LSP Calculations of ConeWire Experiments

1
LSP Calculations of Cone-Wire Experiments

• Presented to
• 8th International Workshop on
• Fast Ignition of Fusion Targets
• Tarragona, Spain
• R. P. J. Town
• AX-Division
• Lawrence Livermore National Laboratory
• June 30, 2005

2
Collaborators

• L. A. Cottrill, M. H. Key, W. L. Kruer, A. B.
  Langdon, B. F. Lasinski, B. C. McCandless, H. S.
  Park, B. A. Remington, R. A. Snavely, C.
  H. Still, M. Tabak, S. C. Wilks, LLNL, Livermore,
  CA, USA.
• J. F. Myatt, LLE, Rochester, NY, USA.
• D. R. Welch, MRC, Albuquerque, NM, USA.

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We have performed LSP calculations of recent RAL
experiments
Summary

• Experiments have been performed at RAL using
  various targets that are important for a range of
  programs
• Ka radiography
• Isochoric heating
• Electron transport for fast ignition and
• Neutron star atmosphere.
• To more directly compare to these experiments we
  have modified LSP to create photons in a
  non-interfering way.
• We have performed LSP simulations of small planar
  and cone-wire targets and calculated the Ka
  yields.

4
We have performed LSP simulations of two types of
low-mass targets

• 1D Flag targets

• 2D Cone-wire targets

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LSP1 can model larger, more dense plasmas for
longer simulation times than explicit PIC codes

• LSP uses
• a direct implicit energy-conserving
  electromagnetic algorithm
• hybrid fluid-kinetic descriptions for electrons
  with dynamic reallocation and
• inter-and intra-species collisions based on
  Spitzer collision frequencies.
• The simulations reported here used
• 2-D cylindrical geometry
• a fixed ionization state throughout the
  simulation
• an ideal gas EOS and
• a hot electron beam created by promotion from the
  background plasma.
• To generate photons a medium must be inserted
  into the plasma. The medium uses the Monte-Carlo
  ITS2 kernel. This model was modified so that the
  only effect of the medium was to generate photons.

1D. R. Welch, et al, Nucl. Inst. Meth. Phys. Res.
A 242, 134 (2001).
2J. A. Halbleib, et al, IEEE Trans. Nucl. Sci.
NS-39, 1025 (1992).
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Currently, we use scaling laws to establish the
hot electron parameters from the laser intensity

• Conversion Efficiency
• ? 0.000175 I(W/cm2)0.2661

• Hot Electron Energy
• Pondermotive scaling
• Thot(MeV) (Il2/(1019W/cm2mm2))1/2
• Beg scaling
• Thot(MeV) 0.1(Il2/(1017W/cm2mm2))1/3

• We have also taken electrons from our explicit
  PIC code, Z3, and used them as the source in LSP.

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These scaling laws were applied to the RAL
Petawatt laser pulse

• Expect 80J of electrons injected (30 conversion
  efficiency)
• Translates into 2.8x1014 electrons with an
  average energy of 1.7 MeV
• The peak beam energy was 6 MeV

Peak of laser pulse
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The hot electrons fill the planar target during
the laser pulse and are confined by the large
surface electric fields
0.25 ps
0.5 ps
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By 1 ps, the hot electrons have flooded the
target and are constrained by the electric field
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Using a non-interfering medium model we can
record the birth position of the photons

• Energy into K-shell photons 2.7 kJ/g of Cu
  fluor.
• K-shell conversion efficiency from electrons
  0.03

Calculated fwhm 18 mm
Ka
Experimental data
Kb
Bremsstrahlung
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50mm radius target simulations confirm the effect
of refluxing

• Energy into K-shell photons 54 kJ/g of Cu fluor
• K-shell conversion efficiency from electrons
  0.04

R50mm 20mm spot size
R200mm 18mm spot size
The smaller target is significantly brighter
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However, these small targets lead to high
background electron temperatures

• The simulation used ideal gas EOS and a fixed
  ionization state.
• We are currently adding QEOS tables to LSP.

Fluid electrons converted to particles
These high temperatures will cause line shifts
13
A non-LTE atomic physics model has recently been
added to LSP to account for thermal shifts

• Time and space integrated spectra have been
  obtained that clearly show shifts in the Ka lines

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LSP simulations have also been performed of the
2-D cone-wire targets

• The bulk of the hot electron travel inside the
  wire, but some ride along the sheath at the
  wires edge.
• The return current heats the background plasma
  electrons to similar temperatures to the planar
  targets.

Time 1.0ps
Time 0.3ps
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Reduced filamentation was observed when a
transverse beam temperature was added
Time 0.3 ps
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K-shell photons are produced along the length of
the wire

• Energy into K-shell photons 26 kJ/g of Cu fluor.
• K-shell conversion efficiency from electrons
  0.03

17
We have performed LSP calculations of recent RAL
experiments
Summary

• Experiments have been performed at RAL using
  various targets that are important for a range of
  programs
• Ka radiography
• Isochoric heating
• Electron transport for fast ignition and
• Neutron star atmosphere.
• To more directly compare to these experiments we
  have modified LSP to create photons in a
  non-interfering way.
• We have performed LSP simulations of small planar
  and cone-wire targets and calculated the Ka
  yields.