Accelerating Ions by Crossing Two Ultraintense Lasers in a Near-Critical Relativistically Transparent Plasma

A new scheme of ion acceleration by crossing two ultraintense laser pulses in a near-critical relativistically transparent plasma is proposed. One laser, acting as a trigger, preaccelerates background ions in its radial direction via the laser-driven shock. Another crossed laser drives a comoving snowplow field which traps some of the preaccelerated ions and then efficiently accelerates them to high energies up to a few giga-electron-volts. The final output ion beam is collimated and quasimonoenergetic due to a momentum-selection mechanism. Particle-in-cell simulations and theoretical analysis show that the scheme is feasible and robust.

Figure 1

Schematic plot. Two ultraintense laser pulses are crossed in a near-critical relativistically transparent plasma. One of them, the trigger laser (from bottom), preaccelerates background ions in its radial direction. Another one, the driving laser (from left), drives a snowplow field which traps and further accelerates some of the preaccelerated ions to high energy.

Figure 2

Snapshots obtained from a 2D simulation by crossing two laser pulses in plasma. (a),(c) Ion density $n_i/n_c$ and (b),(d) the phase space $(x-p_x)$ of ions along the axis of $y=0$ (arbitrary unit) at different moments, (a),(b) 180 fs and (c),(d) 280 fs. The inset in (d) is a close-up of the accelerating bucket and the green dashed lines are predicted by Eqs. (2) and (3). The overlapping delay between the two laser pulses is 60 fs.

Figure 3

The output ion beam at 467 fs obtained from the simulation in Fig. 2. (a) The density distribution ($n_i/n_c$) in space and (b) the line out at the axis $y=0$. (c) The angular-energy distribution of ions, the angular distribution in the inset, and (d) the energy spectrum for ions within the divergence angle 10 degrees (arbitrary units).

Figure 4

The trajectories of six trapped ions (colored solid curves) and one untrapped ion (blue dashed curve) in (a) phase space $(\xi -p_x)$ (see text) and (b) real space ($x-y$) tracked in the simulation in Fig. 2. The gray dotted curves in (a) depict the phase portrait of the Hamiltonian [Eq. (1)], the black dash-dotted and the black dotted curves are for the separatrix ($\mathcal{H}=-0.16$) and the background ions ($\mathcal{H}=0$), respectively.

Figure 5

(a) and (c) Maximum ion momentum along the axis of $y=0$ in the preacceleration stage ($p_s$) and (b),(d) maximum final output ion energy (${\mathcal{E}}_{\mathrm{out}}$) versus (a),(b) peak intensity of trigger laser ($I_t$) for different trigger laser spot sizes, case A, ${\sigma }_t=4\mathrm{\mu }\mathrm{m}$ (black circles) and case B, ${\sigma }_t=2\mathrm{\mu }\mathrm{m}$ (blue crosses), and those versus (c),(d) plasma density $n_0$ for different laser intensities, case C, $I_d=I_t=2.7\times {10}^{22}\mathrm{W}/{\mathrm{cm}}^2$ (black circles) and case D, $I_d=I_t={10}^{22}\mathrm{W}/{\mathrm{cm}}^2$ (blue crosses). Other parameters are the same as those used in Fig. 2. The dashed curves in (a),(c) and (b),(d) are theoretical predictions Eqs. (5) and (4), respectively.