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
Figure 8.55 Undulator trajectories in the cell.
The x-ray diagnostics setup will provide absolute measurements (within 10%) of spectral flux
after each undulator cell. The growth rate could be derived from the measured flux at successive
diagnostics stations. The growth rate of the flux is a valuable means of evaluating and studying
the development of the SASE process.
The spatial flux distribution for one undulator cell is the superposition of radiation from three
undulator segments. In order to evaluate the sensitivity of the x-ray diagnostics to the angular
misalignment of undulator segments, e-beam trajectories in the first and third undulator segments
in the cell are given a missteering angle θmis (Figure 8.55). A series of calculations of
spontaneous radiation for a cell have been performed for a nominal electron beam emittance of
0.05 nm·rad at a photon energy of 8.29 keV and at a distance of 60 m from the undulator cell.
This 8.29 keV energy is detuned slightly, towards higher energy, from the fundamental energy of
8.27 keV, in order to reduce the angular divergence of the undulator radiation. The calculated
spatial intensity distribution is shown in Figure 8.56 for a missteering angle of 4 µrad, and
calculated horizontal profiles are shown in Figure 8.57 for missteering angles of 2, 3, and 4 µrad.
The figures show that a missteering of 4 µrad can be clearly discerned if the diagnostics station is
far enough from the undulator cell.
Figure 8.56 Calculated spatial distribution of the undulator radiation from a three-undulator cell
with a missteering θ mis = 4 µrad, an electron beam emittance of 0.05 nm·rad, and a
photon energy of 8.29 keV, at 60 m from the undulator segments.
8-92 ♦ U N D U L A T O R