W. Gai

Euclid TechLabs, Maryland, United States

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Publications (244)341.75 Total impact

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    ABSTRACT: A tunable energy-chirp compensator was used to remove a correlated energy chirp from the 60-MeV beam at the Brookhaven National Laboratory Accelerator Test Facility. The compensator operates through the interaction of the wakefield of the electron bunch with itself and consists of a planar structure comprised of two alumina bars with copper-plated backs separated by an adjustable beam aperture. By changing the gap size, the correlated energy chirp of the electron bunch was completely removed. Calculations show that this device, properly scaled to account for the electron bunch charge and length, can be used to remove residual correlated energy spread at the end of the linacs used for free-electron lasers. The experimental results are shown to be in good agreement with numerical simulations. Application of this technique can significantly simplify linac design and improve free-electron lasers performance.
    Physical Review Letters 03/2014; 112(11):114801. · 7.94 Impact Factor
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    ABSTRACT: These reports present the results of the 2013 Community Summer Study of the APS Division of Particles and Fields ("Snowmass 2013") on the future program of particle physics in the U.S. Chapter 6, on Accelerator Capabilities, discusses the future progress of accelerator technology, including issues for high-energy hadron and lepton colliders, high-intensity beams, electron-ion colliders, and necessary R&D for future accelerator technologies.
    01/2014;
  • Wei Gai, Wanming Liu
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    ABSTRACT: In this paper, we presented an overview of the positron source for International Linear Collider (ILC). We started with description of the positron source configuration for ILC RDR baseline and SB2009 baseline followed by the status of critical components of ILC positron source R&D. We also presented some parameters of positron source for both RDR and SB2009 baseline.
    10/2013;
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    ABSTRACT: The parameter set for the International Linear Collider (ILC) positron source given in the Technical Design Report (TDR) is more complicated than that presented in the ILC Reference Design Report (RDR). Studies to define and to optimize the parameters for different scenarios of center-of-mass energies have been performed at both Argonne National Lab (ANL) and Deutsches Elektronen Synchrotron (DESY)/Hamburg University. Results from both institutes agree well and are presented in this paper.
    10/2013;
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    ABSTRACT: We propose the conventional positron source driven by a several-GeV electron beam for ILC. Thermal load of the positron production target was a risk of the conventional positron source. To cure it, we employ a 300 Hz electron linac to create positrons with stretched pulse length. In ILC, the bunch timing structures and pulse timing structures can be diffecent in the positron source, in the DR, and in the main linac. We have some flexibility to choose timing structures in positron source and we use it for time stretching. ILC requires about 2600 bunches in a train in the main linac which pulse length is 1 ms. In the conventional source, about 130 positron bunches are created by each pulse of the 300Hz linac. Then 2600 bunches are created in 63 ms. We optimized parameters such as drive beam energy, beam size on the target, and target thickness to maximize the capture efficiency and to mitigate the target thermal load. A slow rotating tungsten disk is employed as positron production target.
    10/2013;
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    ABSTRACT: A strong energy modulation in an electron bunch passing through a dielectric-lined waveguide was recently demonstrated in Antipov et al., Phys. Rev. Lett. 108, 144801 (2012). In this Letter, we demonstrate a successful conversion of this energy modulation into a beam density modulation, and the formation of a series of microbunches with a subpicosecond periodicity by means of magnetic optics (chicane). A strong coherent transition radiation signal produced by the microbunches is obtained and the tunability of its carrier frequency in the 0.68-0.9 THz range by regulating the energy chirp in the incoming electron bunch is demonstrated using infrared interferometry. A tabletop, compact, tunable, and narrowband source of intense THz radiation based on this technology is proposed.
    Physical Review Letters 09/2013; 111(13):134802. · 7.94 Impact Factor
