Many of the most significant advances in accelerator science have been due to improvements in our ability to manipulate beam phase space. Despite steady progress in beam phase-space manipulation over the last several decades, future accelerator applications continue to outpace the ability to manipulate the phase space. This situation is especially pronounced for longitudinal beam phase-space manipulation, and is now getting increased attention. Herein, we report the first experimental demonstration of the double emittance exchange concept, which allows for the control of the longitudinal phase space using relatively simple transverse manipulation techniques. The double emittance exchange beamline enables extensive longitudinal manipulation, including tunable bunch compression, time-energy correlation control, and nonlinearity correction, in a remarkably flexible manner. The demonstration of this new method opens the door for arbitrary longitudinal beam manipulations capable of responding to the ever increasing demands of future accelerator applications.

We report the generation of up to 565 MW, 2.7 ns (FWHM) pulses at 11.7 GHz from a metamaterial structure in test at the Argonne Wakefield Accelerator. The highest power was generated by a train of eight 65 MeV electron bunches spaced at 1.3 GHz with a total charge of 355 nC. The metamaterial structure consists of 100 copper unit cells each consisting of a wagon-wheel plate and a spacer plate with a total structure length of 0.2 m. The 565 MW pulse generates a wakefield with a peak on-axis gradient of 135 MV/m that could be used to accelerate a trailing main bunch. An estimated surface electric field of over 1 GV/m is generated on the metamaterial plates at the peak power level but no evidence of breakdown was observed during testing. Tests with single electron bunches and with trains of bunches of up to 100 nC produced output power levels in excellent agreement with simulations. At higher total bunch charge, offsets of the bunches from the axis resulted in beam interception and reduced output power. Simulations indicate that a perfectly aligned bunch train would generate more than 1 GW of power from the structure.

We introduce a diagnostic for measuring the transverse phase space (TPS) and transverse second moments of a particle beam in a single shot. The diagnostic consists of a slit followed by a simple projection beam line to project the TPS onto an imaging screen. The beam line is composed of a skew quadrupole magnet followed by a normal quadrupole magnet with an imaging screen at the end. The skew quadrupole generates a coupling between the initial $x$ and the final $y$, while the normal quadrupole removes the correlation between the initial $x$ and the initial ${x}^{\ensuremath{'}}$. Thus, the initial $(x,{x}^{\ensuremath{'}})$ is mapped onto the final $(x,y)$. The slit is needed before the projection beam line to remove the initial $(y,{y}^{\ensuremath{'}})$ contribution from the final $(x,y)$ image on the screen. We present simulation studies and a demonstration of the diagnostic with an experiment performed at Argonne Wakefield Accelerator facility. We also discuss the method's limitations.

Received 18 May 2021DOI:https://doi.org/10.1103/PhysRevAccelBeams.24.069901Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeam controlBeam optics transportSingle-particle dynamicsPhysical SystemsPlasma acceleration & new acceleration techniquesAccelerators & Beams

As one of the candidate accelerating structures of the Argonne 500 MeV short pulse Two Beam Wakefield Acceleration Demonstrator, a single cell X-band dielectric disk loaded accelerator (DDA) has been designed, fabricated, and tested at high power at the Argonne Wakefield Accelerator. The DDA should provide a short pulse (∼20 ns) high gradient (>300 MV/m) accelerator while maintaining a reasonable r/Q and high group velocity. This will allow a significantly larger RF-to-beam efficiency than is currently possible for conventional accelerating structures. A low loss barium titantate ceramic, εr = 50, was selected, and a low temperature brazing alloy chosen to preserve the dielectric properties of the ceramic during brazing. High power testing produced breakdown at the triple junction, resulting from the braze joint design. No evidence of breakdown was observed on the iris of the disk, indicating that the maximum surface electric field on the dielectric was not reached. An improved braze joint has been designed and is in production, with high power testing to follow.

We present our recent results generating 510 MW of power at 11.7 GHz using a metamaterial-based metallic power extractor for structure-based wakefield acceleration (SWFA). Implementing metamaterials in a power extractor design allows the structure to overcome some of the challenges faced by other SWFA techniques. Two previous experiments, Stage 1 in 2018 and Stage 2 in 2019, have successfully demonstrated the functionality of this approach by generating high power pulses using the 65 MeV electron beam at the Argonne Wakefield Accelerator (AWA) facility. Here, we discuss the design and results of the Stage 3 experiment. The Stage 3 design includes significant improvements to increase output power, employing an all-copper structure, fully-symmetric coupler design, and breakdown risk-reduction treatment.

