Design of an electro-optical sampling experiment at the AWA facility Page: 1 of 3
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FERM ILAB-CON F-07-257-AD
DESIGN OF AN ELECTRO-OPTICAL SAMPLING EXPERIMENT AT THE
J. Ruan, H. Edwards, Cheng-Yang Tan, R. Thurman-Keup, V. Scarpine, FNAL, Batavia, USA
Y. Li, John Power, ANL, Argonne, IL 60439, U.S.A.
Tim Maxwell, Northern Illinois University, De Kalb, IL 60115, U.S.A.
The free space electro-optical (EO) sampling technique is
a powerful tool for analyzing the longitudinal charge
density of an ultrashort e-beam. In this paper, we present
(i) experimental results for a laser-based mock-up of the
EO experiment  and (ii) a design for a beam-based,
single-shot, EO sampling experiment using the e-beam
from the Argonne Wakefield Accelerator (AWA) RF
photoinjector. For the mock-up, a tabletop terahertz
experiment is conducted in the AWA laser room. The
mock-up uses an IR beam incident on <110> ZnTe crystal
to produce a THz pulse via optical rectification.
Detection is based on the cross correlation between the
THz field and the probe IR laser field in a second <110>
With the advance of electron acceleration techniques,
the bunch length has become shorter over the decades.
Various techniques has been used to measure such ultra
short bunches. One of the common techniques is the
analysis of the incoherent transition radiation using a
streak camera. The resolution of this technique is limited
by the resolution of the streak camera. A transverse
deflecting structure (LOLA) is another powerful tool to
do this measurement and the resolution can be as short as
100 fs.  However both techniques are invasive methods
which make them less useful for real time monitoring.
EO sampling is proving to be a very powerful tool for
longitudinal beam diagnostics. [1,3-7] In particular, it is a
non-destructive method compared to the other techniques,
which makes it very attractive for real time monitoring.
EO techniques are based on the principle of the pockel
cell effect. When an electric field is applied to a
birefringent crystal, the refractive index ellipsoid will
change, introducing an optical phase shift. To detect this
phase shift, an optical beam polarized at 450 to the
transverse axes of the EO crystal is propagated through
the EO crystal during the presence of the electric field.
The phase retardation is converted to an intensity
modulation after a crossed polarizer. Recently several
single shot schemes have been developed to detect an
ultrashort e-bunch down to 150 fs. Comparative studies
show a good agreement between EO techniques and
LOLA measurements. 
In this paper we will report on the effort to build an EO
sampling experiment at the Argonne Wakefield
Accelerator (AWA) lab. We will describe the mock-up
experiment done using a femtosecond laser source first.
Then we will discuss the ongoing EO experiment with a
picosecond laser source. Finally we will discuss the
difference between these two measurements.
Experiment with femtosecond laser
The schematic of the mocked-up femtosecond laser
experiment is shown in figure 1. The 70 fs, 0.78 mJ laser
output of a Ti:sapphire regen amplifier system with the
repetition rates of 1kHz is split into a probe beam and
pump beam. The pump beam, which carries 90% of the
energy from the seed pulse, drives a 3mm thick ZnTe
crystal with (001) cut producing a terahertz (THz)
generator. The laser spot size is 8mm on the crystal
surface. A parabolic mirror is used to focus the terahertz
onto a 1mm thick ZnTe (001) sampling crystal. A single-
shot detection technique based on the cross correlation
between the terahertz field and the probe laser pulse is
adopted . The sampling crystal is sandwiched between
a pair of crossed polarizers with the probe beam imaged
onto a charge-coupled device (CCD) camera. The setup
has an extinction ratio better than 106 before introducing
the sampling crystal and drops to 3500 after the crystal is
inserted due to the residual birefringence in the sampling
P1 P2 F
Figure: Schematic of the experiment. P, C, F, M, D, S
stands for polarizer, crystal, teflon filter, parabolic mirror,
detector (CCD camera) and beam splitter respectively.
More details about this experiment can be found in Ref. 1.
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Ruan, J.; Edwards, H.; Tan, Cheng Yang; Thurman-Keup, R.; Scarpine, V.; /Fermilab et al. Design of an electro-optical sampling experiment at the AWA facility, article, June 1, 2007; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc885814/m1/1/: accessed February 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.