Picosecond Bunch Length and Energy-z
Correlation Measurements at SLAC's A-Line
and End Station A
S. Molloy, P. Emma, J. Frisch, R. Iverson, M. Ross, D. McCormick, M. Woods, SLAC, CA, USA
S. Walston, Lawrence Livermore National Laboratory, CA, USA
V. Blackmore, Oxford University, Oxford, UK
Absolute Bunch Length Measurement
There are no diagnostics in ESA that would allow an absolute measure of the bunch length. The
longitudinal phase space distribution of the beam in an upstream region of the machine could be
propagated to ESA using the known R56 of the A-line.
The spread of the particles longitudinally may be
determined using the LOLA at the end of the SLAC linac.
The energy spread of the bunch may be measured by
monitoring the synchrotron light generated by the beam in the
dispersion of the A-line bend.
Low energy
particles
High energy
particles
Screen
Synch light fan
generated by the
bending magnets.
Screen Calibration
Energy Calibration
• Alter the energy of the beam using the energy feedback.
A combination of these techniques yields an
expansion of the bunch profile in E and z.
• Record the SLM image at each energy set-point.
• Correlate the mean of the image’s horizontal projection with the energy.
z Calibration
• Alter the LOLA phase by small amounts from the zero crossing.
• Record the SLM image at each phase.
• Correlate the mean of the image’s vertical projection with the phase.
• Using the known frequency of LOLA, convert this to a correlation with z.
Determine Bunch Length from Image
LOLA does not perform a 90° rotation of the bunch, so the height of the image will
include contributions from the transverse bunch size, and the tilt of the bunch.
LiTrack simulation of the longitudinal
distribution at the end of the linac.
These are removed by performing a parabolic fit to the image height versus LOLA
amplitude.
Only three points were recorded: LOLA on, LOLA off, and LOLA on with 180° phase
difference. The phase difference was plotted as a negative amplitude.
Zero offset of fit due to transverse height of
bunch.
Asymmetry of plot due to tilt of bunch.
Location of minimum on horizontal axis gives
the LOLA amplitude and phase that would
remove this tilt.
Tilt of the core in z-E generated by linac
wakefields, and the tails that create the “S”
shape generated by linac phasing.
The phasing of the linac may be altered to
adjust the slope of the core. Due to the R56
of the A-line (R56 = 0.465), this results in
difference bunch lengths in ESA.
The extent of the phasing is limited by the
generation of excessive tails.
Abstract
We report on measurements of picosecond bunch lengths and the energy-z correlation of the
bunch with a high energy electron test beam to the A-line and End Station A (ESA) facilities at
SLAC. The bunch length and the energy-z correlation of the bunch are measured at the end of the
linac using a synchrotron light monitor diagnostic at a high dispersion point in the A-line and a
transverse RF deflecting cavity at the end of the linac. Measurements of the bunch length in ESA
were made using high frequency diodes (up to 100 GHz) and pyroelectric detectors at a ceramic
gap in the beamline. Modeling of the beam's longitudinal phase space through the linac and A-line
to ESA is done using the 2-dimensional tracking program LiTrack, and LiTrack simulation results
are compared with data. High frequency diode and pyroelectric detectors are planned to be used
as part of a bunch length feedback system for the LCLS FEL at SLAC. The LCLS also plans precise
bunch length and energy-z correlation measurements using transverse RF deflecting cavities.
The measured longitudinal distribution is converted into
an array of several thousand points that represent the
bunch in E-z space.
 L2   1 R  L1 
56 
 dP   


dP
 2   0 1  1 
P2 
P1 


Using a dedicated longitudinal dynamics software
package (such as LiTrack), this distribution is tracked
around the A-line into ESA.
Since there are no accelerating elements in the A-line,
a simple R-matrix calculation suffices (as shown).
The length of the
bunch at LOLA
and in ESA was
measured for a
series of linac
phase settings.
Relative Bunch Length Measurements
A system that measures the relative bunch length for every pulse could be employed as part of a
feedback system. This system is based on the bunch length measurements of the SLC final
focus system at SLAC.
Principle of Measurement
Experimental Setup
A beam radiates RF as it passes the electrical discontinuity
presented by a ceramic gap. The spectrum of this radiation is
determined by the bunch shape.
Four different diodes were tested.
Thus, shorter bunches will generate higher frequency RF, and
a measurement of the power present in a particular frequency
band will provide a relative measure of the bunch length.
~20 m of WR90 brought a portion of the
radiation from a ceramic gap (see photo) to a
splitter, which then directed the power to two
identical, ~20 GHz diodes, via 16 and 23 GHz
lowpass filters.
~1 m of WR10 (see photo) was used to
channel the power to two 100 GHz diodes.
This experiment used diodes sensitive to different frequency
bands, and a pyro detector to monitor radiation produced at
ceramic gaps in the ESA beamline.
Since the power radiated at a certain frequency P(ω) is
proportional to the square of the charge, it must be
normalised by this value.
Both the 100 GHz and the pyro-detector
respond strongly to changing the linac phase
ramp, indicating a strong response to the
bunch length.
The pyro is capable of measuring shorter
lengths due to its wideband response.
When plotted alongside the bunch length as
measured by LOLA, it can be seen that the
100 GHz diode has a dynamic range of at
least 400 – 800 µm. This is consistent with
the theory.
2 2

 z 
2
P   Q exp   2 
c 