Longitudinal bunch shaping is gaining increasing attention due to the large impact it has on modern accelerators. The existing methods for controlling the density profile fall into three categories: laser shaping, emittance exchange, or correlations between the energy and one of the other coordinates. These methods require either (i) dispersive elements (e.g., dipole magnet), which generate coherent synchrotron radiation, or (ii) shaping near the cathode, which can distort the profile due to space-charge effects. These collective effects significantly decrease shaping quality as the bunch charge increases. None of the existing methods are currently capable of shaping high-charge bunches. In this paper, we introduce a transverse deflecting cavity-based shaping method to avoid the impact of collective effects on the shaping process. In this method, a transverse deflecting cavity introduces the z - x correlation, and a transverse mask tailors the longitudinal density profile. This method provides high-quality shaping regardless of charge level and preserves the beam quality.

We present an experimental study of coherent high-power wakefield generation in a metamaterial (MTM) structure at 11.7GHz by 65MeV electron bunch trains at the Argonne Wakefield Accelerator (AWA), following a previous experiment, the Stage-I experiment, at the AWA. Both the Stage-II experiment, reported in this paper, and the Stage-I experiment were conducted using MTM structures, which are all-metal periodic structures with the period being much smaller than the wavelength. Differences between the two experiments include (1) structure length (Stage-I 8cm and Stage-II 20cm); (2) number of bunches used to excite the structure (Stage-I with two bunches, up to 85 nC of total charge; Stage-II with eight bunches, up to 224 nC of total charge); and (3) highest peak power measured (Stage-I 80MW in a 2ns pulse and Stage-II 380MW in a 10ns pulse). High-power radio frequency pulses were generated by reversed Cherenkov radiation of the electron beam due to the negative group velocity in the MTM structures. Because the radiation is coherent, a train of bunches with a proper spacing can build up to achieve a high peak power. The observed output power levels are very promising for future applications in direct collinear wakefield acceleration or in transfer to a second accelerator for two-beam acceleration.

Structure-wakefield accelerators (SWFAs) have the potential to support TeV-class high-luminosity lepton colliders. SWFAs can be configured in either two-beam acceleration (TBA) or collinear wakefield acceleration (CWA). Enabling high-gradient, efficient SWFAs to produce TeV-class high-quality beams depends on precise control of the beam distribution. This LOI identifies critical beam-dynamics research opportunities relevant to beam-driven wakefield accelerators. Introduction: Beam-driven wakefield accelerators rely on high-charge “drive” bunches [O(10– 100 nC)] passing through slow-wave structures (SWSs) to excite electromagnetic wakefields [1]. The produced wakefields can be directly used to accelerate a delayed “main” bunch (CWA) or be out-coupled and guided to an optimized accelerating structure that accelerates the main bunch (TBA) in a parallel beamline. CWA offers a simpler configuration where both the drive and main bunches are transported along the same beamline; the TBA scheme decouples the drive and bunch beam dynamics at the expense of increased complexity (e.g., two parallel beamlines are required). The beam dynamics associated with the simultaneous transport of the accelerating main bunch and decelerating drive bunch is one of major challenges in CWA. Drive bunches: For a given SWS, precise control of the drive-bunch distribution is critical to maximizing the SWFA efficiency and accelerating field for both TBA and CWA. In CWA (both SWFA and plasma wakefield acceleration [PWFA]) a shaped high-charge drive bunch can significantly enhance the transformer ratio while also enabling large accelerating fields [2]. This area of research has developed significantly over the last decade as a variety of temporal-shaping techniques have been proposed (e.g., the possible use of laser shaping, transverse-to-longitudinal phase space exchangers, or exploiting nonlinear correlations introduced in the longitudinal phase space using nonlinear longitudinal dispersion, multifrequency linear accelerators, or controlling collective effects; see [3]). Some techniques can shape the beam with sub-picosecond resolution consistent with the use of SWS operating in the THz regime, as required for a GV/m accelerating field. In addition, a possible path to increasing the transformer ratio involves the use of multi-channel SWSs where the drive and main bunches are transversely offset but propagate in the same accelerator beamline [4]. This latter path of CWA utilizes a transversely shaped drive bunch (e.g., an annular [5] or segmented bunches). Generating these various bunch distributions involves precise phasespace manipulation discussed in a companion LOI [3], while also relying on a precise analytical understanding and high-fidelity modelling of collective effects. For instance, the bunch shape and charge involved will most likely require more elaborate models (for, e.g., coherent synchrotron radiation [CSR]) [6]. Likewise, understanding and optimizing the drive-bunch beam dynamics during its deceleration in the SWS is critical. An important aspect regards the interplay between the imposed external periodic focusing and transverse wakefield experienced by the