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    ABSTRACT: The International Linear Collider Technical Design Report (TDR) describes in four volumes the physics case and the design of a 500 GeV centre-of-mass energy linear electron-positron collider based on superconducting radio-frequency technology using Niobium cavities as the accelerating structures. The accelerator can be extended to 1 TeV and also run as a Higgs factory at around 250 GeV and on the Z0 pole. A comprehensive value estimate of the accelerator is give, together with associated uncertainties. It is shown that no significant technical issues remain to be solved. Once a site is selected and the necessary site-dependent engineering is carried out, construction can begin immediately. The TDR also gives baseline documentation for two high-performance detectors that can share the ILC luminosity by being moved into and out of the beam line in a "push-pull" configuration. These detectors, ILD and SiD, are described in detail. They form the basis for a world-class experimental programme that promises to increase significantly our understanding of the fundamental processes that govern the evolution of the Universe.
    06/2013;
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    ABSTRACT: The International Linear Collider Technical Design Report (TDR) describes in four volumes the physics case and the design of a 500 GeV centre-of-mass energy linear electron-positron collider based on superconducting radio-frequency technology using Niobium cavities as the accelerating structures. The accelerator can be extended to 1 TeV and also run as a Higgs factory at around 250 GeV and on the Z0 pole. A comprehensive value estimate of the accelerator is give, together with associated uncertainties. It is shown that no significant technical issues remain to be solved. Once a site is selected and the necessary site-dependent engineering is carried out, construction can begin immediately. The TDR also gives baseline documentation for two high-performance detectors that can share the ILC luminosity by being moved into and out of the beam line in a "push-pull" configuration. These detectors, ILD and SiD, are described in detail. They form the basis for a world-class experimental programme that promises to increase significantly our understanding of the fundamental processes that govern the evolution of the Universe.
    06/2013;
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    ABSTRACT: A table top device for producing high peak power (tens of megawatts to a gigawatt) T-ray beams is described. An electron beam with a rectangular longitudinal profile is produced out of a photoinjector via stacking of the laser pulses. The beam is also run off-crest of the photoinjector rf to develop an energy chirp. After passing through a dielectric loaded waveguide, the beam's energy becomes modulated by its self-wake. In a chicane beamline following the dielectric energy-bunching section this energy modulation is converted to a density modulation-a bunch train. The density modulated beam can be sent through a power extraction section, like a dielectric loaded accelerating structure, or simply can intercept a foil target, producing THz radiation of various bandwidths and power levels.
    The Review of scientific instruments 02/2013; 84(2):022706. · 1.52 Impact Factor
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    ABSTRACT: The generation and capture of polarized positrons at a source with a superconducting helical undulator having 4.3 cm period and 500 GeV electron drive beam have been simulated. The positron polarization has been calculated for the different undulator K values (up to K = 2.5). Without applying a photon collimator, the maximal polarization of positrons is about 25% for 231 meters active magnet length of undulator with K = 0.7. Using an undulator with K = 2.5 and a collimator with an aperture radius of 0.9 mm results in increase of positron polarization to 54%. The energy deposition, temperature rise and stress induced by high intense photon beam in the rotated titanium-alloy target have been estimated. The maximal thermal stress in the target is about 224 MPa for the source with photon collimation to achieve a positron polarization of 54%.
    01/2013;
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    ABSTRACT: Efforts by a number of institutions to develop a Dielectric-Loaded Accelerating (DLA) structure capable of supporting high gradient acceleration when driven by an external radio frequency source have been ongoing over the past decade. Single surface resonant multipactor has been previously identified as one of the major limitations on the practical application of DLA structures in electron accelerators. In this paper, we report the results of an experiment that demonstrated suppression of multipactor growth in an X-band DLA structure through the use of an applied axial magnetic field. This represents an advance toward the practical use of DLA structures in many accelerator applications.
    Applied Physics Letters 01/2013; 103(21):213503-213503-5. · 3.79 Impact Factor
