Dielectric Wakefield Accelerator Research Articles

Acceleration and focusing of positron bunch in a dielectric wakefield acceleratorwith plasma in transport channel

The paper presents the results of numerical particle-in-cell simulation of the positron bunch focusing during acceleration in a plasma dielectric wakefield accelerator. The wakefield is excited by drive electron bunch in quartz dielectric tube, embedded in a cylindrical metal waveguide. The internal area of the dielectric tube is filled with plasma having in the general case the paraxial vacuum channel. Two different models of the plasma density radius-relationship are investigated: the homogeneous model and the inhomogeneous dependence characterized the capillary discharge. Results of the numerical particle-in-cell simulation show that a simultaneous acceleration and focusing of the test positron bunch in the wakefield is possible. The dependence of transport and acceleration of the positron bunch with change in the size of vacuum channel, waveguide length and the plasma density-radius model is studied.

Experimental study of transverse effects in planar dielectric wakefield accelerating structures with elliptical beams

The main obstacle to the practical implementation of the dielectric wakefield acceleration (DWA) concept is the development of the beam breakup instability due to transverse dipole wakefields generated when the beam propagates off axis in the accelerating structure. One of the methods to suppress this instability is to elliptically shape the beam and accelerate it in a planar structure. Here, we report a detailed experimental investigation of the transverse dynamics of elliptical beams in planar dielectric structure with parameters mimicking future DWA modules. Both dipole and quadrupole wakefields' effects on beam stability and projected emittance were studied. This study has highlighted the importance of counteracting quadrupole wakefields in future DWA implementations.

Dielectric Wakefield Acceleration with a Laser Injected Witness Beam

Dielectric Wakefield Acceleration with a Laser Injected Witness Beam

First dielectric wakefield experiments at Daresbury Laboratory

The first dielectric wakefield acceleration (DWA) experiments have been conducted at the CLARA/VELA test facility at Daresbury Laboratory, UK. The DWA structures were of planar geometry with variable gap and dielectric thicknesses ranging from 0.025 to 0.2 mm. The facility, in its current state, provided electron bunches with up to 100 pC bunch charge, and variable 0.2-2.0 ps bunch lengths at the beam energy of 35 MeV. All major wakefield effects have been observed including energy modulation in longer bunches, energy dechirping, and transverse streaking and focussing. With a modest bunch charge of ∼ 50 pC, a decelerating field of ~ 8 MV/m was measured. Using this variable gap planar structure, we have also demonstrated generation of continuously tunable narrowband THz. A summary of experimental results and near future developments are presented.

Development of longitudinal beam profile monitor based on Coherent Transition Radiation effect for CLARA accelerator

Coherent Transition Radiation (CTR) has widely been used for longitudinal beam profile diagnostics. Its spectral-spatial properties have been thoroughly theoretically and experimentally investigated. We have decided to develop a longitudinal beam profile diagnostics based on CTR technique to be used for interpreting the experimental results of Dielectric Wakefield Accelerator (DWA) and Coherent Cherenkov Diffraction Radiation (CChDR) experiments. In this paper we present the technical design,tuning and commissioning of the CTR longitudinal bunch profile monitor at CLARA facility.

Shaped Beams from Diamond Field-Emitter Array Cathodes

Diamond field-emitter arrays (DFEAs) are arrays of diamond pyramids with exquisitely sharp tips and micrometer-scale bases that produce high current densities. These arrays can be fabricated in arbitrary shapes, ranging from single tips to many millions of tips, so that they produce an inherently shaped electron beam. Each tip emits a modest current, but the large dense array can produce many Amps. We are investigating these cathodes for use in dielectric wakefield accelerators; however, they may also be applicable to vacuum microwave tubes. Recently, shaped beam production and transport have been demonstrated in the 1.3-GHz RF gun at the Argonne Cathode Test Stand at the Argonne National Laboratory. The charge was measured on a Faraday cup and the beam imaged on a YAG screen with peak electric field gradients on the cathode ranging from 12 to 35 MV/m. Three cathode geometries were tested: one 1-mm equilateral triangle with 7- $\mu \text{m}$ base pyramids and 10- $\mu \text{m}$ pitch, one 1-mm equilateral triangle with 10- $\mu \text{m}$ base and 25- $\mu \text{m}$ pitch, and one sparse $5\times 5$ square array with 20- $\mu \text{m}$ base and 400- $\mu \text{m}$ pitch. The two triangular arrays emitted 35 nC in an RF macropulse at 35 MV/m and 13-nC charge at 27 MV/m, respectively, while the sparse array emitted 0.060-nC charge at 15 MV/m. This article presents the results of the triangular array experiments, including damage due to breakdown in the RF gun and initial models of tip-to-tip shielding.

