Laser-Driven Shock Acceleration of Ion Beams from Spherical Mass-Limited Targets

We report on experimental studies of ion acceleration from spherical targets of diameter $15\mu \mathrm{m}$ irradiated by ultraintense ($1\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$) pulses from a 20-TW Ti:sapphire laser system. A highly directed proton beam with plateau-shaped spectrum extending to energies up to 8 MeV is observed in the laser propagation direction. This beam arises from acceleration in a converging shock launched by the laser, which is confirmed by 3-dimensional particle-in-cell simulations. The temporal evolution of the shock-front curvature shows excellent agreement with a two-dimensional radiation pressure model.

Figure 1

Experimental setup including a characteristic focal spot image (a) and a picture of a typical microsphere target before the shot (b) and at the time of interaction (c). The white regions visible both in (b) and (c) outside the contour of the spherical target arise from inhomogeneities of the probe beam profile, whereas the bright white spot that appears on the target shadow in (c) is generated by self-emission of the target at the second harmonic of the incident pulse. The increased radius of the sphere $\delta r$ results from the deflection of the probe-laser pulse in the preformed plasma.

Figure 2

Proton energies accessible with the applied CR39 stacks (a) and a typical proton beam profile at the highest energy (inset, color bar indicates number of particles in units of ${10}^6/{\mathrm{rad}}^2$, energies represent the center of the respective interval). The fine grid structure is an artifact resulting from double counts in the automated scanning analysis. (b) Proton energy distributions from aluminum foils (black), plastic foils (green), and plastic spheres (red). The curves for the plastic targets are extracted from the CR39 stack to appear as in the case of a spectrometer with small angular acceptance (${10}^{-7}\mathrm{sr}$) placed in laser propagation direction. The dashed lines are a linear extension of the spectra to the next higher energy level in the stack where no proton signal was observed.

Figure 3

Color-scale plot of the proton density (ranging from 0 to $4\times {10}^{22}/{\mathrm{cm}}^3$) obtained from 3D-PIC simulations 133 fs after the arrival of the laser pulse maximum on a sphere (a) and a foil (b) target. The yellow lines indicate the initial target boundaries. Protons with energy in access of 1 MeV are represented by yellow and red dots, marking surface and shock accelerated ions, respectively. The white dashed lines illustrate the shock front as calculated using the analytical model which is described in the text.