​Beams of energetic ions have applications in a diverse variety of fields, ranging from studying astrophysical conditions recreated in laboratories and nuclear fusion to treating deep-seated tumours. High power laser beams such as the CLF's laser system, Gemini, can be used to generate high energy ion beams; laser-driven ion sources offer a more compact alternative to conventional accelerators, which could be beneficial for specialised applications like those in hospitals.

A recent experiment led by the Queens University Belfast has deepened our knowledge of how to gain the best results out of this process. The team used the CLF's Gemini laser to access a new regime – Light Sail - in laser-driven ion acceleration.

In the Light-Sail regime, the ions gain energy rapidly because they harness the extreme radiation pressure exerted by the high intensity laser. It is an exceptionally promising approach to attain energies up in the 100s of MeV/nucleon range and above. Light sail techniques require laser beams of extreme parameters: ultra-high intensities and an exceptional temporal contrast.

Until recently, most experimental research has been devoted to proton/ion acceleration from the rear surface of thin foils. When irradiated with linearly polarised light, the energetic electrons from the plasma produced at the front surface would penetrate the target and create a sheath field behind the target that accelerates protons and ions. At extreme intensities, however, the radiation pressure of the laser can accelerate protons and ions directly from the bulk of the target. This effect is stronger as the target thickness is reduced to a few nanometres. However, target expansion driven by hot electrons normally causes such thin targets to decompress and become transparent to the laser radiation during the irradiation, which stops radiation pressure acceleration.

Using circularly polarised light (as opposed to linear) suppresses hot electron production and allows the radiation pressure to be applied for a longer amount of time, leaving a larger window of time between the initial impact to the target and the target becoming transparent. This means that ions can experience the acceleration fields for enough time to get accelerated to significant energies.

By using circularly polarised light the team has been able to access an experimental regime where radiation pressure acceleration is dominant. For nanometre-thick targets, circular polarisation produces higher energy Carbon ions compared to linearly polarised light, indicating that Light-sail acceleration is dominant. These results represent an important step in the research on generating compact sources of energetic ions for various applications including cancer therapy.

C. Scullion, D. Doria, L. Romagnani, A. Sgattoni, K. Naughton, D. R. Symes, P. McKenna, A. Macchi, M. Zepf, S. Kar, and M. Borghesi

 For more information on this experiment and to the view the image at its source, visit: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.054801

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