Primed plasmas deliver better ion beams

Accelerated ion beams are finding a broad range of applications, including cancer therapy, isotope generation, and art forensics (see Physics Today, January 2012, page 58). One convenient way to generate such beams is to laser ionize a gas so as to initiate a plasma shockwave. Charge-density gradients at the shock front produce an electric field that reflects ions at twice the wave’s propagation speed. (See the article by Chandrashekhar Joshi and Thomas Katsouleas, Physics Today, June 2003, page 47.) Shockwave acceleration, as it’s known, can produce impressively energetic ion beams, but their use has been limited due to their large energy spreads. Earlier this year, Joshi (UCLA), Luis Silva (Instituto Superior Técnico, Lisbon, Portugal), and coworkers reported a curious experimental result: When they initiated a shockwave with a train of short laser pulses instead of a single long one, they produced a beam that was very nearly monoenergetic. In a new paper, they’ve explained how it works: The first couple of pulses in the train serve to prepare the plasma with a smoothly decaying charge-density profile having a peak value near the propagation threshold. When a subsequent pulse instigates the shockwave, the smoothly decaying profile ensures that the shock front travels at a steady speed and imparts a uniform velocity to reflected ions. Simulations suggest the strategy could deliver high-quality proton beams of up to 200 MeV. (D. Haberberger et al., Nat. Phys. 8, 95, 2012; F. Fiuza et al., Phys. Rev. Lett., in press.)—Ashley G. Smart