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    ABSTRACT: We present and discuss the results from the experimental generation of high-charge annular(ring-shaped)electron beams at the Argonne Wakefield Accelerator (AWA). These beams were produced by using laser masks to project annular laser profiles of various inner and outer diameters onto the photocathode of an RF gun. The ring beam is accelerated to 15 MeV, then it is imaged by means of solenoid lenses. Transverse profiles are compared for different solenoid settings. Discussion includes a comparison with Parmela simulations, some applications of high-charge ring beams,and an outline of a planned extension of this study.
    01/2013;
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    ABSTRACT: A recent paper suggested that an axial magnetic field near the cyclotron resonant value at the operating frequency can suppress multipactor loading in dielectric-loaded accelerating (DLA) structures. This occurs because the modification of electron trajectories caused by the magnetic field can reduce the electron impact energy at a given value of rf electric field, causing the electrons to strike the surface at energies below the first crossover of the secondary emission curve, and thus reducing the secondary electron yield below one. This effect should be enhanced at higher values of the rf electric field. We recently tested this prediction in a cylindrical alumina DLA structure using high-power 11.4 GHz radiation in the Magnicon Laboratory at the Naval Research Laboratory. The DLA structure had an inner diameter of 8.5 mm and an overall length of 14 cm, including tapered matching regions at either end, and employed a thick film silver exterior coating to create the external boundary condition for the propagating TM01 mode. The cyclotron resonant magnetic field was ~4 kG. The solenoidal magnet used in the experiment produced a magnetic field that peaked at the center of the DLA structure, and fell to half the central value at ±5 cm. The structure was conditioned at progressively higher powers up to ~9 MV/m, and the multipactor loading determined by measuring the incident, reflected, and transmitted microwave power at values of the applied magnetic field ranging from zero to the cyclotron resonant value. Without a magnetic field, multi-pactor increased at higher accelerating gradients, as seen in earlier experiments. Introducing a magnetic field first increased, and, at higher magnetic fields, decreased the multi-pactor loading, and this effect was most pronounced at higher gradients. However, multipactor loading was never fully suppressed, as predicted by Ref. 1. The persistence of multi-pactor in the structure may be caused by the lower magnetic field and- lower rf electric fields at the tapered ends of the structure. We plan to carry out further experiments to clarify this issue.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: We report on dark current and Schottky-enabled photoemission from a copper photocathode surface. Field-emitted dark current is a major gradient-limiting factor in RF cavities. Field emission is generally attributed to geometrical projections on the bulk surface whose field enhancement factor ({beta}) and the emitting area (A{sub e}) can be extracted from the Fowler-Nordheim (FN) plot. Measurements were made at Tsinghua S-band RF gun facility in two separate experiments. Using the traditional FN formula for RF fields we discovered that field enhancement factor ({beta}) alone cannot explain the full data set. Instead, we found that a low work function at some sites is required. In addition, surface analysis of the cathode after the experiment shows that geometric {beta} indicated would be < 10. Thus we conclude that low work function sites with a small {beta} are responsible for dark current emission and subsequent breakdown in high-gradient structures. The origin of these sites is unclear but could be due to defects or impurities.
    AIP Conference Proceedings. 12/2012; 1507(1).
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    ABSTRACT: Double triangular current profile (DT) gives a high transformer ratio which is the determining factor of the performance of collinear wakefield accelerator. This current profile can be generated using the emittance exchange (EEX) beam line. Argonne Wakefield Accelerator (AWA) facility plans to generate DT using the EEX beam line. We conducted start-to-end simulation for the AWA beam line using PARMELA code. Also, we discuss requirements of beam parameters for the generation of DT.
    AIP Conference Proceedings. 12/2012; 1507(1).
