Intense Laser-to-Fast Electron Coupling in Wedge Shaped Cavity Targets

1
Intense Laser-to-Fast Electron Coupling in Wedge
Shaped Cavity Targets
S. Ivancic, W. Theobald, F. J. Marshall, B.
Eichman, P. M. Nilson, C. Stoeckl, J. F. Myatt,
J. A. Delettrez, C. Ren, J. D. Zuegel, and T. C.
Sangster, University of Rochester, Laboratory for
Laser Energetics V. Ovchinnikov, L. Van Woerkom,
and R. R. Freeman, , Ohio State University,
Department of Physics R. B. Stephens, General
Atomics
Abstract Experiments performed on LLEs
Multi-Terawatt laser (MTW) investigated the laser
to hot electron conversion efficiency in wedge
shaped small mass copper foils. The study
found an increased production of hot electrons
with narrower wedge opening for p-polarized
light. A spherical Bragg crystal imaged x-ray
florescence of the target while a single hit
photon spectrograph and a graphite crystal
spectrometer (HOPG) made absolute measurements of
x-ray photon production. Higher numbers of x-ray
photons are produced when a component of the
laser electric field is normal to the foil
surface (p-polarization) than with
s-polarization. 2-D particle-in-cell simulations
are in good agreement for p-polarized targets,
but not for s-polarized targets.

• Target Dimensions and Laser Parameters
• 30, 45 and 60 degree wedges were tested
• 5J, 1054 nm, 1 ps pulse
• 4µm focus diameter (FWHM)
• Peak intensity 1019 W cm-2
• Targets produced by General Atomics

Motivation In the fast ignition scheme, the hot
spot is not formed via target compression but by
a separate beam of energetic particles. These
particles penetrate into the compressed target
and deposit their energy, triggering ignition and
subsequent nuclear burn. One method of producing
energetic particles is by irradiating solid
targets with ultrashort high intensity laser
pulses. In this experiment we studied the
coupling efficiency between the laser and
production of hot electrons by varying the
geometry of the wedge and the polarization of the
light. The conversion efficiency is inferred by
measuring the x-ray florescence of the target and
comparing to model calculations.
HOPG reflectivity was measured for absolute
calibration

• K-photon generation calculated as in an infinite
  medium

Particle-in-cell Simulation Several
two-dimensional simulations of the target were
carried out using the particle-in-cell code
OSIRIS.2 The number of hot electrons generated
by the laser was counted and the conversion
efficiency was calculated and compared to the
experimental results. The simulation results
show good agreement in the p-polarized case
however, the s-polarization runs show
significantly lower conversion efficiency. The
coupling between laser radiation and hot electron
production also shows sensitivity to the
preplasma formation in the focal spot.
Homogenous emission indicates refluxing in the
target. 1

• Relativistic K-shell-ionization cross sections
  included
• Classical slowing-down approximation (CSDA)
• Exponential hot-electron distribution with
  ponderomotive scaling

• Refluxing targets confine most fast e-

Conclusions The Ka emission from solid Cu
wedge-shaped, small-mass targets was measured for
various opening angles and polarizations.
Intense lasers produce more hot electrons in
narrow wedge-shaped cavity targets than on flat
foils The laser-to-fast-electron coupling
efficiency is higher with p-polarized light in
wedge targets than with s-polarization. 2-D
OSIRIS simulations are in agreement with the
experimental data for p-polarization but not for
s-polarization
Flat
References 1 P. M. Nilson et al., Phys Plasmas
15, 056308 (2008). 2 R. A. Fonseca et al., in
Computational ScienceICCS 2002 (Springer,
Berlin, 2002), p. 343.
p-polarization absorption is higher than
s-polarization due to resonance absorption/Brunel
absorption.