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**15 papers**

title: | Observation of redshifting and harmonic radiation in inverse Compton scattering |

format: | journal article |

publisher: | |

year: | 2015 |

21 authors: | | | | | | | | | | | | | | | | | | | | | |

abstract: |
Inverse Compton scattering of laser photons by ultrarelativistic electron beam provides polarized x- to γ-ray pulses due to the Doppler blueshifting. Nonlinear electrodynamics in the relativistically intense linearly polarized laser field changes the radiation kinetics established during the Compton interaction. These are due to the induced figure-8 motion, which introduces an overall redshift in the radiation spectrum, with the concomitant emission of higher order harmonics. To experimentally analyze the strong field physics associated with the nonlinear electron-laser interaction, clear modifications to the angular and wavelength distributions of x rays are observed. The relativistic photon wave field is provided by the ps CO2 laser of peak normalized vector potential of 0.5 |

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title: | Beyond Injection: Trojan Horse Underdense Photocathode Plasma Wakefield Acceleration |

format: | conference proceeding |

conference: | AAC 2012 15th |

year: | 2012 |

18 authors: | | | | | | | | | | | | | | | | | | |

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. |

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title: | Experimental observation of helical microbunching of a relativistic electron beam |

format: | journal article |

publisher: | Applied Physics Letters |

year: | 2012 |

8 authors: | | | | | | | | |

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. |

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title: | Resonant excitation of coherent Cerenkov radiation in dielectric lined waveguides |

format: | journal article |

publisher: | |

year: | 2011 |

12 authors: | | | | | | | | | | | | |

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. |

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title: | Harmonic radiation of a relativistic nonlinear inverse Compton scattering using two laser wavelengths |

format: | journal article |

publisher: | |

year: | 2011 |

9 authors: | | | | | | | | | |

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 γ-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 aL∼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 aL laser resulting in the generation of the harmonics with the photon energy of 4γ2h̵(ωL,short+nωL,long). 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. |

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title: | Dielectric Wakefield Acceleration of a Relativistic Electron Beam in a Slab-Symmetric Dielectric Lined Waveguide |

format: | journal article |

publisher: | Physical Review Letters |

year: | 15 June 2012 |

13 authors: | | | | | | | | | | | | | |

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 SiO2 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 reconstruction 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 (TM02). 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. |

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title: | Efficient harmonic microbunching in a 7th-order inverse-free-electron laser interaction |

format: | journal article |

publisher: | Physical Review Special Topics- Accelerators and Beams |

year: | 2009 |

8 authors: | | | | | | | | |

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. Threedimensional 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. |

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title: | Observation of Narrow-Band Terahertz Coherent Cherenkov Radiation from a Cylindrical Dielectric-Lined Waveguide |

format: | journal article |

publisher: | |

year: | 2009 |

6 authors: | | | | | | |

abstract: | We report experimental observation of narrow-band coherent Cherenkov radiation driven by a subpico- second 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 resent detailed measurements showing a narrow emission spectrum peaked at 367 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 |

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title: | Inverse Compton Scattering and Characterization of the Source at the Brookhaven National Laboratory Accelerator Test Facility |

format: | thesis |

year: | |

1 author: | |

abstract: | An inverse Compton scattering source at the Accelerator Test Facility has been thoroughly simulated and characterized in preparation for future applications of this unique source of X - ray pho t ons. Simulations were performed using a 3D time and frequency - domain code for various beam parameters achievable at ATF. Calculations show a total flux of 1x10 9 photons per shot is obtainable with photon energies ranging from 5 to 9 keV over the full divergence angle of 8 mrad. The spectral bandwidth is dominated by electron beam angles and energy spread. In experiment, iron and nickel foils were used as an ene rgy diagnostic due to their ix strongly absorbing K - edges near the design central photon energy. Measurements indicate 2x10 6 photons within a 1 mrad emission angle and 4.0±0.8% bandwidth. Simulation is relied upon for information on source size and pulse le ngth and predicts 22 μm and 0.3 - 4 ps, respectively, resulting in a peak brightness of 2.4x10 19 photons/mm 2 /mrad 2 (0.1% bandwidth). A circularly polarized laser resulted in a source energy and angular distribution indicative of circularly polarized x - rays. |

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title: | Beam-Driven Dielectric Wakefield Accelerating Structure as a THz Radiation Source |

format: | preprint |

year: | 2007 |

9 authors: | | | | | | | | | |

abstract: | Experimental work is planned to study the performance of a beam-driven cylindrical dielectric wakefield accelerating structure as a source of THz coherent Cerenkov radiation (CCR). For an appropriate choice of dielectric tube geometry and driving electron bunch parameters, the device operates in a single-mode regime, producing radiation in the THz range. This source can potentially produce high power levels relative to currently available sources, with ~50 uJ radiated energy per pulse achievable using the electron beam currently in operation at the Neptune advanced accelerator laboratory at UCLA (~13 MeV beam energy, ~200 um RMS bunch length, ~500 pC bunch charge). Preparations underway for installation of the experiment are discussed. |

