9 Accelerator Operations
563
for more details). The luminosity optimization in this configuration will be discussed
in Sect. 9.8.5
9.8.4 Collider with Weak Synchrotron Radiation Damping
In circular colliders with weak synchrotron radiation damping (with damping times
much larger than a typical store length—typically the case for hadron colliders
where the relativistic γ factor is small) the amplitude growth of the particle
oscillations within a bunch due to the beam-beam interaction is not stabilized
and leads to emittance growth, the development of tails in the bunch distribution,
particle losses and a reduction of the beam lifetime and possibly an increase of
the experimental background. The above detrimental processes of the beam-beam
interaction become more pronounced the more resonances are covered by the
tune distribution for the particles within a bunch. The beam-beam limit in hadron
colliders is therefore more commonly expressed by the total maximum acceptable
tune spread within a bunch due to the beam-beam interactions rather than the
maximum attainable beam-beam parameter for a single interaction. For hadron
colliders the beam-beam limit is therefore the sum of all beam-beam related tune
spreads. For purely head-on collisions it is proportional to the number of collisions a
single bunch experiences during one revolution. For a collider that features parasitic
long range beam-beam encounters it also includes the tune spread contributions
from the long-range beam-beam interactions.
The maximum acceptable beam-beam related tune spread depends on the
operation point (transverse tune values) and other sources for tune spread in the
collider (e.g. tune spread due to magnetic octupole components) and typically varies
for different Hadron colliders between values of Q = 0.015 and Q = 0.03 (e.g.
RHIC and Tevatron). The ‘precise’ value of the quoted beam-beam limit in a Hadron
collider depends on the acceptable luminosity lifetime and background rates for a
given operation mode and is therefore somewhat less well defined as for the case
of lepton colliders with strong synchrotron radiation. For example, a beam-beam
driven tune spread of Q = 0.03 is in principle attainable in the Tevatron operation
but the normal machine operation rather aims at a smaller beam-beam driven tune
spread of Q = 0.02 (the quoted tune shift refers to the anti-proton beam—the
beam-beam tune shift for the proton beam is ca. five times smaller) [94].
There are currently several studies under way for looking into possibilities
of compensating part of the beam-beam generated tune spread either by the use
of wires with DC or pulsed currents for the compensation of the tune spread
arising from the long-range beam-beam interactions [95] or by the use of electron
lenses for a compensation of the tune spread generated by the head-on beam-beam
collisions [96, 97].
Without damping the beam sizes are mainly determined by the injector complex
and the ability to preserve the injected emittances in the collider during injection
and acceleration. Without natural equilibrium beam sizes, the luminosity with head-
563
for more details). The luminosity optimization in this configuration will be discussed
in Sect. 9.8.5
9.8.4 Collider with Weak Synchrotron Radiation Damping
In circular colliders with weak synchrotron radiation damping (with damping times
much larger than a typical store length—typically the case for hadron colliders
where the relativistic γ factor is small) the amplitude growth of the particle
oscillations within a bunch due to the beam-beam interaction is not stabilized
and leads to emittance growth, the development of tails in the bunch distribution,
particle losses and a reduction of the beam lifetime and possibly an increase of
the experimental background. The above detrimental processes of the beam-beam
interaction become more pronounced the more resonances are covered by the
tune distribution for the particles within a bunch. The beam-beam limit in hadron
colliders is therefore more commonly expressed by the total maximum acceptable
tune spread within a bunch due to the beam-beam interactions rather than the
maximum attainable beam-beam parameter for a single interaction. For hadron
colliders the beam-beam limit is therefore the sum of all beam-beam related tune
spreads. For purely head-on collisions it is proportional to the number of collisions a
single bunch experiences during one revolution. For a collider that features parasitic
long range beam-beam encounters it also includes the tune spread contributions
from the long-range beam-beam interactions.
The maximum acceptable beam-beam related tune spread depends on the
operation point (transverse tune values) and other sources for tune spread in the
collider (e.g. tune spread due to magnetic octupole components) and typically varies
for different Hadron colliders between values of Q = 0.015 and Q = 0.03 (e.g.
RHIC and Tevatron). The ‘precise’ value of the quoted beam-beam limit in a Hadron
collider depends on the acceptable luminosity lifetime and background rates for a
given operation mode and is therefore somewhat less well defined as for the case
of lepton colliders with strong synchrotron radiation. For example, a beam-beam
driven tune spread of Q = 0.03 is in principle attainable in the Tevatron operation
but the normal machine operation rather aims at a smaller beam-beam driven tune
spread of Q = 0.02 (the quoted tune shift refers to the anti-proton beam—the
beam-beam tune shift for the proton beam is ca. five times smaller) [94].
There are currently several studies under way for looking into possibilities
of compensating part of the beam-beam generated tune spread either by the use
of wires with DC or pulsed currents for the compensation of the tune spread
arising from the long-range beam-beam interactions [95] or by the use of electron
lenses for a compensation of the tune spread generated by the head-on beam-beam
collisions [96, 97].
Without damping the beam sizes are mainly determined by the injector complex
and the ability to preserve the injected emittances in the collider during injection
and acceleration. Without natural equilibrium beam sizes, the luminosity with head-