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We report on experiments in nanopatterning electron beams from a photoinjector as a first step toward a compact XFEL (CXFEL). The nanopatterning is produced by Bragg diffraction of relativistic electron beams through a patterned Si crystal consisting of alternating thick and thin strips to produce nanometer-scale electron density modulations. Multi-slice simulations show that the target can be oriented for a two-beam condition where nearly 80% of the elastically scattered electron beam is diffracted into the 220 Bragg peak. An experiment at the two-beam condition measurement has been carried out at the SLAC UED facility showing this effect with 2.26 MeV electrons. We successfully proved a large portion of the main beam is diffracted into 220 spot by tuning the orientation of the sample. Future plans at UCLA are to observe the nanopatterned beam, and to investigate various grating periods, crystal thicknesses, and sample orientations to maximize the contrast in the pattern and explore tuning the period of the modulation. The SLAC measurement results will be presented along with design of the UCLA experiments., Proceedings of the 38th Int. Free Electron Laser Conf., FEL2017, Santa Fe, NM, USA

ASU is pursuing a concept for a compact x-ray FEL (CXFEL) that uses nanopatterning of the electron beam via electron diffraction and emittance exchange to enable fully coherent x-ray output from electron beams with an energy of a few tens of MeV. This low energy is enabled by nanobunching and use of a short-pulse laser field as an undulator, resulting in an XFEL with 10 m total length and modest cost. The method of electron bunching is deterministic and flexible, rather than dependent on SASE amplification, so that the x-ray output is coherent in time and frequency. The phase of the x-ray pulse can be controlled and manipulated with this method so that new opportunities for ultrafast x-ray science are enabled using e.g. attosecond pulses, very narrow linewidths, or extremely precise timing among multiple pulses with different colors. These properties may be transferred to large XFELs through seeding with the CXFEL beam. Construction of the CXFEL accelerator and laboratory are underway, along with initial experiments to demonstrate nanopatterning via electron diffraction. An overview of the methods, project, and new science enabled are presented., Proceedings of the 38th Int. Free Electron Laser Conf., FEL2017, Santa Fe, NM, USA

A kHz repetition rate RF photoinjector with novel features has been designed for the ASU CXLS project. The photoinjector consists of a 9.3 GHz 4.5 cell standing-wave RF cavity that is constructed from 2 halves. The halves are brazed together, with the braze joint bisecting the irises and cells, greatly simplifying its construction. The cathode is brazed onto this assembly. RF power is coupled into the cavity through inline circular waveguide using a demountable TM01 mode launcher. The mode launcher feeds the power through 4 ports distributed azimuthally to eliminate both dipole and quadrupole field distortions. The brazed-in cathode and absence of complex power coupler result in a very inexpensive yet high performance device. The clean design allows the RF cavity to sit entirely within the solenoid assembly. The cathode gradient is 120 MV/m at 3 MW of input power. The cathode cell is just 0.17 RF wavelength so that laser arrival phase for peak acceleration is 70 degrees from zero crossing resulting in exit energy of 4 MeV. The photoinjector will operate with 1μs pulses at 1 kHz, dissipating 3 kW of heat. Details of the design are presented., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

We present the design of a field-emission X-band gun designed to be powered using a solid-state RF source. The source of the electron beam is a field emission nano-tip array. The RF gun is intended to be a beam source for 1 MeV solid-state driven linac for deployment on a satellite to map magnetic fields in the magnetosphere. The gun has to satisfy strict requirements on both average and peak power consumption, as well as rapid turn on time. In order to achieve low power consumption, the RF gun operates at relatively low accelerating gradient of 2 MeV/m. The beam exit energy is ~20 keV for an RF power 1.5 kW. Each cell of the RF gun is separately powered by commercially available, GaN high electron mobility transistors. In proof of principle experiments we successfully powered a 9.3 GHz accelerating cavity with a 100 W transistor and a 1% duty cycle., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

We present single-cell accelerating structures designed for high-gradient testing at 110 GHz. The purpose of this work is to study the basic physics of ultrahigh vacuum RF breakdown in high-gradient RF accelerators. The accelerating structures are pi-mode standing-wave cavities fed with a TM01 circular waveguide. The structures are fabricated using precision milling out of two metal blocks, and the blocks are joined with diffusion bonding and brazing. The impact of fabrication and joining techniques on the cell geometry and RF performance will be discussed. First prototypes had a measured Qo of 2800, approaching the theoretical design value of 3300. The geometry of these accelerating structures are as close as practical to single-cell standing-wave X-band accelerating structures more than 40 of which were tested at SLAC. This wealth of X-band data will serve as a baseline for these 110 GHz tests. The structures will be powered with short pulses from a MW gyrotron oscillator. RF power of 1 MW may allow us to reach an accelerating gradient of 400 MeV/m., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

