O. Williams

University of California, Los Angeles, Los Ángeles, California, United States

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Publications (35)55.11 Total impact

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    ABSTRACT: We report experimental measurements of narrow-band, single-mode excitation, and drive beam energy modulation, in a dielectric wakefield accelerating structure with planar geometry and Bragg-reflector boundaries. A short, relativistic electron beam (∼1 ps) with moderate charge (∼100 pC) is used to drive the wakefields in the structure. The fundamental mode of the structure is reinforced by constructive interference in the alternating dielectric layers at the boundary, and is characterized by the spectral analysis of the emitted coherent Cherenkov radiation signal. Data analysis shows a narrow-band peak at 210 GHz corresponding to the fundamental mode of the structure. Simulations in both 2D and 3D provide insight into the propagating fields and reproduction of the electron beams dynamics observables and emitted radiation characteristics.
    Physical Review Letters 12/2014; 113(26):264801. DOI:10.1103/PhysRevLett.113.264801 · 7.73 Impact Factor
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    ABSTRACT: Compact, table-top sized accelerators are key to improving access to high-quality beams for use in industry, medicine and academic research. Among laser-based accelerating schemes, the inverse free-electron laser (IFEL) enjoys unique advantages. By using an undulator magnetic field in combination with a laser, GeV m(-1) gradients may be sustained over metre-scale distances using laser intensities several orders of magnitude less than those used in laser wake-field accelerators. Here we show for the first time the capture and high-gradient acceleration of monoenergetic electron beams from a helical IFEL. Using a modest intensity (~10(13) W cm(-2)) laser pulse and strongly tapered 0.5 m long undulator, we demonstrate >100 MV m(-1) accelerating gradient, >50 MeV energy gain and excellent output beam quality. Our results pave the way towards compact, tunable GeV IFEL accelerators for applications such as driving soft X-ray free-electron lasers and producing γ-rays by inverse Compton scattering.
    Nature Communications 09/2014; 5:4928. DOI:10.1038/ncomms5928 · 10.74 Impact Factor
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    ABSTRACT: We present here the results of measurements made showing 1 GV/m accelerating fields using a hollow dielectric-lined waveguide. The results are comprised of measurement of the energy loss of a high charge ( 3 nC) ultrashort (200 fs), ultra relativistic (20 GeV) beam and concomitant auto-correlation interferometeric techniques to obtain the frequency content of simultaneously generated coherent Cherenkov radiation (CCR). Experiments were conducted at the Facility for Advanced aCcelerator Experimental Tests (FACET) at the SLAC National Laboratory using metal-coated sub-millimeter diameter, ten-centimeter long fused silica tubes. We present simulation and theoretical results in support of the conclusions reached through experiment. These results build on previous work to provide a path towards high gradient accelerating structures for use in compact accelerator schemes, future linear colliders and free-electron lasers.
    2014 International Particle Accelerator Conference, Dresden, Germany; 06/2014
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    ABSTRACT: Control and manipulation of properties - such as vacuum modal confinement, spatial harmonic content, phase velocity, and group velocity - are reasons why an all-dielectric beam-driven accelerator would be expected to benefit by borrowing from the field of optical bandgap photonics. We outline the general conditions for coherent Cerenkov radiation in a photonic crystal, illustrated by three examples: two Bragg mirrors separated by a vacuum gap, a woodpile with a linear waveguide channel, and a stack of photonic slabs having a planar waveguide channel.
    12/2012; 1507(1). DOI:10.1063/1.4773765
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    ABSTRACT: In this work we present details of planned experiments to investigate certain aspects of the quasi non linear regime (QNL) of plasma wakefield acceleration (PWFA). In the QNL regime it is, in principal, possible to combine the benefits of both nonlinear and linear PWFA. That is, beams of high quality can be maintained through acceleration due to the complete ejection of plasma electrons from beam occupied region, while large energy gains can be achieved through use of transformer ratio increasing schemes, such as ramped bunch trains. With the addition of an short focal length PMQ triplet capable of focusing beams to the few micron scale and the ability to generate tunable bunch trains, the Accelerator Test Facility (ATF) at Brookhaven National Lab offers the unique capabilities to probe these characteristics of the QNL regime.
    12/2012; 1507(1). DOI:10.1063/1.4773767
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    ABSTRACT: Preparations for an inverse free electron laser experiment at Brookhaven National Laboratory's Accelerator Test Facilty are presented. Details of the experimental setup including beam and laser transport optics are first discussed. Next, the driving laser pulse structure is investigated and initial diagnostics are explored and compared to simulations. Finally, planned improvements to the experimental setup are discussed.
