8 Accelerator Engineering and Technology: Accelerator Technology
451
of the spectrum and X-ray detectors can be used [148], for which diffraction effects
become less important.
8.6.5 Beam Loss Monitoring
Beam loss monitors (BLMs) have three main uses in particle accelerators: Damage
prevention, diagnostics and machine optimisation [149]. The job of the BLM system
is to establish the number of lost particles at a certain position within a specified time
interval. Most BLM systems are mounted outside the vacuum chamber, so that the
detector observes the shower caused by the lost particles interacting in the vacuum
chamber walls or in the materials of the magnets. The number of detected particles
and the signal from the BLM should be proportional to the number of lost particles.
This proportionality depends on the position of the BLM with respect to the beam,
the type of lost particles and the intervening material. It also, however, depends
on the momentum of the lost particles, which may vary by a large amount during
the acceleration cycle. One has to distinguish between two types of losses; fast
losses, where a large amount of beam is lost in a short time (for circular machines)
or distance (for LINACs and transport lines) and slow losses, where partial beam
loss occurs over some time or distance. In storage-rings, the lifetime is defined by
slow losses. Superconducting accelerators use the BLM system to prevent beam
loss induced quenches. The fact that BLM systems have to cover both of these
cases means that they are often required to function over a very large dynamic
range, typically in the region of 10 4 to 10 6 . BLMs can be classed into two main
families: global—containing a few long detectors covering most of the accelerator;
distributed—with many units of relatively small active area.
8.6.5.1 Global BLM Systems
The first global system was proposed by Panowsky for SLAC in 1963 [150] and
was composed of a 3.5 km hollow coaxial cable filled with Ar (95%) + CO 2 (5%),
mounted on the ceiling of the LINAC, about 2 m from the beam. When a beam
loss occurs the secondary particles produced traverse the monitor and induce an
electrical signal which propagates to both ends of the cable. Position sensitivity is
achieved by comparing the time delay between the direct pulse from one end and the
reflected pulse from the other. The time resolution achievable with such systems is
about 30 ns (~8 m), which can be reduced to about 5 ns by using shorter cables. This
principle of space resolution works for relativistic linear accelerators and transport
lines with a bunch train much shorter than the machine. For particles travelling
significantly slower than the signal in the cable the resolution of multiple hits in the
cable becomes difficult. In this case, and for circular machines, it is necessary to split
the cable. Each segment has to be read out separately, with a spatial resolution which
becomes approximately equal to their length. A similar measurement can these days
451
of the spectrum and X-ray detectors can be used [148], for which diffraction effects
become less important.
8.6.5 Beam Loss Monitoring
Beam loss monitors (BLMs) have three main uses in particle accelerators: Damage
prevention, diagnostics and machine optimisation [149]. The job of the BLM system
is to establish the number of lost particles at a certain position within a specified time
interval. Most BLM systems are mounted outside the vacuum chamber, so that the
detector observes the shower caused by the lost particles interacting in the vacuum
chamber walls or in the materials of the magnets. The number of detected particles
and the signal from the BLM should be proportional to the number of lost particles.
This proportionality depends on the position of the BLM with respect to the beam,
the type of lost particles and the intervening material. It also, however, depends
on the momentum of the lost particles, which may vary by a large amount during
the acceleration cycle. One has to distinguish between two types of losses; fast
losses, where a large amount of beam is lost in a short time (for circular machines)
or distance (for LINACs and transport lines) and slow losses, where partial beam
loss occurs over some time or distance. In storage-rings, the lifetime is defined by
slow losses. Superconducting accelerators use the BLM system to prevent beam
loss induced quenches. The fact that BLM systems have to cover both of these
cases means that they are often required to function over a very large dynamic
range, typically in the region of 10 4 to 10 6 . BLMs can be classed into two main
families: global—containing a few long detectors covering most of the accelerator;
distributed—with many units of relatively small active area.
8.6.5.1 Global BLM Systems
The first global system was proposed by Panowsky for SLAC in 1963 [150] and
was composed of a 3.5 km hollow coaxial cable filled with Ar (95%) + CO 2 (5%),
mounted on the ceiling of the LINAC, about 2 m from the beam. When a beam
loss occurs the secondary particles produced traverse the monitor and induce an
electrical signal which propagates to both ends of the cable. Position sensitivity is
achieved by comparing the time delay between the direct pulse from one end and the
reflected pulse from the other. The time resolution achievable with such systems is
about 30 ns (~8 m), which can be reduced to about 5 ns by using shorter cables. This
principle of space resolution works for relativistic linear accelerators and transport
lines with a bunch train much shorter than the machine. For particles travelling
significantly slower than the signal in the cable the resolution of multiple hits in the
cable becomes difficult. In this case, and for circular machines, it is necessary to split
the cable. Each segment has to be read out separately, with a spatial resolution which
becomes approximately equal to their length. A similar measurement can these days