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against each other is a similar idea. Recently, there is a growing interest towards
the suppression of multipacting by means of grooves on the chamber wall. This
technique, first tested in simulations, has proven to be efficient in KEKB and CesrTA. Similarly to a rough surface, but in a controlled way and on a macroscopic
scale, these grooves essentially act as electron traps. Angle and depth of the grooves
are key parameters and specifications are different in dipole or field-free regions.
Proper tailoring of the bunch filling patterns (bunch spacing, bunch trains and bunch
charges) is yet another way of achieving an acceptable electron density. Examples
include the actual bunch spacing chosen for PEP-II and KEKB operation, which are
twice or three times the design spacing, and satellite bunches proposed for the LHC
[152]. Gaps within or between trains can lower the density and reset the cloud at
least to some extent. Extensive studies of the electron cloud formation as a function
of the bunch filling patterns were also carried out at RHIC, in which the optimization
could be found using the maps approach to quickly scan the build-up for different
configurations.
The electron cloud causes a large variety of undesired effects. Common
stabilising measures can be taken against the resulting instabilities, which include
transverse bunch-to-bunch feedback, increased chromaticity, Landau-damping
octupoles, intra-bunch head–tail feedback, and linear coupling. All these measures
are anyway necessary when a machine is operating in beam-induced scrubbing
mode. Degradation of diagnostics signals due to impacting electrons can be also
overcome with local solenoid windings.
4.6 Beam–Beam Effects
W. Herr
4.6.1 Introduction
The problem of the beam–beam interaction is the subject of many studies since
the introduction of the first particle colliders [153]. It has been and will be one of
the most important limits to the performance and therefore attracts the interest at
the design stage of a new colliding beams facility. A particle beam is a collection
of a large number of charges and represents an electromagnetic potential for other
charges. It will therefore exert forces on itself and other beams. The forces are most
important for high density beams, i.e. high intensity and small beam sizes, which
are the key to high luminosity.
The electromagnetic forces from particle beams are very non-linear and result
in a wide spectrum of consequences for the beam dynamics. Furthermore, as a
result of the interaction, the charge distribution creating the disturbing fields can
change as well. This has to be taken into account in the evaluation of beam–
E. Metral et al.
against each other is a similar idea. Recently, there is a growing interest towards
the suppression of multipacting by means of grooves on the chamber wall. This
technique, first tested in simulations, has proven to be efficient in KEKB and CesrTA. Similarly to a rough surface, but in a controlled way and on a macroscopic
scale, these grooves essentially act as electron traps. Angle and depth of the grooves
are key parameters and specifications are different in dipole or field-free regions.
Proper tailoring of the bunch filling patterns (bunch spacing, bunch trains and bunch
charges) is yet another way of achieving an acceptable electron density. Examples
include the actual bunch spacing chosen for PEP-II and KEKB operation, which are
twice or three times the design spacing, and satellite bunches proposed for the LHC
[152]. Gaps within or between trains can lower the density and reset the cloud at
least to some extent. Extensive studies of the electron cloud formation as a function
of the bunch filling patterns were also carried out at RHIC, in which the optimization
could be found using the maps approach to quickly scan the build-up for different
configurations.
The electron cloud causes a large variety of undesired effects. Common
stabilising measures can be taken against the resulting instabilities, which include
transverse bunch-to-bunch feedback, increased chromaticity, Landau-damping
octupoles, intra-bunch head–tail feedback, and linear coupling. All these measures
are anyway necessary when a machine is operating in beam-induced scrubbing
mode. Degradation of diagnostics signals due to impacting electrons can be also
overcome with local solenoid windings.
4.6 Beam–Beam Effects
W. Herr
4.6.1 Introduction
The problem of the beam–beam interaction is the subject of many studies since
the introduction of the first particle colliders [153]. It has been and will be one of
the most important limits to the performance and therefore attracts the interest at
the design stage of a new colliding beams facility. A particle beam is a collection
of a large number of charges and represents an electromagnetic potential for other
charges. It will therefore exert forces on itself and other beams. The forces are most
important for high density beams, i.e. high intensity and small beam sizes, which
are the key to high luminosity.
The electromagnetic forces from particle beams are very non-linear and result
in a wide spectrum of consequences for the beam dynamics. Furthermore, as a
result of the interaction, the charge distribution creating the disturbing fields can
change as well. This has to be taken into account in the evaluation of beam–