Ion Acceleration by Collisionless Shocks in High-Intensity-Laser–Underdense-Plasma Interaction

Ion acceleration by the interaction of an ultraintense short-pulse laser with an underdense-plasma has been studied at intensities up to $3\times {10}^{20}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$. Helium ions having a maximum energy of $13.2\pm 1.0\mathrm{M}\mathrm{e}\mathrm{V}$ were measured at an angle of 100° from the laser propagation direction. The maximum ion energy scaled with plasma density as $n_e^{0.70\pm 0.05}$. Two-dimensional particle-in-cell simulations suggest that multiple collisionless shocks are formed at high density. The interaction of shocks is responsible for the observed plateau structure in the ion spectrum and leads to an enhanced ion acceleration beyond that possible by the ponderomotive potential of the laser alone.

Figure 1

Energy spectra of helium ions for (a) high density, $1.4\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$, and (b) low density, $1.7\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$.

Figure 2

Maximum ion energy (MeV per nucleon) as a function of plasma density.

Figure 3

Angular distribution of energetic helium ions. (a) Ions with energy greater than 3.5 MeV at high density $1.4\times {10}^{20}{\mathrm{c}\mathrm{m}}^{-3}$, (b) ions with energy greater than 2 MeV at low density $1\times {10}^{19}{\mathrm{c}\mathrm{m}}^{-3}$, (c) ion phase space $p_y$ vs $p_x$ plot during the shock interaction at $\tau =2.5\mathrm{p}\mathrm{s}$ in the high density ($1.4\times {10}^{20}{\mathrm{c}\mathrm{m}}^{-3}$) simulation and (d) for the low density simulation at the same time. The layout of the ion diagnostics is also shown.

Figure 4

Simulated ion energy spectra for different laser powers and plasma densities.

Figure 5

The ion phase space plots $y$ vs $x$ (a), $p_y$ vs $x$ (c) at $\tau =3.1\mathrm{p}\mathrm{s}$ in the 250 TW case and $y$ vs $x$ (b), $p_y$ vs $x$ (d) at $\tau =2.3\mathrm{p}\mathrm{s}$ in the 1 PW case from the interactions with high density plasma. Ion expulsion from the multishock interaction region is clearly shown.