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eams. The effects of varying the quadrupole strength on the energy recovered
beam, in terms of machine performance, is minimal because the beam is immedi-
ately deflected to the dump. The accelerated beam, however, also feels the effects
of the quadrupole change which creates a mismatch between the beam and the de-
sign machine optics. The consequences of this mismatch is twofold. First, since
the emittance measurement assumes that the beta functions prior to the scanning
quadrupole remain constant, if left uncorrected, the mismatched beam will recircu-
late through the machine and arrive at the quadrupole with different beta functions.
Second, for large excursions of the quadrupole strength the mismatch can become
severe, leading to excessive beam loss and thereby causing the machine to trip off.
The solution for both problems is to produce compensatory optics downstream of
the scanning quadrupole to re-match the beam to the rest of the machine.
Note that placing a quadrupole in the dump beamline would avoid this com-
plication altogether. In this case the quadrupole could be scanned without the need
to produce compensating optics. Unfortunately due to constraints in time and in
the available space on the dump beamline, this option was not feasible.
2.4.2 Measuring Beam Sizes
The beam profile monitor used for the measurement is an intercepting device
known as a wire scanner, although other beam profile monitoring devices such as flu-
orescent screens and OTR monitors could also be used. Wire scanners are routinely
used throughout the CEBAF accelerator to measure the beam profile by recording
the charge and position of a thin wire as it passes through the beam [39]. The
wire scanner consists of an assembly that rigidly holds three 25 µm tungsten wires
oriented along the x, x-y and y axes. An illustration of a 3-wire scanner is shown in
Fig. 2.11.
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