GeV electron acceleration with a femtosecond PW laser and its applications to secondary radiation and laser-electron scattering

Abstract

The availability of ultrashort high intensity laser pulses (I>10^18 W/cm^2, 10-100 fs) has opened up a plethora of new research directions and the discovery that led to all these achievements has been rightfully rewarded with the Physics Nobel Prize in 2018. The “holy grail” of laser science, however, is to push the boundaries of the intensity up to the Schwinger Limit in order to investigate the fundamental nature of elementary particles and of the quantum vacuum. To get there, however, the research community has to dedicate considerable effort to overcome many theoretical, experimental and technological challenges.

The present work provides a study on the approaches and obstacles that stand in the way of investigating experimentally Strong Field Quantum Electrodynamics (SFQED) through laser electron collisions, using laser pulses produced by Petawatt-class lasers. To advance this field several areas of research have to be investigated together: the acceleration of electrons to multi-GeV regime and its stability, the production, characterization and development of novel detection techniques for high energy (>0.1MeV) photon beams, and investigation of the phenomenology of laser-electron collisions, with emphasis on e^- e^+ pair production and radiative losses.

Building on previous work done on particle acceleration, we performed Laser Wakefield Acceleration (LWFA) experiments in the 1-4 GeV energy regime, studied their stability and developed diagnostics and methods to control the acceleration process. We showed that it is possible to produce 4 GeV electron beams without using any form of additional guiding and that we can expect mrad level pointing instability. We developed a convenient technique for automatic calibration of electron beams and performed experiments on production of up to 4 GeV electrons in order to find the optimal parameters for the beam. We observed that the frequency chirp of the laser pulse can help produce energetic electron beams, and in particular positive frequency chirp can produce much better results than negative one. This finding is also supported by optical diagnostic of plasma, which shows that specific spectral lines of He can produce different emission intensity depending on the laser pulse's frequency chirp, implying a stronger transfer of energy from the laser to the plasma.

While electron beams are a crucial component of SFQED experiments, the detection of resulting radiation is of utmost importance. We thus performed experiments on the production of MeV photon beams through the betatron and bremsstrahlung mechanisms. The photon beams were measured with different techniques and we showed that such beams can be applied to perform radiography of dense objects with sub-millimeter spatial resolution. During this work we developed novel detection techniques for photon beams, including an attempt at performing nuclear activation experiments. These studies are supported by simulations, and we were able to highlight limitations and peculiarities on the measurement of multi-MeV photon beams.

The studies performed in this work highlighted the importance of particle-in-cell (PIC) simulations combined with analytical tools in predicting and discovering new phenomena arising from SFQED processes. We implemented various simulations and phenomenological predictions for pair production and radiation reaction experiments. In this research direction we discovered that the asymmetry of a laser pulse, coupled to the radiative losses of the electrons, plays a crucial factor in the production of secondary pairs from the Trident and Breit-Wheeler process. Additionally, we discovered a novel acceleration mechanism during laser-electron collision, which occurs when low energy pairs are trapped and gain multi-GeV energy, forming e^- e^+ jets. Other characteristics of the laser pulse, in particular polarization, has a crucial effect on the spatial distribution of particles. For future radiation reaction experiments, we quantify the radiative losses that one would expect and highlight the specific signatures that one has to look for: energy reduction and smoothing of the spectrum, and the transverse distribution of electrons.

Overall, the results presented in this thesis show that laser-electron scattering experiments on SFQED have to take an integrated approach to the challenges we face: physical (sources of background electrons, x-rays and gamma-rays, instabilities from electron and laser beam, etc.), technological (implementation of novel detectors) and theoretical (inclusion of various models, novel phenomena, collision area detectors, etc.). The experimental and theoretical results obtained in the study show that implementation of future experiments is possible with current laser technology, and that the challenges faced are more pronounced in the areas related to analysis and detection. Moreover, pursuing this research direction in a combined effort can ultimately give rise to the discovery of novel phenomena and technologies.