The laser-to-fast-electron coupling efficency is higher with p-polarized light in wedge targets than with s-polarization. ⢠2-D OSIRIS simulations are in agreement ...
Intense Laser-to-Fast-Electron Coupling Efficiency in Wedge-Shaped Cavity Targets FSC Single-photon spectrograph 8 keV Crystal HOPG spectrometer
Wedge target
18 nm 110 nm
Crystal imager Laser beam
W. Theobald University of Rochester Laboratory for Laser Energetics
50th Annual Meeting of the American Physical Society Division of Plasma Physics Dallas, TX 17–21 November 2008
Summary
Intense lasers produce more hot electrons in narrow wedge-shaped cavity targets than on flat foils FSC • The Ka emission from solid Cu wedge-shaped, small-mass targets was measured for various opening angles and polarizations. • The laser-to-fast-electron coupling efficency 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
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Collaborators FSC B. Eichman, S. Ivancic, 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 Department of Physics, Ohio State University
R. B. Stephens General Atomics
Wedge-cavity targets are used to study the fast-electronconversion efficiency for 2-D cone-like target geometries Front view Laser-beam spot
Side view 45º, s-pol
E-field 18 nm 105 nm
110 nm
30º, s-pol
• One-piece Cu targets with ~100 # 100 # 40-nm3 volume and 30º, 45º, and 60º opening angles • Radius of curvature (~1 nm) smaller than the focal-spot diameter • Wedge target orientation sets laser polarization E17223a
V. M. Ovchinnikov (JP6.00114).
The experiments were performed on the Multi-Terawatt (MTW) Laser Facility at LLE FSC Wedge
z y
Laser
x
OAP
XCCD i = 62.4º z = 59.7º HOPG Imager i = 135º i = 90º z = 0º z = 35.5º
φ Small-mass Cu targets are in the refluxing regime.*
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Increased focus area
Focus position • Laser parameters: m = 1.053 nm 5 J, 1 ps, 5-nm focus diameter (f/2 optics) 1 # 1019 W/cm2 peak intensity • Spatially and temporally averaged laser intensity on target: 2 to 5 # 1018 W/cm2 • OPCPA amplification provides a high temporal contrast (C > 108)
*A. J. MacKinnon et al., Phys. Rev. Lett. 88, 215006 (2002); J. Myatt et al., Phys. Plasmas 14, 056301 (2007); P. M. Nilson et al., Phys Plasmas 15, 056308 (2008).
A spherical Bragg crystal imager recorded spatially resolved the Cu Ka emission Flat Foil
Wedge
100nm
100nm
2125
2117 Homogeneous emission; indicates hot-electron refluxing in the targets.
8-keV Ka fluor CCD
Laser E17224a
S, 60º
Bragg crystal
2.5
XCCD
2.0
Ka1 and Ka2
1.0 5.0
Kb
0 7500
8000 8500 9000 Photon energy (eV)
9500
Photon number (# 1010 1/eV)
Photon number (#109 1/eV)
The single-hit CCD and the HOPG provide absolute photon numbers to infer the conversion efficiency* FSC 4 HOPG Ka1 3 2
Ka2
1 0 8000
Kb 8050 8850 8900 Photon energy (eV)
8950
• K-photon generation calculated as in an infinite medium • Relativistic K-shell-ionization cross sections included • Classical slowing-down approximation (CSDA) • Exponential hot-electron distribution with ponderomotive scaling E17441
*W. Theobald et al., Phys. Plasmas 13, 043102 (2006). P. M. Nilson et al., Phys. Plasmas 15, 056308 (2008).
40
XCCD
p-polarization s-polarization
30 20 10 0
Calibration uncertainty 0
20
Flat foil
60 160 180 200 40 Opening angle (º)
Conversion efficiency (%)
Conversion efficiency (%)
The conversion efficiency of laser-to-fast-electron energy depends on wedge opening angle and laser polarization FSC 50
HOPG
p-polarization s-polarization
40 30 20 Calibration uncertainty
10 0
0
20
40 60 160 180 200 Opening angle (º)
Fast-electron-conversion efficiency increases for the narrow wedge targets compared to flat foils. E17442
Flat foil
Conversion Efficiency (%)
Two-dimensional OSIRIS* simulations are in agreement with the experimental data for p-polarization but not for s-polarization FSC 40 30
Experiment p-polarization s-polarization
20
Simulation p-polarization s-polarization
Ln = 1 m
10 0
0º, Ln = 10 m 5º 0º
0 20 40 60 160 180 200 Cavity wedge opening angle (º)
0 20 40 60 160 180 200 Cavity wedge opening angle (º)
p-polarization absorption is higher than s-polarization due to resonance absorption/Brunel absorption.
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*R. A. Fonseca et al., in Computational Science–ICCS 2002 (Springer, Berlin, 2002), p. 343.
Summary/Conclusions
Intense lasers produce more hot electrons in narrow wedge-shaped cavity targets than on flat foils FSC • The Ka emission from solid Cu wedge-shaped, small-mass targets was measured for various opening angles and polarizations. • The laser-to-fast-electron coupling efficency 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
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• Laser pulse: I = 1019 W/cm2, 1 ps, 4-nm focus, Gaussian profiles in space and time • Transversal: periodic boundary condition for both fields and particles Longitudinal: thermal boundary conditions for particles and open boundary for fields
y-position (nm)
OSIRIS* 2-D PIC simulations were performed for the wedge targets FSC Initial density profile of 45º target 9 7 5 3 1 1 3 5 7 9 11 13 x-position (nm)
• The total laser absorption into electrons with kinetic energy above 8 keV was calculated
Number particles
• Linear-density ramp to 10 ncr in 0.1 nm to 10 nm 107 106 105 104
Wedge, 30º, p-pol, peak laser intensity T = 2.3 MeV 1 T = 8.7 MeV 2
103 102
20 30 1 10 Electron energy (MeV)
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*R. A. Fonseca et al., in Computational Science–ICCS 2002 (Springer, Berlin, 2002), p. 343
Low-density plasma generation from laser prepulses plays an important role in fast-electron generation FSC Linear density ramp from 0 to 10 ncr, 0º incidence
Conversion efficiency (%)
25
– 1-D hydro simulations with laser prepulse show that the angle of incidence affects the plasma profile
20 15
– Larger angles produce a steeper profile between critical and tenth-critical density
10 Conversion Efficiency (%)
5
0
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• 2-D OSIRIS simulations yield higher conversion efficiency for longer density ramps:
• The plasma scale length might change with wedge angle 0
2
4 6 8 10 D e ns ity R a m p (m ic r o n) Density ramp (nm)
12
– 2-D DRACO hydro simulations will be carried out to assess pre-plasma in the cavity
