Spectral Modification of Shock Accelerated Ions Using a Hydrodynamically Shaped Gas Target.

We report on reproducible shock acceleration from irradiation of a λ=10 µm CO_{2} laser on optically shaped H_{2} and He gas targets. A low energy laser prepulse (I≲10^{14} W cm^{-2}) is used to drive a blast wave inside the gas target, creating a steepened, variable density gradient. This is followed, after 25 ns, by a high intensity laser pulse (I>10^{16} W cm^{-2}) that produces an electrostatic collisionless shock. Upstream ions are accelerated for a narrow range of prepulse energies. For long density gradients (≳40 µm), broadband beams of He^{+} and H^{+} are routinely produced, while for shorter gradients (≲20 µm), quasimonoenergetic acceleration of protons is observed. These measurements indicate that the properties of the accelerating shock and the resultant ion energy distribution, in particular the production of narrow energy spread beams, is highly dependent on the plasma density profile. These findings are corroborated by 2D particle-in-cell simulations.

Effect of lattice structure on energetic electron transport in solids irradiated by ultraintense laser pulses.

The effect of lattice structure on the transport of energetic (MeV) electrons in solids irradiated by ultraintense laser pulses is investigated using various allotropes of carbon. We observe smooth electron transport in diamond, whereas beam filamentation is observed with less ordered forms of carbon. The highly ordered lattice structure of diamond is shown to result in a transient state of warm dense carbon with metalliclike conductivity, at temperatures of the order of 1-100 eV, leading to suppression of electron beam filamentation.