LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

B z (kG)
3
2
1
4-2001
8560A91
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
0
0 10 20 30 40 50
z (cm )
Figure 6.19 Axial magnetic field generated using POISSON of the emittance compensation solenoid
as a function of distance z along the axis for excitation current of 221 A. The cathode
surface is located at z=0 mm.
The multi-parameter space including charge, laser spot size, pulse length, solenoid field and
accelerating gradient has been explored for tuning the 1.6-cell S-band rf gun beamline using a
variety of simulation codes. (See Section 6.1.2, Emittance Compensation.) The overall result is
that to produce a beam of the highest possible brightness, a 1-mm radius and 10-ps bunch length
is about optimum for 1 nC of charge if the peak rf field at the cathode is 140 MV/m. Nearly
identical results can be obtained using 120 MV/m if the radius is increased to 1.2 mm. It is also
clear that using spatial and temporal distributions that are uniform (flat top) rather than Gaussian
will improve the resulting transverse emittance. As a practical matter, uniform temporal
distributions can only be approximated. Therefore the PARMELA simulations discussed here
have usually assumed rise times of 0.35 ps or 0.7 ps, which are within the capability of the laser
system described in Section 6.4, Laser System. For the PARMELA simulations, the initial
temporal uniform distribution was generated by stacking 9(17) Gaussian distributions with an rms
width of 0.35(0.7)º S-band phase, each separated by 0.6(1.1)° of S-band phase. The resulting
temporal bunch shape is shown in Figure 6.20 for a rise time of 0.35 ps and in Figure 6.21 for
0.7 ps. A uniform spatial distribution is assumed for all the PARMELA simulations.
The basic layout of Linac 0 (L0) and the Matching Section (MS) are shown in Figure 6.17.
The corresponding input parameters assumed for the PARMELA simulations are summarized in
Table 6.4. A first series of simulations was done at 140 MV/m and injection phase of 32° and is
described in Section 6.6.2, Optimization for 140 MV/m. However, the initial operation of the gun
6-47 ♦ I NJECTOR