Effect of Laser Intensity on Fast-Electron-Beam Divergence in Solid-Density Plasmas

Abstract

Metal foil targets were irradiated with $1\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ wavelength ($\ensuremath{\lambda}$) laser pulses of 5 ps duration and focused intensities ($I$) of up to $4\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{ }\mathrm{W}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$, giving values of both $I{\ensuremath{\lambda}}^{2}$ and pulse duration comparable to those required for fast ignition inertial fusion. The divergence of the electrons accelerated into the target was determined from spatially resolved measurements of x-ray ${K}_{\ensuremath{\alpha}}$ emission and from transverse probing of the plasma formed on the back of the foils. Comparison of the divergence with other published data shows that it increases with $I{\ensuremath{\lambda}}^{2}$ and is independent of pulse duration. Two-dimensional particle-in-cell simulations reproduce these results, indicating that it is a fundamental property of the laser-plasma interaction.