STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

TABLE 2.3: Comparison of Twiss parameters from optics as modeled by Optim and
from measurements for Einj = 56 MeV.
Location Optim Measured
2L21 Quadrupole
2L22 Quadrupole
βx = 32.3 m βx = (33.5 ± 9.6) m
αx = −2.8 αx = (−2.6 ± 0.1)
βy = 39.5 m βy = (37.6 ± 13.5) m
αy = −3.4 αy = (−3.5 ± 0.2)
2.5 Measuring Beam Profiles of the Energy Re-
covered Beam
The standard CEBAF wire scanner measures the induced current on the wire
due to secondary emission of electrons from the wire. These induced currents tend
to be in the nanoampere range and this system is well suited to measuring the
core size of the beam and typically has two orders of magnitude dynamic range. To
improve the dynamic range of the wire scanner for beam profile measurements of the
energy recovered beam, additional instrumentation was added to the wire scanner
just upstream of the beam dump. This instrumentation relies on photomultiplier
tubes (PMTs) to detect the scattered secondary electrons from the incident beam
intercepting the wire. The intensity of the electron spray detected by the PMT is
proportional to the charge that the wire intercepts and a beam profile with two to
three orders of greater dynamic range can be achieved [46]. The beam currents for
the energy recovery experiment are large (tens of µA) compared to those previously
measured using this method in CEBAF’s Hall-B (nA range) where photomultipliers
are routinely used.
To qualitatively characterize beam preservation through the accelerator, the
beam profile data were fit with a Gaussian distribution. The choice of the fitting
function reflects the fact that a typical wire scan will show a narrow Gaussian
distribution representing the core of the beam. Figure 2.16 and Fig. 2.17 display
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