Beamline

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In particle physics, a beamline is that line in a linear accelerator along which a beam of particles travels through, or projected within a particle accelerator. It may also refer to the line of travel within a bending section such as a storage ring or cyclotron used in high energy physics.

In materials science, physics, chemistry and molecular biology a beamline is the experimental end station utilizing synchrotron light obtained from a synchrotron or neutrons obtained from a research reactor or a spallation source.

Beamline in a particle accelerator

In particle accelerators the beam line is that section usually housed in a tunnel and or underground, cased inside a cement housing. The beam line is usually cylindrical metal. Typical names may include, beam pipe, and or a blank section called a drift tube. This entire section must be under a good vacuum in order to have a mean free path for the beam to go through the beampipe, meaning no collisions, or the absence of atmosphere. The better the vacuum, the less chance of beam blowup.

There are specialized devices and equipment on the beamline that is used for maintaining and accelerating a particle beam. These devices may be in close proximity or attached to the beamline, in order to produce, monitor, and maintain the particle beam inside the beam pipe. These devices include sophisticated transducers, such as bending and focusing magnets; diagnostics such as beam position monitors (or BPMs) and wire scanners; lenses; collimators; thermocouples; ion pumps, ion gauges, and ion chambers (sometimes called "beam loss monitors"); vacuum valves ("isolation valves") and gate valves, to mention a few. There are also water cooling devices such as water valves, regulators, etc., to cool the dipole and quadrupole magnets. Positive pressure, such as that provided by compressed air, regulates and controls the vacuum valves and manipulators on the beamline. Image:Beam pipe.JPG

It is imperative and critical to have all beamline sections, magnets, etc, aligned by a survey and alignment crew by using a laser tracker. All beamlines must be within micrometre tolerance. Good alignment helps to prevent beam loss, and beam from colliding with the pipe walls, which creates secondary emissions and/or radiation.

Image:LINAC.jpg

More rarely than occasionally, except at the end of the accelerator, the beamline may have no beam pipe. The beam may go through a metal or glass window which acts as a point of exit to another girder section point of entry, while preserving and/ or isolating the internal vacuum. This section, or cavity is usually an air gap of no more than a few inches in length.

Synchrotron radiation beamline

Beamline may also refer to an end station which uses the synchrotron radiation produced by the bending magnets and insertion devices in the storage ring of a synchtrotron radiation facility. A typical application for this kind of beamline is crystallography, although many other techniques utilising synchrotron radiation exist.

At a large synchrotron facility there will be many beamlines, each optimised for a particular field of research. The differences will depend on the type of insertion device (which, in turn, determines the intensity and spectral distribution of the radiation); the beam conditioning equipment; and the experimental end station. A typical beamline at a modern synchrotron facility will be 25 to 100 m long from the storage ring to the end station, and may cost up to millions of US dollars. For this reason, a synchrotron facility is often built in stages, with the first few beamlines opening on day one of operation, and other beamlines being added later as the budget permits.

Elements that are used in beamlines by experimenters for conditioning beamlines between the storage ring and the end station include the following:

• Windows - thin sheets of metal, often Beryllium, which transmit almost all of the beam, but protect the vacuum within the storage ring from contamination

• Slits - which control the physical width of the beam and its angular spread

• Focusing mirrors - one or more mirrors, which may be flat, bent-flat, or toroidal, which helps to collimate (focus) the beam

• Monochromators - devices based on diffraction by crystals which select particular wavelength bands and absorb other wavelengths, and which are sometimes tunable to varying wavelengths, and sometimes fixed to a particular wavelength

• Spacing tubes - vacuum tubes which provide the proper space between optical elements, and shield any scattered radiation

• Sample stages - for mounting and manipulating the sample under study and subjecting it to various external conditions, such a varying temperature, pressure etc.

• Radiation detectors - for measuring the radiation which has interacted with the sample

The combination of beam conditioning devices controls the thermal load (heating caused by the beam) at the end station; the spectrum of radiation incident at the end station; and the focus or collimation of the beam. Devices along the beamline which absorb significant power from the beam may need to be actively cooled by water, or even by liquid nitrogen. The entire length of a beamline is normally kept under ultra high vacuum conditions.

Neutron beamline

An experimental end station in a neutron facility is called a neutron beamline. Superficially, neutron beamlines differ from synchrotron radiation beamlines mostly by the fact that they use neutrons from a research reactor or a spallation source instead of photons. The experiments usually measure neutron scattering from the sample under study.

See also

• Accelerator physics
• Cyclotron
• Ion beam
• Linear particle accelerator
• List of synchrotron radiation facilities
• Category:Neutron facilities
• Klystron
• Particle accelerator
• Particle beam
• Particle physics
• Quadrupole magnet
• Waveguide

External link

• Macromolecular Crystallography at Synchrotrons: An Historical Introduction