
G are a possibility in field-free regions, which was successfully implemented in
the straight sections of KEKB and in RHIC. The solenoids do not really affect
the photoemission process, but they keep the photoelectrons close to the wall
and, thus, strongly mitigate the subsequent beam–electron interaction. Since the
gas ionization rate is linearly proportional to the vacuum pressure in the beam
chamber, the number of electrons created by gas ionization can only be reduced by
significant factors improving the vacuum. If field ionization is important, however,
a possible cure would be lengthening the bunches, though this will mainly be a
concern for future projects such as linear colliders or X-ray FELs operating with
positrons. Electrons generated by beam loss can be controlled if the localization of
the losses is known with good precision. For example, electrons produced by the
beam losses at a collimator can be controlled by solenoids or clearing electrodes.
A large number of electrons is also generated at the injection stripping foils, for
accelerators employing charge-exchange injection. At the SNS, a 10-kV clearing
voltage is applied to channel the electrons liberated at the stripping foil onto a
collector plate that is monitored by a TV camera, while solenoids are used along
the collimator straights.
The reduction of the SEY of the inner wall of the beam chamber can be achieved
in different manners. First of all, a serendipitous feature of the electron cloud build
up in an accelerator’s chamber is that, while the electrons hit the beam chamber with
high energy and multiply, they also ‘scrub’ the surface by first removing layers of
impurities responsible for high SEY values and eventually graphitising the surface
with a further reduction of the SEY from that of the pure metal [151]. This means
that, if a method is found to run an accelerator with electron cloud and stable beam,
e.g. by stabilising the beam against the electron cloud through appropriate machine
settings, it will be the electron cloud itself to gradually lower the SEY of the inner
wall of the chamber and eventually turn itself off. This obviously relies on that the
final SEY reachable through scrubbing is below the value that sets off the electron
cloud build up in the operational configuration. Besides, it may take a significant
amount of time to reach this condition, because the electron flux is decreasing while
scrubbing and the electron doses required to perform further SEY reduction steps are