8 Accelerator Engineering and Technology: Accelerator Technology
485
be detected and addressed. The LHC collimator controls system allows a synchronization to other accelerator systems like magnets and radio-frequency to the
micro-second level. Motors are used in a dedicated tool to align in parallel several
collimators, at frequencies up to 50 Hz, in a feedback loop that uses 100 Hz beam
loss measurements [251, 252]. The recent addition of orbit measurements [242]
improves further the on-line diagnostic capabilities, allowing an earlier detection of
possibly critical shifts of the local beam orbit.
8.8.3.2 Mechanical Design, Cooling and Vacuum
The demanding requirements in modern high power accelerators were already
shortly reviewed. These requirements translate into important constraints for the
design of collimators. We summarize some of them:
– The design of mechanical movement must be precise, robust and reproducible.
The lifetime of the collimator movement system should be a few 10,000 cycles
for slowly ramped accelerators (like LHC) up to a few million cycles for
rapidly cycled accelerators. The tolerances for mechanical plays and setting
reproducibility can be as small as a few 10 μm.
– The use of traditional grease is often not permitted due to the presence of high
beam losses and radiation. Grease could age and become sticky. Instead, advanced
techniques like dry grease (graphite powder) or surface coatings must be used.
– Thermal heat flow must be carefully designed, from the jaw where heat is
deposited to the cooling pipes that take it out. Heat loads from direct particle
losses and electro-magnetic currents must be taken into account (W to 100 kW).
Thermal expansion coefficients of materials must be well adapted to ensure
minimal deformations during power gradients (few 10 μm for several kW) and
good thermal contact for heat transfer [253].
– The impacting power loads often require powerful cooling of the jaw materials.
High-pressure water flow (20 bar) must be integrated into the jaw design.
Sometimes, it is required to also cool the vacuum tank and flanges.
– Safety and robustness against beam shock impacts (accidents) often requires
that cooling pipes do not have brazed parts under vacuum.
– Many accelerators require ultra-high vacuum (~10 -8 mbar) and the collimators
must not disturb this. As heated materials show outgassing, additional constraints
on maximum jaw temperatures and cooling must be respected.
– Intense beams induce electro-magnetic image currents on the surrounding materials: machine impedance or wakefields are induced [254–256]. As collimator
materials are very close to the beam, their electro-magnetic properties must
be well designed. Critical are good electrical resistivity (~μm) and smooth
geometrical shapes. Tapering of jaws is used to avoid sharp edges close to the
beam. RF fingers need to be used to guide image currents.
– The high irradiation at collimators [257] often requires special measures.
Possibilities include quick plug-ins for cables and water pipes, absence of
485
be detected and addressed. The LHC collimator controls system allows a synchronization to other accelerator systems like magnets and radio-frequency to the
micro-second level. Motors are used in a dedicated tool to align in parallel several
collimators, at frequencies up to 50 Hz, in a feedback loop that uses 100 Hz beam
loss measurements [251, 252]. The recent addition of orbit measurements [242]
improves further the on-line diagnostic capabilities, allowing an earlier detection of
possibly critical shifts of the local beam orbit.
8.8.3.2 Mechanical Design, Cooling and Vacuum
The demanding requirements in modern high power accelerators were already
shortly reviewed. These requirements translate into important constraints for the
design of collimators. We summarize some of them:
– The design of mechanical movement must be precise, robust and reproducible.
The lifetime of the collimator movement system should be a few 10,000 cycles
for slowly ramped accelerators (like LHC) up to a few million cycles for
rapidly cycled accelerators. The tolerances for mechanical plays and setting
reproducibility can be as small as a few 10 μm.
– The use of traditional grease is often not permitted due to the presence of high
beam losses and radiation. Grease could age and become sticky. Instead, advanced
techniques like dry grease (graphite powder) or surface coatings must be used.
– Thermal heat flow must be carefully designed, from the jaw where heat is
deposited to the cooling pipes that take it out. Heat loads from direct particle
losses and electro-magnetic currents must be taken into account (W to 100 kW).
Thermal expansion coefficients of materials must be well adapted to ensure
minimal deformations during power gradients (few 10 μm for several kW) and
good thermal contact for heat transfer [253].
– The impacting power loads often require powerful cooling of the jaw materials.
High-pressure water flow (20 bar) must be integrated into the jaw design.
Sometimes, it is required to also cool the vacuum tank and flanges.
– Safety and robustness against beam shock impacts (accidents) often requires
that cooling pipes do not have brazed parts under vacuum.
– Many accelerators require ultra-high vacuum (~10 -8 mbar) and the collimators
must not disturb this. As heated materials show outgassing, additional constraints
on maximum jaw temperatures and cooling must be respected.
– Intense beams induce electro-magnetic image currents on the surrounding materials: machine impedance or wakefields are induced [254–256]. As collimator
materials are very close to the beam, their electro-magnetic properties must
be well designed. Critical are good electrical resistivity (~μm) and smooth
geometrical shapes. Tapering of jaws is used to avoid sharp edges close to the
beam. RF fingers need to be used to guide image currents.
– The high irradiation at collimators [257] often requires special measures.
Possibilities include quick plug-ins for cables and water pipes, absence of