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  1. Abstract

    Laser wakefield accelerators (LWFAs) have electric fields that are orders of magnitude larger than those of conventional accelerators, promising an attractive, small-scale alternative for next-generation light sources and lepton colliders. The maximum energy gain in a single-stage LWFA is limited by dephasing, which occurs when the trapped particles outrun the accelerating phase of the wakefield. Here, we demonstrate that a single space–time structured laser pulse can be used for ionization injection and electron acceleration over many dephasing lengths in the bubble regime. Simulations of a dephasingless laser wakefield accelerator driven by a 6.2-J laser pulse show 25 pC of injected charge accelerated over 20 dephasing lengths (1.3 cm) to a maximum energy of 2.1 GeV. The space–time structured laser pulse features an ultrashort, programmable-trajectory focus. Accelerating the focus, reducing the focused spot-size variation, and mitigating unwanted self-focusing stabilize the electron acceleration, which improves beam quality and leads to projected energy gains of 125 GeV in a single, sub-meter stage driven by a 500-J pulse.

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  2. 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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    Free, publicly-accessible full text available July 1, 2024
  3. We are developing an X-ray source for radiography of high-energy density (HED) experiments by passing a picosecond, relativistic laser beam through an underdense plasma to generate a relativistic beam of electrons. These electrons, in turn, generate bright, (1010 photon/keV/sr), high energy (10 keV - 1 MeV) X-rays. Over the years, this X-ray platform has been demonstrated on the Titan, Omega EP, and NIF-ARC lasers. This paper gives the present state of the field and argues that the platform has reached a level of maturity where the X-rays produced using this novel platform have the potential to find radiographic applications in a broad range of fields. Index Terms—X-ray, High Energy Density Science (HEDS), Self-Modulated Plasma Instability, NIF, OMEGA, Backlighter 
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    Free, publicly-accessible full text available April 26, 2024
  4. Abstract The longitudinal coherence of X-ray free-electron lasers (XFELs) in the self-amplified spontaneous emission regime could be substantially improved if the high brightness electron beam could be pre-bunched on the radiated wavelength-scale. Here, we show that it is indeed possible to realize such current modulated electron beam at angstrom scale by exciting a nonlinear wake across a periodically modulated plasma-density downramp/plasma cathode. The density modulation turns on and off the injection of electrons in the wake while downramp provides a unique longitudinal mapping between the electrons’ initial injection positions and their final trapped positions inside the wake. The combined use of a downramp and periodic modulation of micrometers is shown to be able to produces a train of high peak current (17 kA) electron bunches with a modulation wavelength of 10’s of angstroms - orders of magnitude shorter than the plasma density modulation. The peak brightness of the nano-bunched beam can be O (10 21 A/m 2 /rad 2 ) orders of magnitude higher than current XFEL beams. Such prebunched, high brightness electron beams hold the promise for compact and lower cost XEFLs that can produce nanometer radiation with hundreds of GW power in a 10 s of centimeter long undulator. 
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  5. Abstract This paper analyses images from 43 to 340 GHz to trace the structure of the Source I (SrcI) disk in Orion-KL with ∼12 au resolution. The data reveal an almost edge-on disk with an outside diameter ∼100 au, which is heated from the inside. The high opacity at 220–340 GHz hides the internal structure and presents a surface temperature ∼500 K. Images at 43, 86 and 99 GHz reveal structure within the disk. At 43 GHz there is bright compact emission with brightness temperature ∼1300 K. Another feature, most prominent at 99 GHz, is a warped ridge of emission. The data can be explained by a simple model with a hot inner structure, seen through cooler material. A wide-angle outflow mapped in SiO emission ablates material from the interior of the disk, and extends in a bipolar outflow over 1000 au along the rotation axis of the disk. SiO v = 0, J = 5–4 emission appears to have a localized footprint in the warped ridge. These observations suggest that the ridge is the working surface of the disk, and heated by accretion and the outflow. The disk structure may be evolving, with multiple accretion and outflow events. We discuss two sources of variability: (1) variable accretion onto the disk as SrcI travels through the filamentary debris from the Becklin–Neugebauer Object-SrcI encounter ∼550 yr ago; and (2) episodic accretion from the disk onto the protostar, which may trigger multiple outflows. The warped inner-disk structure is direct evidence that SrcI could be a binary experiencing episodic accretion. 
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  6. In Dunfield’s catalog of the hyperbolic manifolds in the SnapPy census which are complements of L-space knots in S, we determine that 22 have tunnel number 2 while the remaining all have tunnel number 1. Notably, these 22 manifolds contain 9 asymmetric L-space knot complements. Furthermore, using SnapPy and KLO we find presentations of these 22 knots as closures of positive braids that realize the Morton-Franks-Williams bound on braid index. The smallest of these has genus 12 and braid index 4. 
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