    12/2012; 1507(1). DOI:10.1063/1.4773737
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    ABSTRACT: The X-ray spectrum emitted during relativistic non-linear inverse Compton scattering is angularly resolved, and energy analyzed through the use of K-edge and attenuation filters. A 0.5 nC electron beam of 65 MeV energy was employed, colliding with a highly intense CO{sub 2} laser of normalized vector potential a{sub L}{approx}0.5. In this experiment, 2nd harmonic radiation was clearly observed. In addition, evidence for the 3rd harmonic component of the radiation, having radiation angle of < 1/{gamma} has been found. This measurement represents a significant step forward in experimental understanding the electrodynamics of ICS in the nonlinear regime.
    12/2012; 1507(1). DOI:10.1063/1.4773791
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    ABSTRACT: We report first evidence of wakefield acceleration of a relativistic electron beam in a dielectric-lined slab-symmetric structure. The high energy tail of a $60 MeV electron beam was accelerated by $150 keV in a 2 cm-long, slab-symmetric SiO 2 waveguide, with the acceleration or deceleration clearly visible due to the use of a beam with a bifurcated longitudinal distribution that serves to approximate a driver-witness beam pair. This split-bunch distribution is verified by longitudinal recon-struction analysis of the emitted coherent transition radiation. The dielectric waveguide structure is further characterized by spectral analysis of the emitted coherent Cherenkov radiation at THz frequencies, from a single electron bunch, and from a relativistic bunch train with spacing selectively tuned to the second longitudinal mode (TM 02). Start-to-end simulation results reproduce aspects of the electron beam bifurcation dynamics, emitted THz radiation properties, and the observation of acceleration in the dielectric-lined, slab-symmetric waveguide.
    Physical Review Letters 06/2012; 108,(24):244801. DOI:10.1103/PhysRevLett.108.244801 · 7.73 Impact Factor
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    ABSTRACT: Experimental observation of the microbunching of a relativistic electron beam at the second harmonic interaction frequency of a helical undulator is presented. The microbunching signal is observed from the coherent transition radiation of the electron beam and indicates experimental evidence of a dominantly helical electron beam density distribution. This result is in agreement with theoretical and numerical predictions and provides a proof-of-principle demonstration of proposed schemes designed to generate light with orbital angular momentum in high-gain free-electron lasers. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3690900]
    Applied Physics Letters 02/2012; 100(9):091110. DOI:10.1063/1.3690900] · 3.52 Impact Factor
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    ABSTRACT: We demonstrate multiphoton, single shot diffraction images of x rays produced by inverse Compton scattering a high-power CO 2 laser from a relativistic electron beam, creating a pulse of 8.7 keV x rays. The tightly focused, relatively high peak brightness electron beam and high photon density from the 2 J CO 2 laser yielded 6 Â 10 7 x-ray photons over the full opening angle in a single shot. Single shot x-ray diffraction is performed by passing the x rays though a vertical slit and on to a flat silicon (111) crystal. 10 2 diffracted photons were detected. The spectrum of the detected x rays is compared to simulation. The diffraction and detection of 10 2 x rays is a key step to a more efficient time resolved diagnostic in which the number of observed x rays might reach 10 4 ; enabling a unique, flexible x-ray source as a sub-ps resolution diagnostic for studying the evolution of chemical reactions, lattice deformation and melting, and magnetism.
    Physical Review Special Topics - Accelerators and Beams 02/2012; 15(2):020702. DOI:10.1103/PhysRevSTAB.15.020702 · 1.52 Impact Factor
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    ABSTRACT: It has recently been proposed to use a dispersionless translating section (dogleg) with sextupole correction magnets as a bunch compressor to create longitudinally shaped (linearly ramped) electron bunches. We discuss the experiment soon to be underway at the UCLA Neptune Linear Accelerator Laboratory to test this technique with the 300 pC, 13 MeV electron bunches produced by the Neptune S-Band photoinjector. The experiment will utilize a dipole-mode deflecting cavity, as a temporal diagnostic, and a final focus system of permanent magnet quadrupoles with field gradients of 110 T/m. We also discuss the potential scaling of this technique to bunches of high (i.e. >1nC) charge for the purpose of creating a suitable drive beam for the plasma wakefield accelerator, operating in the blowout regime.
