Metal foil targets were irradiated with $1\mu \mathrm{m}$ wavelength ($\lambda$) laser pulses of 5 ps duration and focused intensities ($I$) of up to $4\times {10}^{19}\mathrm{W}{\mathrm{cm}}^{-2}$, giving values of both $I{\lambda }^2$ 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 $K_{\alpha }$ 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 $I{\lambda }^2$ 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.

• Received 14 May 2007
• Corrected 15 January 2008

DOI:https://doi.org/10.1103/PhysRevLett.100.015003