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Title: Radiation generation and high-field physics phenomena in ultra-intense laser-solid interactions
Author: Duff, Matthew James
ISNI:       0000 0004 8509 9504
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2019
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This thesis reports on experimental and numerical investigations of the radiation produced via ultra-intense laser-solid interactions. Laser intensities in the range 10exp18−10exp24 Wcm−2 are explored, enabling these interactions to be investigated at current laser facilities (at the lower end of this intensity range), and predictions to be made about the properties of the emitted radiation, which will be measured at upcoming multi-petawatt laser facilities (for which intensities exceeding 10exp23 Wcm−2 should be accessible). The associated laser fields have sufficiently strong magnitudes that quantum electrodynamics (QED) effects play a significant role in the interaction. There are two key effects which will be considered within the context of this thesis. The first is prolific production of high energy radiation from electrons that are accelerated by the laser fields. This radiation emission is necessarily accompanied by a back-reaction, or radiation reaction(RR) force. The second key effect is the production of electron-positron pairs, from the interaction of the high energy radiation with the laser fields. These QED effects will, in turn, impact relativistic plasma physics processes, enabling insight to be gained into interactions in the QED-plasma regime. Ion acceleration will be influenced, as well as the production of radiation, which may enable the realisation of ultra-bright radiation sources (with brightness ∼ 1024 ph·s−1·mm−2·mrad−2·(0.1% bandwidth)−1) from laser-solid interactions. Exploiting these QED effects will enable exotic states of matter, such as electron-positron pair-plasmas, to be produced in the laboratory enabling, for example, the study of fundamental astrophysical phenomena. The results presented in this thesis form three distinct studies. The first investigates the process of relativistic self-induced transparency (RSIT) during the interaction of an ultra-intense laser pulse (10exp20 Wcm−2) with an ultra-thin foil target (which is tens of nanometers thick). RSIT is a plasma physics process which is predicted to be heavily influenced by QED effects. During this investigation of RSIT, it was found that the light detected at the rear of ultra-thin foil targets is converted into higher order spatial modes, at both the laser frequency and its second harmonic. It is found that it is possible to produce a radially polarised mode, of high intensity (∼10exp18 Wcm−2). Such a mode has applications in the efficient acceleration of electrons and positrons, and in the generation of radiation sources which are optimised in terms of their average photon energy and beam divergence. It is highly difficult to generate these modes at high intensity, using conventional solid-state optics, given their damage thresholds. The second study is a numerical and analytical investigation of the effects of RSIT and RR on the acceleration of thin foil targets (hundreds of nanometers thick) interacting with ultra-intense laser pulses (10exp23 Wcm−2). It is demonstrated that the magnitude of the RR force is sensitive to the target thickness, thus indicating that it may be possible to control the properties of the emitted radiation, and the partition of laser energy between the plasma species, at upcoming laser facilities. For targets in which the magnitude of the RR force is weak, the emitted radiation is nearly isotropic, whereas for strong RR, distinct peaks appear in the photon angular distribution. Given that the magnitude of the RR force is also reduced by early onset RSIT, it will be possible to diagnose this process via changes in the radiation distribution, at upcoming laser facilities. In this study, an analytical model was developed which, for the first time, describes the target velocity in the light sail regime of radiation pressure acceleration,under the influence of the RR force. The predictions of this model, in terms of the target velocity and the photon conversion efficiency, are found to be in good agreement with numerical simulations. The final study proposes a multi-stage scheme for the investigation of non-linear pair-production, utilising a laser-solid interaction. By employing numerical modelling,it is demonstrated that this set-up enables an enhancement in the number and energy of positrons produced during ultra-intense laser-solid interactions, compared to conventional experimental set-ups. The first stage is the generation and optimisation of aγ-ray beam (in terms of the average photon energy and divergence half-angle) from an ultra-intense (10exp23 Wcm−2) laser-solid interaction. In the second stage, this beam interacts with dual counter-propagating laser pulses, inducing non-linear pair-production. It is shown that a significant number of positrons are produced by employing this scheme, with an anisotropic energy-angle distribution. This study will assist in the design of future experiments aiming to generate QED plasmas in the laboratory.
Supervisor: McKenna, Paul Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral