LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

Figure 9.19 Scattering Foil Imager
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
With a thin (30 micron) polished Be foil, the Scattering Foil Imager is nearly transparent to
the 8 keV radiation and can be used throughout the beam line as a non-intrusive pulse-to-pulse
monitor of the beam energy, shape, and centroid. At 0.8 keV, the foil will not be transparent but it
will be able to withstand the full FEL intensity, so the Scattering Foil Imager can be used
intrusively to measure the pulse-to-pulse statistics of the beam energy, shape, and centroid.
9.4.2.3 Micro-Strip Ion Chamber
The Scattering Foil Imager cannot monitor the 0.8 keV FEL non-intrusively since it is opaque
and even at 8 keV it could introduce unwanted distortion into the beam. A traditional ion
chamber, commonly used at current synchrotrons, is designed to operate at 1 atmosphere gas
pressure, with a fairly low intensity DC beam. The high intensity and pulsed nature of the FEL
require some modifications to the traditional design (Figure 9.20). The current-measuring
electronics of traditional ion chambers must be replaced by pulse processing electronics to
measure the energy in each FEL pulse. The drift region must be carefully designed so that the
photoelectrons from the pulse are efficiently collected at the anode in the time between pulses.
The chamber must be operated at pressures below 1 atmosphere to reduce the instantaneous
charge that must be drifted and collected. At 8 keV the gas can be contained within Be windows,
but for 0.8 keV operation a windowless chamber with differential pumping is required. Finally
low-resolution centroid and shape information can be obtained by segmenting the anode as in a
micro-strip detector.
X - 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 ♦ 9-27