EXTRACTION SYSTEMS

Beam injection and extraction are critical techniques in circular accelerators. Special conditions must be created to inject and extract the beam. An extracted beam may be transferred to a subsequent accelerator, or it may be directed toward a target.

Some experiments do not require beam extraction, most notably when internal targets are used. An internal target may be a puff of gas or a solid material that is moved into the path of the beam. Perhaps the most important type of internal target, however, is a beam traveling in the opposite direction. This configuration is known as colliding beams and is used extensively for high-energy particle experiments.

Single-Turn Extraction

For single-turn extraction, a special magnet, known as a kicker magnet, is switched on to steer the beam away from its normal orbit. The switching must be almost instantaneous; any beam that passes through the kicker before the full field is obtained tends to be lost because it is not bent far enough. The fast-rise time requirement limits the bending that can be obtained, and kickers are frequently paired with a special type of magnet known as the Lambertson magnet (named for the inventor Glen Lambertson). This magnet does not need to be switched. It has a high magnetic field in one region, which is separated by a thin wall of iron (known as a septum) from a field-free region. The beam usually circulates in the field-free region, and the kicker displaces the beam across the septum to be bent further by the strong magnetic field. Single-turn extraction is used almost universally for transferring beams between machines.

Slow Extraction

Single-turn extraction can be used for directing the beam toward an external target, but the duration of the beam pulse is equal to the time that it takes the beam to make a single turn around the accelerator, typically 1 to 10 μ sec for high-energy accelerators. The resulting rate of interactions would generally be too high for effective experimentation. An alternative extraction technique, known as slow extraction, can extend the duration of the extracted beam pulse from milliseconds to many seconds.

During normal accelerator operation, particles oscillate around a nominal closed orbit. When the number of oscillations per turn (the tune) is such that the particle motion repeats after a small number of turns, certain small deviations of the magnetic fields from their nominal values have large cumulative effects after many turns. This condition is known as a "resonance," and particle oscillations may not be stable, that is, they may grow in time. Resonances are usually undesirable but can be controlled and utilized to extract the beam slowly.

Extraction systems have been built using both the 1/2 and 1/3 resonances. The particle motion repeats every two or three turns, respectively, for these resonances. For the 1/2 resonance, quadrupole magnets are introduced deliberately to create deviations from the nominal magnetic field; the 1/3 resonance requires sextupole magnets. A beam may be extracted by changing the accelerator tune toward the resonance. Typically particles with large oscillation amplitudes become unstable first (when the accelerator tune is relatively far from the resonance); particles with smaller oscillation amplitudes become unstable as the tune approaches the resonant value. The rate of change of tune controls the rate at which particle oscillations become unstable and therefore controls the rate of extraction.

If no special measures were taken, the particles with growing amplitudes would eventually run into the accelerator walls and be lost. They are extracted in a controlled way by placing two parallel plates with an electric field between them in the accelerator beam chamber. The growing particle amplitude eventually crosses into the region between the plates and is deflected by the electric field. The plate nearest the circulating beam is known as a septum and is invariably made of a grid of wires to minimize the mass and hence the number of beam particles that interact in the septum. The wire septum performs for slow extraction the same function that the kicker performs for single-turn extraction. A Lambertson magnet is normally used as a second bending device in slow extraction systems.

See also:Accelerator; Accelerators, Colliding Beam: Electron-Positron; Accelerators, Colliding Beam: Electron-Proton; Accelerators, Colliding Beam:Hadron; Accelerators, Early; Accelerators, Fixed-Target: Electron; Accelerators: Fixed-Target: Proton; Beam Transport; Injector System

Bibliography

Edwards, D. A., and Syphers, M. J. An Introduction to the Physics of High Energy Accelerators (Wiley, New York, 1992).

John Marriner