The performance and scientific reach of advanced electron accelerator applications, such as particle colliders, x-ray free electron lasers, and ultrafast electron diffraction, are determined by beam brightness. The beam brightness is constrained by the quality of photocathodes and is associated with low Mean Transverse Energy (MTE) of photoemitted electrons. To meet the requirements for applications demanding a bright electron beam, photocathodes must exhibit ultrasmooth physical and chemical roughness, a long operational lifetime, and robustness under high applied electric fields and laser fluences. In this work, we present the development of an experimental setup for the growth and in-situ characterization of high-quality, low-MTE alkali antimonide photocathodes. Additionally, we describe the modifications made to the Argonne Cathode Test-stand (ACT) at the Argonne Wakefield Accelerator (AWA) Facility, necessary for studying the performance of alkali antimonide photocathodes under real photoinjector conditions.

A Dielectric Disk Accelerator (DDA) is a metallic accelerating structure loaded with dielectric disks to increase its shunt impedance. These structures use short RF pulses of 9 ns to achieve accelerating gradients of more than 100 MV/m. Single cell and multicell clamped structures have been designed and high power tested at the Argonne Wakefield Accelerator. During testing, the single cell clamped DDA structure achieved an accelerating gradient of 102 MV/m with no visible damage in the RF volume region. The minimal damage that was seen outside the RF volume was likely due to RF leakage from uneven clamping during assembly. Based on the success of that experiment, a clamped multicell DDA structure has been designed and tested at high power. Simulation results for this new structure show a 108 MV/m accelerating gradient with 400 MW of input power with high shunt impedance and group velocity. Engineering designs were improved from the single cell structure for a more consistent clamping over the entire structure. Up to this point in the high power experiments, the results show a peak input power of 222 MW correlating to an accelerating gradient of 80 MV/m. Testing of this structure will continue January 2024.

Dielectric-lined waveguides have been extensively studied for high-gradient acceleration in beam-driven dielectric wakefield acceleration (DWFA) and for beam manipulations, including the application of zero transverse force modes in the waveguides. In this paper, we investigate the zero transverse force modes excited by a relativistic electron bunch passing through a dielectric waveguide with a rectangular transverse cross section. Numerical simulations were performed to optimize the start-to-end beamline using Opal-t, ELEGANT, and WARPX. A Longitudinal Phase Space (LPS) measurement system is used to understand the interaction of the beam with the waveguide modes, and analysis of the resolution of the LPS system was conducted. We will discuss preliminary experimental data collected at the Argonne Wakefield Accelerator (AWA) benchmarked with the simulation results.

Metamaterial accelerators driven by nanosecond-long RF pulses show promise to mitigate RF breakdown. Recent high-power tests at the Argonne Wakefield Accelerator (AWA) with an X-band metamaterial structure have demonstrated to achieve a gradient of 190 MV/m, while we also observed a new acceleration regime, the breakdown-insensitive acceleration regime (BIAR), where the RF breakdown may not interrupt acceleration of a main beam. Statistical analysis between different breakdown types reveals that the characteristics of the BIAR breakdown are beneficial to high-gradient acceleration at short pulse lengths.

Dielectric-lined waveguides have been extensively studied for high-gradient acceleration in beam-driven dielectric wakefield acceleration (DWFA) and for beam manipulations, including the application of zero transverse force modes in the waveguides. In this paper, we investigate the zero transverse force modes excited by a relativistic electron bunch passing through a dielectric waveguide with a rectangular transverse cross section. Numerical simulations were performed to optimize the start-to-end beamline using Opal-t, ELEGANT, and WARPX. A Longitudinal Phase Space (LPS) measurement system is used to understand the interaction of the beam with the waveguide modes, and analysis of the resolution of the LPS system was conducted. We will discuss preliminary experimental data collected at the Argonne Wakefield Accelerator (AWA) benchmarked with the simulation results.

Previously we had developed a new method to fabricate corrugated waveguides (CW) operating in sub-THz frequency regime. As the next step, collaborative effort is underway to demonstrate GW-level high-power sub-THz pulse generation using a CW. We plan to fabricate a CW operating at around 400 GHz. This waveguide will be driven by a bunch train including 16 bunches with nanocoulomb-level charges per bunch. We present an overview of project’s current status.

Metamaterial accelerators driven by nanosecond-long RF pulses show promise to mitigate RF breakdown. Recent high-power tests at the Argonne Wakefield Accelerator (AWA) with an X-band metamaterial structure have demonstrated to achieve a gradient of 190 MV/m, while we also observed a new acceleration regime, the breakdown-insensitive acceleration regime (BIAR), where the RF breakdown may not interrupt acceleration of a main beam. Statistical analysis between different breakdown types reveals that the characteristics of the BIAR breakdown are beneficial to high-gradient acceleration at short pulse lengths.

We have fabricated corrugated structures for sub-THz regime by stacking disks. By sending electron beams into the structure, the wakefield of 200 GHz was successfully measured. The frequency and power levels of wakefield were very similar to our design. For the our next target of gigawatts power, we have newly designed a structure of 400 GHz. More precise fabrication is required compared to the 200 GHz structure. The die stamping method was changed to the LIGA process for the production of each disk. And we improved the assembly method as well. In the previous fabrication, the maximum error was around 10 micrometers. The errors may be reduced to one-tenth of the previous one. In this paper, we will introduce the new design.

