Prof Peter Norreys FInstP;

Abstract:

A new physical model of the hosing instability that includes relativistic laser pulses and moderate densities is presented and derives the density dependence of the hosing equation. This is tested against two-dimensional particle-in-cell simulations. These simulations further examine the feasibility of using multiple pulses to mitigate the hosing instability in a Nd:glass-type parameter space. An examination of the effects of planar versus cylindrical exponential density gradients on the hosing instability is also presented. The results show that strongly relativistic pulses and more planar geometries are capable of mitigating the hosing instability which is in line with the predictions of the physical model.

Abstract:

Betatron radiation from direct-laser accelerated electrons is characterized analytically and numerically. It is shown here that the electron dynamics is strongly dependent on a self-similar parameter S(≡ne/nca0). Both the electron transverse momentum and energy are proportional to the normalized amplitude of laser field (a0) for a fixed value of S. As a result, the total number of radiated photons scales as a2/0/√S and the energy conversion efficiency of photons from the accelerated electrons scales as a3/0/S. The particle-in-cell simulations agree well with the analytical scalings. It is suggested that a tunable high-energy and high-flux radiation source can be achieved by exploiting this regime.

Abstract:

Highly anisotropic, beam-like neutron emission with peak flux of the order of 109 n/sr was obtained from light nuclei reactions in a pitcher–catcher scenario, by employing MeV ions driven by a sub-petawatt laser. The spatial profile of the neutron beam, fully captured for the first time by employing a CR39 nuclear track detector, shows a FWHM divergence angle of $\sim 70^\circ $, with a peak flux nearly an order of magnitude higher than the isotropic component elsewhere. The observed beamed flux of neutrons is highly favourable for a wide range of applications, and indeed for further transport and moderation to thermal energies. A systematic study employing various combinations of pitcher–catcher materials indicates the dominant reactions being d(p, n+p)1H and d(d,n)3He. Albeit insufficient cross-section data are available for modelling, the observed anisotropy in the neutrons' spatial and spectral profiles is most likely related to the directionality and high energy of the projectile ions.

Abstract:

© 2015 Elsevier B.V..The AWAKE Collaboration has been formed in order to demonstrate proton-driven plasma wakefield acceleration for the first time. This acceleration technique could lead to future colliders of high energy but of a much reduced length when compared to proposed linear accelerators. The CERN SPS proton beam in the CNGS facility will be injected into a 10 m plasma cell where the long proton bunches will be modulated into significantly shorter micro-bunches. These micro-bunches will then initiate a strong wakefield in the plasma with peak fields above 1 GV/m that will be harnessed to accelerate a bunch of electrons from about 20 MeV to the GeV scale within a few meters. The experimental program is based on detailed numerical simulations of beam and plasma interactions. The main accelerator components, the experimental area and infrastructure required as well as the plasma cell and the diagnostic equipment are discussed in detail. First protons to the experiment are expected at the end of 2016 and this will be followed by an initial three-four years experimental program. The experiment will inform future larger-scale tests of proton-driven plasma wakefield acceleration and applications to high energy colliders.

Abstract:

© Published under licence by IOP Publishing Ltd. We perform relativistic-intensity laser experiments using the Omega EP laser to investigate channeling phenomena and particle acceleration in underdense plasmas. A fundamental understanding of these processes is of importance to the hole-boring fast ignition scheme for inertial confinement fusion. Proton probing was used to image the electromagnetic fields formed as the Omega EP laser pulse generated a channel through underdense plasma. Filamentation of the channel was observed, followed by self-correction into a single channel. The channel radius as a function of time was found to be in reasonable agreement with momentum- conserving snowplough models.