8 Accelerator Engineering and Technology: Accelerator Technology
433
the vacuum system (complex shapes, more pumps, retrofitted modifications, etc.)
and even into performance limitations. Experience has shown that the vacuum engineering shall proceed in parallel on the following topics: expertise provided to beamrelated components (magnets, beam instrumentation, radio-frequency systems, etc.),
engineering of vacuum related components (beampipes, bellows, pumping ports,
etc.) and machine integration including cabling and services.
8.5.4.1 Vacuum Pumping
The pumping scheme is often decided at the early stages of the design since it affects
the overall engineering of the accelerator. Indeed, the pumping scheme will define
the reserved space for the pumping and in case of integration problems will lead to
alternative solutions to preserve the expected performances or to compromise. The
number and types of pumps will allow reaching the design operating pressure which
is linked to the beam lifetime, a parameter in relation with the beam-gas scattering.
As the pumping speeds vary between a few litre-per-seconds to thousands of
litre-per-seconds while the outgassing rates of materials in the vacuum system vary
by up to 8 orders of magnitude, an appropriate selection of material and design is
required to achieve the specified vacuum performances. Said differently, increasing
the installed pumping speed in an accelerator by a factor 2 is already challenging
and costly, whereas losing several orders of magnitude in outgassing rates could
easily result from an inappropriate use of materials or bad vacuum engineering or
cleaning of the materials.
8.5.4.1.1 Discrete Pumping
In particle accelerators, discrete pumping is the most commonly used solution and
is often obtained by ion pumps combined with sublimation or non-evaporable getter
(NEG) cartridge pumps, cryogenic and turbomolecular pumps.
Ion pumps [111] are widely used since they are very reliable and provide a
large pumping speed, up to 800 l/s. In addition, the discharge current of an ion
pump is directly proportional to the pressure, and thereby provides an indication of
pressure for a limited cost. This signal is often used to trigger vacuum interlocks for
machine protection. Ion pumps have also the advantage of pumping all gas species
and once baked-out at 250–300 ◦ C, the ultimate pressure is in the low 10 −10 Pa.
Special attention has to be taken while pumping noble gasses and water. Above a
given quantity of gas pumped, part of the pumped noble gasses will be released as
hydrogen (via dissociation of the water molecules) when pumping water.
Sublimation pumps [112] are often used to speed up the pump down to the UHV
pressure range or as a complement to ion pumps at very low pressures (<10 −10 Pa).
If used at high pressures, the pump’s bodies must be water cooled to prevent the
heating induced by the frequent activations (several per minute). Similarly to NEG
433
the vacuum system (complex shapes, more pumps, retrofitted modifications, etc.)
and even into performance limitations. Experience has shown that the vacuum engineering shall proceed in parallel on the following topics: expertise provided to beamrelated components (magnets, beam instrumentation, radio-frequency systems, etc.),
engineering of vacuum related components (beampipes, bellows, pumping ports,
etc.) and machine integration including cabling and services.
8.5.4.1 Vacuum Pumping
The pumping scheme is often decided at the early stages of the design since it affects
the overall engineering of the accelerator. Indeed, the pumping scheme will define
the reserved space for the pumping and in case of integration problems will lead to
alternative solutions to preserve the expected performances or to compromise. The
number and types of pumps will allow reaching the design operating pressure which
is linked to the beam lifetime, a parameter in relation with the beam-gas scattering.
As the pumping speeds vary between a few litre-per-seconds to thousands of
litre-per-seconds while the outgassing rates of materials in the vacuum system vary
by up to 8 orders of magnitude, an appropriate selection of material and design is
required to achieve the specified vacuum performances. Said differently, increasing
the installed pumping speed in an accelerator by a factor 2 is already challenging
and costly, whereas losing several orders of magnitude in outgassing rates could
easily result from an inappropriate use of materials or bad vacuum engineering or
cleaning of the materials.
8.5.4.1.1 Discrete Pumping
In particle accelerators, discrete pumping is the most commonly used solution and
is often obtained by ion pumps combined with sublimation or non-evaporable getter
(NEG) cartridge pumps, cryogenic and turbomolecular pumps.
Ion pumps [111] are widely used since they are very reliable and provide a
large pumping speed, up to 800 l/s. In addition, the discharge current of an ion
pump is directly proportional to the pressure, and thereby provides an indication of
pressure for a limited cost. This signal is often used to trigger vacuum interlocks for
machine protection. Ion pumps have also the advantage of pumping all gas species
and once baked-out at 250–300 ◦ C, the ultimate pressure is in the low 10 −10 Pa.
Special attention has to be taken while pumping noble gasses and water. Above a
given quantity of gas pumped, part of the pumped noble gasses will be released as
hydrogen (via dissociation of the water molecules) when pumping water.
Sublimation pumps [112] are often used to speed up the pump down to the UHV
pressure range or as a complement to ion pumps at very low pressures (<10 −10 Pa).
If used at high pressures, the pump’s bodies must be water cooled to prevent the
heating induced by the frequent activations (several per minute). Similarly to NEG