STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

the coupling of the HOM coupler is required. Such an undertaking is complicated by
the fact that only simple analytic models have been used to estimate the magnitude
of the steering and coupling effects. For instance, a formula for the magnitude of
the dipole steering exists but assumes that each cavity operates with an accelerating
gradient of 5 MV/m, which is not the case in reality [33]. Estimates of the magnitude
of the coupling due to a single cavity have been made [31], however it neglects
the effects of the betatron phase advance which can play an important role for an
extended linac like in CEBAF [45].
Extracting Twiss Parameters
In addition to measuring the emittance, the Twiss parameters prior to the
scanning quadrupole can be determined. With knowledge of the emittance and
of the fitting coefficients (β1ɛg), (α1ɛg) and (γ1ɛg) the Twiss parameters are easily
calculated and can be compared with the model optics. The linear optics code
Optim was used to model the CEBAF-ER lattice. The results of the model optics
and of the experimental measurements are displayed in Table 2.3. The agreement
is very good, on the level of 5%, and further justifies the choice of data points used
for fitting. The large errors associated with the extracted betas and alphas are the
result of propagating the relatively large errors from the corresponding emittances.
Because a model optics does not exist for the machine configured with 20 MeV
injection energy, the extracted Twiss parameters could not be compared to the
model.
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