Diffraction in the transmission geometry through a single-crystal silicon slab is exploited to control the intensity of a relativistic electron beam. The choice of crystal thickness and incidence angle can extinguish or maximize the transmitted beam intensity via coherent multiple Bragg scattering; thus, the crystal acts as a dynamical beam stop through the Pendel'sung effect, a well-known phenomenon in X-ray and electron diffraction. In an initial experiment, we have measured the ability of this method to transmit or extinguish the primary beam and diffract into a single Bragg peak. Using lithographic etching of patterns in the crystal we intend to use this method to nanopattern an electron beam for production of coherent x-rays. We compare the experimental results with simulations using the multislice method to model the diffraction pattern from a perfect silicon crystal of uniform thickness, considering multiple scattering, crystallographic orientation, temperature effects, and partial coherence from the momentum spread of the beam. The simulations are compared to data collected at the ASTA UED facility at SLAC for a 340 nm thick Si(100) wafer with a beam energy of 2.35 MeV., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

We describe Bragg diffraction of relativistic electron beams through a patterned Si crystal consisting of alternating thick and thin strips to produce nanometer scale electron density modulations. Multi-slice simulations show that a two-beam situation can be set up where, for a particular thickness of Si, nearly 100% of the electron beam is diffracted. Plans are underway to carry out experiments showing this effect in UCLA's ultrafast electron microscopy lab with 3.5 MeV electrons. We will select either the diffracted beam or the primary beam with a small aperture in the diffraction plane of a magnetic lens, and so record either the dark or bright field magnified image of the strips. Our first goal is to observe the nanopatterned beam at the image plane. We will then investigate various crystal thickness and sample orientations to maximize the contrast in the pattern and explore tuning the period of the modulation through varying magnification., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

Recent efforts have demonstrated the possibility of achieving ultralow transverse emittance beams for high brightness light sources and free electron lasers*. While these lower emittances should translate to improved lasing efficiency and higher peak brightness in FELs, these beams are commensurately more vulnerable to coherent synchrotron radiation (CSR) for the selfsame reasons. Conserving these ultralow emittances through the bunch compressors in an FEL given their increased propensity to emit CSR is particularly challenging. We investigate the possibility of imposing a large energy chirp at terahertz wavelengths to reduce the required magnetic fields in the compressor, counteracting the ultralow emittance in the generation of CSR. A second, higher frequency THz chirper would then be used to dechirp the beam after the chicane. Operation at THz as opposed to conventional radiofrequencies offers significantly larger chirp at similar input powers, yet still with wavelengths greater than typical FEL bunch lengths (several femtoseconds). Potential experimental schemes will be suggested in the context of LCLS and their feasibility evaluated., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

We present the design and parameters of single-cell accelerating structures for high-gradient testing at 110 GHz. The purpose of this work is to study the basic physics of ultrahigh vacuum RF breakdown in high-gradient RF accelerators. The accelerating structures consist of pi-mode standing-wave cavities fed with TM01 circular waveguide mode. The geometry and field shape of these accelerating structures is as close as practical to single-cell standing-wave X-band accelerating structures, more than 40 of which were tested at SLAC. This wealth of X-band data will serve as a baseline for these 110 GHz tests. The structures will be powered from a pulsed MW gyrotron oscillator. One MW of RF power from the gyrotron may allow us to reach a peak accelerating gradient of 400 MeV/m., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

We present PIC simulations of electron bunches with nm scale longitudinal modulation produced using a compact 2-20 MeV LINAC. The modulation is initially imparted in the transverse dimension of the electron bunch with a nano-patterned photo-emitter in a X-band RF gun with 2 MeV exit energy. The electron bunch passes through a 1 m standing wave X-band LINAC which can raise the beam energy up to 20 MeV. The transverse modulation is exchanged into the longitudinal dimension using a double dog-leg emittance exchange setup with a 5 cell RF deflector cavity. The modulation pitch can be tuned by adjusting the spacing of the nano-patterned photo-emitter or magnification of the transverse pitch with electron optics. The electron beam parameters are optimized to produce coherent XFEL radiation upon interacting with a “laser undulator”., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

We report the first experimental demonstration of linear electron acceleration using an optically generated single cycle THz pulse centered at 0.45 THz. 7 keV of acceleration is achieved using 10 microJ THz pulses in a 3 mm interaction length. The THz pulse is produced via optical rectification of a 1.2 mJ, 1 micron laser pulse with a 1 kHz repetition rate. The THz pulse is coupled into a dielectric-loaded circular waveguide with 10 MeV/m on-axis accelerating gradient. A 25 fC input electron bunch is produced with a 60 keV DC photo-emitting cathode. The achievable accelerating gradient in the THz structures being investigated will scale rapidly by increasing the IR pulse energy (100 mJ - 1 J) and correspondingly the THz pulse energy. Additionally, with recent advances in the generation of THz pulses via optical rectification, in particular improvements to efficiency and generation of multi-cycle pulses, GeV/m accelerating gradients could be achieved. An ultra-compact high-gradient THz accelerator would be of interest for a wide variety of applications., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany