High Gradient Acceleration Research Articles

Short electron bunch generation using single-cycle ultrafast electron guns

We introduce a solution for producing ultrashort ($\sim$fs) high charge ($\sim$pC) from ultra-compact guns utilizing single-cycle THz pulses. We show that the readily available THz pulses with energies as low as 20 ?J are sufficient to generate multi-10 keV electron bunches. Moreover, It is demonstrated that THz energies of 2mJ are sufficient to generate relativistic electron bunches with higher than 2 MeV energy. The high acceleration gradients possible in the structures provide 30 fs electron bunches at 30 keV energy and 45 fs bunches at 2 MeV energy. These structures will underpin future devices for strong field THz physics in general and miniaturized electron guns, in which the high fields combined with the short pulse duration enable electron beams with ultrahigh brightness.

Electron energy and electron trajectories in an inverse free-electron laser accelerator based on a novel electrostatic wiggler

We expand here a theory of a high-gradient laser-excited electron accelerator based on an inverse free-electron laser (inverse-FEL), but with innovations in the structure and design. The electrostatic wiggler used in our scheme, namely termed the Paul wiggler, is generated by segmented cylindrical electrodes with applied oscillatory voltages over segments. The inverse-FEL interaction can be described by the equations that govern the electron motion in the combined fields of both the laser pulse and Paul wiggler field. A numerical study of electron energy and electron trajectories has been made using the fourth-order Runge–Kutta method. The results indicate that the electron attains a considerable energy at short distances in this device. It is found that if the electron has got sufficient suitable wiggler amplitude intensities, it can not only gain higher energy in longer distances, but also can retain it even after the passing of the laser pulse. In addition, the results reveal that the electron energy gains different peaks for different initial axial velocities, so that a suitable small initial axial velocity of e-beam produces substantially high energy gain. With regard to the transverse confinement of the electron beam in a Paul wiggler, there is no applied axial guide magnetic field in this device.

Tunable High-Intensity Electron Bunch Train Production Based on Nonlinear Longitudinal Space Charge Oscillation.

High-intensity trains of electron bunches with tunable picosecond spacing are produced and measured experimentally with the goal of generating terahertz (THz) radiation. By imposing an initial density modulation on a relativistic electron beam and controlling the charge density over the beam propagation, density spikes of several-hundred-ampere peak current in the temporal profile, which are several times higher than the initial amplitudes, have been observed for the first time. We also demonstrate that the periodic spacing of the bunch train can be varied continuously either by tuning launching phase of a radio-frequency gun or by tuning the compression of a downstream magnetic chicane. Narrow-band coherent THz radiation from the bunch train was also measured with μJ-level energies and tunable central frequency of the spectrum in the range of ∼0.5 to 1.6THz. Our results pave the way towards generating mJ-level narrow-band coherent THz radiation and driving high-gradient wakefield-based acceleration.

Electron-beam manipulation techniques in the SINBAD Linac for external injection in plasma wake-field acceleration

The SINBAD facility (Short and INnovative Bunches and Accelerators at Desy) is foreseen to host various experiments in the field of production of ultra-short electron bunches and novel high gradient acceleration techniques. Besides studying novel acceleration techniques aiming to produce high brightness short electron bunches, the ARD group at DESY is working on the design of a conventional RF accelerator that will allow the production of low charge (0.5pC – few pC) ultra-short electron bunches (having full width half maximum, FWHM, length ≤ 1fs – few fs). The setup will allow the direct experimental comparison of the performance achievable by using different compression techniques (velocity bunching, magnetic compression, hybrid compression schemes). At a later stage the SINBAD linac will be used to inject such electron bunches into a laser driven Plasma Wakefield Accelerator, which imposes strong requirements on parameters such as the arrival time jitter and the pointing stability of the beam. In this paper we review the compression techniques that are foreseen at SINBAD and we underline the differences in terms of peak current, beam quality and arrival time stability.

