Laser polarization boosts quality of proton beams

Simulation of a laser pulse that's created a plasma aperture in a thin foil
Hole punch: simulation of a laser pulse that’s created a plasma aperture in a thin foil

The quality of laser-accelerated proton beams can be improved by controlling the polarization of the incident laser light, researchers in the UK have discovered. The finding could help physicists to create compact sources of proton beams for use in medicine, lithography or even astrophysics.

Beams of protons and other positive ions have a wide range of applications, including particle physics, materials processing and medicine. Proton-beam therapy, for example, is used to destroy some cancerous tumours with a minimum of collateral damage to surrounding healthy tissue. However, the practical use of proton and ion beams is held back by the need for large and expensive particle accelerators to generate high-quality beams.

One way forward is laser-plasma acceleration, in which a high-power laser pulse is fired into a target. This creates a plasma in which the electrons separate from the ions. This creates huge electric fields that are capable of accelerating protons, ions and electrons to very high energies.

Messy process

This can be a very messy process, as Felix Mackenroth of the Max Planck Institute for the Physics of Complex Systems in Dresden, Germany explains: “The beams are very, very poor quality. We just essentially shoot a laser into a foil and hope that something good happens.”

In a previous study, Paul McKenna and colleagues at the University of Strathclyde, together with researchers at Queens University Belfast and the STFC Rutherford Appleton Laboratory in Oxfordshire, looked at the laser acceleration of electrons from a target of ultrathin aluminium foil using the Rutherford Appleton Laboratory’s Gemini laser. A powerful laser pulse from Gemini hits the foil, heating it up so that it becomes a plasma. This disc of plasma in the foil layer is transparent to the pulse and the researchers were able to show that the pulse then diffracts through the plasma disc as it would through a classical aperture. Furthermore, they found that the pattern of electrons ejected from the foil depends on the polarization of the incident laser light.

Water vapour

In this latest research, the team has applied a similar approach to proton acceleration. Even though their experiments are conducted in a vacuum chamber, water vapour naturally condenses on the foil, providing a natural source of protons. The team wanted to see if the structure of the resulting proton beam would be affected by the polarization of the light.

In separate experiments, the researchers irradiated aluminium foils with petawatt pulses of linearly, elliptically and circularly polarized light from Gemini. The pattern of the protons ejected at various energies differed markedly: the lower-energy protons were concentrated into the centre for all the polarizations, for example, but the spread was much tighter for elliptically and circularly polarized light than for linearly polarized light. The higher-energy protons produced by linearly polarized light formed a double-lobe pattern, whereas circularly and elliptically polarized light led to annular density profiles. The experimentally observed patterns closely matched computer simulations, with small deviations fully explained by experimental imperfections, say the researchers.

Black holes

The team plans to “take this research forward with further investigation of approaches to control the ‘relativistic plasma aperture'”, explains McKenna. “This includes polarization control and also control by variation of the intensity profile of the drive laser pulse.” Although the work is currently at the fundamental research stage, McKenna believes it could ultimately have multiple applications for controlling dose deposition in proton-beam therapy, lithography or even astrophysical modelling: “We are exploring the potential applications to other areas of science, including experimental models of astrophysical relativistic plasma jets created by a rotating black-hole accretion disc,” he says.

Other researchers are impressed. “It’s definitely an important contribution,” says Victor Malka of ENSTA Paris-Tech in France, “The quality of the experimental data together with the simulations shows that we [the scientific community] have a very fine understanding of this process.” Felix Mackenroth agrees: “This overall idea that the researchers had is a very neat one and a very significant one as well.” He notes, however, that the proton beam is still divergent, and that the protons produced here are too low in energy – and have too large an energy spread – to be used directly for medical applications.

The research is described in Nature Communications.

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