Beam dynamics studies for beam focusing and solenoid alignment at SINBAD

SINBAD facility (Short INnovative Bunches and Accelerators at DESY) under construction at DESY plans to host several experiments for the production of ultra-short bunches and will be a test facility for high-gradient compact novel acceleration techniques. The ARES (Accelerator Research Experiment at SINBAD) linac is foreseen to produce ultra-short bunches to be injected e.g. into Novel Dielectric Laser Acceleration structures or Laser Wake-Field Acceleration experiments. The work presented in this paper is based on optimization of the focusing system consisting of solenoids for the ARES, which have been studied earlier in detail but is revisited for updated beamline. Moreover tolerances for the possible misalignment of solenoids are presented investigating the effect on the beam properties during the gun commissioning.

Observation of High Transformer Ratio of Shaped Bunch Generated by an Emittance-Exchange Beam Line.

Collinear wakefield acceleration has been long established as a method capable of generating ultrahigh acceleration gradients. Because of the success on this front, recently, more efforts have shifted towards developing methods to raise the transformer ratio (TR). This figure of merit is defined as the ratio of the peak acceleration field behind the drive bunch to the peak deceleration field inside the drive bunch. TR is always less than 2 for temporally symmetric drive bunch distributions and therefore recent efforts have focused on generating asymmetric distributions to overcome this limitation. In this Letter, we report on using the emittance-exchange method to generate a shaped drive bunch to experimentally demonstrate a TR≈5 in a dielectric wakefield accelerator.

Simulation studies for dielectric wakefield programme at CLARA facility

Short, high charge electron bunches can drive high magnitude electric fields in dielectric lined structures. The interaction of the electron bunch with this field has several applications including high gradient dielectric wakefield acceleration (DWA) and passive beam manipulation. The simulations presented provide a prelude to the commencement of an experimental DWA programme at the CLARA accelerator at Daresbury Laboratory. The key goals of this programme are: tunable generation of THz radiation, understanding of the impact of transverse wakes, and design of a dechirper for the CLARA FEL. Computations of longitudinal and transverse phase space evolution were made with Impact-T and VSim to support both of these goals.

Phase control with two-beam interferometry method in a terahertz dielectric wakefield accelerator

High-gradient, beam-driven wakefield acceleration in THz structures is a promising technology for future free electron lasers and colliders. In this scheme, the main beam is accelerated by the wakefield of the high current drive beam. The time separation between the main and drive beams has to be chosen carefully to ensure that the main beam is in an accelerating phase of the drive's wakefield. THz accelerating structures provide high gradient acceleration due to their small apertures, but their phase control is difficult due to the picosecond-scale period. Here, we report on a wakefield acceleration experiment in a 460 GHz dielectric wakefield accelerator (DWA). The optimum phase of the main beam during the experiment is determined with a two-beam wakefield interferometry (TBI) measurement. This is performed without the measurement of the main and drive beam bunch lengths or their separation. In TBI, the interference of the wakefields produced by the drive and main beams is measured with an integrating THz detector. The TBI signal, as a function of separation between the drive and main beams, exhibits a minimum due to destructive interference of these wakefields, which corresponds to maximum acceleration of the main beam as is confirmed by the energy spectrometer measurement. The maximum energy gain of 0.8 MeV and maximum energy loss of 1.2 MeV for the main beam have been measured, which agrees well with theoretical predictions.

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.

Mul'tybanchevyy rezhym zbudzhennya kil'vaternoho polya v plazmovo-dielektrychniy strukturi

The main results of theoretical and experimental researches concerning the physical processes in a dielectric wakefield accelerator based on the accelerating wakefield excitation in a dielectric structure with the use of a long sequence of electron bunches are presented. The enhancement of the excited wakefield amplitude is achieved owing to the coherent summation of wakefields generated by separate bunches. The bunches are accelerated in the total wakefield by arbitrarily dividing the bunch sequence into the exciting and accelerated parts, by using the appropriate frequency detuning of the bunch repetition frequency with respect to the frequency of the excited principal transverse mode. The influence of plasma in the transit channel on the amplitude of excited wakefields and the focusing of driving and accelerated bunches is analyzed.

