Intense Laser-to-Fast Electron Coupling in Wedge Shaped Cavity TargetsS. Ivancic, W. Theobald, F. J. Marshall, B. Eichman, 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 EnergeticsV. Ovchinnikov, L. Van Woerkom, and R. R. Freeman, , Ohio State University, Department of PhysicsR. B. Stephens, General Atomics

MotivationIn the fast ignition scheme, the hot spot is not formed via target compression but by a separate beam of energetic particles. These particles penetrate into the compressed target and deposit their energy, triggering ignition and subsequent nuclear burn. One method of producing energetic particles is by irradiating solid targets with ultrashort high intensity laser pulses. In this experiment we studied the coupling efficiency between the laser and production of hot electrons by varying the geometry of the wedge and the polarization of the light. The conversion efficiency is inferred by measuring the x-ray florescence of the target and comparing to model calculations.

AbstractExperiments performed on LLE’s Multi-Terawatt laser (MTW) investigated the laser to hot electron conversion efficiency in wedge shaped small mass copper foils. The study found an increased production of hot electrons with narrower wedge opening for p-polarized light. A spherical Bragg crystal imaged x-ray florescence of the target while a single hit photon spectrograph and a graphite crystal spectrometer (HOPG) made absolute measurements of x-ray photon production. Higher numbers of x-ray photons are produced when a component of the laser electric field is normal to the foil surface (p-polarization) than with s-polarization. 2-D particle-in-cell simulations are in good agreement for p-polarized targets, but not for s-polarized targets.

Particle-in-cell SimulationSeveral two-dimensional simulations of the target were carried out using the particle-in-cell code OSIRIS.2 The number of hot electrons generated by the laser was counted and the conversion efficiency was calculated and compared to the experimental results. The simulation results show good agreement in the p-polarized case; however, the s-polarization runs show significantly lower conversion efficiency. The coupling between laser radiation and hot electron production also shows sensitivity to the preplasma formation in the focal spot.

Target Dimensions and Laser Parameters• 30, 45 and 60 degree wedges were tested

• 5J, 1054 nm, 1 ps pulse

• 4µm focus diameter (FWHM)

• Peak intensity ~1019 W cm-2

• Targets produced by General Atomics

• Refluxing targets confine most fast e-

Homogenous emission indicates refluxing in the target. 1

References1 P. M. Nilson et al., Phys Plasmas 15, 056308 (2008).2 R. A. Fonseca et al., in Computational Science–ICCS 2002 (Springer, Berlin, 2002), p. 343.

p-polarization absorption is higher than s-polarization due to resonance absorption/Brunel absorption.

• 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

Conclusions• The Ka emission from solid Cu wedge-shaped, small-mass targets was measured for various opening angles and polarizations.

Intense lasers produce more hot electrons in narrow wedge-shaped cavity targets than on flat foils

• The laser-to-fast-electron coupling efficiency 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

Flat

HOPG reflectivity was measured for absolute calibration