Radiation-Pressure Acceleration of Ion Beams Driven by Circularly Polarized Laser Pulses

We present experimental studies on ion acceleration from ultrathin diamondlike carbon foils irradiated by ultrahigh contrast laser pulses of energy 0.7 J focused to peak intensities of $5\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$. A reduction in electron heating is observed when the laser polarization is changed from linear to circular, leading to a pronounced peak in the fully ionized carbon spectrum at the optimum foil thickness of 5.3 nm. Two-dimensional particle-in-cell simulations reveal that those ${\mathrm{C}}^{6+}$ ions are for the first time dominantly accelerated in a phase-stable way by the laser radiation pressure.

Figure 1

(a) Experimentally observed maximum proton (green dotted line, light green dotted line) and carbon ${\mathrm{C}}^{6+}$ (red dotted line, orange dotted line) energies per atomic mass unit over target thickness for linearly and circularly polarized irradiation. Gray dotted lines represent the respective values obtained from 2D PIC simulations. Corresponding electron spectra measured at the optimum target thickness $d=5.3\mathrm{nm}$ are given in (b), showing a strong reduction in electron heating for circularly polarized irradiation.

Figure 2

Experimentally observed proton (green curves) and carbon ${\mathrm{C}}^{6+}$ (red curves) spectra in the case of linear (a) and circular (b) polarized irradiation of a 5.3 nm thickness DLC foil. The corresponding curves as obtained from 2D PIC simulations (c),(d) show excellent agreement with the measured distributions at late times (red curves, $t=221\mathrm{fs}$ after the arrival of the laser pulse maximum at the target). A quasimonoenergetic peak generated by radiation-pressure acceleration is revealed for circular polarization, being still isolated at the end of the laser-target interaction (black curve, $t=45\mathrm{fs}$).

Figure 3

Carbon ion phase space at the end of the laser-target interaction ($t=45\mathrm{fs}$). A significant amount of particles form a distinct loop in the case of circular polarization (b), giving evidence of a phase-stable acceleration driven by the laser radiation pressure.

Figure 4

Cycle-averaged electron (a),(b) and carbon ion (c),(d) density at $t=61\mathrm{fs}$ after the peak of the laser pulse reached the 5.3 nm target initially located at $x=3\lambda$. While linear polarization results in strong expansion of the target caused by hot electrons, for circularly polarized irradiation the foil is accelerated as a dense, quasineutral plasma bunch.