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1. Ponderomotive snowplow electron acceleration with high energy tilted ultrafast laser pulses

   https://doi.org/10.21203/rs.3.rs-4177060/v1  

   Hunt, Patrick; Wilhelm, Alex; Adams, Daniel; Wang, Shoujun; Hollinger, Reed; Shpilman, Ze'ev; Anaraki, Sina; Westlake, Nathaniel; Schmidt, David; Rocca, Jorge; et al (April 2024, Research Square)

   Abstract The application high intensity ultrafast lasers to compact plasma-based electron accelerators has recently been an extremely active area of research. Here, for the first time, we show experimentally and theoretically that carefully sculpting an intense ultrafast pulse in the spatio-temporal domain allows ponderomotive pressure to be used for direct acceleration of electron bunches from rest to relativistic energies. With subluminal group velocity and above-threshold intensity, a laser pulse can capture and accelerate electrons, pushing on them like a snowplow. Acceleration of electrons from rest requires a substantial reduction of group velocity. In this demonstration experiment, we achieve a group velocity of ∼0.6c in a tilted pulse by focusing the output of a novel asymmetric pulse compressor we developed for the petawatt-class ALEPH system at Colorado State University. This direct laser-electron approach opens a route towards exploiting optical spatio-temporal control techniques to sculpt electron beams with desired properties such as narrow energy and angular distributions. The tilted-pulse snowplow technique can be scaled from small-scale to facility-scale amplifiers to produce short electron bunches in the 10 keV−10 MeV range for applications including ultrafast electron diffraction and efficient injection into laser wakefield accelerators for acceleration beyond the GeV level. 

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2. Control of Pulse Front Tilt and Curvature for Ultrafast Ponderomotive Electron Acceleration

   Wilhelm, Alex; Durfee, Charles G. (July 2022, Conference on Lasers and Electrooptics)

   We􀀁explore􀀁an􀀁electron􀀁acceleration􀀁scheme􀀁which􀀁uses􀀁the􀀁ponderomotive􀀁force􀀁of􀀁a􀀁tilted􀀁ultrafast􀀁laser􀀁as􀀁the􀀁drive􀀁mechanism􀀁for􀀁acceleration. The􀀁effect􀀁of􀀁pulse􀀁front􀀁curvature􀀁on􀀁the􀀁acceleration􀀁process􀀁is􀀁also􀀁discussed. 

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3. Coherence effects in nonlinear Thomson scattering by electrons born from the same atom

   https://doi.org/10.1103/PhysRevA.110.043508  

   Sá, Nuno_Barros e; Sun, Yance; Peatross, Justin (October 2024, Physical Review A)

   We investigate theoretically nonlinear Thomson scattering by multiple electrons ionized from individual atoms during a short high-intensity laser pulse. The emitted light is influenced by the distance that the electrons move apart from each other during the passage of the pulse, owing to coherence effects. We examine trajectories of electrons born from the same atom via successive ionizations as the laser pulse ramps up. While the overall trajectory of an individual electron is influenced by the ponderomotive force, we find that the separation between electrons arises mostly from stronger and differing initial drift velocities associated with the moment of ionization in the laser field. In the case of helium, we find that the separation between its two ionized electrons becomes appreciable (compared to emitted wavelengths) primarily along the dimension of laser linear polarization. This distorts the angular emission patterns of nonlinear Thomson scattering in comparison with emission from individual free electrons. Radiation scattered perpendicular to the laser polarization tends to add constructively, while radiation scattered along the direction of linear laser polarization tends to add incoherently. This effect becomes more pronounced for atoms with higher numbers of ionized electrons. The effect influences primarily the lower harmonic orders. 

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4. Accurate simulation of direct laser acceleration in a laser wakefield accelerator

   https://doi.org/10.1063/5.0152383  

   Miller, Kyle G.; Palastro, John P.; Shaw, Jessica L.; Li, Fei; Tsung, Frank S.; Decyk, Viktor K.; Joshi, C.; Mori, Warren B. (July 2023, Physics of Plasmas)

   In a laser wakefield accelerator (LWFA), an intense laser pulse excites a plasma wave that traps and accelerates electrons to relativistic energies. When the pulse overlaps the accelerated electrons, it can enhance the energy gain through direct laser acceleration (DLA) by resonantly driving the betatron oscillations of the electrons in the plasma wave. The traditional particle-in-cell (PIC) algorithm, although often the tool of choice to study DLA, contains inherent errors due to numerical dispersion and the time staggering of the electric and magnetic fields. Furthermore, conventional PIC implementations cannot reliably disentangle the fields of the plasma wave and laser pulse, which obscures interpretation of the dominant acceleration mechanism. Here, a customized field solver that reduces errors from both numerical dispersion and time staggering is used in conjunction with a field decomposition into azimuthal modes to perform PIC simulations of DLA in an LWFA. Comparisons with traditional PIC methods, model equations, and experimental data show improved accuracy with the customized solver and convergence with an order-of-magnitude fewer cells. The azimuthal-mode decomposition reveals that the most energetic electrons receive comparable energy from DLA and LWFA. 

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5. Direct Laser Acceleration of Electrons using a Shaped Tilted Ponderomotive Mirror

   Hunt, Patrick; Adams, Daniel; Durfee, Charles G (October 2022, Bulletin of the American Physical Society)

   While there has been success in Wakefield acceleration of electrons, there are a number of applications that could benefit from acceleration to modest energy (~MeV) by the laser field, for example, ultrafast electron diffraction and injection into higher-energy laser-driven accelerators. Here we outline our scheme for ponderomotive acceleration of electrons (and in principle, positrons) in which we control the group velocity of ultrafast pulses through pulse front tilt. Provided the intensity is above the threshold for capture of electrons, the leading part of the pulse front effectively acts like a moving mirror whose shape is controlled by the spatio-temporal topology of the intensity profile. Our analytic models of the propagation of spatially-chirped beams, simple relativistic single-particle models of the laser-electron interaction and our implementation of these beams in particle-in-cell simulations help to predict the output electron energy and direction. We are preparing experiments on the ALEPH laser system at Colorado State University in which we will use the diagnostic techniques that we have developed to align our scaled-up design of a high-energy pulse compressor that will deliver spatially chirped pulses. 

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