The range of potential applications of compact laser-plasma ion sources motivates the development of new acceleration schemes to increase achievable ion energies and conversion efficiencies. Whilst the evolving nature of laser-plasma interactions can limit the effectiveness of individual acceleration mechanisms, it can also enable the development of hybrid schemes, allowing additional degrees of control on the properties of the resulting ion beam. Here we report on an experimental demonstration of efficient proton acceleration to energies exceeding 94 MeV via a hybrid scheme of radiation pressure-sheath acceleration in an ultrathin foil irradiated by a linearly polarised laser pulse. This occurs via a double-peaked electrostatic field structure, which, at an optimum foil thickness, is significantly enhanced by relativistic transparency and an associated jet of super-thermal electrons. The range of parameters over which this hybrid scenario occurs is discussed and implications for ion acceleration driven by next-generation, multi-petawatt laser facilities are explored.

No abstract

Ion acceleration driven by the interaction of an ultraintense (2 × 10 20 W cm −2 ) laser pulse with an ultrathin (40 nm) foil target is experimentally and numerically investigated. Protons accelerated by sheath fields and via laser radiation pressure are angularly separated and identified based on their directionality and signature features (e.g. transverse instabilities) in the measured spatial-intensity distribution. A low divergence, high energy proton component is also detected when the heated target electrons expand and the target becomes relativistically transparent during the interaction. 2D and 3D particle-in-cell simulations indicate that under these conditions a plasma jet is formed at the target rear, supported by a self-generated azimuthal magnetic field, which extends into the expanded layer of sheath-accelerated protons. Electrons trapped within this jet are directly accelerated to super-thermal energies by the portion of the laser pulse transmitted through the target. The resulting streaming of the electrons into the ion layers enhances the energy of protons in the vicinity of the jet. Through the addition of a controlled prepulse, the maximum energy of these protons is demonstrated experimentally and numerically to be sensitive to the picosecond rising edge profile of the laser pulse.

(2016). Optically controlled dense current structures driven by relativistic plasma aperture-induced diffraction. Nature Physics, 12, 505-512. DOI: 10.1038/nphys3613 General rights Copyright for the publications made accessible via the Queen's University Belfast Research Portal is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights.Take down policy The Research Portal is Queen's institutional repository that provides access to Queen's research output. Every effort has been made to ensure that content in the Research Portal does not infringe any person's rights, or applicable UK laws. If you discover content in the Research Portal that you believe breaches copyright or violates any law, please contact openaccess@qub.ac.uk. AbstractThe collective response of charged particles to intense fields is intrinsic to plasma accelerators and radiation sources, relativistic optics and many astrophysical phenomena. Here we show that the fundamental optical process of diffraction of intense laser light occurs via the self-generation of a relativistic plasma aperture in thin foils undergoing relativistic induced transparency. The plasma electrons collectively respond to the resulting near-field diffraction pattern, producing a beam of energetic electrons with spatial structure which can be controlled by variation of the laser pulse parameters. It is shown that static electron beam, and induced magnetic field, structures can be made to rotate at fixed or variable angular frequencies depending on the degree of ellipticity in the laser polarization. The concept is demonstrated numerically and verified experimentally. It is a viable step towards optical control of charged particle dynamics in laser-driven sources. * Electronic address: paul.mckenna@strath.ac.uk 1 The formation of current structures due to the collective response of charged particles to a perturbation is one of the most fundamental properties of plasma. This is manifest in plasma dynamics ranging from flares and X-ray jets on the sun to disruptive instabilities in fusion plasmas. This feature is also exploited to great effect in the development of compact laser-based particle accelerators and radiation sources, which have wide-ranging potential applications in science, medicine and industry. Controlling the collective motion of plasma electrons in response to perturbation produced by intense laser light is key to the development of these novel sources. Pertinent examples in plasma with density low enough for laser light to propagate (underdense plasma) include the self-generated plasma cavity or 'bubble' produced in laser-driven wakefield acceleration [1] and plasma channels [2]. These structures are formed principally by the ponderomotive force induced by the propagating laser pulse, which expels electrons from the regions of high laser intensity, and by self-generated fields induced by the current displacement [3]. Sh...

Giant electromagnetic pulses (EMP) generated during the interaction of high-power lasers with solid targets can seriously degrade electrical measurements and equipment. EMP emission is caused by the acceleration of hot electrons inside the target, which produce radiation across a wide band from DC to terahertz frequencies. Improved understanding and control of EMP is vital as we enter a new era of high repetition rate, high intensity lasers (e.g. the Extreme Light Infrastructure). We present recent data from the VULCAN laser facility that demonstrates how EMP can be readily and effectively reduced. Characterization of the EMP was achieved using B-dot and D-dot probes that took measurements for a range of different target and laser parameters. We demonstrate that target stalk geometry, material composition, geodesic path length and foil surface area can all play a significant role in the reduction of EMP. A combination of electromagnetic wave and 3D particle-in-cell simulations is used to inform our conclusions about the effects of stalk geometry on EMP, providing an opportunity for comparison with existing charge separation models.