Observation of Skewed Electromagnetic Wakefields in an Asymmetric Structure Driven by Flat Electron Bunches.

Relativistic charged-particle beams that generate intense longitudinal fields in accelerating structures also inherently couple to transverse modes. The effects of this coupling may lead to beam breakup instability and thus must be countered to preserve beam quality in applications such as linear colliders. Beams with highly asymmetric transverse sizes (flat beams) have been shown to suppress the initial instability in slab-symmetric structures. However, as the coupling to transverse modes remains, this solution serves only to delay instability. In order to understand the hazards of transverse coupling in such a case, we describe here an experiment characterizing the transverse effects on a flat beam, traversing near a planar dielectric lined structure. The measurements reveal the emergence of a previously unobserved skew-quadrupolelike interaction when the beam is canted transversely, which is not present when the flat beam travels parallel to the dielectric surface. We deploy a multipole field fitting algorithm to reconstruct the projected transverse wakefields from the data. We generate the effective kick vector map using a simple two-particle theoretical model, with particle-in-cell simulations used to provide further insight for realistic particle distributions.

Attosecond-Angstrom free-electron-laser towards the cold beam limit.

Electron beam quality is paramount for X-ray pulse production in free-electron-lasers (FELs). State-of-the-art linear accelerators (linacs) can deliver multi-GeV electron beams with sufficient quality for hard X-ray-FELs, albeit requiring km-scale setups, whereas plasma-based accelerators can produce multi-GeV electron beams on metre-scale distances, and begin to reach beam qualities sufficient for EUV FELs. Here we show, that electron beams from plasma photocathodes many orders of magnitude brighter than state-of-the-art can be generated in plasma wakefield accelerators (PWFAs), and then extracted, captured, transported and injected into undulators without significant quality loss. These ultrabright, sub-femtosecond electron beams can drive hard X-FELs near the cold beam limit to generate coherent X-ray pulses of attosecond-Angstrom class, reaching saturation after only 10 metres of undulator. This plasma-X-FEL opens pathways for advanced photon science capabilities, such as unperturbed observation of electronic motion inside atoms at their natural time and length scale, and towards higher photon energies.

Single Shot Characterization of High Transformer Ratio Wakefields in Nonlinear Plasma Acceleration.

Plasma wakefields can enable very high accelerating gradients for frontier high energy particle accelerators, in excess of 10 GeV/m. To overcome limits on single stage acceleration, specially shaped drive beams can be used in both linear and nonlinear plasma wakefield accelerators (PWFA), to increase the transformer ratio, implying that the drive beam deceleration is minimized relative to acceleration obtained in the wake. In this Letter, we report the results of a nonlinear PWFA, high transformer ratio experiment using high-charge, longitudinally asymmetric drive beams in a plasma cell. An emittance exchange process is used to generate variable drive current profiles, in conjunction with a long (multiple plasma wavelength) witness beam. The witness beam is energy modulated by the wakefield, yielding a response that contains detailed spectral information in a single-shot measurement. Using these methods, we generate a variety of beam profiles and characterize the wakefields, directly observing transformer ratios up to R=7.8. Furthermore, a spectrally based reconstruction technique, validated by 3D particle-in-cell simulations, is introduced to obtain the drive beam current profile from the decelerating wake data.

Conductivity Induced by High-Field Terahertz Waves in Dielectric Material.

An intense, subpicosecond, relativistic electron beam traversing a dielectric-lined waveguide generates very large amplitude electric fields at terahertz (THz) frequencies through the wakefield mechanism. In recent work employing this technique to accelerate charged particles, the generation of high-power, narrow-band THz radiation was demonstrated. The radiated waves contain fields with measured amplitude exceeding 2 GV/m, orders of magnitude greater than those available by other THz generation techniques at a narrow bandwidth. For fields approaching the GV/m level, a strong damping has been observed in SiO_{2}. This wave attenuation with an onset near 850 MV/m is consistent with changes to the conductivity of the dielectric lining and is characterized by a distinctive latching mechanism that is reversible on longer timescales. We describe the detailed measurements that serve to clarify the underlying physical mechanisms leading to strong field-induced damping of THz radiation (hω=1.59 meV, f=0.38 THz) in SiO_{2}, a bulk, wide band-gap (8.9 eV) dielectric.

Advanced schemes for underdense plasma photocathode wakefield accelerators: pathways towards ultrahigh brightness electron beams.

