LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

7.4.1.1 Overview and Parameters
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
The chicane design is set by the need to reduce transverse emittance dilution due to coherent
synchrotron radiation (CSR) in the bends, which is most pronounced for short bunches [19], [20],
[21], [22], [23]. Since the dominant component of the energy spread generated by CSR is
correlated along the bunch, only the projected transverse emittance is altered. The emittance of
the bunch slices is typically unchanged. The effect can be minimized by using a weak chicane
and a large initial correlated energy spread. A symmetric double-chicane is also possible [16] and
can be used to reduce the projected emittance growth. Unfortunately, the added bending for the
double-chicane increases the possibility of a CSR-induced micro-bunching instability [3]. For this
reason single chicanes are used in the present design.
Table 7.11 Parameters of 1 st bunch compressor chicane, BC1.
Parameter Symbol Unit Value
Beam energy E GeV 0.250
Initial rms bunch length σz i mm 0.83
Final rms bunch length σz f mm 0.19
RMS total incoming relative energy spread (at 250 MeV) σδ % 1.78
RMS uncorrelated relative energy spread (at 250 MeV) σδ u
Momentum compaction R 56 mm −35.9
Second order momentum compaction T 566 mm +53.9
Total chicane length (1 st bend to last) Ltotal m 6.56
Floor length of each of four dipole magnets L B m 0.20
Floor length of drift between first two and last two dipoles ∆L m 2.60
Floor length of drift between center two dipoles ∆Lc m 0.50
Bend angle of each dipole |θ B| deg 4.62
Magnetic field of each dipole |B| kG 3.36
Maximum dispersion in chicane center (≈ beamline excursion) |η max| m 0.229
Projected CSR emittance dilution (γε 0 = 1 µm) ∆ε CSR/ε 0 % 5
CSR-induced relative energy spread (at 250 MeV) σδ CSR % 0.029
CSR- induced relative energy loss (at 250 MeV) δ CSR % −0.068
Motivations and quantitative arguments for the choices of these parameters are described in
the following sections. Results of longitudinal beam dynamics simulations have been used to set
the final BC1 bunch length at 195 µm rms. The bunch length is adjustable using the R56 of BC1
and the rf phase of L1. The center two dipoles of the chicane will be placed on remotely movable
10 −5
A C C E L E R A T O R ♦ 7-35
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