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) photoemission gun (Xgun) is powered by high-power, short RF pulses (9 ns) generated by the AWA drive beam in a wakefield structure. In the first series of experiments, the Xgun demonstrated peak fields of ~400 MV/m on the photocathode surface. As a first step towards achieving a complete understanding of the Xgun’s performance in the high-field regime, we studied the photoemission mechanism by measuring the quantum efficiency (QE) and thermal emittance across a large range of operating fields on the photocathode surface from 60 MV/m to values exceeding 300 MV/m. In this work, we will present the results of our experimental measurements and simulation studies on examining photoemission at high fields on the photocathode surface.

Phase space reconstruction using Neural-Networks and differentiable simulation* is a robust beam diagnostic method to obtain complete 4D phase space including the coupling terms such as (x-y’) and (y-x’). In the first experimental demonstration, it was verified that the RMS beam envelope and normalized emittance from the reconstructed phase space are quantitatively similar to those from the conventional beam diagnostics such as quadrupole scan. In addition, here we show the demonstration of the phase space for the i) flat and ii) magnetized beam where the beam has i) very large ratio in between horizontal and vertical emittances (e.g., enx/eny >>1) and ii) transverse coupling induced by non-zero solenoid magnetic field at the cathode (known-as canonical momentum-dominated beam). Through the demonstrations using the experimental data achieved at the Argonne Wakefield Accelerator Facility (AWA), we successfully obtained the information such that the measured flat beam indeed has the emittance ratio larger than 70 with minimized transverse coupling. In addition, we were able to obtain the magnetization from the reconstructed phase space. Moreover, we will compare the beam parameters obtained from the phase space reconstruction and conventional diagnostics and discuss the uncertainty of the parameters.

A very high electron peak current is needed in many applications of modern electron accelerators. To achieve this high current, a large energy chirp must be imposed on the bunch so that the electrons will compress when they pass through a chicane. In existing linear accelerators (LINACs), this energy chirp is imposed by accelerating the beam off-crest from the peak fields of the RF cavities, which increases the total length and power requirements of the LINAC. A novel concept known as the Transverse Deflecting Cavity Based Chirper (TCBC) [1] can be used to actively impose a large energy chirp onto an electron beam in an accelerator, without the need for off-crest acceleration. The TCBC consists of 3 transverse deflecting cavities, which together impose an energy chirp while cancelling out the transverse deflection. An experiment is being developed to demonstrate this concept at the Argonne Wakefield Accelerator (AWA) facility. Here we explain the concept, show preliminary simulations of the experiment, and report on progress related to implementation of the experiment at AWA.

Ultrafast Microscopy using MeV beam has made significant progress in the past 5 years. However, in order to push to atomic level resolution, other than the requirements of beam source, there are also high demands in high strength focusing elements. In comparison of commercial 100s KeV level electron microscopes, an MeV imaging beamline requires Tesla level lenses, preferably round solenoid lens. Tesla class DC solenoids are prohibitively bulky and heavy, and superconducting solenoids are not cost effective. We have developed a novel miniature flux concentrator based solenoid lens system for MeV UED/UEM applications. It can reach 2-Tesla with 1e-5 level stability (depending on the pulsed current source). Here we will present detailed development process and experimental results.

The performance and scientific reach of advanced electron accelerator applications, such as particle colliders, x-ray free electron lasers, and ultrafast electron diffraction, are determined by beam brightness. The beam brightness is constrained by the quality of photocathodes and is associated with low Mean Transverse Energy (MTE) of photoemitted electrons. To meet the requirements for applications demanding a bright electron beam, photocathodes must exhibit ultrasmooth physical and chemical roughness, a long operational lifetime, and robustness under high applied electric fields and laser fluences. In this work, we present the development of an experimental setup for the growth and in-situ characterization of high-quality, low-MTE alkali antimonide photocathodes. Additionally, we describe the modifications made to the Argonne Cathode Test-stand (ACT) at the Argonne Wakefield Accelerator (AWA) Facility, necessary for studying the performance of alkali antimonide photocathodes under real photoinjector conditions.

Single-pulse, high dynamic range BPM signal detection has been at the top of the Argonne Wakefield Accelerator (AWA) Test Facility's most-wanted list for many years. The AWA beamline's unique capabilities require BPM instrumentation with an unprecedented dynamic range, making it challenging to design and prototype a cost-effective solution. We have prototyped many different approaches over the years. Finally, a recent prototype shows the long-sought solution for AWA's low-cost button BPM signal detection is becoming feasible. This paper shares the design and test results of this prototype.

Particle accelerators based on a two-beam acceleration scheme offer a path to high-accelerating fields by powering the accelerating structures using short (nanosecond) radiofrequency pulses. At the Argonne Wakefield Accelerator (AWA) facility, this approach was recently applied to an X-band radiofrequency gun and demonstrated an unprecedented electric field on the photocathode of ~400 MV/m. In the next phase, a short X-band linac will be added to boost the beam energy up to ~10 MeV. This paper examines the linac optimization and beam dynamics in this integrated system over a wide range of operating parameters. Planned experiments will also be discussed along with possible applications of the setup to compact light sources (e.g. inverse Compton scattering) or electron scattering experiments (e.g. microscopy or diffraction).