Energy transport experiments on VULCAN PW

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Energy transport experiments on VULCAN PW

• Dr Kate Lancaster
• Central Laser Facility
• CCLRC Rutherford Appleton Laboratory

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Acknowledgements
K. L. Lancaster, P.A.Norreys, J. S. Green ,
Gianlucca Gregori, R. Heathcote Central Laser
Facility, CCLRC Rutherford Appleton Laboratory,
UK. C. Gregory Department of Physics, University
of York, Uk. K. Krushelnick Blackett
Laboratory, Imperial College, UK M. H.
Key Lawrence Livermore National Laboratory, CA,
USA Also at University of California, Davis M.
Nakatsustumi T. Yabuuchi H. Habara, M. Tampo, R.
Kodama, Institute of Laser Engineering, Osaka
University, Japan R.Stephens General Atomics,
San Diego, CA, USA C. Stoeckl, W. Theobald, M.
Storm Laboratory of Laser Energetics, University
of Rochester, NY, USA R.R. Freeman, L. Van
Workem, R. Weber, K. Highbarger, D. Clark, N.
Patel Ohio State University, Columbus, Ohio,
USA S. Chen, F. Beg University of California,
San Diego
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Overview

• Motivation for the work
• Experimental arrangements and diagnostics
• XUV imaging data
• Shadowgraphs
• Al Spectroscopy data
• Atomic Kinetic code modelling and results
• Vlasov-Fokker-Plank modelling and results
• Conclusions

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Purpose of work
Ultra intense laser
Cone / Shell
Hot electrons are generated when an ultra intense
laser is focused into the gold cone. Goal is to
investigate how energy is transported to the
compressed deuterium fuel via the hot electrons
and ions.
Hot electrons
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Experimental setup
Targets
256 eV XUV multilayer mirror
2w probe system
2w probe system
CH-Al-CH targets with and without CH 40o flare
angel cone
Laser 300J, 1ps, l1.05mm I5x1020 Wcm-2
Assuming 30 energy contained in 7mm spot.
Parabola
X-ray crystal spectrometer
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XUV imaging
Multilayer mirror
Target
A Spherical multilayer mirror images rear surface
emission on to a Princeton Instruments large area
16 bit CCD camera.
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Aluminium x-ray spectroscopy
Centre of crystal
source
Central radius
Detector plane
Crystal centre
Target
12.5cm
12.5cm
Hall configuration conical crystal
spectrometer CsAP conically curved crystal
range 6.2 8.4 A Detector Fuji-film BAS image
plate with Be Filter
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Transverse optical probe
Part of the main beam was frequency doubled laser
and used to probe the interaction in the
transverse direction. This was split and used as
dual probe system to allow probing at 0 and 40
degrees Scattered and collimated light imaged on
to 16 bit Andor CCD camera
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256eV XUV images
No cone
Cone
Average FWHM 69 mm
Average FWHM 38 mm
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Shadowgraphs of rear surface
No cone
Cone
85mm
370mm
CH-Al-CH (4-0.2-4mm), no cone, t0 400ps
CH-Al-CH (4-0.2-4mm), CH cone, t0 400ps
Shadowgraph of slab without cone geometry shows
regular expansion pattern of transverse size
370mm. Shadowgraph of slab with cone geometry
shows a smaller transverse region of expansion of
size 85mm although longitudinal extent is
approximately the same.
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Discussion of cone geometry
Including cone geometry changes the transport
pattern somewhat in both shape and lateral
extent The extra density of the cone wall that
the lateral fast electrons travel through should
not effect the rear expansion much There may
therefore be fields due to the cone geometry
which act to confine the energy at the cone tip
Focusing effects were reported by Sentoku et al
where quasi-static magnetic and electrostatic
sheath fields guide electron flow
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Aluminium spectra
Ly a
He a
From the spectra the Lyman a line drops with the
addition of a cone This suggests the temperature
of the Al layer falls in this situation
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Modelling of spectra
The synthetic spectra for single temperatures and
densities were generated using a code that
combines collisional radiative atomic kinetics
with spectroscopic quality radiation transport
and stark broadening effects
T 610 eV, n1024 el/cc
cone
No cone
T 790 eV, n7x1023 el/cc
Under these conditions the code failed to
reproduce the line profiles of the He b and He g
lines
U. Andiel et al, Europhysics letters 60 861
2002
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Revised atomic model

• To try to reproduce the He b and He g lines it
  was necessary to implement new physics in the
  collisional radiative atomic kinetics code
• Effects of Li-like Hollow atom states
• Non-thermal electron distributions
• Atomic structure and processes calculated using
  Flexible Atomic Code (FAC)
• It is proposed that non-thermal electron
  distributions in combination with hollow atom
  states may act as a conduit to enhanced He b and
  He g lines

M. F. Gu, Astrophysical Journal 582 1241 2003
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Distribution of return current may be
non-Maxwellian
The best fit to the spectra was produced when a
two temperature electron distribution was used
with Tc100 eV and TH800ev (where 40 of the
population was at TH).
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KALOS simulations

• In order to examine the distribution of electrons
  in the return current modeling was performed with
  KALOS
• KALOS was in this case a1D 2P relativistic
  Vlasov-Fokker-Planck code (for details see
  A.R.Bell et al PPCF 48 2006 R37).
• Simulation conditions
• Fast electron generation consistent with an
  intensity 3.5 x 1020 Wcm-2 in 700fs
• Reflective rear boundary
• Fast electron distribution relativistic
  maxwellian
• Fully ionised slab at 100ev initial temp

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KALOS results
The buried Al layer is raised to a temperature of
720 eV, in agreement with the experimental
result The return current departs from a Spitzer
description at the edges of the buried layer This
is due to non-Maxwellian component in the return
current This may help to explain the enhanced He
b and He g emission
Dotted line without enhanced ne Solid line
with enhanced ne
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Conclusions
Experiments were performed using buried CH-Al-CH
slabs with and without CH cone geometry XUV
images and Shadowgraphs reveal that the transport
pattern changes between the two geometries from a
ring structure with no cone to a smaller solid
emission region with a cone. This may be due to
self generated fields causing the electrons to
concentrate at the cone tip Al spectroscopy of
the buried layer reveals a slight drop in
temperature in going from no-cone geometry (790
eV) to cone geometry (610 eV) Enhanced He b and
He g emission suggest that new physics must be
considered when modelling PW laser interactions
such as non-maxwellian return currents and hollow
atom states. A VFP code shows that the buried
layer causes a departure from Spitzer behaviour
at the layer edges that is due to a
non-maxwellian component of the return current.