O. Williams

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

Are you O. Williams?

Claim your profile

Publications (31)36.07 Total impact

  • [Show abstract] [Hide abstract]
    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.
    AIP Conference Proceedings. 12/2012; 1507(1).
  • [Show abstract] [Hide abstract]
    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.
    AIP Conference Proceedings. 12/2012; 1507(1).
  • [Show abstract] [Hide abstract]
    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.
    AIP Conference Proceedings. 12/2012; 1507(1).
  • [Show abstract] [Hide abstract]
    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.
    AIP Conference Proceedings. 12/2012; 1507(1).
  • [Show abstract] [Hide abstract]
    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). · 1.13 Impact Factor
  • [Show abstract] [Hide abstract]
    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). · 1.13 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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 01/2012; 108,:244801. · 7.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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 01/2012; 15:020702. · 1.57 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3690900] Free-electron lasers (FELs) rely on highly relativistic electron beams (e-beams) to generate coherent, high-brightness light pulses. FELs have enormous flexibility in that the character of the emitted light is determined by the properties of the e-beam: the frequency is tunable via its quadratic dependence on the e-beam energy, and the trans-verse optical mode structure is governed both by the har-monic frequency content 1,2 and by the correlated spatial distribution of electrons. In Ref. 3, a scenario referred to as high-gain high-mode generation (HGHMG) was described, where the FEL is tailored to emit light that carries an effec-tive component of orbital angular momentum (OAM) (Ref. 4) using an e-beam with a helical density modulation about its propagation axis. The high power coherent FEL light has a corresponding helical transverse phase that is useful for a broad scope of research. 5–7 Experimental realization of the HGHMG OAM scheme relies on the ability to generate the required helical e-beam density distribution. The electrons must be arranged into a spiral staircase structure with the same longitudinal periodic-ity as the wavelength of the FEL light (helical microbunch-ing). A simple method was proposed in Ref. 8, where the e-beam is naturally helically modulated by a laser tuned to one of the harmonics, h, of the resonant interaction in a heli-cal undulator (inverse FEL, or IFEL). The helical e-beam ge-ometry emerges because the energy kick given to an electron depends on the field gradient of the transversely Gaussian laser profile, which varies azimuthally. The excited azi-muthal e-beam mode l depends on the harmonic number via the relation jlj ¼ h À 1. Thus, a simple Gaussian laser pulse profile interacting at the second harmonic is predicted to gen-erate the required helical microstructure as a seed for jlj ¼ 1 OAM light in a downstream FEL. Here we present experimental evidence of induced heli-cal microbunching in the helical interaction experiment (HE-LiX) at the Neptune Laboratory at UCLA. Results indicate a dominant l ¼ À1 helical distribution imposed on the e-beam density structure at the second harmonic by the transversely Gaussian seed laser. The primary components of the HELiX are illustrated in Fig. 1. A short, r z =c ¼ 3 ps rms bunch length, 12–12.5 MeV (c ¼ 24–25), initially unmodulated e-beam propagates through a k w ¼ 2p=k w ¼ 1.9 cm period helical undulator co-axially with a k ¼ 2p=k ¼ 10.6 lm wavelength, 100 ps long CO 2 laser pulse. The interaction is tuned to be near the h ¼ 2 (second harmonic) resonance k ¼ k w 2hc 2 ð1 þ K 2 Þ, where K ¼ eBk w =2pmc ¼ 0.58 is the nor-malized field strength of the undulator magnetic field B, Àe and m are the electron charge and mass, and c is the speed of light. The laser pulse reaches up to 30 MW (3 mJ) of peak power and comes to a measured w 0 ¼ 400 lm axisymmetric waist 10 cm inside the N w ¼ 12 period undulator. The e-beam had a fixed charge of eN e ¼ 24069 pC, 0.035% slice RMS energy spread, and 1.3% FWHM relative energy spread due to the correlations imposed by the s-band injector. Microbunching was diagnosed by measuring the total coherent transition radiation (CTR) emitted by the e-beam as it passed through a 10 lm thick, 1.5 cm square aluminum foil positioned flush against the 1 cm round undulator exit aper-ture to minimize the large laser background. The helical e-beam bunching factor is defined as b l ¼ N À1
    Applied Physics Letters 01/2012; 100:091110. · 3.79 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
  • [Show abstract] [Hide abstract]
    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).
  • Source
    [Show abstract] [Hide abstract]
    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 01/2011; 14:120702. · 1.57 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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. © 2011 American Institute of Physics. doi:10.1063/1.3592579 Dielectric lined cylindrical waveguides have long been studied for use in advanced acceleration applications, as they provide the potential high accelerating gradients for the dielectric wakefield accelerator DWA, in a relatively compact form in the difficult to access terahertz THz spec-tral region. 1,2 Initial experiments using dielectric loaded waveguides, at longer wavelengths, measured the wake po-tentials of a driving bunch for different materials, 3 while more recent THz excitation experiments using single, ex-tremely short, very high charge electron pulses demonstrated GV/ m fields before structure breakdown. 