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 8.39 An optical ray diagram for imaging of OTR when the sensor is positioned at the focal
plane and image plane. Such imaging provides the OTR angular distribution pattern or
the beam profile, respectively.
8.11.5 Cherenkov Detectors
A series of Cherenkov detectors will be placed at all diagnostics stations between undulator
segments. These devices are similar to those used at the PEP-II project as beam loss monitors
[53]. These devices are quite simple conceptually, involving a small piece of fused silica glass,
which emits Cherenkov radiation when traversed by high energy charged particles. A miniature
photomultiplier tube is used to amplify these small pulses, which are then transmitted to data
acquisition electronics, e.g. a gated integrator followed by a digitizer. To prevent FEL x- rays
from corrupting the bremsstrahlung gamma ray signal, the fused silica pellet is embedded in a
lead-lined housing such that only very high energy photons can penetrate. Refinements to the
PEP-II mechanical design and data acquisition have been carried out recently at the APS [54].
8.11.6 Current Monitoring Toroids
High quality pulsed current monitoring toroids are commercially available, together with
front end electronics, if so desired. It is important to measure the charge entering and exiting the
undulator to provide information for normalization of data sets and to tune for maximum
transport efficiency. It should be straightforward to monitor a nominal 1 nC pulse with a few
percent accuracy shot to shot. Data acquisition is provided.
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