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High-energy Picosecond Laser Pulse Recirculation for Compton Scattering*
I. Jovanovic, S. G. Anderson, S. M. Betts, C. Brown, D. J. Gibson,
F. V. Hartemann, J. E. Hernandez, M. Johnson, D. P. McNabb, M. Messerly,
J. Pruet, M. Y. Shverdint, A. M. Tremaine, C. W. Siders, C. P. J. Barty
Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
Frequency upconversion of laser-generated photons by
inverse Compton scattering for applications such as nu-
clear spectroscopy and gamma-gamma collider concepts
on the future ILC would benefit from an increase of av-
erage source brightness. The primary obstacle to higher
average brightness is the relatively small Thomson scatter-
ing cross section. It has been proposed that this limitation
can be partially overcome by use of laser pulse recircu-
lation. The traditional approach to laser recirculation en-
tails resonant coupling of low-energy pulse train to a cavity
through a partially reflective mirror . Here we present an
alternative, passive approach that is akin to "burst-mode"
operation and does not require interferometeric alignment
accuracy. Injection of a short and energetic laser pulse is
achieved by placing a thin frequency converter, such as
a nonlinear optical crystal, into the cavity in the path of
the incident laser pulse. This method leads to the increase
of x-ray/gamma-ray energy proportional to the increase in
photon energy in frequency conversion. Furthermore, fre-
quency tunability can be achieved by utilizing parametric
amplifier in place of the frequency converter.
RECIRCULATION INJECTION BY
NONLINEAR GATING CONCEPT
Many applications of high intensity lasers such as
Compton-scattering based light sources, high-harmonics
generation, laser produced plasmas and Thomson scatter-
ing are limited by low conversion efficiencies. The ef-
ficiency of nonlinear process induced by interaction of a
short intense laser pulse with an optically thin medium
could be increased by reusing the laser photons after each
interaction. Current pulse recirculation schemes are based
either on resonant cavity coupling [2, 3] or active (electro-
optic) pulse switching [4, 5] into and out of the resonator.
Here, we describe an alternative efficient pulse trapping
scheme based on nonlinear frequency conversion, termed
recirculation injection by nonlinear gating (RING). In the
simplest implementation of this technique, the incident
laser pulse at the fundamental frequency enters the res-
onator and is efficiently frequency doubled. The resonator
* This work was performed under auspices of the U.S. Department of
Energy by University of California, Lawrence Livermore National Labo-
ratory under Contract W-7504-Eng-48.
mirrors are dichroic, coated to transmit the (Lw) light and
reflect the 2nd harmonic (see Fig. 1). The upconverted
2w pulse becomes trapped inside the cavity. After many
roundtrips, the laser pulse decays primarily due to Fresnel
losses at the crystal faces and cavity mirrors. The major ad-
vantage of the outlined recirculation scheme compared to
active (electro-optic or accousto-optic) pulse switching is
that the pulse traverses a significantly thinner optical mate-
rial. Conversion efficiency, r/ c IL2, where I is the pulse
intensity and L is the crystal thickness. A 1 mm thick BBO
crystal efficiently frequency doubles pulses at incident in-
tensities of X10 GW/cm2. A typical thickness for a Pock-
els cell and a waveplate is 1 cm. For short, high peak
power pulses, nearly an order of magnitude decrease in the
length of the traversed medium reduces pulse dispersion
and nonlinear phase accumulation that ultimately leads to
beam break-up. Resonant cavity coupling techniques have
so far been demonstrated for low peak power pulses.
Figure 1: RING cavity principle
We have completed a proof of principle experiment
which demonstrates the RING scheme [Fig 2(a)] . The
experiment was conducted at the Advanced Petawatt Con-
cepts facility at LLNL . The incident pulse was at
1053 nm, 10 Hz, chirped from its 250 fs transform limit
to 10 ps, with a pulse energy of 1 mJ. The spatial profile of
the incident beam was nearly gaussian in space, with a spot
size of 3 mm and nearly flat-top in time. A lens telescope
collimated the laser beam before the resonator. 80 pJ of
2o light was generated with an anti-reflection (AR) coated
1.5 mm type I BBO crystal. Flat 1" dichroic cavity mirrors
were e-beam coated by Lattice Electro-Optics to achieve
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Jovanovic, I; Anderson, S G; Betts, S M; Brown, C; Gibson, D J; Hartemann, F V et al. High-energy Picosecond Laser Pulse Recirculation for Compton Scattering, article, June 12, 2007; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc899629/m1/3/: accessed November 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.