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title: | Beam-driven Dielectric Wakefield Accelerating Structure as a Thz Radiation Source |

format: | conference proceeding |

conference: | PAC 2007 |

year: | 2007 |

9 authors: | | | | | | | | | |

abstract: | Experimental work is planned to study the performance of a beam-driven cylindrical dielectric wakefield accelerat- ing structure as a source of THz coherent Cerenkov radi- ation (CCR). For an appropriate choice of dielectric tube geometry and driving electron bunch parameters, the de- vice operates in a single-mode regime, producing radiation in the THz range. This source can potentially produce high power levels relative to currently available sources, with ?50 ?J radiated energy per pulse achievable using the elec- tron beam currently in operation at the Neptune advanced accelerator laboratory at UCLA ( ?13 MeV beam energy, ?200 ?m RMS bunch length, ?500 pC bunch charge). Preparations underway for installation of the experiment are discussed. |

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title: | Limits on production of narrow band photons from inverse Compton scattering |

format: | conference proceeding |

conference: | AAC 2006 12th |

year: | 2006 |

2 authors: | | |

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 |

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title: | Dielectric Wakefield Accelerating Structure as a Source of Terahertz Coherent Cerenkov Radiation |

format: | conference proceeding |

conference: | AAC 2006 12th |

year: | 2006 |

8 authors: | | | | | | | | |

abstract: | We discuss future experimental work proposed to study the performance of a cylindrical dielectric wakefield accelerating structure as a coherent Cerenkov radiation source at the Neptune laboratory at UCLA. The Cerenkov wakefield acceleration experiment carried out recently by UCLA/SLAC/USC, using the ultrashort and high charge beam (Q = 3 nC, RMS bunch length = 20 micron) at the SLAC FFTB, demonstrated electromagnetic wakes at the few GV/m level. The motivation of our prospective experiment is to investigate the operation of a similar scenario using the comparatively long pulse, low charge beam (Q = 0.5 nC, RMS bunch length = 200 micron) at UCLA Neptune. The field amplitude produced in this setup would be one to two orders of magnitude lower, at the few tens to few 100 MV/m level. Such a decelerating field would extract a significant amount of energy from a low-energy beam in a distance on the order of a few centimeters, allowing the use of short dielectric structures. We discuss details of the geometry and composition of the structures to be used in the experiment. We also examine the possibility of a future dedicated facility at UCLA Neptune based on a hybrid photoinjector currently in development. The intrinsic bunch compression capabilities and improved beam parameters (RMS bunch length = 100 micron, Q = 1 nC) of the photoinjector would allow the creation of a high power radiation source in the terahertz regime. |

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title: | Investigation of X-Ray Harmonics in the Polarized Nonlinear Inverse Compton Scattering Experiment at UCLA |

format: | conference proceeding |

conference: | FEL 2005 27th |

year: | 2005 |

8 authors: | | | | | | | | |

abstract: | An Inverse Compton Scattering (ICS) experiment, which will investigate nonlinear properties of scattering utilizing a terawatt CO2 laser system with various polarizations, is ongoing at the UCLA Neptune Laboratory. When the normalized amplitude of the incident laser's vector potential ao is larger than unity the scattering occurs in the nonlinear region; therefore, higher harmonics are also produced. ICS can be used, e.g., for a polarized positron source by striking a thin target (such as tungsten) with the polarized X-rays. As such, it is critical to demonstrate the production of polarized scattered photons and to investigate the ICS process as it enters the nonlinear regime. We present the description of the experimental set up and equipment utilized, including diagnostics for electron and photon beam detection. We present the current status of the experiment. electron beam focusing and recollimating systems consist of 5 equal strength and length PMQs each, set up as a modified FF-DD-F triplet. The high power CO2 laser is focused by an off-axis parabolic mirror with a 12.7 cm diameter and 17 cm focal length. Since the divergence of the scattered photons is quite large (-I/, where 28) it is necessary to place the detector (soft X-ray camera and/or micro-channel plate) very close to the IP. This requires bending the electron beam immediately after the recollimating PMQ system, in a very short bending radius. To this end, we have designed and built a permanent magnet dipole (PMD) with a -60 mm bending radius for this purpose. Table 1: Electron and Laser Beam Parameters |

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title: | Study of X-ray Harmonics of the Polarized Inverse Compton Scattering Experiment at UCLA |

format: | journal article |

publisher: | AIP Conference Proceedings |

year: | 2004 |

8 authors: | | | | | | | | |

abstract: | We propose an Inverse Compton Scattering (ICS) experiment, which will investigate nonlinear properties of the scattering utilizing the terawatt CO2 laser system with various polarizations in Neptune Laboratory at UCLA. When the normalized amplitude of the vector potential a0 is larger than unity the scattering occurs in the nonlinear region; therefore, higher harmonics are also produced. We present a calculation tool for the Double Differential Spectrum (DDS) distribution and total number of photons produced for both head-on and 90 scattering. We decided to do the experiment at 90 to avoid complications due to strong diffraction of the incoming laser. We discuss the electron and laser beam parameters for the experiment. 2004 American Institute of Physics |

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