    International Journal of Modern Physics A 01/2012; 22(23). DOI:10.1142/S0217751X07037706 · 1.09 Impact Factor
  • J.rosenzweig, O.williams
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    ABSTRACT: In using the inverse Compton scattering (ICS) interaction as a high brilliance, short wavelength radiation source, one collides two beams, one an intense laser, and the other a high charge, short pulse electron beam. In order to maximize the flux of photons from ICS, one must focus both beams strongly, which implies both use of short beams and the existence of large angles in the interaction. One aspect of brilliance is the narrowness of the wavelength band emitted by the source. This paper explores the limits of ICS-based source brilliance based on inherent wavelength broadening effects that arise due to focal angles, laser energy density, and finite laser pulse length effects. It is shown that for a nominal 1% desired bandwidth, that one obtains approximately one scattered photon per electron in a head-on collision geometry.
    International Journal of Modern Physics A 01/2012; 22(23). DOI:10.1142/S0217751X07037871 · 1.09 Impact Factor
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    ABSTRACT: An Inverse Compton Scattering (ICS) experiment investigating the polarized harmonic production in the nonlinear regime has begun which will utilize the existing terawatt CO2 laser system and 15 MeV photoinjector in the Neptune Laboratory at UCLA. A major motivation for a source of high brightness polarized x-rays is the production of polarized positrons for use in future linear collider experiments. Analytical calculations have been performed to predict the angular and frequency spectrums for various polarizations and different scattering angles. We report on the experimental set-up and status.
    International Journal of Modern Physics A 01/2012; 22(23). DOI:10.1142/S0217751X07037895 · 1.09 Impact Factor
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    ABSTRACT: An overview on the underlying principles of the hybrid plasma wakefield acceleration scheme dubbed 'Trojan Horse' acceleration is given. The concept is based on laser-controlled release of electrons directly into a particle-beam-driven plasma blowout, paving the way for controlled, shapeable electron bunches with ultralow emittance and ultrahigh brightness. Combining the virtues of a low-ionization-threshold underdense photocathode with the GV/m-scale electric fields of a practically dephasing-free beam-driven plasma blowout, this constitutes a 4th generation electron acceleration scheme. It is applicable as a beam brightness transformer for electron bunches from LWFA and PWFA systems alike. At FACET, the proof-of-concept experiment 'E-210: Trojan Horse Plasma Wakefield Acceleration' has recently been approved and is in preparation. At the same time, various LWFA facilities are currently considered to host experiments aiming at stabilizing and boosting the electron bunch output quality via a trojan horse afterburner stage. Since normalized emittance and brightness can be improved by many orders of magnitude, the scheme is an ideal candidate for light sources such as free-electron-lasers and those based on Thomson scattering and betatron radiation alike.
    AIP Conference Proceedings 01/2012; 1507:570. DOI:10.1063/1.4773760
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    ABSTRACT: The use of two different wavelength lasers in the nonlinear regime of the inverse Compton scattering interaction is proposed in order to provide a new strategy for controlling scattered photon energy distributions in the x-ray to -ray spectral region. In this nonlinear interaction, the component of the relativistic electron's trajectory driven by a longer-wavelength laser with the normalized vector potential a L $ 1 is a large oscillatory figure-8; in the proposed scenario a rapid small-amplitude oscillation induced by a shorter-wavelength laser is superimposed upon this figure-8. Thus, the electron's momentum is mainly supplied from longer-wavelength laser, while the high-frequency part of the acceleration is given by shorter-wavelength laser. In this way, the harmonics radiated at high frequency from the oscillating electron can be strongly modified by the nonlinear motion initiated by the low frequency, large a L laser resulting in the generation of the harmonics with the photon energy of 4 2 @ð! L;short þ n! L;long Þ. In this paper, the electron's kinetics in the two-wavelength laser field and the concomitant emitted radiation spectrum are examined, with numerical illustrations based on a classical Lienard-Wiechert potential formalism provided.