The 'Trojan Horse' underdense plasma photocathode scheme applied to electron beam-driven plasma wakefield acceleration has opened up a path which promises high controllability and tunability and to reach extremely good quality as regards emittance and five-dimensional beam brightness. This combination has the potential to improve the state-of-the-art in accelerator technology significantly. In this paper, we review the basic concepts of the Trojan Horse scheme and present advanced methods for tailoring both the injector laser pulses and the witness electron bunches and combine them with the Trojan Horse scheme. These new approaches will further enhance the beam qualities, such as transverse emittance and longitudinal energy spread, and may allow, for the first time, to produce ultrahigh six-dimensional brightness electron bunches, which is a necessary requirement for driving advanced radiation sources. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

Directions in plasma wakefield acceleration.

This introductory article is a synopsis of the status and prospects of particle-beam-driven plasma wakefield acceleration (PWFA). Conceptual and experimental breakthroughs obtained over the last years have initiated a rapid growth of the research field, and increased maturity of underlying technology allows an increasing number of research groups to engage in experimental R&D. We briefly describe the fundamental mechanisms of PWFA, from which its chief attractions arise. Most importantly, this is the capability of extremely rapid acceleration of electrons and positrons at gradients many orders of magnitude larger than in conventional accelerators. This allows the size of accelerator units to be shrunk from the kilometre to metre scale, and possibly the quality of accelerated electron beam output to be improved by orders of magnitude. In turn, such compact and high-quality accelerators are potentially transformative for applications across natural, material and life sciences. This overview provides contextual background for the manuscripts of this issue, resulting from a Theo Murphy meeting held in the summer of 2018. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

An inverse free electron laser acceleration-driven Compton scattering X-ray source.

The generation of X-rays and γ-rays based on synchrotron radiation from free electrons, emitted in magnet arrays such as undulators, forms the basis of much of modern X-ray science. This approach has the drawback of requiring very high energy, up to the multi-GeV-scale, electron beams, to obtain the required photon energy. Due to the limit in accelerating gradients in conventional particle accelerators, reaching high energy typically demands use of instruments exceeding 100's of meters in length. Compact, less costly, monochromatic X-ray sources based on very high field acceleration and very short period undulators, however, may enable diverse, paradigm-changing X-ray applications ranging from novel X-ray therapy techniques to active interrogation of sensitive materials, by making them accessible in energy reach, cost and size. Such compactness and enhanced energy reach may be obtained by an all-optical approach, which employs a laser-driven high gradient accelerator based on inverse free electron laser (IFEL), followed by a collision point for inverse Compton scattering (ICS), a scheme where a laser is used to provide undulator fields. We present an experimental proof-of-principle of this approach, where a TW-class CO2 laser pulse is split in two, with half used to accelerate a high quality electron beam up to 84 MeV through the IFEL interaction, and the other half acts as an electromagnetic undulator to generate up to 13 keV X-rays via ICS. These results demonstrate the feasibility of this scheme, which can be joined with other techniques such as laser recirculation to yield very compact photon sources, with both high peak and average brilliance, and with energies extending from the keV to MeV scale. Further, use of the IFEL acceleration with the ICS interaction produces a train of high intensity X-ray pulses, thus enabling a unique tool synchronized with a laser pulse for ultra-fast strobe, pump-probe experimental scenarios.

Experimental Characterization of Electron-Beam-Driven Wakefield Modes in a Dielectric-Woodpile Cartesian Symmetric Structure.

Photonic structures operating in the terahertz (THz) spectral region enable the essential characteristics of confinement, modal control, and electric field shielding for very high gradient accelerators based on wakefields in dielectrics. We report here an experimental investigation of THz wakefield modes in a three-dimensional photonic woodpile structure. Selective control in exciting or suppressing of wakefield modes with a nonzero transverse wave vector is demonstrated by using drive beams of varying transverse ellipticity. Additionally, we show that the wakefield spectrum is insensitive to the offset position of strongly elliptical beams. These results are consistent with analytic theory and three-dimensional simulations and illustrate a key advantage of wakefield systems with Cartesian symmetry: the suppression of transverse wakes by elliptical beams.

Compact and tunable focusing device for plasma wakefield acceleration.

Plasma wakefield acceleration, either driven by ultra-short laser pulses or electron bunches, represents one of the most promising techniques able to overcome the limits of conventional RF technology and allows the development of compact accelerators. In the particle beam-driven scenario, ultra-short bunches with tiny spot sizes are required to enhance the accelerating gradient and preserve the emittance and energy spread of the accelerated bunch. To achieve such tight transverse beam sizes, a focusing system with short focal length is mandatory. Here we discuss the development of a compact and tunable system consisting of three small-bore permanent-magnet quadrupoles with 520 T/m field gradient. The device has been designed in view of the plasma acceleration experiments planned at the SPARC_LAB test-facility. Being the field gradient fixed, the focusing is adjusted by tuning the relative position of the three magnets with nanometer resolution. Details about its magnetic design, beam-dynamics simulations, and preliminary results are examined in the paper.

