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
The emittance compensating solenoid is physically 22.5 cm long. For the simulations it is
placed against the downstream edge of the rf gun, which results in the center of the solenoidal
field being 19.1 cm from the cathode surface. The field map (Figure 6.19) indicates that for this
configuration the field at the cathode is essentially zero without the use of a bucking coil.
6.6.2 Optimization for 140 MV/m
Using only the gun, solenoid, and the immediately following drift space (i.e., no booster), a
minimum value for the first emittance minimum along the drift was obtained by varying the
solenoidal field and beam spot radius. A value of Bz = 3.15 kG and hard-edge radius of 1 mm was
found to be optimum. The emittance minimum very nearly coincides with that of the new
working point described in Section 6.1.3, Design Principles. The slightly larger value of BZ here
is consistent with the solenoid being displaced somewhat downstream because of the physical
interference with the gun structure.
Next, including both the boosters (with an accelerating gradient of 24.1 MV/m) and the
second solenoid (S2), the emittance at the booster exit was minimized by varying the booster
locations (keeping the drift distance between the two sections fixed at 0.5 m. The results are
summarized in Figure 6.22, which is really a compilation of 12 independent figures. The
optimum position of the entrance to the first section was found to be 1.4 m from the cathode (the
S2 field was -1.5 kG). Note that as the booster is moved toward the position for the minimum
emittance, the emittance decreases more gently and eventually monotonically, approximating the
shape shown in Figure 6.3. Finally the emittance was minimized by varying the field and
position of S2. An emittance minimum was found for a field of -1.75 kG and by positioning the
start of the solenoid at 1.43 m with respect to the cathode. Finally, a thermal emittance of 0.3 µm
was added to the PARMELA deck [8]. Using these parameters and an input pulse rise time of
0.35 ps, a final emittance of 0.8 µm was obtained for a 20 K-particle run. See Figure 6.23. Using
the same parameters but substituting a rise time of 0.7 ps for the input pulse increased the
emittance by about 10%.
6-50 ♦ I NJECTOR