Influence of emittance on transverse dynamics of accelerated bunches in the plasma–dielectric wakefield accelerator

We study theoretically transverse dynamics of the bunch of charged particles with the finite emittance in the plasma–dielectric wakefield accelerator. Parameters of bunches are chosen the same as available from the 15MeV Argonne Wakefield Accelerator beamline. The goal of the paper is to study the behavior of bunches of charged particles with different emittances while accelerating these bunches by wakefields in plasma–dielectric structures. Obtained results allow us to determine the limits of the emittance of the bunch where dynamics of the accelerated particles remains stable.

Towards a Practical Multi-Meter Long Dielectric Wakefield Accelerator: Problems and Solutions

A multi-meter long collinear dielectric wakefield accelerator is considered, and it is shown that a single bunch breakup instability is a major limiting factor for obtaining highly efficient energy transfer from the drive bunch to the main bunch. Different methods for instability suppression are studied. Numerical simulations using a 6D particle tracking computer code are performed and tolerances to various errors are defined.

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.

Investigations of the concept of a multibunch dielectric wakefield accelerator

Theoretical and experimental investigations of the physical principles of multibunch dielectric wakefield accelerator concept based on the wakefield excitation in the dielectric structure by a sequence of relativistic electron bunches are presented. The purpose of the concept is to enhance the wakefield intensity by means of the multibunch coherent excitation and wakefield accumulation in a resonator. The acceleration of bunches is achieved at detuning of bunch repetition frequency relative to the frequency of the excited wakefield. In such a way the sequence of bunches is divided into exciting and accelerated parts due to displacing bunches into accelerating phases of wakefield excited by a previous part of bunches of the same sequence. Besides the change of the permittivity and loss tangent of dielectrics under the irradiation by 100MeV electron beam is studied.

Real-time diagnostic for charging and damage of dielectrics in accelerators

We report on the progress made during the initial stage of our research to study charging rate and charge distribution in a thin walled dielectric wakefield accelerator (DWA) from a passing charge bunch and the physics of conductivity and discharge phenomena in dielectric materials useful in accelerator applications. The issue is the role played by the beam halo and intense wakefields in charging the dielectric, possibly leading to undesired deflection of charge bunches and degradation of the dielectric material: the effects that may grow over many pulses, albeit perhaps differently at different repetition rates. During the initial stage of development, a microwave apparatus was built and signal processing was developed to observe time-dependent charging of dielectric surfaces and/or plasmas located on or near the inner surface of a thin-wall hollow dielectric tube. Three frequencies were employed to improve the data handling rate and the signal-to-noise. The test and performance results for a plasma test case are presented; in particular, the performance of the test unit shows capability to detect small changes ~0.1% of a dielectric constant, which would correspond to the scraping-off of only 0.3nC to the walls of the dielectric liner inside the cavity from the passing charge bunch.

A preliminary design of the collinear dielectric wakefield accelerator

A preliminary design of the multi-meter long collinear dielectric wakefield accelerator that achieves a highly efficient transfer of the drive bunch energy to the wakefields and to the witness bunch is considered. It is made from ~0.5m long accelerator modules containing a vacuum chamber with dielectric-lined walls, a quadrupole wiggler, an rf coupler, and BPM assembly. The single bunch breakup instability is a major limiting factor for accelerator efficiency, and the BNS damping is applied to obtain the stable multi-meter long propagation of a drive bunch. Numerical simulations using a 6D particle tracking computer code are performed and tolerances to various errors are defined.

Potential applications of the dielectric wakefield accelerators in the SINBAD facility at DESY

Short, high-brightness relativistic electron bunches can drive ultra-high wakefields in the dielectric wakefield accelerators (DWFAs). This effect can be used to generate high power THz coherent Cherenkov radiation, accelerate a witness bunch with gradient two or three orders of magnitude larger than that in the conventional RF linear accelerators, introduce energy modulation within the driving bunch itself, etc. The paper studies potential applications of the DWFAs in the SINBAD facility at DESY. The simulations show that the ultra-short relativistic bunches from the SINBAD injector ARES can excite accelerating wakefields with peak amplitudes as high as GV/m at THz frequencies in proper DWFA structures. In addition, it illustrates that the DWFA structure can serve as a dechirper to compensate the correlated energy spread of the bunches accelerated by the laser plasma wakefield accelerator.

Dielectric Wakefield Accelerators

Dielectric-structure-based wakefield acceleration provides a viable approach to achieving the luminosity, efficiency, and cost requirements of a future linear collider as well as future x-ray light sources. This technology is capable of accelerating electrons and positrons at the substantially high gradients needed. Important progress in the development of dielectric wakefield accelerators has been made both experimentally and theoretically in the past few years. In this article we provide an overview of the basics of dielectric wakefield acceleration and major developments to date.