Single-stage plasma-based correlated energy spread compensation for ultrahigh 6D brightness electron beams.

Plasma photocathode wakefield acceleration combines energy gains of tens of GeV m-1 with generation of ultralow emittance electron bunches, and opens a path towards 5D-brightness orders of magnitude larger than state-of-the-art. This holds great promise for compact accelerator building blocks and advanced light sources. However, an intrinsic by-product of the enormous electric field gradients inherent to plasma accelerators is substantial correlated energy spread-an obstacle for key applications such as free-electron-lasers. Here we show that by releasing an additional tailored escort electron beam at a later phase of the acceleration, when the witness bunch is relativistically stable, the plasma wave can be locally overloaded without compromising the witness bunch normalized emittance. This reverses the effective accelerating gradient, and counter-rotates the accumulated negative longitudinal phase space chirp of the witness bunch. Thereby, the energy spread is reduced by an order of magnitude, thus enabling the production of ultrahigh 6D-brightness beams.

Generation of Ramped Current Profiles in Relativistic Electron Beams Using Wakefields in Dielectric Structures.

Temporal pulse tailoring of charged-particle beams is essential to optimize efficiency in collinear wakefield acceleration schemes. In this Letter, we demonstrate a novel phase space manipulation method that employs a beam wakefield interaction in a dielectric structure, followed by bunch compression in a permanent magnet chicane, to longitudinally tailor the pulse shape of an electron beam. This compact, passive, approach was used to generate a nearly linearly ramped current profile in a relativistic electron beam experiment carried out at the Brookhaven National Laboratory Accelerator Test Facility. Here, we report on these experimental results including beam and wakefield diagnostics and pulse profile reconstruction techniques.

Laser-plasma-based Space Radiation Reproduction in the Laboratory.

Space radiation is a great danger to electronics and astronauts onboard space vessels. The spectral flux of space electrons, protons and ions for example in the radiation belts is inherently broadband, but this is a feature hard to mimic with conventional radiation sources. Using laser-plasma-accelerators, we reproduced relativistic, broadband radiation belt flux in the laboratory, and used this man-made space radiation to test the radiation hardness of space electronics. Such close mimicking of space radiation in the lab builds on the inherent ability of laser-plasma-accelerators to directly produce broadband Maxwellian-type particle flux, akin to conditions in space. In combination with the established sources, utilisation of the growing number of ever more potent laser-plasma-accelerator facilities worldwide as complementary space radiation sources can help alleviate the shortage of available beamtime and may allow for development of advanced test procedures, paving the way towards higher reliability of space missions.

Observation of acceleration and deceleration in gigaelectron-volt-per-metre gradient dielectric wakefield accelerators.

There is urgent need to develop new acceleration techniques capable of exceeding gigaelectron-volt-per-metre (GeV m(-1)) gradients in order to enable future generations of both light sources and high-energy physics experiments. To address this need, short wavelength accelerators based on wakefields, where an intense relativistic electron beam radiates the demanded fields directly into the accelerator structure or medium, are currently under intense investigation. One such wakefield based accelerator, the dielectric wakefield accelerator, uses a dielectric lined-waveguide to support a wakefield used for acceleration. Here we show gradients of 1.347±0.020 GeV m(-1) using a dielectric wakefield accelerator of 15 cm length, with sub-millimetre transverse aperture, by measuring changes of the kinetic state of relativistic electron beams. We follow this measurement by demonstrating accelerating gradients of 320±17 MeV m(-1). Both measurements improve on previous measurements by and order of magnitude and show promise for dielectric wakefield accelerators as sources of high-energy electrons.

High-quality electron beams from a helical inverse free-electron laser accelerator.

Compact, table-top sized accelerators are key to improving access to high-quality beams for use in industry, medicine and academic research. Among laser-based accelerating schemes, the inverse free-electron laser (IFEL) enjoys unique advantages. By using an undulator magnetic field in combination with a laser, GeV m(-1) gradients may be sustained over metre-scale distances using laser intensities several orders of magnitude less than those used in laser wake-field accelerators. Here we show for the first time the capture and high-gradient acceleration of monoenergetic electron beams from a helical IFEL. Using a modest intensity (~10(13) W cm(-2)) laser pulse and strongly tapered 0.5 m long undulator, we demonstrate >100 MV m(-1) accelerating gradient, >50 MeV energy gain and excellent output beam quality. Our results pave the way towards compact, tunable GeV IFEL accelerators for applications such as driving soft X-ray free-electron lasers and producing γ-rays by inverse Compton scattering.

