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
beam on the center OTR profile monitor of the ED0 emittance diagnostic section (at S ≈ 14.1 m in
Figure 7.33). With a beam waist (αx = αy = 0) nominally located at this monitor, a quadrupole
gradient scan can be done while measuring the sliced horizontal beam size. A simulation of this
‘streak’ effect is shown in Figure 7.68, for nominal quadrupole settings. In addition to slice
emittance, the horizontal Twiss parameter variations along the bunch length can also be
measured. Of course, the absolute bunch length and temporal profile can also be measured here.
Figure 7.68 Simulated profile monitor in middle of ED0 emittance diagnostic section with 55-cm rf
deflector OFF (left) and with voltage set to 1 MV (right). The deflector ‘streaks’ the beam
vertically such that the horizontal slice emittance can easily be measured.
7.8.2.2 RF Zero-Phasing Technique
Beyond the rf-deflectors, CSR detectors will be used as relative bunch length monitors,
calibrated using a streak camera for the 10-psec range, and calibrated using the rf-deflector, or a
“zero-phasing technique” [50] for the 2-psec and 0.2-psec ranges.
The zero-phasing technique is another way to measure the micro-bunch in an absolute sense,
by employing an accelerating rf system at zero-crossing angle to generate correlated energy
spread. The transverse beam size is measured in a dispersive region to extract the bunch length. In
the LCLS, the entire L3-linac is used as the rf system. The measurement is invasive and
performed infrequently to calibrate or crosscheck the CSR detectors. The micro-bunch is
transported to the energy spread measuring profile monitor in DL2 (PR31) with a +90° L3 rf
phase (i.e., zero crossing producing no acceleration in L3). The measurement is then repeated at a
–90° rf phase. The bunch length is then given by the average of the two energy-spread values.
λ E
σ
i
z ≈
4π
Ef − Ei
( σδ+ σδ2)
1
(7.28)
Here λ is the rf wavelength (105 mm), Ei is the L3 injection energy (4.54 GeV), Ef is the L3 final
energy obtained with the L3 rf phase at crest-phase (14.6 GeV), and σδ 1 and σδ 2 are the two
energy spread measurements at +90° and –90°.
A C C E L E R A T O R ♦ 7-97