Laser plasma accelerators 1 have produced high-quality electron beams with GeV energies from cm-scale devices 2 and are being investigated as hyperspectral fs light sources producing THz to γ-ray radiation 3-5 , and as drivers for future highenergy colliders 6,7 . These applications require a high degree of stability, beam quality and tunability. Here we report on a technique to inject electrons into the accelerating field of a laser-driven plasma wave and coupling of this injector to a lower-density, separately tunable plasma for further acceleration. The technique relies on a single laser pulse powering a plasma structure with a tailored longitudinal density profile, to produce beams that can be tuned in the range of 100-400 MeV with per-cent-level stability, using laser pulses of less than 40 TW. The resulting device is a simple stand-alone accelerator or the front end for a multistage higher-energy accelerator.Producing high-quality electron beams from an accelerator requires electron injection into the accelerating field to be localized in time and space. For laser plasma accelerators (LPAs) that rely on homogeneous plasmas driven with single laser pulses, continuous injection can occur when driving large-amplitude plasma waves (wakefields), resulting in large energy spread. Lower energy spread can be achieved through termination of injection by operating near the injection threshold or by injecting enough charge to suppress the wake amplitude (that is, beam loading). Subsequent termination of the accelerating process at dephasing (that is, when electrons are starting to outrun the accelerating wave) minimizes energy spread. These mechanisms have produced per-cent-level energy-spread beams 2,8-10 , but small changes in parameters can result in large changes in beam quality. As a result, tunability has been limited, necessitating the development of a simple, robust and controlled injection technique combined with an independently controllable accelerating stage.In general, injection of electrons into a plasma wave occurs when the velocity of background electrons approaches the wake phase velocity. Laser-based methods for boosting the electron velocity have been proposed 11,12 and implemented 13,14 to achieve tunable electron beams, but require sophisticated alignment and synchronization of the multiple laser pulses. Injection can also be triggered by introducing electrons into the correct phase of the wake through ionization 15 , but so far the technique has resulted in broad energy-spread beams with high divergence 16,17 . A different approach, that relies on a single laser pulse for powering the LPA, is to momentarily slow down the wake phase velocity to facilitate trapping 18 . The control of the wake phase velocity can be achieved by tailoring the nonlinear plasma wavelength λ p (z) along the longitudinal coordinate z, through control of the electron density n e and the laser parameters. Specifically, λ p (z) = λ p0 (z)F , where the linear plasma wavelength λ p0 (µm) ≈ 3.3 × 10 10 / √ n e (cm −3 ) and F ...

A new ion acceleration method, namely, phase-stable acceleration, using circularly-polarized laser pulses is proposed. When the initial target density n(0) and thickness D satisfy a(L) approximately (n(0)/n(c))D/lambda(L) and D>l(s) with a(L), lambda(L), l(s), and n(c) the normalized laser amplitude, the laser wavelength in vacuum, the plasma skin depth, and the critical density of the incident laser pulse, respectively, a quasiequilibrium for the electrons is established by the light pressure and the space charge electrostatic field at the interacting front of the laser pulse. The ions within the skin depth of the laser pulse are synchronously accelerated and bunched by the electrostatic field, and thereby a high-intensity monoenergetic proton beam can be generated. The proton dynamics is investigated analytically and the results are verified by one- and two-dimensional particle-in-cell simulations.

Lithium metal anodes can largely enhance the energy density of rechargeable batteries because of the high theoretical capacity and the high negative potential. However, the problem of lithium dendrite formation and low Coulombic efficiency (CE) during electrochemical cycling must be solved before lithium anodes can be widely deployed. Herein, a new atomic layer deposition (ALD) chemistry to realize the low-temperature synthesis of homogeneous and stoichiometric lithium fluoride (LiF) is reported, which then for the first time, as far as we know, is deposited directly onto lithium metal. The LiF preparation is performed at 150 °C yielding 0.8 Å/cycle. The LiF films are found to be crystalline, highly conformal, and stoichiometric with purity levels >99%. Nanoindentation measurements demonstrate the LiF achieving a shear modulus of 58 GPa, 7 times higher than the sufficient value to resist lithium dendrites. When used as the protective coating on lithium, it enables a stable Coulombic efficiency as high as 99.5% for over 170 cycles, about 4 times longer than that of bare lithium anodes. The remarkable battery performance is attributed to the nanosized LiF that serves two critical functions simultaneously: (1) the high dielectric value creates a uniform current distribution for excellent lithium stripping/plating and ultrahigh mechanical strength to suppress lithium dendrites; (2) the great stability and electrolyte isolation by the pure LiF on lithium prevents parasitic reactions for a much improved CE. This new ALD chemistry for conformal LiF not only offers a promising avenue to implement lithium metal anodes for high-capacity batteries but also paves the way for future studies to investigate failure and evolution mechanisms of solid electrolyte interphase (SEI) using our LiF on anodes such as graphite, silicon, and lithium.

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