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Accelerator Operations Orientation

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An Accelerator Overview

The CEBAF accelerator is a superconducting radio frequency (RF) electron accelerator that was commissioned during the early 1990s and produced the first experimental beam in October of 1994.

Date Milestone
1976 Facility envisioned and requested by physicists as a necessary tool to answer emerging questions about the quark structure of matter
1980 Initial design developed
1983 The SURA proposal was selected by the Department of Energy after competition with Massachusetts Institute of Technology, University of Illinois, Argonne National Laboratory, and the National Bureau of Standards
1984 The Newport News site was selected and the initial federal funding was received for research, development and design
1985 Superconducting electron accelerating technology adopted
1987 Accelerator construction underway
1990 Prototype injector installed in tunnel
1990-1993   Accelerator commissioning with beam
1994 First physics experiments started
1995 Design energy of 4 GeV reached
1997 Simultaneous 5-pass, 4-GeV, 3-beam separation to all three Halls
1999 Machine energy at 5.5 GeV with 3-hall operation

The accelerator uses a state-of-the-art photocathode gun system that is capable of delivering beams of high polarization and high current to Hall A and Hall C while maintaining high polarization low current beam delivery to Hall B. An RF chopping system operating at 499 MHz is used to develop a 3-beam 1497 MHz bunch train at 100 keV. The beam is then longitudinally compressed in the bunching section to provide 2 picosecond bunches, which are then accelerated to just over 1% of the total machine energy in the remaining injector section. The beam polarization, optics and energy are verified in the injector matching region prior to injection into the main machine. The beam from the injector is accelerated through a unique recirculating beamline that looks something like a "racetrack", with two linear accelerators joined by two 180° arcs with a radius of 80 meters. Twenty cryomodules, each containing eight superconducting niobium cavities, line the two linear accelerators. Liquid helium, produced at the Lab's Central Helium Liquefier (CHL), keeps the accelerating cavities superconducting at a temperature of 2 Kelvin. The linac energies are each set identically and the RF cavities are phased to provide maximum acceleration. Subsequent passes through the accelerator are phased to maximum energy gain by adjusting the length of travel in the dogleg section of the preceding arc. Quadrupole and dipole magnets in the tunnel steer and focus the beam as it passes through each arc. More than 2,200 magnets are necessary to keep the beam on a precise path and tightly focused. Beam is directed into a hall's transport channel using magnetic or RF extraction. The RF scheme uses 499 MHz cavities, which kick every third bunch out of the machine. The accelerator can deliver the first four passes to one hall only. The fifth pass can be sent to all three halls simultaneously.