Generation and characterization of electron bunches with ramped current profiles in a dual-frequency superconducting linear accelerator

Fabrication of THz corrugated wakefield structure and its high power test

We present overall process for developing terahertz (THz) corrugated structure and its beam-based measurement results. 0.2-THz corrugated structures were fabricated by die stamping method as the first step demonstration towards GW THz radiation source and GV/m THz wakefield accelerator. 150-[Formula: see text]m thick disks were produced from an OFHC (C10100) foil by stamping. Two types of disks were stacked alternately to form 46 mm structure with [Formula: see text] 170 corrugations. Custom assembly was designed to provide diffusion bonding with a high precision alignment of disks. The compliance of the fabricated structure have been verified through beam-based wakefield measurement at Argonne Wakefield Accelerator Facility. Both measured longitudinal and transverse wakefield showed good agreement with simulated wakefields. Measured peak gradients, 9.4 MV/m/nC for a long single bunch and 35.4 MV/m/nC for a four bunch trains, showed good agreement with the simulation.

FLASHForward: plasma wakefield accelerator science for high-average-power applications

The FLASHForward experimental facility is a high-performance test-bed for precision plasma wakefield research, aiming to accelerate high-quality electron beams to GeV-levels in a few centimetres of ionized gas. The plasma is created by ionizing gas in a gas cell either by a high-voltage discharge or a high-intensity laser pulse. The electrons to be accelerated will either be injected internally from the plasma background or externally from the FLASH superconducting RF front end. In both cases, the wakefield will be driven by electron beams provided by the FLASH gun and linac modules operating with a 10 Hz macro-pulse structure, generating 1.25 GeV, 1 nC electron bunches at up to 3 MHz micro-pulse repetition rates. At full capacity, this FLASH bunch-train structure corresponds to 30 kW of average power, orders of magnitude higher than drivers available to other state-of-the-art LWFA and PWFA experiments. This high-power functionality means FLASHForward is the only plasma wakefield facility in the world with the immediate capability to develop, explore and benchmark high-average-power plasma wakefield research essential for next-generation facilities. The operational parameters and technical highlights of the experiment are discussed, as well as the scientific goals and high-average-power outlook. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

Passive Ballistic Microbunching of Nonultrarelativistic Electron Bunches Using Electromagnetic Wakefields in Dielectric-Lined Waveguides

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 ultrafast 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 nonultrarelativistic 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.

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.

Precision Control of the Electron Longitudinal Bunch Shape Using an Emittance-Exchange Beam Line

We report on the experimental generation of relativistic electron bunches with a tunable longitudinal bunch shape. A longitudinal bunch-shaping (LBS) beam line, consisting of a transverse mask followed by a transverse-to-longitudinal emittance exchange (EEX) beam line, is used to tailor the longitudinal bunch shape (or current profile) of the electron bunch. The mask shapes the bunch's horizontal profile, and the EEX beam line converts it to a corresponding longitudinal profile. The Argonne wakefield accelerator rf photoinjector delivers electron bunches into a LBS beam line to generate a variety of longitudinal bunch shapes. The quality of the longitudinal bunch shape is limited by various perturbations in the exchange process. We develop a simple method, based on the incident slope of the bunch, to significantly suppress the perturbations.

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.