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
phosphor screen profile monitor, a BPM, and a horizontal beam collimator will be included at the
chicane center.
In addition, a short one-period, superconducting wiggler will be located just upstream of the
BC2 chicane. This wiggler is used to increase the intrinsic (uncorrelated) energy spread of the
beam in order to damp CSR micro-bunching effects in the BC2 chicane [3]. The relative energy
spread due to incoherent synchrotron radiation (ISR), σδ ISR , per bend magnet is given by
1
−11 2 −553
σ δ ≈ ( 4.13× 10 m ⋅ GeV ) E θ
ISR
B , (7.20)
L
B
where LB is the magnet length, θB is the bend angle, and E is the beam energy (4.54 GeV in the
wiggler). If we use two 7-cm long coils sandwiching two 10-cm long coils, with the field profile
shown in Figure 7.24, then a 6-Tesla peak field increases the incoherent energy spread to 3×10 −5
(rather than 3×10 −6 without wiggler). This is enough to reduce the CSR-induced micro-bunching
effect to tolerable levels (see below). The wiggler is then ~60 cm in physical length and
composed of superconducting magnets with a local liquid He cryogenics supply in or near sector-
24. This scenario has been used in the past to supply the superconducting spin-rotator solenoids
presently installed in the SLAC linac at north damping ring entrance and exit.
B y [T]
6
4
2
0
−2
−4
−6
0 0.2 0.4 0.6 0.8
s [m]
Figure 7.24 Magnetic field profile of one-period superconducting wiggler placed just upstream of
BC2 chicane to increase incoherent energy spread to 3×10 −5 rms at 4.54 GeV, which
damps CSR micro-bunching.
The wiggler has almost no effect on bunch length, and CSR effects are small since the bunch
length is ~200 µm long throughout the wiggler. The ‘slice’ horizontal emittance is, however,
increased by the spontaneous radiation in the wiggler, where the dispersion is not zero. The beta
7-44 ♦ A C C E L E R A T O R