STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

FIG. 2.9: Images of the beam at various locations in the machine. Two co-propagating
beams in arc 1 (left), the south linac (middle) and the energy recovered beam image at
the dump (right).
vertical) corrector. While this effect presented operational challenges, it did not
limit machine performance.
The difficulties in steering the beam were compounded by the lack of functional
beam diagnostics in the energy recovery regime [36]. The beam viewers in the
linacs have holes to pass the primary beam so later passes can be seen as shown
in Fig. 2.9. Viewers in the arcs, however, were never intended to see multiple
beams and have no holes. They are only useful for looking at first pass beam.
Furthermore, standard arc BPMs were incapacitated during energy recovery due to
the destructive interference of the two out-of-phase beams. The only real-time, cw
diagnostics capable of measuring the energy recovered beam were the synchrotron
light monitor (SLM) at the midpoint of arc 1A and the dump beamline diagnostics.
These problems arose simply because the diagnostics for CEBAF were designed for
a different set of requirements, namely operating in a non-energy recovery regime.
Despite the operational difficulties, the beam was energy recovered and steered
to the dump. The following days were dedicated to making a number of measure-
ments which will be described in the following sections. After measurements were
completed using the 56 MeV injection energy, the injector was set to provide beam
at 20 MeV and the measurements were repeated.
35