STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

upon re-entry into the north linac, the beam was 180 ◦ out of phase with respect to
the accelerating RF waveform and was decelerated to 556 MeV. After traversing
arc 1 a second time the beam entered the south linac - still out of phase with the
RF accelerating field - and was decelerated to 56 MeV at which point the energy
recovered beam was deflected to a dump. Upon configuring the machine for energy
recovery, measurements were performed to characterize the beam phase space at
various points in the machine. These will be discussed in detail in Sections 2.4, 2.5
and 2.6. Once satisfactory measurements were obtained using the 56 MeV injection
energy, the measurements were repeated for a lower injection energy of 20 MeV to
study the parametric dependence on high maximum-to-injection energy ratios.
While the modifications required to transform CEBAF into an ERL-based ac-
celerator are relatively minor, the fact that CEBAF was not originally designed
with the intention of performing energy recovery presents challenges. The following
sections will discuss some of those issues and challenges.
2.2.1 Phase Delay Chicane and Beam Dump
The purpose of the phase delay chicane is to provide a path length differential
such that the beam enters the north linac on the second pass 180 ◦ out of phase
with respect to the accelerating RF field. The wavelength corresponding to the
fundamental frequency of 1497 MHz is given by
λRF = c
fRF
25
= 0.2 m (2.1)
where c is the speed of light in vacuum. For energy recovery a path length differential
of λRF /2 = 10 cm is required. The phase delay chicane consists of four dipole
magnets and is achromatic, which means that the beam transport does not depend
on beam momentum. Because the chicane is installed in a non-dispersive region, it