Spectral modification of shock accelerated ions using hydrodynamically shaped gas target

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.

Buffered spectrally-peaked proton beams in the relativistic-transparency regime

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 beams 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.

Optical probing of shocks driven into overdense plasmas by laser hole-boring

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.

Monoenergetic proton beams accelerated by a radiation pressure driven shock

High energy ion beams (> MeV) generated by intense laser pulses promise to be viable alternatives to conventional ion beam sources due to their unique properties such as high charge, low emittance, compactness and ease of beam delivery. Typically the acceleration is due to the rapid expansion of a laser heated solid foil, but this usually leads to ion beams with large energy spread. Until now, control of the energy spread has only been achieved at the expense of reduced charge and increased complexity. Radiation pressure acceleration (RPA) provides an alternative route to producing laser-driven monoenergetic ion beams. In this paper, we show the interaction of an intense infrared laser with a gaseous hydrogen target can produce proton spectra of small energy spread (~ 4%), and low background. The scaling of proton energy with the ratio of intensity over density (I/n) indicates that the acceleration is due to the shock generated by radiation-pressure driven hole-boring of the critical surface. These are the first high contrast mononenergetic beams that have been theorised from RPA, and makes them highly desirable for numerous ion beam applications.