Particle-beam-driven plasma wakefield acceleration (PWFA) enables various novel high-gradient techniques for powering future compact light-source and high-energy physics applications. Here, a driving particle bunch excites a wakefield response in a plasma medium, which may rapidly accelerate a trailing witness beam. In this Letter, we present the measurement of ratios of acceleration of the witness bunch to deceleration of the driver bunch, the so-called transformer ratio, significantly exceeding the fundamental theoretical and thus far experimental limit of 2 in a PWFA. An electron bunch with ramped current profile was utilized to accelerate a witness bunch with a transformer ratio of 4.6_{-0.7}^{+2.2} in a plasma with length ∼10  cm, also demonstrating stable transport of driver bunches with lengths on the order of the plasma wavelength.

Temporally-modulated electron beams have a wide array of applications ranging from the generation of coherently-enhanced electromagnetic radiation to the resonant excitation of electromagnetic wakefields in advanced-accelerator concepts. Likewise producing low-energy ultrashort microbunches could be useful for ultra-fast electron diffraction and new accelerator-based light-source concepts. In this Letter we propose and experimentally demonstrate a passive microbunching technique capable of forming a picosecond bunch train at ∼ 6 MeV. The method relies on the excitation of electromagnetic wakefields as the beam propagates through a dielectric-lined waveguide. Owing to the non-ultrarelativistic nature of the beam, the induced energy modulation eventually converts into a density modulation as the beam travels in a following free-space drift. The modulated beam is further accelerated to ∼ 20 MeV while preserving the imparted density modulation. 41.75.Fr Forefront applications of electron beams call for increasingly precise spatio-temporal control over the beam phase-space distribution. Beam-manipulation techniques to tailor the distributions of electron bunches have flourished over the last decade and include various degrees of complexity [1][2][3][4][5]. Recently, methods to passively shape the temporal (or current) distribution of an electron beam have emerged [6][7][8]. In essence, this class of techniques uses a dielectric-lined waveguide (DLW) to impart an arbitrary time-energy correlation along an electron bunch; subsequently a suitable beamline converts the induced energy correlations into a temporal distribution i.e. current profile. The techniques successfully demonstrated so far [6,8] were realized at relativistic energies and use a dispersive section composed of a magnetic chicane [9] to manipulate the current profile.In this Letter we demonstrate that a DLW located directly downstream of a photoemission electron source supports the formation of a current-modulated beam over a drift in free space, thereby avoiding a magnetic-based dispersive section and associated dilution of the phasespace distribution in the bending-plane degree of freedom [10]. The formed current-modulated beams could be injected in a subsequent linear accelerator to allow for further tailoring. Additionally, the availability of shaped low-energy modulated beams [11] could have direct application to THz light sources [12,13] or ultra-fast electron diffraction [14,15].In order to quantify the proposed self-bunching mechanism, we model the electron bunch as a line-charge distribution and analyze the dynamics of the electrons in the longitudinal phase space (LPS) with coordinates (ζ, δ) where ζ refers to the axial position of an electron with re-spect to the bunch's center and δ ≡ p/ p −1 ∆p z / p z is the fractional momentum offset of an electron; here p represents the bunch mean momentum (p z refers to the longitudinal momentum). The axial field associated to the wakefield generated by the electron bunch is given by E z (ζ) = ζ −∞ Λ(...

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