STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
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identified, although the phase delay chicane and/or steering errors may have con-<br />
tributed. The dispersion was never fully suppressed and this may have led to the<br />
spurious results in arc 2. Recall that the emittance measurement explicitly assumes<br />
zero dispersion. Moreover, measuring the emittance in arc 2 was difficult even with<br />
zero dispersion (as was the case for the 20 MeV setup), due to the close proximity of<br />
the wire scanner to the 2E02 quadrupole used for generating multiple optics. With<br />
the wire scanner only 1 m downstream of the scanning quadrupole, the optics could<br />
not be changed sufficiently for a good measurement (analogous to not sweeping<br />
through a minimum of the beam size for the single quadrupole scanning method).<br />
These two conditions working together may account for the spurious data.<br />
Even when the arc 2 measurement is disregarded, the measurements show that<br />
the normalized emittance grows as it is transported through the machine. Possible<br />
sources of the emittance growth can be traced back to the SRF induced effects<br />
discussed in Section 2.2.3.<br />
Simulations using the present RF feed configuration in CEBAF show that a<br />
single pass through the machine could cause the projected horizontal normalized<br />
emittance to grow by up to 1 mm-mrad due solely to the effects of the dipole<br />
mode driven head-tail steering. Fields also exist which cause growth of the vertical<br />
emittance, however the effect is small in comparison.<br />
Simulations were also performed with DIMAD [44] in an effort to understand<br />
the role of the HOM coupler induced transverse coupling in the observed emittance<br />
growth. The simulation modeled the coupling by introducing a thin skew quadrupole<br />
element at the location of each cavity’s HOM coupler. Results of the simulation<br />
showed the horizontal normalized emittance exhibiting growth of 0.5 mm-mrad but<br />
failed to explain the observed behavior of the vertical emittance in Table 2.1 and<br />
Table 2.2.<br />
Ultimately, a simulation that incorporates the dipole steering of the FPC and<br />
50