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
Table 8.12 Initial launch conditions at undulator entrance before and after step-1, Table 8.11
correction.
Launch parameter Before Step-1, Table 8.11 After Step-1, Table 8.11
〈x〉 310 µm 14 µm
〈x’〉 14 µrad 3.5 µrad
〈y〉 250 µm 13 µm
〈y’〉 17 µrad −1.7 µrad
Step-2, Table 8.11, involves a weighted steering procedure using the undulator quadrupole
magnet movers where the absolute readings of the BPMs are minimized with respect to a 50 µm
weighting and the applied magnet mover changes are simultaneously minimized with respect to a
50 µm weighting. Without this weighting the magnet movers can move by up to ~1 mm in order
to exactly zero each BPM reading which, due to the large BPM offsets, is an unnecessary and
undesirable steering accuracy at this early stage. With the weighting included, the movers change
by ~50 µm, which is sufficient to correct the large trajectory deviations of Figure 8.43. Figure
8.44 shows the trajectory after the step-2 weighted steering is applied. This step simply improves
the trajectory before the energy is changed.
X/µm
Y/µm
1000
500
0
−500
〈X 2 〉 1/2 = 162.96 µm
−1000
0 20 40 60 80 100 120
1000
500
0
−500
〈Y 2 〉 1/2 = 73.17 µm
−1000
0 20 40 60 80 100 120
S/m
Figure 8.44 Trajectory (blue-solid) at 14.3 GeV after application of step-2, Table 8.11, weighted
steering. BPM readbacks (green-circle) and applied magnet mover changes are both
minimized with respect to a 50 µm weighting. The post-steering quadrupole positions
(red-cross) are also shown. These BPM readings are saved for analysis.
These 14.3-GeV BPM data are saved and the energy is then lowered to 10 GeV by switching
off half of the klystrons in linac-3. The fields of the magnets upstream of the undulator are scaled
to the new energy and any beam position differences upstream of the undulator, with respect to
U N D U L A T O R ♦ 8-81