Transition of Proton Energy Scaling Using an Ultrathin Target Irradiated by Linearly Polarized Femtosecond Laser Pulses

Particle acceleration using ultraintense, ultrashort laser pulses is one of the most attractive topics in relativistic laser-plasma research. We report proton and/or ion acceleration in the intensity range of $5\times {10}^{19}$ to $3.3\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$ by irradiating linearly polarized, 30-fs laser pulses on 10-to 100-nm-thick polymer targets. The proton energy scaling with respect to the intensity and target thickness is examined, and a maximum proton energy of 45 MeV is obtained when a 10-nm-thick target is irradiated by a laser intensity of $3.3\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$. The proton acceleration is explained by a hybrid acceleration mechanism including target normal sheath acceleration, radiation pressure acceleration, and Coulomb explosion assisted-free expansion. The transition of proton energy scaling from $I^{1/2}$ to $I$ is observed as a consequence of the hybrid acceleration mechanism. The experimental results are supported by two- and three-dimensional particle-in-cell simulations.

Figure 1

Proton and ${\mathrm{C}}^{6+}$ energies measured from the Thomson parabola. (a) Energy spectra of the protons and carbon ions obtained from a 10-nm-thick target irradiated at $3.3\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$. (b) Maximum proton energies obtained for different target thickness and intensities.

Figure 2

Energy spectra of protons and ${\mathrm{C}}^{6+}$ ions obtained from 10- and 100-nm-thick targets. (a) Proton and (b) ${\mathrm{C}}^{6+}$ energy spectrum obtained with an intensity of $3.3\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 3

Maximum proton energy as a function of the laser peak intensity for linearly polarized laser pulses. (a) Maximum proton energy for 10-, 20-, and 30-nm-thick polymer targets. (b) Comparison between the simulated and experimental results.

Figure 4

PIC simulation results. Temporal evolutions of the number density of (a) protons and (b) its line profile over the central region with a $1\mathrm{\text{−}}\mu \mathrm{m}$ radius from a 10-nm target irradiated at $3.0\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$. (c) The corresponding proton phase space distribution. (d) Temporal evolutions of the longitudinal electrostatic field for a 10-nm target irradiated at $3.0\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$ and a 30-nm target irradiated at $1.0\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$. Results are shown for $t=24$, 36, and 60 fs after the interaction.