E-320 experiment at FACET-II

The present proposal is intended to support the participation of Dr. Gianluca Sarri and his PhD student, Mr. Niall Cavanagh, of The Queen's University of Belfast to the experimental campaign E-320 at the FACET-II beamline of the Stanford Linear Accelerator (SLAC). The investigator of this proposal is a central member of an international collaboration of world-leading Universities and research institutes, including Princeton, Stanford, Ecole Polytechnique, the Max-Planck institute for Nuclear Physics, and UCLA that recently was awarded, in an extremely competitive environment, beam time at FACET-II for a series of ground-breaking experiments on Strong-Field Quantum ElectroDynamics (SFQED). In a nutshell, the experiments aim at studying the dynamics of the 13 GeV electron beam provided by the FACET-II accelerator as it propagates through the focus of a 20 TW laser system, already installed and operational. Peak intensities exceeding 1x1020 Wcm^-2 can be realistically achieved, allowing to reach a peak electric field, in the rest frame of the electrons, of the order of 70% of the Schwinger field, or critical field of QED. This value significantly exceeds what previously obtained at SLAC in the late 90s and, more recently, by the PI and collaborators using the Gemini laser at the Central Laser Facility in the UK.
We plan to further advance previous work by performing precision measurements and collecting extensive data sets to provide sufficient statistics to study in detail iconic phenomena in SFQED, such as quantum radiation reaction, strongly non-linear Compton scattering, and pair production. The collected data will shed light at the current intensity frontier of high-power lasers, allowing one not only to test numerical and analytical models currently used in this physical regime, but also to optimise experimental techniques for the operation of the next generation of ultra-intense laser facilities.

The next generation of ultra-intense laser facilities will allow achieving unprecedented laser intensities and will open up exciting and unexplored avenues of research in the response of matter to these ultra-high fields. Besides the fundamental interest in probing physics at this intensity frontier, there are also attractive practical applications since particle and photon beams with unique properties will be generated. The applications of these beams to areas as diverse as manufacturing, medicine, and homeland security are virtually limitless.

However, the response of particles to such ultra-intense fields cannot be treated classically and naturally falls within the realm of QED Whilst QED is well understood in a low-field regime, little is yet experimentally known of its strong-field behaviour (SFQED). A vast wealth of theoretical work has been, and continues to be, developed to understand and model this extreme experimental conditions. Several numerical codes have been developed worldwide, including CAIN [29], GUINEA-PIG [30], OSIRIS [33], and the UK-developed code EPOCH [32]. These models already include SFQED corrections to the particle dynamics but, to date, lack a detailed experimental validation. Providing large-statistics and precise datasets in this regime is thus crucial for the correct operation of the next generation of ultra-intense lasers. Not only, experimentally accessing high-energy density environments will have ample resonance in several others scientific areas. For instance, it will help understanding the dynamics and radiative properties of astrophysical plasmas in proximity of ultra-massive objects and it will help towards the design of the next generation of particle colliders (CLIC and ILC will reach the critical field at the interaction point). Finally, the practical work involved in setting up this class of experiments will result in the development of novel detectors and techniques that will be instrumental for running high energy-density experiments.

To ensure maximum academic impact of our results, we identify three main routes of dissemination of our results. The primary means of dissemination remains publication in internationally leading scientific journals. We will target different journals, depending on the scientific nature of the results to be presented. We will submit our main results to high impact factor journals such as Physical Review Letters, journals of the Nature family, and Applied Physics Letters. For the more technical aspects of the work, we will target more specialised journals. Within the collaboration, we have agreed to list all authors in alphabetical order under the general name "SFQED collaboration".

We will also present our results to national and international conferences. We will present our results at world-leading conferences in the field such as the Plasma Physics European and American physical conferences, the European Advanced Accelerator Concepts, SPIE, and PQE. The investigator has already a long track record of invitations in these conferences and he is also an active member of European and national projects on using laser-driven secondary sources for practical applications, such as EuPRAXIA, PWASC, and ALEGRO. The results of this project will then also be shown at the regular international meetings of these large-scale collaborations, in order to boost this emerging area of laser-driven strong field QED. Finally, our collaboration is planning to setup a freely accessible database of the results obtained, so that they can be used worldwide for the benchmarking of numerical codes and the development of more refined analytical models in strong-field QED.