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
measurement is derived from the spatial dependence of the image intensity. Metal screens are
typically prompt (subpicosecond) emitters of optical transition radiation (OTR), making them
ideal light sources for preserving the axial structure (intensity) of the incident electron bunch
within the emitted optical pulse. An apparatus or procedure capable of resolving both axial and
transverse variations in OTR intensity enables a slice emittance measurement. Streak cameras
with temporal resolution better than two picoseconds have been available for some time. In a
slice emittance application, an image of a line segment on the OTR screen is made at the narrow
(20 µm) slit entrance to the streak camera. Preserved in the OTR pulse, the streak tube output
displays on its horizontal axis the electron beam intensity along this line segment, and on its
vertical axis the temporal dependence of this intensity. A charged coupled device (CCD) image of
tube output is ideal for analyzing the beamwidth of different slices, which are represented by
some number of adjacent pixel rows.
The feasibility of using a streak camera and OTR optical system as shown in Figure 6.18 as a
backup for the rf deflector to measure the slice emittance at 150 MeV is being studied.
6.5.3 Temporal Pulse Shape
Features of the photoinjector laser system described in Section 6.4, Laser System, that tailor it
for reliable electron production also facilitate applications of the laser to novel electron beam
diagnostics. Stable, unconverted laser light (infrared and visible) constitutes a diagnostic beam
(probe) for applying electro-optic sampling techniques to the measurement of the temporal shape
of the electron bunch. Temporal resolution of sampling measurements is determined by the
duration of the probe pulse and its timing jitter relative to the UV pulse (which is used for
photoelectron production). Nanosecond delay times can be set with subpicosecond stability for
picosecond probe pulses. Probe pulses of millijoule energy are available.
The positive uniaxial crystals LiNbO3 and LiTaO3 are suitable candidates for electro-optic
beam sampling. In the linear or Pockels regime, bias fields generated by the electron beam do not
alter the crystal anisotropy. In previous work with 16-MeV electrons, wakefield-induced phase
retardation in LiTaO3 has been demonstrated with resolution of order 10 -1 radians with wakefield
sensitivity of order 1 radian-m/MV [50]. More recent work has demonstrated single-shot bunch
shape measurements using a wavelength-chirped laser pulse [51].
The electron beam longitudinal distribution and bunch length will be monitored
noninvasively using beam wakefield components as a Pockels-effect bias to induce accumulated
phase retardation of a probe pulse as it propagates through the crystal. The wakefield-induced
Pockels effect generates a linear response that is determined by wakefield dynamics. In a standard
configuration using cross-polarized optics, a null signal is set for zero wakefield amplitude; i.e.,
when the probe waveform and beam wakefield are not coincident. Incident and transmitted probe
pulses are transported to and from the crystal location by polarization-preserving optical fiber. In
general, picosecond or nanosecond (i.e., uncompressed) probe durations can be used. In the
picosecond case, signals can be scanned by varying the relative probe-beam timing. This
6-45 ♦ I NJECTOR