Transcript interview thing - University of Rochester

Energy transport experiments on
VULCAN PW
Dr Kate Lancaster
Central Laser Facility
CCLRC Rutherford Appleton Laboratory
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
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
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
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, l=1.05mm
I=5x1020 Wcm-2
Assuming 30% energy
contained in 7mm spot.
Parabola
X-ray crystal
spectrometer
XUV imaging
28o
Multilayer
mirror
Target
Large area
CCD
A Spherical multilayer mirror images rear surface emission on to a
Princeton Instruments large area 16 bit CCD camera.
Aluminium x-ray spectroscopy
Centre of crystal
Detector
plane
Crystal centre
Central radius
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
source
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
256eV XUV images
No cone
Average FWHM – 69 mm
Cone
Average FWHM – 38 mm
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.
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 quasistatic magnetic and electrostatic
sheath fields guide electron flow
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
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*
cone T = 610 eV, n=1024 el/cc
No cone
T = 790 eV, n=7x1023 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
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
Distribution of return current may be nonMaxwellian
The best fit to the spectra was produced when a two temperature electron
distribution was used with Tc=100 eV and TH=800ev (where 40% of the
population was at TH).
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 VlasovFokker-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
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
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 nonmaxwellian 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.