Collisionless Shock Acceleration of High-Flux Quasimonoenergetic Proton Beams Driven by Circularly Polarized Laser Pulses

We present experimental studies on ion acceleration using an 800-nm circularly polarized laser pulse with a peak intensity of $6.9\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$ interacting with an overdense plasma that is produced by a laser prepulse ionizing an initially ultrathin plastic foil. The proton spectra exhibit spectral peaks at energies up to 9 MeV with energy spreads of 30% and fluxes as high as $3\times {10}^{12}\mathrm{protons}/\mathrm{MeV}/\mathrm{sr}$. Two-dimensional particle-in-cell simulations reveal that collisionless shocks are efficiently launched by circularly polarized lasers in exploded plasmas, resulting in the acceleration of quasimonoenergetic proton beams. Furthermore, this scheme predicts the generation of quasimonoenergetic proton beams with peak energies of approximately 150 MeV using current laser technology, representing a significant step toward applications such as proton therapy.

Figure 1

(a) Schematic of the experimental setup. Image plate data of Thomson parabola and electron spectrometer obtained from 40-nm plastic foils irradiated by (b) CP and (c) LP laser pulses with a peak intensity of $6.9\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 2

Processed energy spectra of (a) protons and (b) electrons in the case of CP (red, light red lines) and LP (black, gray lines) laser pulses with a peak intensity of $6.9\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$ irradiating 40-nm plastic foils. (c) Details of proton spectra on three different laser shots for CP laser with intensities of $2.7\times {10}^{19}$ (yellow line), $3.6\times {10}^{19}$ (green line), and $5.5\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$ (blue line). (d) Experimental (red squares) and simulated (blue circles) results of the peak energy of quasimonoenergetic protons as a function of laser intensity, where the vertical bars represent the FWHM energy spread of each shot.

Figure 3

Simulation results from CP laser of $a_0=4$ and LP laser of $a_0=5.6$ interacting with overdense plasmas of $n_{e,\max }=4n_c$. (a) Proton spectra for CP (red line) and LP (blue line) cases at 377 fs after the start of laser-plasma interaction. (b) Evolution of the Mach number (red, light red squares) with average electron temperature (black, gray triangles) in the upstream. The black dashed line represents the beginning of shock formation. (c)–(d) Corresponding proton phase spaces at 377 fs. (e)–(f) Longitudinal electric field $E_x$ (red, light red lines) and transversely averaged proton density (black, gray lines) at two different times, 163 and 297 fs.

Figure 4

Simulated proton spectrum obtained from CP laser with $a_0$ of 28 interacting with an exploded plasma of a peak density $n_{e,\max }=16n_c$. The plasma density profile is identical to that in the $a_0=4$ case. The inset represents the corresponding proton phase space.