An intense laser can ionize matter and accelerate those ions into a beam. Such laser-driven ion beams could be useful for a range of applications that benefit from short high-flux pulses of ions, including research on radiotherapy. The trouble is that the target materials inevitably contain contaminants. Lighter ions, protons in particular, dominate the beam and prevent heavier ions from accelerating. That effect limits the potential applications.

Now Marco Borghesi and Aodhan McIlvenny of Queen’s University Belfast in the UK and their colleagues have developed a method to selectively excite certain ion species. Their complementary simulations explain how the shape of their laser pulse leads protons to move out of the way so that carbon ions can be accelerated.

Using a 40 fs pulse of circularly polarized IR light, Borghesi and his colleagues irradiated amorphous carbon foil targets that were 2–100 nm thick. They were searching for the maximum energy of the ejected carbon ions as a function of target thickness and found that it occurred in a 15-nm-thick target. What the researchers didn’t expect was that for the same foil thickness, the proton contaminants had a local energy minimum and reached energies per nucleon only about half those of the carbon ions. That behavior was unusual- In all previous experiments, the proton energies per nucleon always dwarfed those of the carbon.

To understand the atypical behavior, the researchers performed particle-in-cell simulations of the carbon and proton energies as a function of laser intensity for an idealized pulse and one matching the experimental pulse. In the case of an idealized Gaussian temporal profile, the carbon ions’ and protons’ energies have roughly matching trends, although the protons’ energies are higher.

Their realistic laser profile, on the other hand, has shoulders before the peak intensity. That pulse shape creates three stages in the laser–foil interaction. First, the low-intensity shoulder causes the target to expand radially out from the center of the laser spot. Protons, with their lighter masses, expand more than carbon. Next, the rising intensity causes the material in the central region to contract, but few protons remain in that region by that point. Finally, the peak arrives and accelerates ions in the central region, which are primarily carbon.

The simulations showed that the same species-selective acceleration should be possible with a pair of pulses. In the future, the technique could potentially be used for research on carbon radiotherapy to treat cancerous tumors. Compared with traditional ion sources, which dose the tumor over minutes, the emerging approach’s shorter dose could cause less damage to nearby healthy cells. (A. McIlvenny et al., Phys. Rev. Lett. 127, 194801, 2021.)