LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

L C L S C O N C E P T U A L D E S I G N R E P O R T
model, which has been seen to somewhat overestimate the effect as compared to the 3D model. In
addition, the projected emittance growth is due to slice transverse offsets, which dominantly
occur at the extreme head and tail of the bunch due to the very high current spikes at these
locations. The projected emittance growth calculated by integrating over only the central core of
the beam with 75% of the particles is actually only 60%, and this is also only the projected
emittance, not the slice.
The total energy loss due to CSR is −3.2 MeV (or −0.071%). The change in central trajectory
produced by the energy loss in the bends has been corrected in the tracking by steering so that the
electron beam does not pass off-center through quadrupoles. The mismatch effect on the
horizontal beta and alpha functions at the end of the chicane is significant ([i.e., ζ ≈ 1.23; see Eq.
(7.26)]), but this is not a mismatch of the individual slices. It is a mismatch of the projected phase
space, which should have a limited effect on the SASE FEL gain.
Finally, it is also possible to generate ‘slice’ emittance growth with the effects of CSR. The
slice growth is generated when the longitudinal CSR wakefield has a significant transverse
gradient over the width of the bunch [30]. These effects have been studied using TraFiC4 with a
previous, stronger design for BC2, with R56 ≈ −30 mm. The slice emittance growth seen in these
calculations was too small to resolve numerically (i.e.,