Laser-Accelerated Ions from a Shock-Compressed Gas Foil

We present results of energetic laser-ion acceleration from a tailored, near solid density gas target. Colliding hydrodynamic shocks compress a pure hydrogen gas jet into a $70\mu \mathrm{m}$ thick target prior to the arrival of the ultraintense laser pulse. A density scan reveals the transition from a regime characterized by a wide angle, low-energy beam (target normal sheath acceleration) to one of a more focused beam with a high-energy halo (magnetic vortex acceleration). In the latter case, three-dimensional simulations show the formation of a $Z$ pinch driven by the axial current resulting from laser wakefield accelerated electrons. Ions at the rear of the target are then accelerated by a combination of space charge fields from accelerated electrons and Coulombic repulsion as the pinch dissipates.

Figure 1

Illustration of the acceleration process. (a) An intense laser pulse drives a self-modulated wakefield, injecting and accelerating a high-energy electron beam from the ambient plasma. A $Z$ pinch of the trailing electrons and ions is driven by the high current beam. (b) The large magnitude and gradient of the magnetic field leads to an axial guiding center drift for a secondary electron beam moving out of the plasma. (c) The space-charge fields of the exiting electrons accelerate ambient plasma ions. The $Z$-pinched compressed ions explode out radially, while cold ions collectively escape with the electron beam.

Figure 2

A schematic of the experimental setup including the CR-39 detector stack. A 1 mm supersonic gas jet delivers the neutral gas while laser ignited strong hydrodynamic shocks act to shape it. The accelerated ions are then detected by CR-39 plates with varying Al filters.

Figure 3

False-color image of $10\times 5{\mathrm{cm}}^2$ CR-39 plates for the cases of $0.3n_{\mathrm{crit}}$ (left) and $0.6n_{\mathrm{crit}}$ (right). The detector covers a solid angle of 0.25 sr. The laser axis and vacuum spot size are included on both. Dashed horizontal lines represent the location of the filters and their lower-energy bound is listed on the left. On the bottom are plots of simulated test particles projected to the plane of the detector color coded to the experimental energy buckets. See Fig. 4 for contour plots of the simulated proton distribution.

Figure 4

The spatial distribution of test particles as projected to the plane of the detector for $0.3n_{\mathrm{crit}}$ (left) and $0.6n_{\mathrm{crit}}$ (right). The gray box indicated the region extracted in Fig. 3. Inset below are contour plots of the proton spatial distributions corresponding to the various energy buckets. Inset above are proton density plots taken along the polarization plane for each case. Note that for the $0.3n_{\mathrm{crit}}$ case the cavitation region extends the entire length of the plasma while it only extend halfway through for the $0.6n_{\mathrm{crit}}$ case.

Figure 5

3D volumetric renderings of the electron density and azimuthal fields with proton test particles included. The test particles are color coded by their kinetic energy. Note that test particles have a radial energy distribution with lowest energy on axis and the highest off axis. A movie of these simulation results is provided as Supplemental Material [17].

Figure 6

Plot comparing experimental to simulated proton energy spectrum for $n_e=0.3n_{\mathrm{crit}}$. Proton distribution asymmetries, simulated experimental parameters, or sampling statistics with PIC macroparticles may be the cause of discrepancies at higher energies.