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 m base pyramids and 10-mu m pitch, one 1-mm equilateral triangle with 10-mu m base and 25-mu m pitch, and one sparse 5 x 5 square array with 20-mu m base and 400-mu 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.

Dielectric loaded structures are promising candidates for use in the structure wakefield acceleration (SWFA) technique, for both the collinear wakefield and the two-beam acceleration (CWA and TBA respectively) approaches, due to their low fabrication cost, low rf losses, and the potential to withstand high gradient. A short pulse (<=20 ns) TBA program is under development at the Argonne Wakefield Accelerator (AWA) facility where dielectric loaded structures are being used for both the power extractor/transfer structure (PETS) and the accelerator. In this study, an X-band 11.7 GHz dielectric PETS was developed and tested at the AWA facility to demonstrate high power wakefield generation. The PETS was driven by a train of eight electron bunches separated by 769.2 ps (9 times of the X-band rf period) in order to achieve coherent wakefield superposition. A total train charge of 360 nC was passed through the PETS structure to generate ~200 MW, ~3 ns flat-top rf pulses without rf breakdown. A future experiment is being planned to increase the generated rf power to approximately ~1 GW by optimizing the structure design and improving the drive beam quality.

Temporally-shaped laser pulse are desirable in various applications including emittance reduction and beam-driven acceleration. Pulse-shaping techniques enable flexible controls over the longitudinal distribution of electron bunches emitted from photocathode. While direct manipulation and measurement of ultrashort pulses can be challenging in the time domain, both actions can be performed in the frequency domain. In this paper, we report investigations toward the development of laser shaper and diagnostics at the Argonne Wakefield Accelerator. Simulations are presented to describe the shaping and measurement process based on a digital mask and a frequency resolved optical gating method

Quantum efficiency (QE), intrinsic emittance, and robustness are the three most important figures of merit for photocathodes, the first two determine the ultimate achievable brightness of an electron beam, and the third one directly correlates with the complications of a beamline design. Nitrogen-incorporated ultrananocrystalline diamond [(N)UNCD] materials are promising candidates for photocathode applications due to their remarkable electron emission performance as well as the moderate vacuum requirement. Two (N)UNCD photocathodes have been characterized in a realistic RF gun environment with the nicely balanced performance of all three figures of merit. The QE of the first (N)UNCD cathode (stored in air for two years before the test) was found to be 3.8 × 10−4 using a 262 nm UV laser and a cathode surface field of 30 MV/m. It was found that the QE of the second (N)UNCD sample (grown days before the test) was nearly the same and, therefore, demonstrates the exceptional environmental tolerance of the material. The intrinsic emittance of (N)UNCD was measured to be 1.00 μm/mm.

Plasma wakefields can enable very high accelerating gradients for frontier high energy particle accelerators, in excess of 10  GeV/m. To overcome limits on single stage acceleration, specially shaped drive beams can be used in both linear and nonlinear plasma wakefield accelerators (PWFA), to increase the transformer ratio, implying that the drive beam deceleration is minimized relative to acceleration obtained in the wake. In this Letter, we report the results of a nonlinear PWFA, high transformer ratio experiment using high-charge, longitudinally asymmetric drive beams in a plasma cell. An emittance exchange process is used to generate variable drive current profiles, in conjunction with a long (multiple plasma wavelength) witness beam. The witness beam is energy modulated by the wakefield, yielding a response that contains detailed spectral information in a single-shot measurement. Using these methods, we generate a variety of beam profiles and characterize the wakefields, directly observing transformer ratios up to R=7.8. Furthermore, a spectrally based reconstruction technique, validated by 3D particle-in-cell simulations, is introduced to obtain the drive beam current profile from the decelerating wake data.

