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
[10], for a 1-nC uniform-charge distribution and a 10-ps long bunch with 1-mm hard-edge radius,
good emittance performance and high peak current at the exit of the gun can be obtained with a
peak field on the cathode of about 140 MV/m, an injection phase (with respect to the rf zero
crossing) on the order of 35°, and a moderate solenoid field strength of about 3.0 kG. Earlier
design studies for the LCLS injector made use of a low-gradient booster as was then the standard
[4]. In the following analysis, these parameters, which in this context will be called the standard
or old working point, are taken as the starting condition for a new parameter-space search.
When observing envelope and emittance behavior while scanning the gun solenoid field
strength using HOMDYN, an interesting feature can be seen that appears to be a very effective
new working point for a split (gun separate from booster) rf photoinjector [11]. By increasing the
solenoid strength, the emittance evolution shows a double minimum behavior in the drift region
following the gun. For a unique value of the solenoid strength (3.1 kG in this case) the envelope
waist occurs where the emittance has its relative maximum (z≈1.5 m in this case) as shown by the
red (bold) lines in Figs. 6.1 and 6.2. The solenoid value that produces this unique coincidence of
waist and emittance relative maximum is a key constituent of the new working point.
The performance using the new working point relies on this feature of the emittance
oscillation. If the booster entrance is located at z ≈ 1.5 m, where the beam laminar waist occurs,
and if simultaneously the other IE matching condition is satisfied, i.e., Eq. (6-3), the second
emittance minimum can be shifted to higher energy and frozen at a lower level, taking advantage
of the additional emittance compensation occurring in the booster.
σx (mm)
4
3
2
1
4-2001
8560A94
0
0 50 100 150 200 250
Z (cm)
Figure 6.1 Beam envelope versus z. Each curve is for a different solenoid strength, i.e., values
between 0.26 and 0.33 T in equal increments.
6-6 ♦ I NJECTOR