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Abstract:

The Galactic cosmic ray spectrum is a remarkably straight power law. Our current understanding is that the dominant sources that accelerate cosmic rays up to the knee (3 × 1015 eV) or perhaps even the ankle (3 × 1018 eV), are young Galactic supernova remnants. In theory, however, there are various reasons why the spectrum may be different for different sources, and may not even be a power law if non-linear shock acceleration applies during the most efficient stages of acceleration.We show how the spectrum at the accelerator translates to the spectrum that makes up the escaping cosmic rays that replenish the Galactic pool of cosmic rays. We assume that cosmic ray confinement, and thus escape, is linked to the level of magnetic field amplification, and that the magnetic field is amplified by streaming cosmic rays according to the non-resonant hybrid or resonant instability. When a fixed fraction of the energy is transferred to cosmic rays, it turns out that a source spectrum that is flatter than E-2 will result in an E-2 escape spectrum, whereas a steeper source spectrum will result in an escape spectrum with equal steepening. This alleviates some of the concern that may arise from expected flat or concave cosmic ray spectra associated with non-linear shock modification. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

Previous work on the interaction of high power lasers with high density targets have identified that emission is primarily from interaction within the skin layer at the target front (e.g. Ridgers et al 2012 Phys. Rev. Lett. 108 165006). This mechanism is inefficient when compared to Reinjected Electron Synchrotron Emission (RESE) for laser interaction with low density solids (Brady et al 2012 Phys. Rev. Lett. 109 245006). However, these detailed analyses of the emission mechanisms were mainly based on 1D simulations and so did not incorporate some important 2D effects. In this paper these 1D emission mechanisms are confirmed to still exist in 2D with comparable properties and a new, intrinsically 2D, emission mechanism, termed edgeglow, is described which can convert 4-5% of the laser energy into gamma-ray energy. © 2013 IOP Publishing Ltd.

Abstract:

Pulses from a 10 PW laser are predicted to produce large numbers of gamma-rays and electron-positron pairs on hitting a solid target. However, a pair plasma, if it accumulates in front of the target, may partially shield it from the pulse. Using stationary, one-dimensional solutions of the two-fluid (electron-positron) and Maxwell equations, including a classical radiation reaction term, we examine this effect in the hole-boring scenario. We find the collective effects of a pair plasma 'cushion' substantially reduce the reflectivity, converting the absorbed flux into high-energy gamma-rays. There is also a modest increase in the laser intensity needed to achieve threshold for a non-linear pair cascade. © 2013 IOP Publishing Ltd.

Abstract:

Electron-positron plasmas are a prominent feature of the high energy Universe. In the relativistic winds from pulsars and black holes it is thought that non-linear quantum electrodynamics (QED) processes cause electromagnetic energy to cascade into an e-e+ plasma. We show that next-generation 10PW lasers, available in the next few years, will generate such a high density of pairs that they create a micro-laboratory for the first experimental study of a similarly generated e-e+ plasma. In the first simulations of a 10PW laser striking a solid we demonstrate the production of a pure electron-positron plasma of density 1026m-3. This is seven orders of magnitude denser than currently achievable in the laboratory and is comparable to the critical density for commonly used lasers, marking a step change to collective e-e+ plasma behaviour. Furthermore, a new ultraefficient laser-absorption mechanism converts 35% of the laser energy to a burst of gamma-rays of intensity 1022Wcm-2, potentially the most intense gamma-ray source available in the laboratory. This absorption results in a strong feedback between both pair and gamma-ray production and classical plasma physics leading to a new physical regime of QED-plasma physics. In this new regime the standard particle-in-cell (PIC) simulation approach, which has been the dominant kinetic simulation tool in plasma physics for 50 years, is inadequate. We have developed a new approach (QED-PIC) which will provide a powerful new modelling tool essential to the future advancement of the field of high intensity laser-plasma interactions. © 2013 SPIE.