Beam-driven wakefield accelerators offer the potential to achieve accelerating fields approaching the sub-GV/m range, paving the way for compact and cost-effective next-generation high-energy colliders. This method typically employs a two-bunch system: a drive bunch that excites an electromagnetic wakefield in a solid-state or plasma medium, and a witness bunch that extracts energy from the wake for acceleration. Our research focuses on two key challenges in wakefield acceleration using structures. First, we investigate strategies to suppress transverse instabilities by employing non-cylindrical geometries that damp dipole modes. Second, we explore methods to precisely terminate the wakefield after the interaction region, thereby improving the efficiency of energy transfer. This paper presents detailed simulation results and the corresponding experimental design developed to validate these concepts. The proposed experiment will be carried out at the Argonne Wakefield Accelerator (AWA) facility, focusing on controlling wakefield duration through tailored drive and witness bunch configurations and evaluating dielectric-lined structures with non-cylindrical cross sections.

Two-beam acceleration is a powerful method to generate high accelerating fields by utilizing short radiofrequency pulses. The Argonne Wakefield Accelerator facility is applying a two-beam acceleration approach to an X-band radiofrequency gun. This gun has experimentally demonstrated an electric field on the photocathode of similar to 400 MV/m. The next phase of this experiment will involve adding a short X-band linac to boost the beam energy up to 10 MeV. This paper summarizes the optimization of the linac and beam dynamics simulations in the integrated system over a wide range of operating parameters and demonstrates that the available setup will support the generation of bright or ultrashort beams with possible applications to compact light sources including inverse Compton scattering.

Magnetized beams - beam with significant canonical angular momentum - are critical to electron cooling of hadron beams such as contemplated in next-generation hadron and electron-ion colliders. The transport of magnetized electron beams over long distances in a locally non-axisymmetric external field is challenging. An alternative is to transform the beam into an uncoupled "flat beam", transport the produced "flat'' beam over a long distance, and reintroduce the cross-plane coupling to "re-magnetize"the beam. In this paper, we demonstrate via numerical simulations and laboratory experiments such a cascaded-transformation approach.

The Argonne Wakefield Accelerator (AWA) facilitates research on advanced acceleration, beam manipulation, and beam production aiming to develop the next generation of accelerators for energy frontier applications. Additionally, this research is synergistic with R&D efforts on compact X-ray light sources. This paper discusses near-term upgrade plans to improve beam brightness and stability. Furthermore, we describe longer-term upgrades aimed at increasing the beam energy to enable next-generation beam-driven wakefield accelerators. These upgrades will ultimately enable the generation of bright 500 MeV electron bunches using the two-beam accelerator concept and exploring collinear-wakefield acceleration in the THz regime.

Achieving high-gradient acceleration is critical to enabling future linear colliders, free-electron lasers, and other compact accelerator applications. The Argonne Wakefield Accelerator (AWA) group has pioneered short-pulse structure wakefield acceleration technology, which has shown remarkable promise for surpassing the long-standing barrier of ~100 MV/m in X-band normal-conducting structures. Recent experiments have demonstrated the feasibility of this approach, with gradients exceeding 300 MV/m in a variety of X-band accelerating structures and an X-band photogun. Experimental results indicate that the empirical scaling law used to estimate the RF breakdown rate (BDR ~ E^30 * t^5) may be too conservative for RF pulse durations below 10 ns. Potential advanced accelerator designs based on short-pulse acceleration will also be presented, including a conceptual design for an ultra-compact XFEL.

This document outlines a community-driven Design Study for a 10 TeV pCM Wakefield Accelerator Collider. The 2020 ESPP Report emphasized the need for Advanced Accelerator R&D, and the 2023 P5 Report calls for the “delivery of an end-to-end design concept, including cost scales, with self-consistent parameters throughout." This Design Study leverages recent experimental and theoretical progress resulting from a global R&D program in order to deliver a unified, 10 TeV Wakefield Collider concept. Wakefield Accelerators provide ultra-high accelerating gradients which enables an upgrade path that will extend the reach of Linear Colliders beyond the electroweak scale. Here, we describe the organization of the Design Study including timeline and deliverables, and we detail the requirements and challenges on the path to a 10 TeV Wakefield Collider.

