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As part 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, µ_{r} = 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., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

We present our recent results generating 510 MW of power at 11.7 GHz using a metamaterial-based metallic power-extractor for application in structure-based wakefield acceleration (SWFA). SWFA is a novel acceleration scheme in which high-charge electron bunches are passed through a power extractor structure to produce a high-intensity wakefield. This wakefield can then be used to accelerate a witness bunch in the same beamline or passed to a separate acceleration beamline. MIT’s approach uses a specialized metamaterial for the power extractor design. By using a metamaterial, we can overcome some of the challenges faced by other SWFA techniques. Here, we discuss the Stage 3 experiment. The Stage 1 and Stage 2 experiments successfully demonstrated the functionality of the metamaterial approach by generating high power RF pulses using the 65 MeV electron beam at the Argonne Wakefield Accelerator (AWA) facility. The 510 MW result from Stage 3 experiment is the highest power generated to-date for SWFA at AWA, and was enable by significant design improvements, including an all-copper structure, fully-symmetric coupler design, and breakdown risk-reduction treatment., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

Emittance exchange beamline provides a unique correlation between the upstream transverse momentum and downstream longitudinal timing. Similar to the bunch train generation concept using energy modulation and chicane, the emittance exchange beamline can convert the transverse momentum modulation to the temporal modulation at the end of the beamline. The beam can obtain this transverse modulation from alternating magnet array (e.g. 90 degree rotated undulator). While most of other methods provide only one knob to control both micro-bunch length and bunch-to-bunch spacing or hard to control one of the knobs, this method provides separated knobs for the micro-bunch length and spacing and they are easy to control. These knobs enable to separately control the fundamental frequency of the radiation and its bandwidth. We plan to demonstrate this method at Argonne Wakefield Accelerator facility (AWA). This poster present progress on this new method and its demonstration at AWA., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

Planar nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, is a unique and attractive field emission source because of the capability to generate high charge beam, the simplicity of production without shaped emitters, and the ease of handling with moderate vacuum requirement. In the presented study using an L-band normal conducting single-cell rf gun, a (N)UNCD cathode has been conditioned to 42 MV/m with a well-controlled manner and reached a maximum charge of 15 nC and an average emission current of 6~mA during a 2.5 us emission period. The systematic study of emission properties during the rf conditioning process illustrates the tunability of (N)UNCD in a wide range of surface gradients. This research demonstrates the versatility of (N)UNCD cathode which could enable multiple designs of field emission rf injector for industrial and scientific applications., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

We present initial measurements from a UCLA-Argonne Wakefield Accelerator collaborative plasma wakefield acceleration (PWFA) experiment aimed at demonstrating the dependence of transformer ratio on longitudinal beam shape. The transformer ratio or the ratio between the maximum acceleration of the witness and the maximum deceleration of the drive beam, is key to a mature, beam-based, plasma wakefield accelerator design. Utilizing the unique capabilities of the emittance exchange (EEX) beamline, we may obtain transformer ratios in excess of six in PWFA. We present the experimental beamline design, relevant beam diagnostics and explore preservation of the longitudinal beam profile., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

An emittance exchange (EEX) provides a precise longitudinal phase space manipulation of electron bunch. It has been studied for an easy and precise control of temporal distribution, but controls of energy distribution have not been explored. Since the energy control using EEX is under the identical principle to the temporal control, the EEX beamline can control a correlated energy spread of the electron bunch. This would benefit accelerator facilities requiring a low energy spread such as X-ray Free Electron Laser Oscil-lator (XFELO). In this paper, we present principle and preliminary simulation work on the suppression of correlated energy spread using the EEX beamline. ing the EEX beamline., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

The RF system of Argonne Wakefield Accelerator (AWA) facility has grown over the years from one RF power station into 4 RF power stations. The demand for RF power keeps growing as the capability of AWA continues to grow. Now the 5th RF station is needed to fulfill the RF power needs of AWA facility. Some details regarding the construction and commissioning of the 5th RF station of AWA facility are documented in this paper., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

Systematic differences between the linear and nonlinear regimes of plasma wakefield acceleration from electron beams are manifested in the plasma response. Typically, the ratio of peak beam density to nominal plasma density determines operation in the linear or nonlinear regime. Previous reports have shown that a the cross-over into the nonlinear regime is associated with an increase in the wakefield amplitude, as well as sawtooth-like shape. In this paper, we present preliminary measurements of quasi-nonlinear wakefields driven by a linearly ramped beam, with a maximum charge close to the unperturbed plasma density. We also demonstrate nonlinear wakefield behavior in a probe bunch using a single shot, multi-period wakefield measurement and its dependency on plasma density., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

