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
In addition, a 55-cm long S-band transverse RF deflecting structure will be located just after
the L0-linac (see “RF Kicker” in Figure 7.32). This system will be used to ‘streak’ the bunch for
bunch-length, slice emittance, and slice energy spread measurements using screens in the DL1
system. This diagnostic is described in more detail in Section 7.8.2.
An insertable tune-up dump will also be included after DL1 in order to allow invasive tuning
of the DL1 and upstream systems. The dump will need to handle a 1-nC beam at 120 Hz and
150 MeV or 18-W of average power.
7.5.2 High-Energy Dog-Leg
The requirements for beam transport from the L3-linac to the LCLS undulator are fairly
simple. The transport line must:
• Include bends to introduce precise energy and energy spread measurement capability
without generating significant CSR or other emittance dilution effects,
• Include precise transverse emittance and matching diagnostics for final
verification/tuning prior to undulator,
• Provide adjustable undulator-input beta-matching for the various beam energies (i.e.,
various radiation wavelengths) desired,
• Not alter the bunch length (must be nearly isochronous),
• Make use of the existing FFTB tunnel and its components wherever possible, as long
as the performance of the transport line is not compromised,
• Adjust the vertical beamline angle to remove the 0.3˚ downward linac angle so that
experimental areas do not need to be located below ground level.
Energy and energy spread diagnostics are built into a four dipole horizontal inflector
beamline (DL2) where the first bend is located just inside the beginning of the undulator hall,
which previously housed the Final Focus Test Beam (FFTB). Primarily to meet the small energy
spread measurement capability, a doublet of Chasman-Green [35] type cells is used. A cell
consists of a dipole pair sandwiching a quadrupole triplet. The horizontal dispersion function in
the center of each cell reaches a maximum, while the horizontal beta function converges towards
a minimum. A horizontal OTR monitor here (ηx ≈ 50 mm, βx ≈ 1.6 m) is capable of measuring an
rms energy spread of 0.03% at 14.3 GeV with a nominal betatron beam size contribution of only
10% at γεx ≈ 1 µm (see Secs. 7.8.2 and 7.8.2.3).
The Chasman-Green type cells are advantageous since they introduce very little path length
energy dependence and generate minimal emittance dilution due to synchrotron radiation. The net
system forms a 4-dipole dog-leg (DL2) displacing the beamline horizontally toward the south by
0.45 m. The net R56 for the 4-dipole system is set to zero by allowing the dispersion function to
reverse sign in half of the bends (see Figure 7.36).
Bends of θ B ≈ 0.65° and L B ≈ 2.62 m produce R56 ≈ 0, and a second order term of
T566 ≈ 73 mm, which is, for a worst-case energy spread of ~0.1%, completely isochronous (i.e.,
7-60 ♦ A C C E L E R A T O R