Low divergence proton beams from a laser-plasma accelerator at kHz repetition rate

Proton beams with up to 100 pC bunch charge, 0.48 MeV cutoff energy, and divergence as low as 3° were generated from solid targets at kHz repetition rate by a few-mJ femtosecond laser under controlled plasma conditions. The beam spatial profile was measured using a small aperture scanning time-of-flight detector. Detailed parametric studies were performed by varying the surface plasma scale length from 8 to 80 nm and the laser pulse duration from 4 fs to 1.5 ps. Numerical simulations are in good agreement with observations and, together with an in-depth theoretical analysis of the acceleration mechanism, indicate that high repetition rate femtosecond laser technology could be used to produce few-MeV proton beams for applications.

Figure 1

Experimental setup.

Figure 2

TPS proton spectra in arbitrary units and logarithmic scale, for ${\tau }_{\mathrm{las}}=27\mathrm{fs}$ and varying plasma scale length (a), and for 0 prepulse delay and varying pulse duration (b). Angular-spectral proton number distributions measured with the calibrated TOF detector for 4 fs (c), 27 fs (d), and 200 fs (e).

Figure 3

(a) Maximum proton energy as a function of laser pulse duration obtained experimentally (red dots), in PIC simulations (blue diamonds), from numerical integration of proton motion with Eq. (1) and (2) (black solid line), and the scaling Eq. (3) (dashed line). (b–d) Final angular spectral distributions of protons for 30 fs (b), 100 fs (c) and 250 fs (d) laser pulse durations.

Figure 4

Final angular-spectral distribution of protons for a laser energy of 30 mJ and 70 fs pulse duration.