8 Accelerator Engineering and Technology: Accelerator Technology
421
coulomb scattering), intensity limitation by pressure instabilities (ionization) and,
for positive beams only, electron (ionization) induced instabilities, e.g. beam blow
up.
Beam-gas scattering can also increase the background to the detectors in the
experimental areas (non-captured particles or nuclear cascade generated by the lost
particles upstream the detectors) and the radiation dose rates in the accelerator
tunnels. Thus leading to material activation, dose rates to intervention crews,
premature degradation of tunnel infrastructures like cables and electronics and
finally higher probability of electronic single events induced by neutrons which can
destroy the electronics in the tunnel but also in the service galleries.
In addition, the design of an accelerator vacuum system must observe severe
additional constraints which have to be considered at the design stage since
retrofitting mitigation solutions is often impossible or very expensive. Among
them, the vacuum system has to be designed to minimise beam impedance and
radiofrequency higher-order-modes (HOM) generation as well as to optimise the
beam aperture in particular in the magnets. It also must provide enough ports for the
pumps and for the vacuum diagnostics and allow for bake-outs in order to achieve
Ultra-High Vacuum (UHV) pressures (<10 −8 Pa). The impact of other constraints
like integration, safety (material and personnel), operational issues (conditioning of
RF and HV devices) and costs often lead to a compromise in performances of all
systems of an accelerator. This explains why these issues must be addressed at the
design stage [91].
For accelerators operating at cryogenic temperatures [92], the heat load induced
by scattered beam particles and synchrotron radiations can also be an issue for the
cryo-magnets since local heat loads can lead to a magnet quench i.e. a transition
from the superconducting to the normal state. The heavy gases are the most
dangerous because of their higher ionisation cross-sections. Thus, the beam-pipes
shall be designed to intercept heat loads induced by synchrotron radiation, energy
loss by nuclear scattering, image currents, energy dissipated during the development
of electron clouds. In the LHC, these constraints required, for the first time in a
particle accelerator, the use of a beam screen [93]. Increasing further the luminosity
of a storage ring operating at cryogenic temperature, such as in the High-Luminosity
LHC (HL-LHC), the very large production of collision debris must be intercepted
at the level of the beam screen by heavy material, e.g. tungsten. This shielding is
designed to offer a protection of the superconducting coil against radiation induced
ageing and extract the beam induced heat load at an elevated temperature, e.g. 60 K,
to optimise the Carnot efficiency [a].
8.5.2 Vacuum Fundamentals
Vacuum is defined as the absence of matter or a space empty of matter. These
are idealistic definitions since a perfect vacuum is a theoretical limit. Realistically,
whenever a pressure in an enclosed space is less than the pressure of the surrounding
421
coulomb scattering), intensity limitation by pressure instabilities (ionization) and,
for positive beams only, electron (ionization) induced instabilities, e.g. beam blow
up.
Beam-gas scattering can also increase the background to the detectors in the
experimental areas (non-captured particles or nuclear cascade generated by the lost
particles upstream the detectors) and the radiation dose rates in the accelerator
tunnels. Thus leading to material activation, dose rates to intervention crews,
premature degradation of tunnel infrastructures like cables and electronics and
finally higher probability of electronic single events induced by neutrons which can
destroy the electronics in the tunnel but also in the service galleries.
In addition, the design of an accelerator vacuum system must observe severe
additional constraints which have to be considered at the design stage since
retrofitting mitigation solutions is often impossible or very expensive. Among
them, the vacuum system has to be designed to minimise beam impedance and
radiofrequency higher-order-modes (HOM) generation as well as to optimise the
beam aperture in particular in the magnets. It also must provide enough ports for the
pumps and for the vacuum diagnostics and allow for bake-outs in order to achieve
Ultra-High Vacuum (UHV) pressures (<10 −8 Pa). The impact of other constraints
like integration, safety (material and personnel), operational issues (conditioning of
RF and HV devices) and costs often lead to a compromise in performances of all
systems of an accelerator. This explains why these issues must be addressed at the
design stage [91].
For accelerators operating at cryogenic temperatures [92], the heat load induced
by scattered beam particles and synchrotron radiations can also be an issue for the
cryo-magnets since local heat loads can lead to a magnet quench i.e. a transition
from the superconducting to the normal state. The heavy gases are the most
dangerous because of their higher ionisation cross-sections. Thus, the beam-pipes
shall be designed to intercept heat loads induced by synchrotron radiation, energy
loss by nuclear scattering, image currents, energy dissipated during the development
of electron clouds. In the LHC, these constraints required, for the first time in a
particle accelerator, the use of a beam screen [93]. Increasing further the luminosity
of a storage ring operating at cryogenic temperature, such as in the High-Luminosity
LHC (HL-LHC), the very large production of collision debris must be intercepted
at the level of the beam screen by heavy material, e.g. tungsten. This shielding is
designed to offer a protection of the superconducting coil against radiation induced
ageing and extract the beam induced heat load at an elevated temperature, e.g. 60 K,
to optimise the Carnot efficiency [a].
8.5.2 Vacuum Fundamentals
Vacuum is defined as the absence of matter or a space empty of matter. These
are idealistic definitions since a perfect vacuum is a theoretical limit. Realistically,
whenever a pressure in an enclosed space is less than the pressure of the surrounding