Dominance of Radiation Pressure in Ion Acceleration with Linearly Polarized Pulses at Intensities of ${10}^{21}\mathrm{W}{\mathrm{cm}}^{-2}$

A novel regime is proposed where, by employing linearly polarized laser pulses at intensities ${10}^{21}\mathrm{W}{\mathrm{cm}}^{-2}$ (2 orders of magnitude lower than discussed in previous work [T. Esirkepov et al., Phys. Rev. Lett. 92, 175003 (2004)]), ions are dominantly accelerated from ultrathin foils by the radiation pressure and have monoenergetic spectra. In this regime, ions accelerated from the hole-boring process quickly catch up with the ions accelerated by target normal sheath acceleration, and they then join in a single bunch, undergoing a hybrid light-sail–target normal sheath acceleration. Under an appropriate coupling condition between foil thickness, laser intensity, and pulse duration, laser radiation pressure can be dominant in this hybrid acceleration. Two-dimensional particle-in-cell simulations show that 1.26 GeV quasimonoenergetic ${\mathrm{C}}^{6+}$ beams are obtained by linearly polarized laser pulses at intensities of ${10}^{21}\mathrm{W}{\mathrm{cm}}^{-2}$.

Figure 1

Three regimes for ion acceleration from thin foils by LP laser pulses, based on Eqs. (2, 3, 4, 5): RPA-dominated (blue and purple lines), competing of RPA and TNSA (black and purple), and TNSA-dominated (purple and green), where the cross point corresponds to the transition time from HB RPA to LS RPA.

Figure 2

(a)–(d) Electron ($n_e$) and ${\mathrm{C}}^{6+}$ ($n_i$) densities at, respectively, $t=15$, 23, 32, and 53 fs for a 80 nm fully ionized carbon foil irradiated by LP laser pulses at intensity $I_0=3\times {10}^{21}\mathrm{W}/{\mathrm{cm}}^2$, wavelength $\lambda =1.0\mu \mathrm{m}$, and duration ${\tau }_L=20{\tau }_0=67\mathrm{fs}$, where the foil electron density is $600n_c$; (e),(f) the longitudinal profile of $n_i$ and electrostatic field $E_z$ at $t=15$ and 23 fs, which shows that ions are accelerated by a hybrid effect of the charge separation and rear sheath fields, i.e., undergoing hybrid RPA-TNSA; (g),(h) ${\mathrm{C}}^{6+}$ phase space distributions at $t=23$ and 32 fs for particles with $|x|<2\mu \mathrm{m}$, which show typical dynamics of hybrid RPA-TNSA acceleration in the LS stage.

Figure 3

(a) ${\mathrm{C}}^{6+}$ density at $t=83\mathrm{fs}$ when the laser pulse ends for the acceleration in Fig. 2. (b) ${\mathrm{C}}^{6+}$ energy spectra at $t=53$ (black line), 68 (blue line), and 83 fs (red line) for Fig. 2, which have a well-defined peak as RPA is dominant in the hybrid acceleration. (c),(d) ${\mathrm{C}}^{6+}$ density and spectra obtained by increasing foil thickness to $l_0=200\mathrm{nm}$ with the condition (4) violated, showing no pronounced spectral peak as RPA is nondominant. (e) shows the scaling of the required foil thickness for the RPA-dominated regime with the laser power by fixing laser pulse duration at 66 fs, where the foil density is the same as in Fig. 2. The corresponding points of the spectral cases of (b) and (d) are, respectively, marked in (e).

Figure 4

Ion acceleration from a 80 nm fully ionized copper-carbon mixed foil target by LP laser pulse, where the density ratio between ${\mathrm{Cu}}^{24+}$ and ${\mathrm{C}}^{6+}$ is $5:1$. The total mass density and other parameters are the same as those in Fig. 2. (a) is the plot of both ${\mathrm{Cu}}^{24+}$ and ${\mathrm{C}}^{6+}$ densities at $t=83\mathrm{fs}$ when the laser pulse ends; (b) shows the corresponding energy spectra. The dashed blue and green lines are the energy spectra for ${\mathrm{C}}^{6+}$ in the cases of thicker foil with $l_0=200$ and 800 nm, respectively.