High transformer ratio of multi-channel dielectric wakefield structures

Dielectric wakefield (DWA) accelerator concepts are receiving attention on account of their promising performance, mechanical simplicity, and anticipated low cost. Interest in DWA physics directed toward an advanced high-gradient accelerator has been enhanced by a finding that some dielectrics can withstand very high fields (>1GV/m) for the short times during the passage of charged bunches along dielectric-lined channels. In a two-channel structure, a drive bunch train propagates in a first channel, and in the second adjacent channel where a high gradient wakefield develops, a witness bunch is accelerated. Compared with single-channel DWA׳s, a two-beam accelerator delivers a high transformer ratio, and thereby reduces the number of drive beam sections needed to achieve a given final test beam energy. An overview of multi-channel DWA structures will be given, with an emphasis on two-channel structures, presenting their advantages and drawbacks, and potential impact on the field. Studies aimed to examine charging rate and charge distribution in a thin walled dielectric wakefield accelerator from a passing charge bunch and the physics of conductivity and discharge phenomena in dielectric materials useful for such accelerator applications are presented in a separate paper in the EAAC-2015 conference proceedings.

A Traveling-Wave Forward Coupler Design for a New Accelerating Mode in a Silicon Woodpile Accelerator

Silicon woodpile photonic crystals provide a base structure that can be used to build a 3-D dielectric waveguide system for high-gradient laser-driven acceleration. A new woodpile waveguide design that hosts a phase synchronous centrally confined accelerating mode is proposed. Comparing with previously discovered silicon woodpile accelerating modes, this mode shows advantages in terms of better electron beam loading and higher achievable acceleration gradient. Several traveling-wave coupler design schemes developed for multicell RF cavity accelerators are adapted to the woodpile power coupler design for this new accelerating mode. Design of a forward coupled highly efficient silicon woodpile accelerator is achieved. Simulation shows high efficiency of over 75% of the drive laser power coupled to this fundamental accelerating mode, with less than 15% backward wave scattering. The estimated acceleration gradient, when the coupler structure is driven at the damage threshold fluence of silicon at its operating 1.506-μm wavelength, can reach 185 MV/m. A 17-layer woodpile waveguide structure was successfully fabricated, and the measured bandgap is in excellent agreement with simulation.

Betatron radiation based diagnostics for plasma wakefield accelerated electron beams at the SPARC_LAB test facility

Recent progress with wake-field acceleration has shown a great potential in providing high gradient acceleration fields, while the quality of the beams remains relatively poor. Precise knowledge of the beam size at the exit from the plasma and matching conditions for the externally injected beams are the key for improvement of beam quality. Betatron radiation emitted by the beam during acceleration in the plasma is a powerful tool for the transverse beam size measurement, being also non-intercepting. In this work we report on the technical solutions chosen at SPARC_LAB for such diagnostics tool, along with expected parameters of betatron radiation.

High frequency single mode traveling wave structure for particle acceleration

The development of the new high frequency slow traveling wave structures is one of the promising directions in accomplishment of charged particles high acceleration gradient. The disc and dielectric loaded structures are the most known structures with slowly propagating modes. In this paper a large aperture high frequency metallic two-layer accelerating structure is studied. The electrodynamical properties of the slowly propagating TM01 mode in a metallic tube with internally coated low conductive thin layer are examined.

Interaction of an Ultrarelativistic Electron Bunch Train with a W-Band Accelerating Structure: High Power and High Gradient.

Electron beam interaction with high frequency structures (beyond microwave regime) has a great impact on future high energy frontier machines. We report on the generation of multimegawatt pulsed rf power at 91GHz in a planar metallic accelerating structure driven by an ultrarelativistic electron bunch train. This slow-wave wakefield device can also be used for high gradient acceleration of electrons with a stable rf phase and amplitude which are controlled by manipulation of the bunch train. To achieve precise control of the rf pulse properties, a two-beam wakefield interferometry method was developed in which the rf pulse, due to the interference of the wakefields from the two bunches, was measured as a function of bunch separation. Measurements of the energy change of a trailing electron bunch as a function of the bunch separation confirmed the interferometry method.

