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1. Experimental signatures of direct-laser-acceleration-assisted laser wakefield acceleration

   Shaw, J.L.; Lemos, N.; Marsh, K.A.; Froula, D.H.; and Joshi, C. (February 2018, Plasma physics and controlled fusion)
2. Undepleted direct laser acceleration

   https://doi.org/10.1126/sciadv.adk1947  

   Cohen, Itamar; Meir, Talia; Tangtartharakul, Kavin; Perelmutter, Lior; Elkind, Michal; Gershuni, Yonatan; Levanon, Assaf; Arefiev, Alexey V; Pomerantz, Ishay (January 2024, Science Advances)

   Intense lasers enable generating high-energy particle beams in university-scale laboratories. With the direct laser acceleration (DLA) method, the leading part of the laser pulse ionizes the target material and forms a positively charged ion plasma channel into which electrons are injected and accelerated. The high energy conversion efficiency of DLA makes it ideal for generating large numbers of photonuclear reactions. In this work, we reveal that, for efficient DLA to prevail, a target material of sufficiently high atomic number is required to maintain the injection of ionization electrons at the peak intensity of the pulse when the DLA channel is already formed. We demonstrate experimentally and numerically that, when the atomic number is too low, the target is depleted of its ionization electrons prematurely. Applying this understanding to multi-petawatt laser experiments is expected to result in increased neutron yields, a perquisite for a wide range of research and applications. 

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3. Electron acceleration using twisted laser wavefronts

   https://doi.org/10.1088/1361-6587/ac318d  

   Shi, Yin; R Blackman, David; Arefiev, Alexey (November 2021, Plasma Physics and Controlled Fusion)

   Abstract Using plasma mirror injection we demonstrate, both analytically and numerically, that a circularly polarized helical laser pulse can accelerate highly collimated dense bunches of electrons to several hundred MeV using currently available laser systems. The circular-polarized helical (Laguerre–Gaussian) beam has a unique field structure where the transverse fields have helix-like wave-fronts which tend to zero on-axis where, at focus, there are large on-axis longitudinal magnetic and electric fields. The acceleration of electrons by this type of laser pulse is analyzed as a function of radial mode number and it is shown that the radial mode number has a profound effect on electron acceleration close to the laser axis. Using three-dimensional particle-in-cell simulations a circular-polarized helical laser beam with power of 0.6 PW is shown to produce several dense attosecond bunches. The bunch nearest the peak of the laser envelope has an energy of 0.47 GeV with spread as narrow as 10%, a charge of 26 pC with duration of ∼ 400 as, and a very low divergence of 20 mrad. The confinement by longitudinal magnetic fields in the near-axis region allows the longitudinal electric fields to accelerate the electrons over a long period after the initial reflection. Both the longitudinal E and B fields are shown to be essential for electron acceleration in this scheme. This opens up new paths toward attosecond electron beams, or attosecond radiation, at many laser facilities around the world. 

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4. Beam dynamics in dielectric laser acceleration

   https://doi.org/10.1088/1748-0221/17/05/P05014  

   Niedermayer, U.; Leedle, K.; Musumeci, P.; Schmid, S.A. (May 2022, Journal of Instrumentation)

   Abstract We discuss recent developments and challenges of beam dynamics in Dielectric Laser Acceleration (DLA), for both high and low energy electron beams. Starting from ultra-low emittance nanotip sources the paper follows the beam path of a tentative DLA light source concept. Acceleration in conjuction with focusing is discussed in the framework of Alternating Phase Focusing (APF) and spatial harmonic ponderomotive focusing. The paper concludes with an outlook to the beam dynamics in laser driven nanophotonic undulators, based on tilted DLA grating structures. 

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5. 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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