Electron, photon, and ion beams from the relativistic interaction of Petawatt laser pulses with solid targets

In recent Petawatt laser experiments at Lawrence Livermore National Laboratory, several hundred joules of 1 μm laser light in 0.5–5.0-ps pulses with intensities up to ${\text{3×10}}^{20} {\mathrm{W cm}}^{\mathrm{-2}}$ were incident on solid targets and produced a strongly relativistic interaction. The energy content, spectra, and angular patterns of the photon, electron, and ion radiations have all been diagnosed in a number of ways, including several novel (to laser physics) nuclear activation techniques. About 40%–50% of the laser energy is converted to broadly beamed hot electrons. Their beam centroid direction varies from shot to shot, but the resulting bremsstrahlung beam has a consistent width. Extraordinarily luminous ion beams (primarily protons) almost precisely normal to the rear of various targets are seen—up to ${\text{3×10}}^{13}$ protons with ${\mathrm{kT}}_{\mathrm{ion}}\sim \mathrm{several MeV}$ representing ∼6% of the laser energy. Ion energies up to at least 55 MeV are observed. The ions appear to originate from the rear target surfaces. The edge of the ion beam is very sharp, and collimation increases with ion energy. At the highest energies, a narrow feature appears in the ion spectra, and the apparent size of the emitting spot is smaller than the full back surface area. Any ion emission from the front of the targets is much less than from the rear and is not sharply beamed. The hot electrons generate a Debye sheath with electrostatic fields of order MV per micron, which apparently accelerate the ions.