STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

FIG. 5.1: Successive frames in time (progressing from left to right) from a movie of the
synchrotron light monitor in the second endloop at the onset of BBU.
5.1 Overview
While the remainder of this chapter is dedicated to describing the details of
the quantitative measurements, Fig. 5.1 illustrates a qualitative characterization of
BBU. Figure 5.1 shows a series of frames from a recording of a synchrotron light
monitor located in the second recirculation arc of the FEL Driver. During the
recording, the average beam current was being slowly increased until beam breakup
developed. The instability clearly manifests itself as vertical growth which continues
until beam losses become large enough to trip the machine off. The time elapsed
from the first to the last frame of Fig. 5.1 is approximately 0.25 s.
From the point of view of a machine operator in the control room, the only
indication that the operating current is approaching the threshold current are obser-
vations of the SLM image growing as depicted in Fig. 5.1. Additional characteristics
of the presence of BBU - from an operator’s perspective - are single, hard machine
trips at one particular beam loss monitor (BLM) location. In most instances, these
BLM trips occurred in the 5F region of the Driver where the recirculated beam is
re-injected through the linac and the beam envelopes are largest.
Clearly a more quantitative method is needed to confirm that a machine trip
is due to BBU. Additional measurements are required to ascertain which cavity
contains the dangerous HOM and to determine the frequency of the mode. Fur-
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thermore, adequately benchmarking BBU codes requires accurately measuring the