LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

economic reasons.
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
7.6 Six-Dimensional Particle Tracking Studies
In this section the electron beam quality is evaluated by slicing the beam longitudinally after
detailed 6D tracking through the injector, compressors, and main linac. The final electron beam
density is used to estimate the FEL performance for the nominal LCLS undulator parameters (see
also [36]).
7.6.1 Electron Beam Evaluation
The entire LCLS accelerator, from rf-gun to undulator entrance, has been tracked in six
dimensions using Parmela for the injector, up to 150 MeV, and then tracking these same 2×10 5
macro-particles using Elegant [10] for the linac and compressors, up to undulator entrance at
14.35 GeV. The tracking calculations include the following effects:
• An estimated thermal emittance included at the cathode (see Chapter 6),
• Space charge forces up to 150 MeV for the LCLS gun and injector,
• Longitudinal and transverse geometric wakefields of the S-band and X-band
accelerating structures (transverse wakes are only applied past the L0-linac),
• Bunch compression including all linear and non-linear energy correlations induced in
the linacs and compressors,
• Transverse misalignments (past L0) of BPMs, quadrupoles and all 3-meter
accelerating structures (BPMs: 150 µm rms, quadrupoles: 150 µm rms, structures:
300 µm rms; all gaussian distributions with 3-σ cuts),
• Trajectory correction (past L0) applied using existing (and planned) steering
elements and misaligned BPMs,
• Coherent synchrotron radiation in all bends, with a transient field model integrated
over the ‘real’ evolving non-gaussian temporal bunch distribution, and including
radiation effects between and after bend magnets [27], [28],
• Incoherent synchrotron radiation effects in all bends, which adds slice emittance and
slice energy spread,
• First and second-order lumped optics of each half-magnet (every magnet is split into
two pieces),
• Resistive-wall longitudinal wakefields of the micro-bunch in several long sections of
1-inch diameter stainless-steel and aluminum vacuum chambers which are located in
the L3-linac and DL2 beamlines.
An example of a final steered trajectory and the related emittance growth over the entire
LCLS is shown in Figure 7.44. The normalized rms projected emittance here does not include the
effects of CSR (see below), and no ‘emittance-bump’ corrections [18] have been applied to
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