Amplification and generation of ultra-intense twisted laser pulses via stimulated Raman scattering.

Twisted Laguerre–Gaussian lasers, with orbital angular momentum and characterized by doughnut-shaped intensity profiles, provide a transformative set of tools and research directions in a growing range of fields and applications, from super-resolution microcopy and ultra-fast optical communications to quantum computing and astrophysics. The impact of twisted light is widening as recent numerical calculations provided solutions to long-standing challenges in plasma-based acceleration by allowing for high-gradient positron acceleration. The production of ultra-high-intensity twisted laser pulses could then also have a broad influence on relativistic laser–matter interactions. Here we show theoretically and with ab initio three-dimensional particle-in-cell simulations that stimulated Raman backscattering can generate and amplify twisted lasers to petawatt intensities in plasmas. This work may open new research directions in nonlinear optics and high–energy-density science, compact plasma-based accelerators and light sources.

Staging optics considerations for a plasma wakefield acceleration linear collider

Plasma wakefield acceleration offers acceleration gradients of several GeV/m, ideal for a next-generation linear collider. The beam optics requirements between plasma cells include injection and extraction of drive beams, matching the main beam beta functions into the next cell, canceling dispersion as well as constraining bunch lengthening and chromaticity. To maintain a high effective acceleration gradient, this must be accomplished in the shortest distance possible. A working example is presented, using novel methods to correct chromaticity, as well as scaling laws for a high energy regime.

Laser pulse shaping for high gradient accelerators

In many high gradient accelerator schemes, i.e. with plasma or dielectric wakefield induced by particles, many electron pulses are required to drive the acceleration of one of them. Those electron bunches, that generally should have very short duration and low emittance, can be generated in photoinjectors driven by a train of laser pulses coming inside the same RF bucket. We present the system used to shape and characterize the laser pulses used in multibunch operations at Sparc_lab. Our system gives us control over the main parameter useful to produce a train of up to five high brightness bunches with tailored intensity and time distribution.

Statistical model for field emitter activation on metallic surfaces used in high-gradient accelerating structures

Both super- and normal-conducting high-gradient linear accelerators are limited by enhanced field emission (EFE) in the accelerating structures, e.g. due to power loss or ignition of discharges. We discuss the dependence of the number density of typical emitters, i.e. particulates and surface defects, on the electric field level at which they are activated for naturally oxidized metallic surfaces. This activation is explained by the transition of a metal–insulator interface into geometric features that enhance the EFE process. A statistical model is successfully compared to systematic studies of niobium and copper relevant for recent and future linear accelerators. Our results show that the achievable surface quality of Nb might be sufficient for the suppression of EFE in the superconducting accelerating structures for the actual European XFEL but not for the planned International Linear Collider. Moreover, some effort will be required to reduce EFE and thus the breakdown rate of the normal conducting Cu structures for the Compact Linear Collider.

Dielectric laser acceleration of sub-100 keV electrons with silicon dual-pillar grating structures.

We present the demonstration of high-gradient laser acceleration and deflection of electrons with silicon dual-pillar grating structures using both evanescent inverse Smith-Purcell modes and coupled modes. Our devices accelerate subrelativistic 86.5 and 96.3 keV electrons by 2.05 keV over 5.6 μm distance for accelerating gradients of 370 MeV/m with a 3 nJ mode-locked Ti:sapphire laser. We also show that dual pillars can produce uniform accelerating gradients with a coupled-mode field profile. These results represent a significant step toward making practical dielectric laser accelerators for ultrafast, medical, and high-energy applications.

