Acceleration of particle in plasmas exploits the ultralarge electric fields (up to 1013 V/m) that can be generated by inducing a local charge separation. This is at the basis of novel approaches for acceleration of electrons and ions which are pursued by many groups and projects worldwide. These activities are motivated by the search for compact alternatives to high-energy Radiofrequency accelerators, but also by novel applications facilitated by some of the peculiar properties of laser-accelerated beams.
At QUB, we pursue, in particular, research in the acceleration of ions, and we have contributed over the year some major developments to this field of research. In the framework of sheath acceleration processes on foil targets driven by intense, ultrashort laser pulses, experiments have demonstrated, over a wide range of laser and target parameters, the generation of multi-MeV proton and ion beams with ultrashort duration, high brilliance, and low emittance. Recent activities have focused on advanced acceleration mechanisms which aim to harness the enormous radiation pressure associated to ultra-intense, short laser pulses to accelerate ions directly from the bulk of ultrathin foils (10-100 nm thick). Expansion of the foils during the irradiation may lead to regimes where an enhanced coupling and complex interplay between laser radiation, electron and ions in the plasma can also enhance the acceleration efficiency. We are also engaged in the development of target-based techniques which act on the ions after their acceleration from a foil, in order to optimize their properties (such as divergence and energy spectrum) towards applicative use of the ions.
These activities are currently pursued in the framework of the A-SAIL project, a QUB-led, UK-wide consortium aimed to explore and develop the potential of laser-driven sources for medical applications.
Laser-accelerated particles are also used to generate secondary particle sources, generated through the interaction of laser-accelerated electrons and ions with suitable targets.
Beams of neutrons are produced through nuclear reactions initiated by the ions, either inside the laser-irradiated primary target or when directed to a suitably chosen secondary target, in the so-called pitcher-catcher scheme. The reactions produce directional beams of neutrons with MeV energies and short pulse duration (typically containing 109-1010 neutrons in a nanosecond pulse), which can be further moderated to the epithermal and thermal regimes in compact arrangements.
Positrons are generated by directing ultrarelativistic electron beams (accelerated by intense femtosecond laser pulses in gaseous media trough wakefield processes) onto high-Z targets. The electrons initiate electromagnetic cascades, which lead to the emission of bright fluxes of gamma photons and high-energy positrons. Highly directional, high brightness beams of 100s MeV positrons have been demonstrated so far, with options for further acceleration and use in higher energy physics applications. By tuning the generation conditions, it is possible to obtain neutral electron-positron beams, which are of high relevance for laboratory astrophysics applications.
KEY RECENT PUBLICATIONS:
1. A. Higginson, R. J. Gray, M. King, R. J. Dance, S. D. R. Williamson, N. M. H. Butler, R. Wilson, R. Capdessus, C. Armstrong, J. S. Green, S. J. Hawkes, P. Martin, W. Q. Wei, S. R. Mirfayzi, X. H. Yuan, S. Kar, M. Borghesi, R. J. Clarke, D. Neely, and P. McKenna, Near-100 MeV protons via an enhanced hybrid laser-ion acceleration mechanism, Nature Comm., 9, 724 (2018)
2. C. Scullion, D. Doria, L. Romagnani, A. Sgattoni, K. Naughton, D.R. Symes, P. McKenna, A. Macchi, M. Zepf, S. Kar, and M. Borghesi, Polarization dependence of bulk ion acceleration from ultrathin foils irradiated by high intensity, ultrashort laser pulse, Phys. Rev. Lett. 119, 054801 (2017)
3. S. Kar, H. Ahmed, R. Prasad, M. Cerchez, S. Brauckmann, B. Aurand, G. Cantono, P. Hadjisolomou, C.L. Lewis, A. Macchi, G. Nersisyan, A. P.L. Robinson, A. M. Schroer, M. Swantusch, M. Zepf, O. Willi, M.Borghesi, Guided post-acceleration of laser-driven ions by a miniature modular structure, Nature Communications, 7, 10792 (2016)
4. S. Kar, A. Green, A. Alejo, H. Ahmed, A.P.L. Robinson, M. Cerchez, R. Clarke, D. Doria, S. Dorkings, J. Fernandez, S. R. Mirfayzi, P. McKenna, K. Naughton, D. Neely, P. Norreys, C. Peth, J.A.Ruiz, J. Swain, O. Willi, M. Borghesi, Beamed neutron source generation employing laser driven light ions in a beam-fusion scenario, New J. Phys. 18, 053002 (2016)
5. G. Sarri, W. Schumaker, A. Di Piazza, M. Vargas, B. Dromey, M. E. Dieckmann, V. Chvykov, A. Maksimchuk, V. Yanovsky, Z.H. He, B.X, Hou, J.A. Nees, A.G.R. Thomas, C.H. Keitel, M. Zepf, K. Krushelnick, Table-top laser-based source of femtosecond, collimated, ultrarelativistic positron beams, Phys. Rev. Lett., 110, 255002 (2013)
Advanced laser-ion acceleration strategies towards next generation healthcare, EPSRC EP/K022415/1 (Programme Grant, 2013-2020)
Laboratory studies of neutral and collimated electron-positron beams, EPSRC EP/N027175/1 (2016-2020)
Laser-driven multi-modal beams for nuclear waste inspection, STFC ST/P000177/1 (2016-2019)
Novel approach to laser-driven multistage particle accelerator, EPSRC 1782782 (2017-2021, PhD studentship)
Compact, laser-driven ion beamlines for interdisciplinary applications, EPSRC 2114405 (2018-2022, PhD studentship)