Planar-dielectric-wakefield accelerator structure using Bragg-reflector boundaries.

We report experimental measurements of narrow-band, single-mode excitation, and drive beam energy modulation, in a dielectric wakefield accelerating structure with planar geometry and Bragg-reflector boundaries. A short, relativistic electron beam (∼1 ps) with moderate charge (∼100 pC) is used to drive the wakefields in the structure. The fundamental mode of the structure is reinforced by constructive interference in the alternating dielectric layers at the boundary, and is characterized by the spectral analysis of the emitted coherent Cherenkov radiation signal. Data analysis shows a narrow-band peak at 210 GHz corresponding to the fundamental mode of the structure. Simulations in both 2D and 3D provide insight into the propagating fields and reproduction of the electron beams dynamics observables and emitted radiation characteristics.

Generation of coherent broadband photon pulses with a cascaded longitudinal space-charge amplifier.

The longitudinal space-charge amplifier has been recently proposed by Schneidmiller and Yurkov as an alternative to the free-electron laser instability for the generation of intense broadband radiation pulses [Phys. Rev. ST Accel. Beams 13, 110701 (2010)]. In this Letter, we report on the experimental demonstration of a cascaded longitudinal space-charge amplifier at optical wavelengths. Although seeded by electron beam shot noise, the strong compression of the electron beam along the three amplification stages leads to emission of coherent undulator radiation pulses exhibiting a single spectral spike and a single transverse mode. The on-axis gain is estimated to exceed 4 orders of magnitude with respect to spontaneous emission.

Single-shot coherent diffraction imaging of microbunched relativistic electron beams for free-electron laser applications.

With the advent of coherent x rays provided by the x-ray free-electron laser (FEL), strong interest has been kindled in sophisticated diffraction imaging techniques. In this Letter, we exploit such techniques for the diagnosis of the density distribution of the intense electron beams typically utilized in an x-ray FEL itself. We have implemented this method by analyzing the far-field coherent transition radiation emitted by an inverse-FEL microbunched electron beam. This analysis utilizes an oversampling phase retrieval method on the transition radiation angular spectrum to reconstruct the transverse spatial distribution of the electron beam. This application of diffraction imaging represents a significant advance in electron beam physics, having critical applications to the diagnosis of high-brightness beams, as well as the collective microbunching instabilities afflicting these systems.

Using the relativistic two-stream instability for the generation of soft-x-ray attosecond radiation pulses.

In this Letter we discuss a novel method for generating ultrashort radiation pulses using a broadband two-stream instability in an intense relativistic electron beam. This method relies on an electron beam having two distinct two-energy bands. The use of this new high brightness electron beam scenario, in combination with ultrashort soft x-ray pulses from high harmonic generation in gas, allows the production of high power attosecond pulses for ultrafast pump and probe experiments.

Stable charged-particle acceleration and focusing in a laser accelerator using spatial harmonics.

Regarding the laser-driven acceleration of charged particles in photonic systems, a central unmet challenge is the achievement of simultaneous transverse and longitudinal stability at nonultrarelativistic energies. At such energies, Earnshaw's theorem [S. Earnshaw, Trans. Cambridge Philos. Soc. 7, 97 (1842)] indicates that a synchronous accelerating wave gives a defocusing effect. We present a scheme in which particles are accelerated by interaction with a resonant spatial harmonic and are focused by strong ponderomotive interaction with nonresonant spatial harmonics. We show that this scheme exhibits net transverse focusing and longitudinal stability, and we discuss its use in a compact laser accelerator.

Dielectric wakefield acceleration of a relativistic electron beam in a slab-symmetric dielectric lined waveguide.

We report first evidence of wakefield acceleration of a relativistic electron beam in a dielectric-lined slab-symmetric structure. The high energy tail of a ∼60 MeV electron beam was accelerated by ∼150 keV in a 2 cm-long, slab-symmetric SiO2 waveguide, with the acceleration or deceleration clearly visible due to the use of a beam with a bifurcated longitudinal distribution that serves to approximate a driver-witness beam pair. This split-bunch distribution is verified by longitudinal reconstruction analysis of the emitted coherent transition radiation. The dielectric waveguide structure is further characterized by spectral analysis of the emitted coherent Cherenkov radiation at THz frequencies, from a single electron bunch, and from a relativistic bunch train with spacing selectively tuned to the second longitudinal mode (TM02). Start-to-end simulation results reproduce aspects of the electron beam bifurcation dynamics, emitted THz radiation properties, and the observation of acceleration in the dielectric-lined, slab-symmetric waveguide.