TeV/m nano-accelerator: Investigation on feasibility of CNT-channeling acceleration at Fermilab

The development of high gradient acceleration and tight phase-space control of high power beams is a key element for future lepton and hadron colliders since the increasing demands for higher energy and luminosity significantly raise costs of modern HEP facilities. Atomic channels in crystals are known to consist of 10–100V/Å potential barriers capable of guiding and collimating a high energy beam providing continuously focused acceleration with exceptionally high gradients (TeV/m). However, channels in natural crystals are only angstrom-size and physically vulnerable to high energy interactions, which has prevented crystals from being applied to high power accelerators. Carbon-based nano-crystals such as carbon-nanotubes (CNTs) and graphenes have a large degree of dimensional flexibility and thermo-mechanical strength, which could be suitable for channeling acceleration of MW beams. Nano-channels of the synthetic crystals can accept a few orders of magnitude larger phase-space volume of channeled particles with much higher thermal tolerance than natural crystals. This paper presents the current status of CNT-channeling acceleration research at the Advanced Superconducting Test Accelerator (ASTA) in Fermilab.

Longitudinal dynamics of twin electron bunches in the Linac Coherent Light Source

The recent development of two-color x-ray free-electron lasers, as well as the successful demonstration of high-gradient witness bunch acceleration in a plasma, have generated strong interest in electron bunch trains, where two or more electron bunches are generated, accelerated and compressed in the same accelerating bucket. In this paper we analyze a twin-bunch technique in a high-energy linac, such as the Linac Coherent Light Source. This method allows the generation of two electron bunches with high peak current and independent control of time delay and energy separation. We find that the wakefields in the accelerator structures play an important role in the twin-bunch compression, and they can be used to extend the available time delay range. Based on the theoretical model and simulations we propose several methods to achieve larger time delay.

Plasmonic metasurface for efficient ultrashort pulse laser-driven particle acceleration

A laser-driven particle accelerator based on plasmonic nano-antennas is proposed and analyzed. The concept utilizes the enhancement and localization of the electric field by nano-antennas to maximize the acceleration gradient and to overcome potential metallic losses. The structure is optimized for accelerating relativistic particles using a femto-second laser source operating at 800nm, and is shown to support the bandwidth of ultra-short laser pulses (up to 16fsec) while providing a high acceleration gradient potentially reaching 11.6GV/m.

Parametric generation of energetic short mid-infrared pulses for dielectric laser acceleration

Laser-driven high-gradient electron acceleration in dielectric photonic structures is an enabling technology for compact and robust sources of tunable monochromatic x-rays. Such advanced x-ray sources are sought in medical imaging, security, industrial, and scientific applications. The use of long-wavelength pulses can mitigate the problem of laser-induced breakdown in dielectric structures at high optical intensities, relax the structure fabrication requirements, and allow greater pulse energy to be injected into the structure. We report on the design and construction of a simple and robust, short-pulse parametric source operating at a center wavelength 5 μm, to be used as a pump for a dielectric photonic structure for laser-driven acceleration. The source is based on a two-stage parametric downconversion design, consisting of a β-BaB2O4-based 2.05 μm optical parametric amplifier (OPA) and a ZnGeP2-based 5 μm OPA. The 2.05 μm OPA is presently pumped by a standard Ti:sapphire chirped-pulse amplified laser, which will be replaced with direct laser pumping at wavelengths in the future. The design and performance of the constructed short-pulse mid-infrared source are described. The demonstrated architecture is also of interest for use in other applications, such as high harmonic generation and attosecond pulse production.

Beam-driven acceleration in ultra-dense plasma media

Accelerating parameters of beam-driven wakefield acceleration in an extremely dense plasma column has been analyzed with the dynamic framed particle-in-cell plasma simulator, and compared with analytic calculations. In the model, a witness beam undergoes a TeV/m scale alternating potential gradient excited by a micro-bunched drive beam in a 1025 m−3 and 1.6 × 1028 m−3 plasma column. The acceleration gradient, energy gain, and transformer ratio have been extensively studied in quasi-linear, linear-, and blowout-regimes. The simulation analysis indicated that in the beam-driven acceleration system a hollow plasma channel offers ∼20% higher acceleration gradient by enlarging the channel radius (r) from 0.2 λp to 0.6 λp in a blowout regime. This paper suggests a feasibility of TeV/m scale acceleration with a hollow crystalline structure (e.g., nanotubes) of high electron plasma density.

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.