Isochoric heating from fast electrons using mass limited targets

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Isochoric heating from fast electrons using mass
limited targets
Michel Koenig - LULI Warm Dense Matter
Workshop Porquerolles, France - June 13-16 2007
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OUTLINE
Introduction
Principles and set-up of the experiment
Results
Conclusions
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How can we move in the phase diagram ?
Idea is to change initial conditions P0, T0, ?0
Different initial density Porous material
shocks
Precompressed target (Collins et al. 2001)
Isentropic Compression Experiment
Isochoric heating Proton beam Laser beam Electron
beam
Requires short pulses
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Can we trap the electrons in the target ?
Hea
ions
e-
accelerated ions
Ka
e- refluxes

• If the target is isolated, most of the fast
  electrons should stay confined in and around the
  target by the electrostatic potential
• The fast electrons energy is transferred
• to heat the target bulk
• to ion acceleration and radiation
• Smaller is the target higher is the expected
  temperature

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What have been obtained up to now ?
Temperatures in the range of a few eV have been
measured using optical diagnostics (rear side
self-emission reflectivity) or aluminium
shifted K? spectra
E. Martinolli et al., Phys. Rev. E 73, 046402
(2006)
Heating was obtained with massive targets We
measured 2 eV/J
R. Kodama et al., Nature 418, 933 (2002)
Using small isolated targets, maximum
temperatures increases up to a few hundred eV
S. Wilks et al., IFSA 2005 K. Akli et al., PoP,
14 023102 (2007) W. Theobald et al., PoP, 13
043102 (2006)
Measurement was obtained "indirectly" without any
detailed x-ray spectra
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Different target masses
Al
Cu
V
Laser
The vanadium layer prevents from irradiating
directly Cu It acts as a converter layer for the
electrons
Two "types" of targets
Same thicknesses 0.2 µm V- 5 µm Cu - 5 µm Al
The surface varies from ?300 down to ?50
mass varies from 4 µg down to 0.1 µg
Different Cu thicknesses 5 - 10 - 20 µm
Targets were electrically isolated by using
either SiO2 or C thin stalks
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Experimental set-up
Cu-K? 2D imager (8.04 KeV) Spherical Bragg
crystal
X-CCD
Laser ? 1.057µm E 17 J (on target) I
2.1019 Wcm-2 Focal Spot 10µm f/3 off axis
parabola
Gated Optical Imager ?? 420500 nm
Target

• 2 X-ray spectrometers
• (time integrated resolved)
• Conical Bragg crystals
• ? 7.58.4 Å

Contrast -gtA.S.E. Iase/I 10-7 500ps
before the pulse
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2D Cu K? imaging
Spherical quartz cristal
200 x 200 µm
M8
Resolution was lt 10 µm
This diagnostic gives
Size of the source
Intensity at a given energy (Cu K? 8.05 keV)
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2D Cu results
20 µm Cu - 20 µm Al
For "massive" targets The total intensity is
almost constant
For low mass targets The total intensity
decreases
5 µm Cu - 5 µm Al
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Low mass targets Cu2D intensity
Our spherical crystal selects a short energy
range 10 eV due to Bragg reflection
When the target mass decreases, the temperature
rises. The "cold" K? line becomes weaker and
weaker because ionisation becomes important.
The mean target temperature can be deduced
K. Akli et al., PoP, 14 023102 (2007)
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Conical spectrometer
7.6Å
Cu Ka
This diagnostic gives
Al lines at 1st order K? He-?
Al Ka
Cu K? lines at 5th order
Time resolution
8.4Å
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Time integrated spectra
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Deduced temperature from Cu spectra
We did calculations using FLYCHK with a hot
electron temperature Thot 500 keV and a ratio
of electron density Nehot/Ne 0.1
We do observe lines which only occur at very high
temperatures for the ?50 target Are features due
to direct Cu heating by laser or hydrodynamic
expansion effect or non uniform energy deposition
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Test on small Cu target
? 300 µm - 5 µm Cu
K?
K?
No "hot" lines come from the front surface
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Time resolved spectra
We compare He-? time emission for two different
target masses keeping the same area
5 µm Cu
? 50 µm
20 µm Cu
The streak time resolution is lt 2 ps
Reducing the mass (0.4 down to 0.1 µg), the He-?
time duration is almost doubled
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Time resolved spectra
We also compare K? time emission for the same
targets
5 µm Cu
? 300 µm
20 µm Cu
5 ps
The K? time emission remains identical regarding
the mass 5 ps
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Hydrodynamic expansion
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Time Integrated spectra
Hydrodynamic expansion after a few ps cannot be
ignored
With hydro expansion taken into account for a 450
eV initial solid target
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Hybrid simulation shows a non uniform heating
Simulation of 1J of electron at 400 keV injected
in 5µm Cu / 5µm Al target

• Pâris simulation shows that the fast electron
  energy is transferred by collision e--e- to the
  target.
• With 1J of electron the target temperature reach
  200eV
• Energy deposition is non uniform

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Conclusions
Using isolated mass limited targets, solid
targets were heated up to 400 eV
As Cu is heated, ionization balance changes and
cold K? broadens and disappears Hydrodynamic
expansion causes decompression of Al diagnostic
layer in a few ps timeframe of observation
Lower density expanding plasma dominates the
measured Al spectra gt Emission of Al He-?
appears optically thin

• Optically thin tracer layers are needed to
  determine more accurately the temperature
• Spatial resolution of the spectra will allow to
  infer the energy transfer process
• Dynamics of deposited energy -gt fast e- transport

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COLLABORATORS

• S. Baton, P. Guillou, J. Fuchs, P. Audebert, L.
  Lecherbourg, S. Bastiani, B. Barbrel, T. Vinci
• C. Rousseaux, L. Gremillet
• C. Back
• P. Patel