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
compression, the L3 longitudinal wakefield will now add to the correlated energy spread, rather
than canceling it. Thus, a large correlated electron energy spread (chirp) of ~2% FWHM can be
generated at 14.35 GeV. The over-compression in the BC2 chicane, however, forces the bunch to
pass through its minimum length (a few microns) inside the chicane, which may destroy the
horizontal emittance with the potentially stronger CSR forces. This case requires a subtle
calculation including the transverse beam dimensions and cannot, at present, be relied upon to
accurately predict machine performance. Therefore, over-compression is presently viewed as a
possible option needing experimental verification before it is seen as a realistic chirp strategy.
The most promising method of producing a significant chirp is to reduce the bunch charge
from 1 nC to 0.6 nC and operationally re-configure the linac parameters. The reduced bunch
charge makes the L3 wakefields weaker, which helps to leave some chirp in the electron beam
after L3. The re-configuration is used to further amplify the chirp to ~1% FWHM. This 1%
electron chirp may then be used to compress the x-ray pulse by a factor of ~5, to a pulse length of
~50 fsec FWHM (see Chapter 9).
The reduced charge also allows the initial bunch length, prior to BC1, to be reduced from
0.83 mm to 0.71 mm rms, and the emittance to be slightly reduced from 1.2 µm to 0.9 µm. This
keeps SASE saturation at ~87 meters with a 2.1-kA peak current [see Eqs. (7.10) and (7.11)]. The
machine parameters and sensitivities associated with this configuration are shown in Table 7.8
along side the nominal parameters. The final ‘slice’ energy spread in both cases is