It is critical to maintain a stable laser phase for RF photocathode electron gun to achieve high beam stability. In order to achieve a higher beam stability for AWA(Argonne Wakefield Accelerator) beamline, a novel laser phase monitor has been designed to allow us to monitor and feedback on. Both the design and its applications at AWA are presented in this paper., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

The short-pulse two-beam acceleration approach is a promising candidate to meet the cost and luminosity requirements for future linear colliders. Dielectric-loaded structure has been intensely investigated for this approach because of its low fabrication cost, low RF loss, and potential to withstand GV/m gradient. An X-band 11.7~GHz dielectric-loaded accelerator (DLA) has been designed for high power test with short RF pulses (3~ns) generated from a power extractor driven by high charge bunches at Argonne Wakefield Accelerator (AWA) facility. The gradient is expected to be over 100~MV/m with the maximum input power of 400~MW., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

Temporally shaped laser pulses 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 the photocathode. While direct manipulation and measurement of an ultrashort pulse can be challenging in the time domain, both actions can be performed in the frequency domain. In this paper, we report the study and development of laser shaper and diagnostics at Argonne Wakefield Accelerator (AWA). Simulations of the shaping process for several sought-after shapes are presented along with the temporal diagnosis. Status of the experiment at the AWA facility is also discussed., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

Argonne Wakefield Accelerator (AWA) group demonstrated arbitrary longitudinal shaping capability of thee emittance exchange (EEX) beamline in 2016. Several different transverse masks were used to shape the beam transversely, and the transmission through the mask was around 40%. The masking is one of the easiest ways to control the profile, but this low transmission would make significant drop of the beam quality due to a higher charge requirement in the gun, and it can make thermal issues for high repetition rate or high intensity beams. At the same time, it only controls the profile not a 2D phase space. We recently proposed a scheme to generate a tunable bunch train using a EEX beamline with a transverse wiggler. This wiggler provides a sinusoidal magnetic field which makes a sinusoidal modulation on the transverse phase space. If the beam passes series of transverse wigglers with different period and strength, one can make arbitrary correlation on the horizontal position and momentum. It opens up totally new way to control all longitudinal properties including arbitrary current profile shaping without charge loss. In this poster, we present the concept of the work and plan., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

We present experimental studies of metamaterial (MTM) structures for wakefield acceleration. The MTM structure is an all-metal periodic structure with its period much smaller than the wavelength at X-band. The fundamental TM mode has a negative group velocity, so an electron beam traveling through the structure radiates by reversed Cherenkov radiation. Two experiments have been completed at the Argonne Wakefield Accelerator (AWA), namely the Stage-I and Stage-II experiments. Differences between the two experiments include: (1) Structure length (Stage-I 8 cm, Stage-II 20 cm); (2) Bunch number used to excite the structure (Stage-I up to 2 bunches, Stage-II up to 8 bunches). In the Stage-I experiment, two bunches with a total charge of 85 nC generated 80 MW of RF power in a 2 ns long pulse. In the Stage-II experiment, the highest peak power reached 380 MW in a 10 ns long pulse from a train of 8 bunches with a total charge of 224 nC. Acceleration of a witness bunch has not been demonstrated yet, but the extracted power can be transferred to a separate accelerator for two-beam acceleration or directly applied to a trailing witness bunch in the same structure for collinear acceleration., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

We present experimental emission results from arrays of diamond field emitter tips operating in an RF gun at the Argonne Cathode Test-stand. Results from various arrays will be presented with different spacing between array elements. Very high charge densities were produced at various field gradients. The maximum field gradient for a particular geometry was discovered and break-down effects will be presented. Cathode lifetime was preliminarily studied. Further experiments are being planned and work on the cathode design optimization to produce higher quality beams will be discussed., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

Diamond Field Emitter Array (DFEA) cathodes are arbitrarily shaped arrays of sharp (~50 nm tip size) nano-diamond pyramids with bases on the order of 3 to 25 microns and pitches 5 microns and greater. These cathodes have demonstrated very high bunch charge in tests at the L-band RF gun at the Argonne National Laboratory (ANL) Advanced Cathode Test Stand (ACTS). Intrinsically shaped electron beams have a variety of applications, but primarily to achieve high transformer ratios for Dielectric Wakefield Accelerators (DWA) when used in conjunction with Emittance Exchange (EEX) systems. Here we will present results from a number of recent cathode tests including bunch charge and YAG images. We have demonstrated shaped beam transport down the 2.54-meter beamline. In addition we will present emission simulations that demonstrate shielding effects for this geometry., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

