We report on reproducible shock acceleration from irradiation of a $lambda = 10$ $mu$m CO$_2$ laser on optically shaped H$_2$ and He gas targets. A low energy laser prepulse ($Ilesssim10^{14}, {rm Wcm^{-2}}$) was used to drive a blast wave inside the
gas target, creating a steepened, variable density gradient. This was followed, after 25 ns, by a high intensity laser pulse ($I>10^{16}, {rm Wcm^{-2}}$) that produces an electrostatic collisionless shock. Upstream ions were accelerated for a narrow range of prepulse energies ($> 110$ mJ & $< 220$mJ). For long density gradients ($gtrsim 40 mu$m), broadband beams of He$^+$ and H$^+$ were routinely produced, whilst for shorter gradients ($lesssim 20 mu$m), quasimonoenergetic acceleration of proton was 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 PIC simulations.
Spectrally-peaked proton beams ($E_{p}approx 8$ MeV, $Delta Eapprox 4$ MeV) have been observed from the interaction of an intense laser ($> 10^{19 }$ Wcm$^{-2}$) with ultrathin CH foils, as measured by spectrally-resolved full beam profiles. These be
ams are reproducibly generated for foil thicknesses (5-100 nm), and exhibit narrowing divergence with decreasing target thickness down to $approx 8^circ$ for 5 nm. Simulations demonstrate that the narrow energy spread feature is a result of buffered acceleration of protons. Due to their higher charge-to-mass ratio, the protons outrun a carbon plasma driven in the relativistic transparency regime.
Observations of the interaction of an intense {lambda}0 approx 10 {mu}m laser pulse with near-critical overdense plasmas (ne = 1.8 - 3 nc) are presented. For the first time, transverse optical probing is used to show a recession of the front surface
caused by radiation pressure driven hole-boring by the laser pulse with an initial velocity > 10^6 ms-1, and the resulting collisionless shocks. The collisionless shock propagates through the plasma, dissipates into an ion-acoustic solitary wave, and eventually becomes collisional as it slows further. These conclusions are supported by PIC simulations which show that the initial evolution is dominated by collisionless mechanisms.
