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.

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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(...

Self-modulation of an electron beam in a plasma has been observed. The propagation of a long (several plasma wavelengths) electron bunch in an overdense plasma resulted in the production of multiple bunches via the self-modulation instability. Using a combination of a radio-frequency deflector and a dipole spectrometer, the time and energy structure of the self-modulated beam was measured. The longitudinal phase space measurement showed the modulation of a long electron bunch into three bunches with an approximately 200  keV/c amplitude momentum modulation. Demonstrating this effect is a breakthrough for proton-driven plasma accelerator schemes aiming to utilize the same physical effect.

To understand astrophysical magnetic-field amplification, we conducted a feasibility study for a laboratory experiment of a nonresonant streaming instability at the Photo Injector Test Facility at DESY, Zeuthen site (PITZ). This non-resonant streaming instability, also known as Bell's instability, is generally regarded as a candidate for the amplification of interstellar magnetic field in the upstream region of supernova-remnant shocks, which is crucial for the efficiency of diffusive shock acceleration. In the beam-plasma system composed of a radio-frequency electron gun and a gas-discharge plasma cell, the goal of our experiment is to demonstrate the development of the non-resonant streaming instability and to find its saturation level in the laboratory environment. Since we find that the electron beam will be significantly decelerated on account of an electrostatic streaming instability, which will decrease the growth rate of desired non-resonant streaming instability, we discuss possible ways to suppress the electrostatic streaming instability by considering the characteristics of a field-emission-based quasi continuous-wave electron beam.