The Accelerator

The initial concept for the 4 GeV accelerator at Jefferson Lab was a pulsed conventional linac with a ring to stretch the short beam pulses it produced into an essentially continuous beam. However, in 1985, shortly after the laboratory began to take shape in Newport News, newly arrived director Hermann A. Grunder conducted a reassessment of available accelerator technologies. Superconducting radio-frequency (SRF) technology was determined to have matured sufficiently for large-scale application. By February 1986 a recirculating SRF design was approved, and by February 1987--when the first PAC meeting convened in Newport News -- accelerator construction was underway.

A schematic layout of the accelerator is shown in Figure 5. Two anti-parallel 400 MeV superconducting linacs (see Figure 6), connected in a "racetrack" configuration by recirculation arcs, accelerate the electrons in five passes to a final energy of 4 GeV. Each linac consists of twenty cryomodules, and each cryomodule contains four pairs of five-cell, 1497 MHz niobium accelerating cavities of a design originally developed at Cornell University. The cavities, shown in the inset in Figure 6 operate at 2 K, with cooling provided by a 4.5 kW plant that represents over half the world's superfluid helium refrigeration capacity. Two and one-quarter additional cryomodules are used in the 45 MeV injector linac, which can deliver unpolarized electrons from a conventional thermionic gun or polarized electrons from a laser-driven photocathode source.

The overall operational complexity of the accelerator is on the scale of CERN's Large Electron-Positron Collider. The Experimental Physics and Industrial Control System (EPICS), developed in a multi-laboratory collaboration, handles some 130,000 control records. With nine beam lines in two recirculation arcs (see Figure 7), the accelerator's magnet count exceeds 2200.

Typical experiments require months of beam time due to the small electromagnetic interaction cross sections. To enhance the productivity of the facility, the accelerator design incorporates RF separators as extraction elements in the second recirculation arc, providing the capability for delivering three simultaneous 499 MHz, cw electron beams of independent current and independent but correlated energy (integer multiples of the energy gain per pass through the two linacs), permitting three experiments to run in parallel. This capability was first demonstrated in August 1996, with the first two-beam split providing electrons simultaneously for both a Hall C experiment and Hall A commissioning.

The installed SRF cavities exceed their specifications. The accelerator has already been run at 1 GeV for one pass (25% above the nominal gradient), and the average Q of the cavities is roughly double the design value, providing "headroom" for higher-gradient operation. Minor upgrades to the beam transport system and refinements in the RF control system are expected to provide beam energies between 5 and 5.5 GeV by the end of 1997. Later, the accelerator can be upgraded simply and cost-effectively to support operation at beam energies in the 8-10 GeV range.