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B. J. Holzer et al.
The number of bunches in a single-ring collider is limited by the possibilities
for separating the opposing beams at unwanted encounters, e.g., by local or longrange (“pretzel scheme”) electrostatic orbit bumps [53]. Collective effects limiting
the single-bunch intensity (bunch-lengthening, transverse mode-coupling, see Chap.
4) are a major concern. In recent double-ring colliders, many more bunches can be
stored. A crossing angle at the collision point separates the beams at encounters
in the adjacent common section of the beam pipe. In recent years, the highest
luminosity collider designs have adopted a new scheme described in the following
section.
Multi-bunch collective effects and other limits related to total beam current (e.g.,
component heating by wakefields or synchrotron radiation, beam-loading, electroncloud, ion-trapping [53]) tend to dominate. The impedance and surface properties
of the vacuum chamber are critical.
Integrated luminosity can be further maximised in moderate energy rings for
which a full-energy injector is available by topping up the intensity of the stored
beam rather than dumping and refilling. The static magnetic configuration (no ramp
and squeeze cycle) simplifies operation dramatically.
The arcs of collider rings are usually composed of FODO cells whose length and
phase advance determine the emittance through Eq. (6.58). To minimise radiation
power, the bending magnets are made as long as possible. In the highest energy
rings, the quadrupoles must also be lengthened.
Low-β insertions (Sect. 6.2.1) provide small values of β ∗
y at the interaction
point(s) of the experiment(s) in long straight sections. These can also accommodate
the accelerating cavities of the RF system, beam instrumentation and wiggler
magnets and are connected to the arcs via dispersion suppressors.
Wiggler magnets modify the radiation damping, bunch length and/or emittance
by contributing additional terms [53] with large |G| to the integrals in Eqs. (6.57) and
(6.58), so providing additional flexibility to maximise performance (e.g., at lower
energy).
Sextupoles incorporated in the arcs must correct the large chromatic aberrations
generated in the low-β quadrupoles while preserving adequate dynamic aperture
(Sect. 3.4.4).
Many variations on this classical e + e − collider design are possible with new
interaction region concepts showing promise (Sect. 6.4) in overcoming the need for
ever-increasing beam current and ever-shorter bunches.
At higher intensities, phenomena such as the Touschek effect and intra-beam
scattering [53], sometimes in combination with non-linear single particle dynamics
or beam-beam effects, can reduce the lifetime below the values implied by Eq.
(6.60); see Chap. 3 and Sect. 4.6.
B. J. Holzer et al.
The number of bunches in a single-ring collider is limited by the possibilities
for separating the opposing beams at unwanted encounters, e.g., by local or longrange (“pretzel scheme”) electrostatic orbit bumps [53]. Collective effects limiting
the single-bunch intensity (bunch-lengthening, transverse mode-coupling, see Chap.
4) are a major concern. In recent double-ring colliders, many more bunches can be
stored. A crossing angle at the collision point separates the beams at encounters
in the adjacent common section of the beam pipe. In recent years, the highest
luminosity collider designs have adopted a new scheme described in the following
section.
Multi-bunch collective effects and other limits related to total beam current (e.g.,
component heating by wakefields or synchrotron radiation, beam-loading, electroncloud, ion-trapping [53]) tend to dominate. The impedance and surface properties
of the vacuum chamber are critical.
Integrated luminosity can be further maximised in moderate energy rings for
which a full-energy injector is available by topping up the intensity of the stored
beam rather than dumping and refilling. The static magnetic configuration (no ramp
and squeeze cycle) simplifies operation dramatically.
The arcs of collider rings are usually composed of FODO cells whose length and
phase advance determine the emittance through Eq. (6.58). To minimise radiation
power, the bending magnets are made as long as possible. In the highest energy
rings, the quadrupoles must also be lengthened.
Low-β insertions (Sect. 6.2.1) provide small values of β ∗
y at the interaction
point(s) of the experiment(s) in long straight sections. These can also accommodate
the accelerating cavities of the RF system, beam instrumentation and wiggler
magnets and are connected to the arcs via dispersion suppressors.
Wiggler magnets modify the radiation damping, bunch length and/or emittance
by contributing additional terms [53] with large |G| to the integrals in Eqs. (6.57) and
(6.58), so providing additional flexibility to maximise performance (e.g., at lower
energy).
Sextupoles incorporated in the arcs must correct the large chromatic aberrations
generated in the low-β quadrupoles while preserving adequate dynamic aperture
(Sect. 3.4.4).
Many variations on this classical e + e − collider design are possible with new
interaction region concepts showing promise (Sect. 6.4) in overcoming the need for
ever-increasing beam current and ever-shorter bunches.
At higher intensities, phenomena such as the Touschek effect and intra-beam
scattering [53], sometimes in combination with non-linear single particle dynamics
or beam-beam effects, can reduce the lifetime below the values implied by Eq.
(6.60); see Chap. 3 and Sect. 4.6.