STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

and using the bunch length derived in the 20 MeV calculation and assuming an RF
phase error of 0.03 ◦ gives 0.01 × 10 −3 which agrees with the measured data.
2.7 Response of the RF System
In addition to characterizing the beam properties, the RF system’s response
to energy recovery can be measured. A typical measurement is shown in Fig. 2.19
which illustrates the RF system gradient modulator drive signal during pulsed beam
operation for cavity 7 in the 2L02 region in the south linac. This signal, locally called
the GASK, is part of the low-level RF control system used to maintain the amplitude
and phase of the cavity fields. Without energy recovery the signal is nonzero when a
250 µs long beam pulse enters the RF cavity, indicating that power is drawn. With
energy recovery, the signal is zero once the initial transient passage of the leading
edge of the pulse is over, thereby showing that no additional power draw is required
by the cavity.
The macropulse draws power on the first pass through the cavity since the RF
system does not see the effects of energy recovery until the beam arrives on the
second pass 180 ◦ out of phase. The inset of Fig. 2.19 shows that power is drawn for
a time of 4.3 µs which corresponds to the recirculation time for one pass through
CEBAF.
As an aside, the GASK signals have a practical function at the FEL Upgrade
Driver where they are routinely used during operations as a diagnostic to properly
energy recover the machine. Changing the path length until the GASK signal is
zero ensures the two beams are 180 ◦ out of phase.
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