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

We report on reproducible shock acceleration from irradiation of a $\lambda =10\mu \mathrm{m}$ ${\mathrm{CO}}_2$ laser on optically shaped ${\mathrm{H}}_2$ and He gas targets. A low energy laser prepulse ($I\lesssim {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$) 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 ($I>{10}^{16}\mathrm{W}{\mathrm{cm}}^{-2}$) that produces an electrostatic collisionless shock. Upstream ions are accelerated for a narrow range of prepulse energies. For long density gradients ($\gtrsim 40\mu \mathrm{m}$), broadband beams of ${\mathrm{He}}^{+}$ and ${\mathrm{H}}^{+}$ are routinely produced, while for shorter gradients ($\lesssim 20\mu \mathrm{m}$), 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.

Figure 1

Electron plasma density from He targets, $n_e$, from interferometry (a) immediately before and (b) 300 ps after the LPI, for $E_{\text{pp}}\approx 220\mathrm{mJ}$. (c) $n_e$ and (d) total helium atomic density (irrespective of ionization), $n_{\mathrm{He}}$, from hydrodynamic simulations after 25 ns of expansion for 3 mJ absorbed energy.

Figure 2

Interferograms 300 ps after the LPI with He (a) without a prepulse, (b) with a prepulse ideal for ion acceleration, $E_{\text{pp}}\approx 150\mathrm{mJ}$, and (c) with a prepulse too large for ion acceleration, $E_{\text{pp}}\approx 1.27\mathrm{J}$. (d)–(f) Corresponding $n_e/n_{\text{cr}}$.

Figure 3

(a) Average flux of accelerated helium ions $({\mathrm{sr}}^{-1})>0.1\mathrm{MeV}$ as sampled by the spectrometer (color scale) as a function of prepulse energy $E_{\text{pp}}$ and main pulse $a_0$. The white region indicates laser parameters tested experimentally. (b) Proton spectra for a ${\mathrm{H}}_2$ target with $E_{\text{pp}}\approx 4$ and 23 mJ, and ${\mathrm{He}}^{+}$ spectrum for a He target with $E_{\text{pp}}\approx 150\mathrm{mJ}$ (solid lines). The dotted lines are the detection thresholds, and the dashed red line is the deconvolved spectrum for $E_{\text{pp}}\approx 4\mathrm{mJ}$.

Figure 4

Results of PIC simulations. Ion density map 12 ps after LPI for (a) no prepulse, (b) the experimental initial profile for $E_{\text{pp}}=150\mathrm{mJ}$ obtained from Fig. 2 (the inset is optical shadowgraphy showing shock remnants), and (c) $z\text{−}{p}_z$ phase space. (d) The $z\text{−}{p}_z$ phase space for hydrogen plasma with exponential ramp ($l_{\exp }=20\mu \mathrm{m}$).

Figure 5

(a) Critical surface (red) and shock (blue) position with time for He simulation are shown in Figs. 4 and 4, and shock position for H simulation is shown in Fig. 4. (b) Comparison among ${\mathrm{He}}^{2+}$ spectra for an $800\mu \mathrm{m}$ linear ramp (no prepulse, green), He with $l_{\exp }=40\mu \mathrm{m}$ (prepulse, blue), and H with $l_{\exp }=20\mu \mathrm{m}$ (black).