    Physical Review Special Topics - Accelerators and Beams 12/2011; 14(12):120702. DOI:10.1103/PhysRevSTAB.14.120702 · 1.52 Impact Factor
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    ABSTRACT: We report our recent progress using a high-power, picosecond CO{sub 2} laser for Thomson scattering and ion acceleration experiments. These experiments capitalize on certain advantages of long-wavelength CO{sub 2} lasers, such as their high number of photons per energy unit and beneficial wavelength- scaling of the electrons' ponderomotive energy and critical plasma frequency. High X-ray fluxes produced in the interactions of the counter-propagating laser- and electron-beams for obtaining single-shot, high-contrast images of biological objects. The laser, focused on a hydrogen jet, generated a monoenergetic proton beam via the radiation-pressure mechanism. The energy of protons produced by this method scales linearly with the laser's intensity. We present a plan for scaling the process into the range of 100-MeV proton energy via upgrading the CO{sub 2} laser. This development will enable an advance to the laser-driven proton cancer therapy.
    AIP Conference Proceedings 05/2011; 1336(1). DOI:10.1063/1.3586126
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    ABSTRACT: We report the observation of coherent Cerenkov radiation in the terahertz regime emitted by a relativistic electron pulse train passing through a dielectric lined cylindrical waveguide. We describe the beam manipulations and measurements involved in repetitive pulse train creation including comb collimation and nonlinear optics corrections. With this technique, modes beyond the fundamental are selectively excited by use of the appropriate frequency train. The spectral characterization of the structure shows preferential excitation of the fundamental and of a higher longitudinal mode. (C) 2011 American Institute of Physics. [doi:10.1063/1.3592579]
    Applied Physics Letters 05/2011; 98(20):202901. DOI:10.1063/1.3592579 · 3.52 Impact Factor
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    ABSTRACT: It has long been noted that the nonlinear “blowout” regime of the PWFA has certain critical aspects for producing high quality beams that are owed to the elimination of electron density and current inside of the beam‐occupied region: time‐independent, linear ion‐focusing, and acceleration independent of transverse position. Unfortunately, in applying this scheme to a linear collider, efficiency considerations strongly encourage use of pulse trains, in which one superimposes the wakes of driving and accelerating beams in turn. This implies that one needs to maintain stable wakes with the ability to maintain a resonant response which, given the presence of wave‐breaking and amplitude dependent frequency, is not straightforward in the nonlinear regime. Here we propose a solution to this problem: operation in the quasi‐nonlinear regime, where one uses beams with relatively low charge and transverse beam size much smaller than a plasma skin‐depth. In this case, the beam density may exceed that of the plasma, producing blowout, but due to the small total charge, producing a disturbance that behaves in many ways as linear, having frequency essentially that of linear plasma oscillations. To illustrate the salient characteristics of this regime we present the results of single and multi‐pulse simulations, and give a theoretical analysis for the preservation of the plasma frequency. We discuss a proposal for testing this regime at the BNL ATF.
    11/2010; 1299(1):500-504. DOI:10.1063/1.3520373
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    ABSTRACT: Given the recent success of >GV/m dielectric wakefield accelerator (DWA) breakdown experiments at SLAC, and follow-on coherent Cerenkov radiation (CCR) production at the UCLA Neptune, a UCLA-USC-SLAC collaboration is now implementing a new set of experiments that explore various DWA scenarios. These experiments are motivated by the opportunities presented by the approval of the FACET facility at SLAC, as well as unique pulse-train wakefield drivers at BNL. The SLAC experiments permit further exploration of the multi-GeV/m envelope in DWAs, and will entail investigations of novel materials (e.g. CVD diamond) and geometries (Bragg cylindrical structures, slab-symmetric DWAs), and have an over-riding goal of demonstrating >GeV acceleration in ~33 cm DWA tubes. In the nearer term before FACET's commissioning, we are performing measurements at the BNL ATF, in which we drive ~50-200 MV/m fields with single pulses or pulse trains, and observe resonantly driven CCR as well as deflection modes. These experiments are of high relevance to enhancing linear collider DWA designs, as they will demonstrate potential for high efficiency operation with pulse trains, and explore transverse modes for the first time.
    11/2010; DOI:10.1063/1.3520344
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    ABSTRACT: Inverse compton scattering (ICS) x-ray sources are of current interest in biomedical imaging. We present an experimental demonstration of inline phase contrast imaging using a single picosecond pulse of the ICS source located at the BNL Accelerator Test Facility. The phase contrast effect is clearly observed. Its qualities are shown to be in agreement with the predictions of theoretical models through comparison of experimental and simulated images of a set of plastic wires of differing composition and size. Finally, we display an application of the technique to a biological sample, confirming the possibility of time-resolved imaging on the picosecond scale.
    Applied Physics Letters 10/2010; 97(13-97):134104 - 134104-3. DOI:10.1063/1.3491430 · 3.52 Impact Factor