4 Follow-on ex-periments have demonstrated their applicability as sources of narrow-band, coherent THz radiation. 5 The characteristics of the emitted radiation depend on the dielectric tube geometry, dielectric material choice, inner/outer dielectric tube bound-ary, as well as driving electron beam properties such as charge, and bunch length or time structure. In preliminary experiments, Cook et al. 5 demonstrated single-mode excita-tion of the fundamental mode 0.3 THz limited by the moderate single bunch length 1 ps and choice of wave-guide parameters restricted by the beam width at the lower energy of 11 MeV. Further applications of coherent radiation sources are accessible using the higher energies and pulse train produc-tion scheme employed at the Brookhaven National Labora-tory Accelerator Test Facility BNL ATF. 6 This pulse train scheme employs a rigid mask inserted at a high-dispersion point along the beamline, that introduces a periodic
    Applied Physics Letters 01/2011; 98:202901. · 3.79 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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.
    AIP Conference Proceedings. 11/2010; 1299(1):500-504.
  • [Show abstract] [Hide abstract]
    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;
  • Source
    [Show abstract] [Hide abstract]
    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; · 3.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Polarized X-ray pulses at 0.6 Å have been generated via head-on collision of a laser pulse from the high-field laser facility at Daresbury with a 30 MeV electron bunch in the ALICE energy recovery linear accelerator. The angular distribution of the backscattered X rays was obtained in single-shot using a scintillation screen. The temporal profile of the X ray yield as a function of the time delay between the laser pulse and electron bunch was measured and agreed well with that expected from the collision point dependence of the laser-electron beam longitudinal overlap.
    Proc SPIE 08/2010;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The use of coherent transition radiation autocorrelation methods to determine bunch length and profile information is examined with the compressed electron beam at the BNL ATF. A bi-gaussian fit is applied to coherent transition radiation auto-correlation data to extract the longitudinal current distribution. The effects of large transverse beam sizes are studied in theory and compared to experimental results. A suitable form of the correction factor is derived for beams with large transverse-longitudinal aspect ratios. Comment: 5 pages, 1 figures, submitted to The Second Workshop on High Average Power & High Brightness Beams (January 2009)
    02/2010;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have shown that a seventh-order inverse-free-electron laser (IFEL) interaction, where the radiation frequency is the seventh harmonic of the fundamental resonant frequency, can microbunch a beam of relativistic electrons inside an undulator. Using coherent transition radiation (CTR) emitted by the bunched 12.3 MeV beam as a diagnostic, strong microbunching of the beam is inferred from the observation of CTR at the first, second, and third harmonics of the seed 10 m radiation. Three-dimensional IFEL simulations show that the observed harmonic ratios can be explained only if transverse spatial distribution of the steepened bunched beam is taken into account. There is a growing interest in developing new light sources in the x-ray range with improved spatial and spec-tral coherence properties compared to the present-day synchrotron radiation sources. Many proposals and several ongoing national projects exist worldwide to build x-ray or UV free-electron lasers (FELs) in which a high-brightness, multi-GeV electron beam has a resonant interaction with radiation in the presence of a periodic magnetic field of an undulator [1]. Because of the practical limit on the size and strength of undulator magnets, which all have periods in the cm range, the electron beam energy on the GeV scale represents one of the main constraints on the shortest reachable wavelength. To circumvent this limitation, a high-order FEL interaction has been proposed recently as a way to reduce the required beam energy [2]. In addition to FELs, inverse-free-electron laser (IFEL) acceleration [3], longitudinal current modulation of an electron beam (microbunching) [4], and creation of very short electron bursts (femtoslicing) [5] are other applications, which can potentially benefit from the high-order resonant beam-radiation interaction in an undulator. High-order FEL and IFEL interactions can be under-stood by looking closely at the resonant condition on the axis of a planar undulator: r ¼ u 2n 2
    Physical Review Special Topics - Accelerators and Beams 09/2009; 12:050703. · 1.57 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report experimental observation of narrow-band coherent Cherenkov radiation driven by a subpicosecond electron bunch traveling along the axis of a hollow cylindrical dielectric-lined waveguide. For an appropriate choice of dielectric wall thickness, a short-pulse beam current profile excites only the fundamental mode of the structure, producing energetic pulses in the terahertz range. We present detailed measurements showing a narrow emission spectrum peaked at 367 + or - 3 GHz from a 1 cm long fused silica capillary tube with submillimeter transverse dimensions, closely matching predictions. We demonstrate a 100 GHz shift in the emitted central frequency when the tube wall thickness is changed by 50 microm. Calibrated measurements of the radiated energy indicate up to 10 microJ per 60 ps pulse for an incident beam charge of 200 pC, corresponding to a peak power of approximately 150 kW.
    Physical Review Letters 08/2009; 103(9):095003. · 7.73 Impact Factor