The construction of 10 PW class laser facilities with unprecedented intensities has emphasized the need for a thorough understanding of the radiation reaction process. We describe simulations for a recent all-optical colliding pulse experiment, where a GeV scale electron bunch produced by a laser wakefield accelerator interacted with a counter-propagating laser pulse. In the rest frame of the electron bunch, the electric field of the laser pulse is increased by several orders of magnitude, approaching the Schwinger field and leading to substantial variation from the classical Landau-Lifshitz model. Our simulations show how the final electron and photon spectra may allow us to differentiate between stochastic and semi-classical models of radiation reaction, even when there is significant shot-to-shot variation in the experimental parameters. In particular, constraints are placed on the maximum energy spread and shot-to-shot variation permissible if a stochastic model is to be proven with confidence.
An investigation of the effects of the radiation reaction force on radiation pressure acceleration is presented. Through 1D(3V) PIC code simulations, it is found that radiation reaction causes a decrease in the target velocity during the interaction of an ultra-intense laser pulse with a solid density thin foil of varying thickness. This change in the target velocity can be related to the loss of backwards-directed electrons due to cooling and reflection in the laser field. The loss of this electron population changes the distribution of the emitted synchrotron radiation. We demonstrate that it is the emission of radiation which leads to the observed decrease in target velocity. Through a modification to the light sail equation of motion (which is used to describe radiation pressure acceleration in thin foils), which accounts for the conversion of laser energy to synchrotron radiation, we can describe this change in target velocity. This model can be tested in future experiments with ultra-high intensity lasers, and will lead to a better understanding of the process of relativistically induced transparency in the new intensity regime.
The radiation pressure of next generation high-intensity lasers could efficiently accelerate ions to GeV energies. However, nonlinear quantum-electrodynamic effects play an important role in the interaction of these lasers with matter. We show that these quantum-electrodynamic effects lead to the production of a critical density pair-plasma which completely absorbs the laser pulse and consequently reduces the accelerated ion energy and efficiency by 30-50%.
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 10<sup>26</sup>m<sup>-3</sup>. 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
10<sup>22</sup>Wcm<sup>-2</sup>, 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.