Short pulse two-beam acceleration (TBA) is a structure wakefield acceleration (SWFA) approach aiming to achieve gradient above 250 MV/m using rf pulses less than 20 ns. An X-band 11.7 GHz metallic power extractor has been developed as the power source to test accelerating structures in this extreme regime. The power extractor is designed to be driven by high charge bunches separated by 769.2 ps (9 times the X-band period) on an L-band 1.3 GHz beamline. In the recent experiment, ~280 MW rf pulses with 3 ns flat-top have been measured by a coaxial rf pickup when driven by 8-bunch trains with a total charge of ~500 nC. The power level is ~50% lower than the theoretical prediction and simulation. Experimental investigation suggests that the missing power was mainly caused by the multipacting issue inside the rf pickup, which could be eliminated by a newly-designed directional coupler., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

Advanced CW accelerators are one of high priority directions identified by Advanced Accelerator Concepts Research Roadmap Workshop Report *. High transformer ratio of beam-driven accelerators is critically important for cost-effective FEL systems. We present a proposed experiment for demonstrating a high transformer ratio CW dielectric accelerator using operational SRF accelerator built for Coherent electron Cooling experiment. This accelerator operates with CW electron beam comprised of 78 kHz train of electron bunches. Electron bunches with controllable longitudinal and charge up to 10 nC per bunch are generated in 1.25 MV SRF photo-electron gun. This bunches are ballistically compressed to duration of 10-to-30 psec and accelerated to 15 MeV in SRF linac**. Such bunches would be excellent drivers of high-transformer ratio DWA accelerators. In this paper we present expected performance of proposed CW DWA accelerator., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

Nonlinearities on the longitudinal phase space in-duce a double-horn current profile when the bunch is compressed strongly. Since this double-horn can de-grade the performance of FELs due to the CSR it makes, the suppression of the double-horn is one of important beam dynamics issues. Emittance exchange (EEX) can be interesting option for this issue due to its longitudinal controllability. Since EEX exchanges the longitudinal phase space and transverse phase space, higher order magnets such as octupole can control the nonlinearity. In this paper, we present simulation re-sults on the suppression of the double-horn current profile using EEX based bunch compression. We use a double EEX beamline installed at the Argonne Wake-field Accelerator facility for the simulation., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

Emittance exchange (EEX) provides a powerful method of controlling the longitudinal phase space using the relatively simpler methods of transverse control. An EEX beamline was installed at the Argonne Wakefield Accelerator (AWA) facility in 2015. Several experiments important to the wakefield acceleration, such as a high transformer ratio from shaped bunches, have already been demonstrated. We are currently developing several applications of the EEX beamline including temporal profile shaping, THz radiation generation, time-energy correlation control, diagnostic uses of EEX etc. We will present the on-going EEX program for longitudinal phase space control taking place at the AWA facility, and discuss recently discovered new opportunities., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

RF breakdown is one of the major limitations to achieve high gradient acceleration for future structure-based normal conducting linear colliders. Previous statistic research shows that the breakdown rate is proportional to E_{a}³⁰ * t_{p}⁵, which indicates that the accelerating gradient E_{a} could be improved by using shorter RF pulses (t_{p}). An X-band 11.7~GHz metallic single-cell structure has been designed for RF breakdown study up to 273~MV/m using short pulses (~3ns) generated by a 400~MW power extractor at Argonne Wakefield Accelerator (AWA) facility. The structure has also been scaled to 11.424~GHz for the long pulse (100-1500~ns) breakdown study driven by a klystron and a pulse compressor at Tsinghua X-band High Power Test-stand (TPoT-X), with the gradient up to 246~MV/m with 200~MW input power., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

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-of-concept experiment is planned to take place at the Argonne Wakefield Accelerator., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

Beam position monitor (BPM) is widely used in accelerator facilities worldwide. It is a device which is capable of providing, non-destructively, accurate beam centroid and charge information of a passing charged beam. A typical BPM system contains customized hardware and specialized processing electronics. The cost is often too high for small facilities to afford them. As a small facility, Argonne Wakefield Accelerator (AWA) decided to develop a solution with high cost-efficiency to fit in its budget. Some details about the development are presented in this paper., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

