LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

Pulse Split/Delay
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
This system will use crystal diffraction to split the FEL pulse, direct the two x-ray pulses
around unequal path lengths, and bring them back onto the primary beam path with a time delay
between them. Figure 9.15 shows the proposed scheme. The beam-splitting is accomplished by a
very thin (10 µm) silicon crystal. The radiation within the bandwidth for Bragg diffraction from
this crystal is reflected with high efficiency (80%), whereas the radiation outside the Bragg
bandwidth is efficiently transmitted (75% at 8 keV). By orienting the crystals in the two beam
paths to reflect slightly different x-ray energies, the pulse is effectively split and sent around two
separate paths. A simple translation can then be used to change the relative path lengths, and thus
the pulse delay. The overall efficiency of the system for each path is about 30% at 8 keV. The
bandwidth (δE/E) for each crystal reflection is about 2.5×10 -5 , so two pulse energies can easily be
selected from the LCLS bandwidth of about 10 -3 .
Figure 9.15 Pulse split and delay technique. Delay values of several hundred picoseconds can be
achieved, with accuracy of a few femtoseconds.
Focusing System
Focusing in Hutch B2 will use a Kirkpatrick-Baez mirror system identical to that used in
Hutch A2. If only one such mirror system is available initially, it can be transported and installed
in either Hutch A2 or Hutch B2 as needed.
Apertures
The apertures used in Hutch B2 will be very similar to those used in Hutch A2.
Attenuator
Some experiments require a local attenuator for calibration and to prevent damage to
sensitive components during alignment. The local attenuator will have a design very similar to the
solid attenuator located in the Front End Enclosure.
9-22 ♦ — R A Y B E A M T R A N S P O R T A N D D I A G N O S T I C S