Laser acceleration of quasi-monoenergetic MeV ion beams

Abstract

Acceleration of particles by intense laser–plasma interactions is a promising alternative to conventional particle accelerators. Two groups this week report advances in this field that bring the prospect of monoenergetic ion beams closer. Such beams are necessary for various potential applications including medical proton and heavy-ion therapy. Hegelich et al. produced laser-driven C5+ions with a vastly reduced energy spread compared to previous experiments. And Schwoerer et al. produced quasi-monoenergetic proton beams from intense laser irradiation of solid microstructured targets. Acceleration of particles by intense laser–plasma interactions represents a rapidly evolving field of interest, as highlighted by the recent demonstration1,2,3,4 of laser-driven relativistic beams of monoenergetic electrons. Ultrahigh-intensity lasers can produce accelerating fields of 10 TV m-1 (1 TV = 1012 V), surpassing those in conventional accelerators by six orders of magnitude. Laser-driven ions with energies of several MeV per nucleon have also been produced5,6,7,8,9. Such ion beams exhibit unprecedented characteristics—short pulse lengths, high currents and low transverse emittance10—but their exponential energy spectra have almost 100% energy spread. This large energy spread, which is a consequence of the experimental conditions used to date, remains the biggest impediment to the wider use of this technology. Here we report the production of quasi-monoenergetic laser-driven C5+ ions with a vastly reduced energy spread of 17%. The ions have a mean energy of 3 MeV per nucleon (full-width at half-maximum ∼0.5 MeV per nucleon) and a longitudinal emittance of less than 2 × 10-6 eV s for pulse durations shorter than 1 ps. Such laser-driven, high-current, quasi-monoenergetic ion sources may enable significant advances in the development of compact MeV ion accelerators11, new diagnostics12,13, medical physics14, inertial confinement fusion and fast ignition15,16,17.