A program is underway at the Argonne Wakefield Accelerator (AWA) facility, in collaboration with Euclid Techlabs and Northern Illinois University (NIU), to develop a GV/m-scale photocathode gun to produce bright electron bunches. The novel X-band (11.7 GHz) photogun (Xgun), powered by a short radiofrequency (RF) pulse (9 ns), has already demonstrated peak electric fields of 400 MV/m in previous work. A detailed description of the Xgun RF design is introduced with a focus on the transient nature associated with the use of a short RF pulse to power the cavity and the pulsed heating effects in non-steady-state conditions. Additionally, an overview of the mechanical design is presented, including basic tolerance checks and their potential effects on RF performance. Then, to gain a comprehensive understanding of the Xgun's photoemission performance in the high-field regime, a series of phase scans (or Schottky scans) was performed across a large range of operating fields from 60 MV/m to 320 MV/m on the cathode surface. Systematic beam simulations were conducted to interpret the measured phase scan data and provide further insights into the Xgun's photoemission performance at high gradients.

Precise measurement of an electron bunch’s longitudinal profile is critical for wakefield accelerators as shaped electron bunches can improve transformer ratios in collinear wakefield acceleration. Electro-Optic sampling of terahertz (THz) radiation from the bunch is one of the most attractive approaches to provide a view into the structure of a relativistic electron bunch due to its non-destructive nature. Recent developments in spectral encoding methods have shown that Phase Diversity Electro-Optic Sampling (DEOS) can accurately retrieve profiles from both sub-picosecond bunches and those requiring long sampling time windows near the traditional resolution limits. We report the progress on DEOS measurements using coherent transition radiation (CTR), as well as simulations of retrieved THz fields from arbitrary shaped electron bunches using various crystal and probe-laser configurations.

This paper presents a study of the radiofrequency (RF) characteristics and beam dynamics of an X-band photogun (Xgun) operating in the transient state. The photoinjector is designed to operate with short RF pulses (9 ns) to achieve high accelerating gradients. Short-pulse operation potentially reduces breakdown risks, as experimentally demonstrated by achieving a gradient exceeding 350 MV/m. However, the short pulse duration causes the cavity to operate in a transient regime where the electromagnetic field deviates from the conventional steady-state condition. To investigate the effects of deviations from steady-state operation, the time-dependent spatial evolution of the cavity fields was examined using both 9-ns and 50-ns RF pulses. The 50-ns pulse served as a reference to characterize the cavity behavior under a fully filled steady-state condition. Beam dynamics simulations were conducted to explore the impact of transient RF effects on beam kinetic energy, transverse emittance, and bunch length; these simulations employed a new dynamic field mapping approach to model transient RF fields.

Higher-Order Modes (HOMs) in superconducting radiofrequency (SRF) cavities are traditionally considered detrimental to efficient operation. They are often associated with beam instabilities and are actively damped. However, these “harmful” HOMs, if used strategically, can be transformed into a tool for providing extra control over the beam, which can introduce new opportunities that are not easily achievable by conventional SRF cavity-based systems. Particularly, we have investigated the feasibility of boosting ballistic bunch compression using HOMs in SRF gun. The proposed idea will be presented with preliminary simulation results. The 185 MHz SRF gun cavity used for the simulation study was modelled using the ACE3P software suite and further modelling of the compression scheme was performed using the GPT code.

Cooling of beams circulating in storage rings is critical for many applications including particle colliders and synchrotron light sources. A method enabling unprecedented beam -cooling rates, optical stochastic cooling (OSC), was recently demonstrated in the Integrable Optics Test Accelerator (IOTA) electron storage ring at Fermilab [J. Jarvis et al., Nature (London) 608, 287 (2022)]. This paper describes the numerical implementation of the OSC process in the particle -tracking program ELEGANT and discusses the validation of the developed model with available experimental data. The model is also employed to highlight some features associated with different modes of operation of OSC. The developed simulation tool should be valuable in guiding future configurations of optical stochastic cooling and, more broadly, modeling self -field -based beam manipulations.

In a linear collider, the colliding beam has to be flat in the transverse plane to suppress energy spread by Beamstrahlung and to maximize the luminosity, simultaneously. In the current design of ILC, the flat beam is realized by the asymmetric emittance generated by the radiation-damping effect. We propose to generate the equivalent beam directly in the injector linac employing the emittance repartitioning technique. As an experimental demonstration, a beam experiment was carried out at KEK-STF. We present the experimental results.

Intense THz pulses synchronized with X-rays are expected to enable a broad range of experiments at XFELs. This contribution discusses our progress in developing such a THz source driven by relativistic low-energy 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.

Beams with cross-plane coupling or extreme asymmetries between the two transverse phase spaces are often encountered in particle accelerators. Flat beams with large transverse-emittance ratios are critical for future linear colliders. Similarly, magnetized beams with significant cross-plane coupling are expected to enhance the performance of electron cooling in hadron beams. Preparing these beams requires precise control and characterization of the four-dimensional transverse phase space. In this study, we employ generative phase-space reconstruction techniques to rapidly characterize magnetized and flat-beam phase-space distributions using a conventional quadrupole-scan method. The reconstruction technique is experimentally demonstrated on an electron beam produced at the Argonne Wakefield Accelerator and successfully benchmarked against conventional diagnostics techniques. Specifically, we show that predicted beam parameters from the reconstructed phase-space distributions (e.g., as magnetization and flat-beam emittances) are in excellent agreement with those measured from the conventional diagnostic methods.

Particle accelerators based on a two-beam acceleration scheme offer a path to high-accelerating fields by p owering the a ccelerating structures using s hort (nanosecond) radiofrequency pulses. In this contribution, we discuss a compact X-ray free-electron laser (FEL) concept based on a ∼ 0 . 5 GeV accelerator. We present the optimization of the accelerator and FEL process. We also discuss ongoing and planned experiments toward developing an integrated proof-of-principle experiment at Argonne National Laboratory in the vacuum and extreme ultraviolet (V/EUV) regimes.

We report the development of a multileaf collimator (MLC) for charged particle beams, based on independently actuated tungsten strips which can selectively scatter unwanted particles. The MLC is used in conjunction with an emittance exchange beamline to rapidly generate highly variable longitudinal bunch profiles. The developed MLC consists of 40 independent leaves that are 2 mm wide and can move up to 10 mm, and operates in an ultra high vacuum environment, enabled by novel features such as magnetically coupled actuation. An experiment at the Argonne Wakefield Accelerator, which previously used inflexible, laser-cut masks for beam shaping before an emittance exchange beamline, was conducted to test functionality. The experiment demonstrated myriad transverse mask silhouettes, as measured on a scintillator downstream of the MLC and the corresponding longitudinal profiles after emittance exchange, as measured using a transverse deflecting cavity. Rapidly changing between mask shapes enables expeditious execution of various experiments without the downtime associated with traditional methods. The many degrees of freedom of the MLC can enable optimization of experimental figures of merit using feed-forward control and advanced machine learning methods.

Optical Stochastic Cooling (OSC), a beam cooling technique based on Stochastic Cooling, is in the early stages of experimental development. It uses radiation produced by the beam in an undulator magnet (the pickup) to correct the momentum deviation of particles downstream in another undulator (the kicker). The OSC mechanism was recently demonstrated at Fermilabʼs IOTA ring using a passive configuration. However, the cooling rate of OSC can be dramatically increased by first amplifying the undulator radiation before applying the corrective kick. In collaboration with the IOTA experiment, we developed a computational model of the OSC mechanism. This work presents beam‐dynamics simulations of the amplified‐OSC configuration. We implement a model of intrabeam scattering and study the effects on beam equilibrium and diffusion rate as a function of bunch charge. Finally, we investigate the phase‐space dynamics with various coupling configurations between the transverse and longitudinal planes.

The current designs of future electron-positron linear colliders incorporate large and complex damping rings to produce asymmetric beams for beamstrahlung suppression. Here, we present the design of an electron injector capable of delivering flat electron beams with phase-space partition comparable to the electron-beam parameters produced downstream of the damping ring in the proposed International Linear Collider (ILC) design. Our design does not employ a damping ring but is instead based on cross-plane phase-space manipulation techniques. The performance of the proposed configuration, its sensitivity to jitter along with its impact on spin-polarization are investigated. The proposed paradigm could be adapted to other linear collider concepts under consideration and offers a path toward significant cost and complexity reduction.