LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

εn,x (µm)
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3
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4-2001
8560A95
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 50 100 150 200 250
Z (cm)
Figure 6.2 Beam emittance versus z. Each curve is for a different solenoid strength, i.e., values
between 0.26 and 0.33 T in equal increments.
Since the minimum rms spot size in Figure 6.1 for the conditions of the new working point is
σ=0.41 mm, the current averaged over the slices is 96 A, and the average slice energy is 6.4
MeV, the matched accelerating gradient of the traveling wave (TW) booster is required to be 35
MV/m. Two SLAC 3-m accelerating structures are required to drive the beam out of the space
charge dominated regime, resulting in an energy of 216 MeV in an 8-m long injector line
(assuming a 0.5-m long drift in between the two structures and not including the gun structure
upstream of the cathode). As expected, the second emittance minimum, which is 0.5 µm, now
occurs downstream of the booster structures, at z=10 m. (This location will be taken as a
reference position to quote emittance at the injector exit).
Despite the good emittance resulting from this design, the necessary gradient to match the
beam to the booster exceeds the limit of reliable performance by available SLAC 3-m structures.
One solution is to shift the solenoid location downstream and set the solenoid strength so as to
recover the new working point conditions. By doing so the resulting spot size at the waist is
bigger and thus, from Eq. (6.3), a lower matched gradient is required.
A lower gradient solution can also be achieved by increasing the focusing properties of the
booster [12]. This can be done by means of standing wave (SW) structures or equivalently by a
long solenoid around the first TW structure [11]. The second solution is chosen for the LCLS
design to simplify the rf system. Setting the desired accelerating field of the TW sections to 26
MV/m and scanning the long solenoid strength, a very good working point is indicated for a
longitudinal field of Bz=800 G. A beam dynamics simulation using HOMDYN indicates a very
low emittance value, of 0.2 µm (thermal emittance not included) at 160 MeV as shown in
Figure 6.3, while at the same time the other relevant LCLS requirements are very nearly fulfilled.
For example, the peak current of 95 A is only slightly below the desired value. The energy spread
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