Ion Acceleration Using Relativistic Pulse Shaping in Near-Critical-Density Plasmas

Ultraintense laser pulses with a few-cycle rising edge are ideally suited to accelerating ions from ultrathin foils, and achieving such pulses in practice represents a formidable challenge. We show that such pulses can be obtained using sufficiently strong and well-controlled relativistic nonlinearities in spatially well-defined near-critical-density plasmas. The resulting ultraintense pulses with an extremely steep rising edge give rise to significantly enhanced carbon ion energies consistent with a transition to radiation pressure acceleration.

Figure 1

(a) Laser transmittance measured through CNF (red circles) and DLC (blue squares). Main sources of errors are uncertainties in the areal density (weight) measurement and background subtraction. The triangles with different colors represent the transmission extracted from 3D PIC simulations performed for electron densities of $n_e/n_c=1$, 3, and 5. (b) Measured and simulated temporal intensity distribution evidencing pulse front steepening through a $7\mu \mathrm{m}$ thick CNF target (corresponding to an areal density of $\sigma =5.2\mu \mathrm{g}/{\mathrm{cm}}^2$).

Figure 2

On-axis intensity-envelope distribution extracted from 3D PIC simulations at different times. The circularly polarized ASTRA Gemini-laser pulse propagates through a NCD plasma with electron density $n_e=2n_c$ (yellow area). The pulse rise time shortens to $\sim 4\mathrm{fs}$. The peak intensity multiplies by a factor of 10, mainly due to relativistic self-focusing as shown in the 2D intensity map at $t=125\mathrm{fs}$ (inset). Placing a micron-scale layer of NCD in front of a DLC foil (inset) allows the enhanced pulse to be exploited for laser ion acceleration.

Figure 3

(a) Energy distributions of ${\mathrm{C}}^{6+}$ ions (solid curves) and protons (dashed curves) registered from DLC foils combined with CNF targets of varying thicknesses irradiated by circularly polarized laser pulses. (b) ${\mathrm{C}}^{6+}$ ion spectrum under best conditions for CP (red) and LP (blue).

Figure 4

3D PIC simulation results. (a) The carbon energy increases with NCD length. (b) Simulations with three different electron densities evidence that self-focusing length optimizes ion acceleration. (c) 2D density map of ${\mathrm{C}}^{6+}$ ions and laser field distribution at $t=100\mathrm{fs}$ for the $5\mu \mathrm{m}$ NCD case, best performance in experiments. (d) ${\mathrm{C}}^{6+}$ ion energy spectra from simulation of $5\mu \mathrm{m}$ $\mathrm{NCD}+\mathrm{DLC}$ (red) and from the plain DLC foil (black) considering all the ions (solid) and ions originating from the back facing $1/3$ of the foil in the focal volume (dashed). The measured spectrum (blue dashed) is shown for comparison.