Beams with large transverse emittance ratios (flat beams)have received renewed interest for their possible applications in future linear colliders and advanced accelerators. A flat beam can be produced by generating a magnetized beam and then repartitioning its emittance using three skew quadrupoles. In this paper, we report on the experimental generation of∼1nC flat beams at the Argonne WakefieldAccelerator (AWA). The emittance ratio of the flat beam is demonstrated to be continuously variable by adjusting the magnetic field on the cathode., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

The Argonne Cathode Test-stand (ACT) is a unique testbed to develop cathodes and to conduct fundamental surface study under ultra-high rf field (up to 700 MV/m with pin-shaped cathodes). The test-stand consists of an L-band 1.3 GHz single-cell photocathode rf gun and a field emission (FE) imaging system to locate emitters with a resolution of ∼20 𝜇m. In the recent upgrade, UV laser has been introduced to improve the imaging system and to significantly expand the ACT towards photoemission and laser-assisted field emission research. In addition, a load-lock system has been added to the beam line to expedite the cathode switching period. The paper will present details of the upgrade as well as experiments planned in the near future., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

Argonne Flexible Linear Collider (AFLC), a proposed 3 TeV electron-positron linear collider based on two-beam acceleration (TBA) scheme, applies a short pulse length (∼20 ns) to obtain a high accelerating gradient (267 MV/m) and a compact footprint (∼18 km). The baseline design of the main accelerator section adopts 26 GHz K-band traveling-wave dielectric-loaded accelerators (DLA) with an rf to beam efficiency 𝜂𝑟𝑓 −𝑏𝑒𝑎𝑚 of 27%. Recently, an alternative structure which is similar to a metallic disk-loaded one but with dielectric disks, noted as dielectric disk accelerator (DDA), has been investigated and optimized, leading to ∼45% improvement in 𝜂𝑟𝑓 −𝑏𝑒𝑎𝑚. To demonstrate the key technologies, an X-band prototype structure has been designed and will be tested at Argonne Wakefield Accelerator (AWA) facility with a 300 MW metallic power extractor. Detailed comparison between K-band DLA and DDA for AFLC main accelerator as well as the preliminary design of the X-band DDA prototype will be presented in this paper., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

Two-Beam Acceleration (TBA) is a structure-based wakefield acceleration method with the potential to meet the luminosity and cost requirements of a TeV class linear collider. The Argonne Wakefield Accelerator (AWA) facility is developing a dielectric-based short pulse TBA scheme with the potential to withstand high acceleration gradients and to achieve low fabrication cost. Recently, the dielectric short pulse TBA technology was successfully demonstrated using K-band 26 GHz structures, achieving 55 MW output power from the power extractor and 28 MeV/m gradient in the accelerator. To improve the generated rf power, an X-band 11.7 GHz power extractor has been developed, which obtained 10⁵ MW in the high power test. In addition, a novel dielectric disk accelerator (DDA) is currently under investigation to significantly increase the efficiency of linear colliders based on short pulse TBA. Details of these research will be presented in this paper., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

Sub-fs electron bunch has been pursued in the last decade using several different methods. These methods rely on one of the velocity difference or path length difference to compress a long bunch to sub-fs bunch. Here, we introduce a new method to generate the compression. Emittance Exchange (EEX) beamline makes transverse-to-longitudinal exchange of phase space. In this beamline, a transverse focusing at the upstream introduces a longitudinal compression at the downstream due to the exchange. Since this exchange scheme does not rely on the velocity or the path length differences, it does not require any longitudinal manipulation (e.g. chirp), and it could generate a short bunch with well-controlled nonlinear effects using nonlinear magnets. We present preliminary simulation results of EEX based bunch compression and sub-fs bunch generation., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

A beam driven acceleration experiment in a photonic band gap (PBG) structure is planned at Argonne wakefied accelerator (AWA) facility at Argonne National Laboratory. We plan to pass a high charge (drive) beam through a travelling wave 11.7 GHz PBG structure and generate a wakefield. This wakefield will be probed by a low charge (witness) beam to demonstrate wakefield acceleration and deceleration. The drive and witness bunches will be accelerated to above 60 MeV in the main accelerator at AWA which has frequency of 1.3 GHz. The charges used in this experiment could be as high as 20 nC. To measure the exclusive effect of PBG the structure on acceleration and deceleration of the witness bunch we have to exclude the effect of beam loading of the main AWA accelerator structure. To understand the wakefield effect in AWA, we conducted an experiment where we passed the high charge (10 nC) beam through the accelerator structure which was followed by a 2 nC witness beam separated by 4 wavelength. The energy of witness beam was measured in the presence and absence of the drive beam. The beam loading was observed and quantified. The results of this work will be presented in the conference., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada