Compact, laser-driven ion beamlines for interdisciplinary applications

The interaction of an intense laser pulse with various target types produces beams of ions with MeV scale energies. The first experiments reporting the acceleration of multi-MeV protons due to these interactions were in 2000, and since then heavier ions have also been accelerated. Various target configurations have been employed in experiments, such as thick (of a scale on the order of a hundred micrometres) metal foils, in which the sources of protons are naturally occurring adsorbed hydrocarbons and water on the foil surface, gas jets, and cryogenic hydrogen and deuterium ribbons. Most of the work focusses on the target normal sheath acceleration (TNSA) mechanism, in which ions are accelerated due to the large potential set up by electrons escaping from the target during the interaction. However, there are also other acceleration mechanisms, such as the radiation pressure acceleration (RPA), in which acceleration occurs due to the pressure exerted by the incident laser photons. The resulting ion beams have potential applications such as hadron therapy in cancer treatment, and as ignitor beams in fast ignition inertial confinement fusion experiments. In the example of hadron therapy, the laser-driven ion accelerator can potentially be built to be more compact than a conventional synchrotron accelerator, thus increasing the practicality of the deployment of this treatment in hospitals.

With potential applications in mind, experiments aim not only to explore and understand the physics of the acceleration mechanisms, but also to optimise the beams produced. For example, the TNSA mechanism provides a beam with a high brightness, however the beam diverges and has a broad exponential energy spectrum. There is also the requirement in hadron therapy that the proton beam has a high energy (of the order of 100s of MeV). Recently, acceleration of protons to energies exceeding 94MeV was achieved using a hybrid mechanism of radiation pressure and sheath acceleration by the irradiation of ultra-thin foils by Higginson et al. (2018), and is reported in Nature Communications. There has also been work to collimate and enhance the energy of proton beams produced by TNSA, such as that presented in Nature Communications by Kar et al. (2016). Here, a helical coil structure is employed on the rear side of a solid target, through which the electromagnetic (EM) pulse generated during the laser interaction can propagate. The electric field in the coil due to this EM pulse acts to collimate the proton beam and was shown to enhance the proton energy by around 5MeV over less than a centimetre of propagation.

The project will aim to develop further the target apparatus such as the helical coil. It will look to assess the stability of the process, such as investigating the directionality and collimation of the resulting beam, as well as determining the optimum coil configuration. It will also be of use to consider the possibility of moving away from a single-shot target, in order that the repetition rate in the experiments, and thus the rate at which the ions may be delivered, can be increased. Indeed, this may necessitate a target design in which the coil is detached from the irradiated foil, and an area of research would be the transport of the EM pulse across this gap from the target to the coil. In addition to this, further beamline apparatus could be deployed, such as quadrupole magnets, to aid in the focussing of the ion beam. The research will rely on the simulation of these processes using Monte Carlo codes, as well as practical experiments carried out in-house on a university scale laser system, as well as at larger laboratories such as the Central Laser Facility. Ultimately, the research will contribute to the development of practical, compact laser-driven ion accelerators for use in a number of applications.