Studies of the influence of laser focusing and plasma profile on Direct Laser Acceleration

Louise Willingale

University of Michigan

Tuesday, May 14, 2024

12:00pm

NW17-218 Hybrid

PSFC Seminars

Abstract: Direct Laser Acceleration (DLA) of electrons is a mechanism for superponderomotive energy gain during relativistically intense laser-plasma interactions. As laser facilities reach multi-petawatt powers, DLA will be increasingly important as the main energy exchange mode between the laser and the plasma. The ponderomotive force of the laser pulse acts on the plasma to form a channel with transverse electric and azimuthal magnetic fields that enable energy exchange from the laser to the electrons. We investigate DLA using experiments performed at the OMEGA EP laser facility and particle-in-cell simulations using the code OSIRIS 4.0. The experimental variables include the plasma density and profile, the laser pulse focusing, energy and pulse duration. Matching the focal spot to the size of the oscillations the electrons perform within the channel is most favorable for achieving the highest energies. Therefore an optimum focal spot size is found to be a function of density and laser pulse power. Additionally, it was found an extended density gradient at the plasma rear created conditions that both improved the channeling of the laser over longer distances to enhance DLA, and inhibited the formation of a sheath field to allow more electrons to escape into the vacuum. Applications of DLA are for bright directional sources of x-rays, or for secondary interactions to create Bremsstrahlung-photons or positrons.

Acknowledgements: This material is based upon work supported by the Department of Energy / NNSA under Award Number DE-NA0004030. The experiment was conducted at the Omega Laser Facility at the University of Rochester's Laboratory for Laser Energetics with the beam time through the National Laser Users' Facility (NLUF) Program supported by DOE/NNSA.     The authors would like to acknowledge the OSIRIS Consortium, consisting of UCLA and IST (Lisbon, Portugal) for the for providing access to the OSIRIS 4.0 framework. Work supported by NSF ACI-1339893.

Bio: Louise Willingale is an Associate Professor at the University of Michigan in the Electrical and Computer Engineering department and the Associate Director for the NSF ZEUS facility at the Gérard Mourou Center for Ultrafast Optical Science. Prof. Willingale researches experimental high-intensity laser-plasma interactions, with a focus on relativistic electron heating, ion acceleration, proton radiography, magnetic-field generation, and reconnection. She received a MSci in Physics (2003) and a PhD in Plasma Physics (2007) from Imperial College London. In 2008, she moved to the University of Michigan, first as a Postdoctoral Researcher, then as an Assistant Research Scientist, before becoming an Assistant Professor in 2014. In 2018, she received a Faculty Early Career Development (CAREER) Award from the NSF to study laser-driven magnetic reconnection and was elected Fellow of the American Physical Society (APS) in 2022 and is a Kavli Fellow.