ELI Facilities publish joint study to predict a new way of accelerating monoenergetic protons

The research paper ‘Controlled transition to different proton acceleration regimes: Near-critical-density plasmas driven by circularly polarized few-cycle pulses’ was recently published in the journal of Matter and Radiations at Extreme. This is a result of joint efforts by the ELI ALPS and ELI Beamlines teams made possible by the IMPULSE project. The findings of the paper might open new experimental avenues to control the spectral shape and energy of laser driven ion beams, and has significant implications for applications such as cancer therapy.

The paper investigates the controlled transition between two different ion acceleration mechanisms using relativistically intense circularly polarized laser pulses interacting with thin near-critical-density plasma targets. The aim is to achieve different ion energies and spectral features within the same experimental configuration, depending on the region of operation.

The study first provides a detailed background on the notoriously complex process of ion acceleration in these different regimes. The authors then show systematically that the plasma density, plasma thickness, exponential density gradient (plasma steepness), and laser frequency chirp (laser structure) can be controlled to switch the interaction from the transparent operating regime to the opaque one, thereby enabling the choice of a Maxwellian-like ion energy distribution with a cutoff energy in the relativistically transparent regime or a quasimonoenergetic spectrum in the opaque regime.

The paper also establishes that a multispecies target configuration can be used effectively for optimal generation of quasi-monoenergetic ion bunches of a desired species. Finally, the feasibility is demonstrated for generating monoenergetic proton beams with energy peak at E≈20–40 MeV and a narrow energy spread of ΔE/E≈18%–28.6% confined within a divergence angle of ∼175 mrad at a reasonable laser peak intensity of I0 ≃ 5.4 × 10²⁰ W/cm².

This has significant implications for applications such as cancer therapy, where proton beams are used to target cancer cells. The ability to generate monoenergetic proton beams with a narrow energy spread could lead to more precise targeting of cancer cells, reducing the damage to healthy tissue. This is part of a larger motivation at ELI to use lasers and their secondary sources for advancing radation therapy methods and technologies.

The possibility of controlling the transition between different ion acceleration mechanisms within the same experimental setup can pave the way to optimising the ion energies and spectral features depending on the region of operation. Also, the possibility of controlled experiments in this direction might open the door to studying the correlation of ion acceleration with proposed cross-disciplinary studies such as strong-field plasma optics.