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
First HOMDYN was used to explore the transverse parameter space for a square pulse of
10 ps. The parameters that were examined included the gun solenoid (S1) field value, the
injection phase, the linac position and gradient and the second solenoid (S2) field value. For a
thermal emittance of 0.5 µm, the minimum normalized transverse emittance obtained was 0.6 µm
rms using an injection phase of 20°, S1 field of 2.71 kG, and a linac field of 24 MV/m (33 MV/m
in HOMDYN units) with the entrance to L0-1 located 1.4 m from the cathode.
When the same parameters were used in PARMELA, including a square pulse of 10 ps, an
emittance of 0.99 µm was obtained. The higher emittance with PARMELA confirms that the
parameter space near the working point as determined with HOMDYN must be further optimized
with PARMELA. The parameter space was explored using PARMELA for a pulse with a finite
rise time of 0.7 ps. (HOMDYN uses only absolutely square pulses.) The PARMELA optimization
was initiated with parameter values obtained from the optimization with HOMDYN. The initial
pulse profile is shown in Figure 6.21. It is built with 9 Gaussians of width 0.7 ps rms separated
by 1.1 ps.
A first series of simulations was performed with a spot-size radius of 1 mm. The best
emittance was obtained by placing L0-1 2.2 m away from the cathode. This solution was
unsatisfactory as the same linac section is located 1.42 m from the cathode when parameters are
optimized with the cathode peak rf field at 140 MV/m.
To keep the distance cathode-to-Linac constant, the spot radius was increased from 1 mm to
1.2 mm. This larger radius reduces the space charge effect without introducing excessive
additional RF defocusing effects. Also, the field in L0-1 was decreased from 24.1 MV/m to
18 MV/m following the criteria presented in Eq. 6.3. This results in an energy at the exit of L0-1
of 58.25 MeV instead of 78.18 MeV . By running L0-2 at 24.1 MV/m and 30.5 MV/m, the energy
at the end of the beamline is 130 MeV and 150 MeV respectively. No emittance growth in the
Matching Section appears for either a 130 MeV or 150 MeV beam.
With the parameters presented in Table 6.4 for 10 K particles and 0.7 ps rise time, the beam
is smoothly converging along the beamline as presented in Figure 6.32. The projected emittance
is 0.93 µm.
6-60 ♦ I NJECTOR