We have demonstrated generation and transport of a patterned electron beam from a Diamond Field-Emitter Array (DFEA) cathode in a radio frequency (rf) gun. DFEAs are arrays of micrometer-scale pyramids with nanometer-scale tips. They can be fabricated with base widths ranging from 3 μm to 25 μm and pitches as small as 5 μm. They have an inherent 1:0.7 base to height ratio. DFEAs operate as field-emitter cathodes and potentially produce intrinsically shaped electron beams, which are of interest for a number of accelerator applications. We report on the results of a recent experiment in which a beam, consisting of several beamlets, was produced from a DFEA cathode in an rf gun and transported 2.54 m along a beam line. A macrobunch charge of 60 pC was measured at a cathode field gradient of 15.1 MV/m.

Thermal emittance and quantum efficiency (QE) are key figures of merit of photocathodes, and their uniformity is critical to high-performance photoinjectors. Several QE mapping technologies have been successfully developed; however, there is still a dearth of information on thermal emittance maps. This is because of the extremely time-consuming procedure to gather measurements by scanning a small beam across the cathode with fine steps. To simplify the mapping procedure, and to reduce the time required to take measurements, we propose a new method that requires only a single scan of the solenoid current to simultaneously obtain thermal emittance and QE distribution by using a pattern beam with multiple beamlets. In this paper, its feasibility has been confirmed by both beam dynamics simulation and theoretical analysis. The method has been successfully demonstrated in a proof-of-principle experiment using an L-band radiofrequency photoinjector with a cesium telluride cathode. In the experiment, seven beamlets were generated from a microlens array system and their corresponding thermal emittance and QE varied from 0.93 to 1.14 $\mu$m/mm and from 4.6 to 8.7%, respectively. We also discuss the limitations and future improvements of the method in this paper.

The generation of high-brightness beams with ultra-low emittance using the plasma photocathode technique has gained significant traction in recent years. The practical execution of a combined plasma wakefield acceleration section and a laser injected typically requires a dual gas medium for precision ionization of low and high ionization thresholds. The concept can be partially simplified in experiment by replacing the plasma wakefield acceleration component with a dielectric wakefield acceleration scheme, sacrificing field gradient but maintaining low emittance beam generation. In this paper, we describe the progress on the design of a hybrid scheme, using laser injection in a gas medium within a dielectric wakefield accelerator structure. The proof-ofconcept experiment is planned to take place at the Argonne Wakefield Accelerator.

An emittance exchange (EEX) beamline can manipulate longitudinal phase space via control of the transverse one at the entrance to the beamline. This EEX beamline can compress a bunch length to a sub-fs level when a specific transverse focusing is applied to the small beam at the entrance of the beamline. The Argonne Wakefield Accelerator Facility (AWA) plans to generate the sub-fs bunch for applications to wakefield accelerators. A preliminary experiment has been performed using the existing EEX beamline at AWA. In this poster, results of the experiment and feasibility of sub-fs bunch compression with a modified EEX beamline are presented.

We present the first demonstration of high-power, reversed-Cherenkov wakefield radiation by electron bunches passing through a metamaterial structure. The structure supports a fundamental transverse magnetic mode with a negative group velocity leading to reversed-Cherenkov radiation, which was clearly verified in the experiments. Single 45 nC electron bunches of 65 MeV traversing the structure generated up to 25 MW in 2 ns pulses at 11.4 GHz, in excellent agreement with theory. Two bunches of 85 nC with appropriate temporal spacing generated up to 80 MW by coherent wakefield superposition, the highest rf power that metamaterial structures ever experienced without damage. These results demonstrate the unique features of metamaterial structures that are very attractive for future high-gradient wakefield accelerators, including two-beam and collinear accelerators. Advantages include the high shunt impedance for high-power generation and high-gradient acceleration, the simple and rugged structure, and a large parameter space for optimization.