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
Table 7.28 Summary of modified, removed or added S-band rf sections for the LCLS design. The
energy loss quoted here assumes that the rf power inputs are re-configured.
Linac section reason for modification net ∆L removed/added
[m]
net energy loss
[MeV]
21-1b,c shorten sections for L1 quads −0.35 −8
21-2a,b,c,d remove for BC1 chicane and X-band −12.2 −235
21-3a remove for ED1 emittance diagnostic −3.0 −34
24-3d,4d,5d remove to add L2-ED profile monitors −9.1 −103
24-7a,b,c,d remove for BC2 chicane −12.2 −235
24-8a,b,c,d remove for BC2 chicane −12.2 −235
25-1c replace missing 3-m section (NPI) +3.0 +34
25-5a remove to add transverse rf deflector −3.0 −34
27-6d remove to add L3-ED profile monitors −3.0 −34
total = — −52.1 −885
7.11 Operational Issues
The LCLS will operate at 120 Hz concurrent with PEP-II B-Factory operations. An electron
beam can be accelerated through the entire SLAC linac to 50 GeV by switching off and
straightening the bunch compressor chicanes, or by providing adequate aperture in the chicane
bends. This second option, however, compromises the ability to provide a high-resolution BPM
in the center of the chicane, which is a critical requirement for the bunch length and energy
feedback systems.
With the chicanes switched off, the DL1 bends are also switched off, as are the other
dedicated LCLS focusing magnets in the L1 and BC1 area. This re-configuration will require a
switching time on the order of a few minutes at best. The possibility of fast, pulse-to-pulse
switching between LCLS and 30-GeV electrons in the end-station can probably only be realized
by building a by-pass beamline from sector-21 to sector-30, as is done in the PEP-II injection
scenario.
A fast-pulsed kicker in the beam switchyard, just downstream of the linac and well before the
undulator, should be installed in order to dump the electron beam during conditions of
exceptionally poor beam quality. This will help to preserve the permanent magnet undulator
fields, and to provide a ‘veto’ for unwanted pulses. It will also allow more invasive tuning with
the electron beam passing through the bulk of the accelerator. The kicker will, of course, only be
fast enough to dump the next pulse(s), after detection of a poor beam-quality trigger. Beam
collimators just upstream of the undulator will provide protection from the first poor-quality
pulse. Such poor quality conditions can be produced by klystron trips (especially in L1 or L2),
the firing of the transverse rf deflector in sector-25, or any number of previously determined
7-112 ♦ A C C E L E R A T O R