An intense laser can ionize matter and accelerate the resulting ions into a beam. Creating and accelerating ions in a single compact system offers an advantage over the bulky conventional combination of a separate ion source and a cyclotron or synchrotron. Such laser-driven ion beams could be useful in various applications, including radiotherapy, that benefit from short high-flux pulses of ions.1 (See the article by Jerimy Polf and Katia Parodi, Physics Today, October 2015, page 28.)

But controlling what ions end up in the beam can be difficult in laser-acceleration systems. The problem is that the target materials inevitably contain contaminants, primarily hydrogen. When the laser hits, protons from the ionized hydrogen dominate the beam and prevent heavier ions from accelerating.

Although proton therapy is currently the most widespread ion radiotherapy, a treatment that employs larger ions, such as carbon, interests medical researchers because the ions’ extra...

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