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In particle physics, a beamline is the line in a linear accelerator along which a beam of particles travels. It may also refer to the line of travel within a bending section such as a storage ring or cyclotron, or an external beam extracted from a cyclic accelerator.

In materials science, physics, chemistry, and molecular biology a beamline leads to the experimental endstation utilizing particle beams from a particle accelerator, synchrotron light obtained from a synchrotron, or neutrons from a spallation source or research reactor.

Beamline in a particle accelerator
It is impossible to see the beam pipe on this beamline. However the section of the big beam pipe is used with a grid system for alignment with a laser, known as the laser pipe. This particular beamline is approximately 3 kilometers long.

In particle accelerators the beamline is usually housed in a tunnel and/or underground, cased inside a cement housing. The beamline is usually cylindrical metal. Typical names 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 large mean free path for the beam.

There are specialized devices and equipment on the beamline that is used for producing, maintaining, monitoring, and accelerating the particle beam. These devices may be in proximity or attached to the beamline. These devices include sophisticated transducers, diagnostics (position monitors and wire scanners), lenses, collimators, thermocouples, ion pumps, ion gauges, ion chambers (sometimes called "beam loss monitors"), vacuum valves ("isolation valves"), and gate valves, to mention a few. There are also water cooling devices 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.

It is imperative 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.
[edit] Synchrotron radiation beamline
The exposed workings of a soft x-ray beamline and endstation at the Australian Synchrotron
Inside the Optical Diagnostic Beamline (ODB) hutch at the Australian Synchrotron; the beamline terminates at the small aperture in the back wall

Regarding synchrotrons, beamline may also refer to the instrumentation that carries beams of synchrotron radiation to an experimental end station, which uses the radiation produced by the bending magnets and insertion devices in the storage ring of a synchrotron radiation facility. A typical application for this kind of beamline is crystallography, although many other techniques utilising synchrotron light 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 funding permits.

The beamline elements are located in radiation shielding enclosures, called hutches, which are the size of a small room (cabin). A typical beamline consists of two hutches, an optical hutch for the beam conditioning elements and an experimental hutch, which houses the experiment. Between hutches, the beam travels in a transport tube. Entrance to the hutches is forbidden when the beam shutter is open and radiation can enter the hutch. This is enforced by the use of elaborate safety systems with redundant interlocking functions, which make sure that no one is inside the hutch when the radiation is turned on. The safety system will also shut down the radiation beam if the door to the hutch is accidentally opened when the beam is on. In this case, the beam is turned off by dumping the electron beam circulating in the synchrotron, which means that all of the beamlines in the facility are shut down.

Elements that are used in beamlines by experimenters for conditioning the radiation beam 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 maintaining 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 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.