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 7.16 Dispersion correction quadrupoles for BC2 chicane for horizontal emittance correction of up
to 250% (with ∆ε/ε ≈ 1% step size control).
Maximum Pole-Tip Field [kG] Quantity Step Size [kG] Pole Radius [mm] Length [m]
2.2 2 0.14 50 0.05
An insertable tune-up dump will also be included after BC2 in order to allow invasive tuning
of the BC2 and upstream systems. The dump will need to handle a 1-nC beam at 120 Hz and
4.54 GeV, or 550 W of average power.
Finally, a phosphor screen profile monitor is included in the center of the BC2 chicane where
the dispersion is large (σx ≈ 2.6 mm). This device allows measurement of the correlated energy
spread and therefore also reveals the temporal distribution of the bunch as it enters the chicane. A
horizontal collimator just upstream of the profile monitor will be used to diagnose beam tails, and
one BPM of ≤40-µm resolution will be located in the chicane center. The BPM reading provides a
high-resolution relative energy measurement (δ ≈ 1.2×10 −4 ) per beam pulse (see Section 7.8.4).
7.5 Beam Transport Lines
This section discusses the two beam transport lines. The first is a low-energy bend system
(DL1) used to transport the electrons from the off-axis injector into the main linac. The second is
the high-energy dog-leg (DL2) used for L3-to-undulator transport, as well as energy and energy
spread analysis. The DL2 beamline horizontally displaces the undulator axis from that of the
main linac in order to protect the undulator from potential beam halo and dark current. In
addition, a short vertical bending system (VB) removes the slight (~0.3°) downward slope of the
accelerator at the entrance to the undulator. This leveling-bend allows the experimental areas to
be located closer to ground level.
7.5.1 Low-Energy Dog-Leg
The function of the low-energy ‘dog-leg’ (DL1) is to transport 150-MeV electrons from the
new injector linac (L0) into the existing SLAC linac. While it is possible to design the dog-leg as
a first bunch compression stage, this necessitates a large incoming correlated energy spread of 1-
2%. In this case, the chromaticity of the quadrupole magnet within the dog-leg, required for a
linear achromat, will generate large second order dispersion which needs sextupole
compensation. Due to this, and also the need for easy R56 tuning (not natural in a dog-leg), DL1 is
designed as a simple transport line. Its design requirements are:
• Provide a horizontal beamline deflection of 35° over a short distance,
• Should not alter the bunch length (i.e., should be nearly isochronous),
• Should introduce no significant transverse emittance dilution,
• Should provide a dispersive section for energy and energy spread measurement.
7-54 ♦ A C C E L E R A T O R