Hot Electron Temperature and Coupling Efficiency Scaling with Prepulse for Cone-Guided Fast Ignition

The effect of increasing prepulse energy levels on the energy spectrum and coupling into forward-going electrons is evaluated in a cone-guided fast-ignition relevant geometry using cone-wire targets irradiated with a high intensity (${10}^{20}\mathrm{W}/{\mathrm{cm}}^2$) laser pulse. Hot electron temperature and flux are inferred from $K\alpha$ images and yields using hybrid particle-in-cell simulations. A two-temperature distribution of hot electrons was required to fit the full profile, with the ratio of energy in a higher energy (MeV) component increasing with a larger prepulse. As prepulse energies were increased from 8 mJ to 1 J, overall coupling from laser to all hot electrons entering the wire was found to fall from 8.4% to 2.5% while coupling into only the 1–3 MeV electrons dropped from 0.57% to 0.03%.

Figure 1

(a) Schematic of the cone-wire target and diagnostic geometry. (b) Example of the 2D spatially resolved image of $K\alpha$ emission along the wire.

Figure 2

Transversely integrated lineouts along the Cu wire from the $K\alpha$ imager for 17 mJ of prepulse (pink), 100 mJ (green), 500 mJ (blue), and 1 J (orange). All profiles are normalized to their peak. (inset) Total $K\alpha$ conversion efficiencies in the wire as a function of prepulse. The statistical shot-to-shot variation, as well as the systematic error in the absolute $K\alpha$ yield were added in quadrature to provide the $\sim \pm 40\%$ error bars applied to all points.

Figure 3

(a) Single-temperature distribution fits (dotted) cannot fully capture the pattern of $K\alpha$ emission along the 1.5 mm long wire. A two-temperature electron distribution (solid) is required to fit the experimental profile (solid, bold). (b) The inferred overall laser-to-electron and laser-to-1–3 MeV electron conversion efficiency over prepulse energies ranging from 10–1000 mJ.

Figure 4

(a) Hot electron number density contour at 6 ps. The trajectory of a 2 MeV test particle injected at the wire edge ($z=600$, $r=19\mathrm{um}$) is shown up to 6 ps with the white line. The electron is seen to surf along the wire and is turned at the rear surface by the ${\mathbf{E}}_{\mathbf{z}}$ and ${\mathbf{B}}_{\theta }$ fields. (b) Radial $\mathbf{E}$ and azimuthal $\mathbf{B}$ fields before and after the electron bunch has reached the end of the wire.