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
power is at angles greater than 1/γ. For this reason, the OTR is not diffraction-limited at 1/γ, but
limited only by the aperture of the collecting optics for a given wavelength. Figure 8.38 shows a
schematic of the OTR angular distribution features at 40 MeV. For a wide-aperture optical
system, sub-10 µm resolution is expected. At APS, B. Yang has recently tested in the lab a
camera-lens package with sub-10 µm resolution for the chicane beam size monitor.
Figure 8.37 A schematic of forward OTR and backward OTR generation for interfaces normal to the
beam (left) and at 45º to the beam (right). The dielectric constants for vacuum (ε 0) and
the medium (ε) are indicated.
In addition to the beam profile function, one may take advantage of the laser-like angular
distribution of OTR to measure e-beam angle. As has been done in previous experiments in the
1980's, the camera-lens system can be set so the sensor is at the focal plane of the lens. In this
configuration far-field or angular distribution information is obtainable as schematically
illustrated in Figure 8.39. Since in this case 1/γ≈35 µrad, the optics and distances would have to
be chosen appropriately. Alternatively, at a 10-m distance the angular divergence contribution
from OTR would give a projected spot size of ~700 µm with no lens. The centroid of such an
image is related to beam direction and angle. An angle sensitivity of several micro-radians should
be possible.
8-70 ♦ U N D U L A T O R