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    ABSTRACT: We report on investigations into the fundamental surface emission parameters, the geometric field enhancement factor (β) and the work function (ϕ), by making both field emission and Schottky-enabled photoemission measurements. The measurements were performed on a copper surface in the Tsinghua University S-band RF gun in two separate experiments. Fitting our data to the models for each experiment indicate that the traditionally assumed high value of β(≈50-500) does not provide a plausible explanation of the data, but incorporating a low value of ϕ at some sites does. In addition, direct measurements of the surface conducted after the experiment show that β is on the order of a few, consistent with our understanding of the electron emission measurements. Thus we conclude that the dominant source of electron emission in high gradient RF cavities is due to low ϕ sites, as opposed to the conventionally assumed high β sites. The origin of low ϕ at these sites is unclear and should be the subject of further investigation.
    Physical Review Letters 11/2012; 109(20):204802. · 7.94 Impact Factor
  • W. GAI, J. G. POWER, C. JING
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    ABSTRACT: We are exploring a new parameter space of the two-beam acceleration (TBA) scheme based on an ultra-short (~20 ns) rf pulse in a dielectric TBA. All two-beam accelerators (TBAs) use an electron drive beam to generate high-power rf in a decelerator and extract this power to drive an accelerating structure to high gradient. Typically, the rf pulse is on the order of hundreds of ns or greater in order to maintain good rf-to-beam efficiency. However, recent scaling arguments show that the rf breakdown threshold improves with decreasing rf pulse length, so it desirable to find a way to run at short-pulse length with good efficiency. In this paper, we discuss how we chose the design parameters of a short-pulse TBA for a TeV linear collider module. We then present plans for an experimental program to demonstrate TBA at Argonne wakefield accelerator (AWA) facility including high-power rf generation, high-gradient acceleration, and staging.
    Journal of Plasma Physics 08/2012; · 0.76 Impact Factor
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    ABSTRACT: We report the observation of a strong wakefield induced energy modulation in an energy-chirped electron bunch passing through a dielectric-lined waveguide. This modulation can be effectively converted into a spatial modulation forming microbunches with a periodicity of 0.5-1 ps and, hence, capable of driving coherent terahertz radiation. The experimental results agree well with theoretical predictions.
    Physical Review Letters 04/2012; 108(14):144801. · 7.94 Impact Factor
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    ABSTRACT: Experimental results are reported for test beam acceleration and deflection in a two-channel, cm-scale, rectangular dielectric-lined wakefield accelerator structure energized by a 14-MeV drive beam. The dominant waveguide mode of the structure is at ∼30 GHz, and the structure is configured to exhibit a high transformer ratio (∼12∶1). Accelerated bunches in the narrow secondary channel of the structure are continuously energized via Cherenkov radiation that is emitted by a drive bunch moving in the wider primary channel. Observed energy gains and losses, transverse deflections, and changes in the test bunch charge distribution compare favorably with predictions of theory.
    Physical Review Special Topics - Accelerators and Beams 03/2012; 15(3). · 1.57 Impact Factor
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    ABSTRACT: We have directly measured THz wakefields induced by a subpicosecond, intense relativistic electron bunch in a diamond loaded accelerating structure via the wakefield acceleration method. We present here the beam test results from the first diamond based structure. Diamond has been chosen for its high breakdown threshold and unique thermoconductive properties. Fields produced by a leading (drive) beam were used to accelerate a trailing (witness) electron bunch which followed the drive bunch at a variable distance. The energy gain of a witness bunch as a function of its separation from the drive bunch describes the time structure of the generated wakefield.
    Applied Physics Letters 01/2012; 100(13). · 3.79 Impact Factor

Publication Stats

371 Citations
341.75 Total Impact Points

Institutions

  • 2007–2013
    • Euclid TechLabs
      Maryland, United States
    • University of Chicago
      Chicago, Illinois, United States
  • 1990–2013
    • Argonne National Laboratory
      • Division of High Energy Physics
      Lemont, Illinois, United States
  • 2005–2009
    • Illinois Institute of Technology
      • Department of Biological and Chemical Sciences
      Chicago, Illinois, United States
    • Fermi National Accelerator Laboratory (Fermilab)
      Batavia, Illinois, United States
  • 1995
    • University of California, Los Angeles
      • Department of Physics and Astronomy
      Los Angeles, CA, United States
  • 1991
    • Stanford University
      • Stanford Linear Accelerator Center
      Palo Alto, CA, United States