  As compared to electron storage rings, one advantage of recirculating linear
accelerators is that the beam properties at target are no longer dominated by
the equilibrium between quantum radiative diffusion and radiation damping
because new beam is continually injected into the accelerator. This allows the
energy spread from a CEBAF-type machine to be relatively small; the measured
energy spread from CEBAF at 4 GeV is less than 100 parts per million
accumulated over times of order several days. In this paper, the various
subsystems contributing to the energy spread of a CEBAF-type accelerator are
reviewed, as well as the machine diagnostics and controls that are used in
CEBAF to ensure that a small energy spread is provided during routine running.
Examples of relevant developments are (1) stable short bunches emerging from
the injector, (2) precision timing and phasing of the linacs with respect to
the centroid of the beam bunches on all passes, (3) implementing 2 kHz sampling
rate feedback systems for final energy stabilization, and (4) continuous beam
energy spread monitoring with optical transition radiation devices. We present
measurement results showing that small energy spreads are achieved over
extended periods.
