8 Accelerator Engineering and Technology: Accelerator Technology
489
It can be seen that long collimator jaws often seem beneficial. It is, however,
important to take into account the edge feature of beam collimation. Most particles
will impact close to the jaw edge. The typical impact parameters on primary
collimators, for a circular machine like the LHC, can range from 100 nm to a
few μm. Surface roughness and MCS will result in many particles exiting from
the jaw before its full length has been traversed. It is seen that longer collimators
jaws are often not useful for interception of primary beam particles. In addition,
longer jaws also require a better design to control static and dynamic deformations
to the required accuracy.
The optimum choices of collimator jaw length and material must be based
on simulations with special programs [258–263, 271]. These programs take into
account the beam properties (beam type, energy, impact parameters), the accelerator type (single-pass or multiple-pass) and the material properties (geometry,
nuclear properties). Results from beam simulations are then put into programs for
calculating energy deposition [264]. Figure 8.87 shows a FLUKA energy deposition
calculation for 1m long carbon block.
A few common directions in collimator design are described:
– Short, high-Z collimators (for example W) are often used as primary scatterers,
which increase particle amplitudes via MCS, while still limiting the energy
deposited on the jaws. Length can range from a few 100 μm to a few cm.
– Long, high Z collimators (for example W) are often used as absorbers, which
effectively stop the particles that hit with large impact parameters. Length can
range from 10’s of cm to several meters.
– Long, low Z collimators (for example C) are used as primary scatterers when
energy deposition with short, high Z collimators would be too high for material
survival. The use of special materials (for example fiber-reinforced graphite CFC)
can maximize the robustness. Such solutions are important for high power beams.
Length can range from 10’s of cm to several meters.
It is noted that research on modern composite materials is ongoing [249, 272]
and can provide new directions and solutions in the future. A key aspect for this
development is to take into account effects related to specific aspect of materials’
response to beam losses [273] while optimizing the machine impedance. Reviews
of recent developments that are already being implemented for the upgrade of the
LHC can be found in [236]. Promising results were achieved recently that identified
valid solutions for a new generation of secondary collimators for the HL-LHC made
of a novel Molybdenum-Graphite composite [274], possibly Mo-coated, as well as
for metallic composites suitable for tertiary collimators, but about 15 times more
robust against beam impact than the tungsten alloy used presently at the LHC [275,
276] (Fig. 8.90).
489
It can be seen that long collimator jaws often seem beneficial. It is, however,
important to take into account the edge feature of beam collimation. Most particles
will impact close to the jaw edge. The typical impact parameters on primary
collimators, for a circular machine like the LHC, can range from 100 nm to a
few μm. Surface roughness and MCS will result in many particles exiting from
the jaw before its full length has been traversed. It is seen that longer collimators
jaws are often not useful for interception of primary beam particles. In addition,
longer jaws also require a better design to control static and dynamic deformations
to the required accuracy.
The optimum choices of collimator jaw length and material must be based
on simulations with special programs [258–263, 271]. These programs take into
account the beam properties (beam type, energy, impact parameters), the accelerator type (single-pass or multiple-pass) and the material properties (geometry,
nuclear properties). Results from beam simulations are then put into programs for
calculating energy deposition [264]. Figure 8.87 shows a FLUKA energy deposition
calculation for 1m long carbon block.
A few common directions in collimator design are described:
– Short, high-Z collimators (for example W) are often used as primary scatterers,
which increase particle amplitudes via MCS, while still limiting the energy
deposited on the jaws. Length can range from a few 100 μm to a few cm.
– Long, high Z collimators (for example W) are often used as absorbers, which
effectively stop the particles that hit with large impact parameters. Length can
range from 10’s of cm to several meters.
– Long, low Z collimators (for example C) are used as primary scatterers when
energy deposition with short, high Z collimators would be too high for material
survival. The use of special materials (for example fiber-reinforced graphite CFC)
can maximize the robustness. Such solutions are important for high power beams.
Length can range from 10’s of cm to several meters.
It is noted that research on modern composite materials is ongoing [249, 272]
and can provide new directions and solutions in the future. A key aspect for this
development is to take into account effects related to specific aspect of materials’
response to beam losses [273] while optimizing the machine impedance. Reviews
of recent developments that are already being implemented for the upgrade of the
LHC can be found in [236]. Promising results were achieved recently that identified
valid solutions for a new generation of secondary collimators for the HL-LHC made
of a novel Molybdenum-Graphite composite [274], possibly Mo-coated, as well as
for metallic composites suitable for tertiary collimators, but about 15 times more
robust against beam impact than the tungsten alloy used presently at the LHC [275,
276] (Fig. 8.90).