| title: | Faraday Cup Beam Dumps for the UCLA PBPL |
| format: | tech note |
| year: | May 16, 1994 |
| 1 author: | |
| abstract: | Two identical Faraday cups have been designed and built to stop a 20 meV, 1 nC electron pulse beam, accelerated at the UCLA Particle Beam Physics Laboratory (PBPL). The dumps were also designed to serve as charge measuring diagnostics. Each device is a simple design using a carbon graphite core, Delrin insulation, vacuum tubing components, and lead brick shielding. The graphite core stops the beam while producing manageable secondary radiation sources. The overall design considerations and final solutions are summarized. |
| keywords: | pbpl Diagnostics |
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| title: | Mitigation of RF Gun Breakdown by Removal of Tuning Rods in High Field Regions |
| format: | conference procceeding |
| conference: | 2007 Particle Accelerator Conference |
| year: | |
| 5 authors: | | | | | |
| abstract: | The pi-mode resonant frequency of the 1.6 cell SLAC/BNL/UCLA style RF photoinjector electron gun is conventionally tuned using cylindrical copper tuning pieces that extend into the full-cell cavity through holes in the side of the gun. This design begins to fail in many versions of this popular gun design at higher voltage levels, when the cavity undergoes electric breakdown in the vicinity of the tuners. In order to remove the tuners from the region of high electric field, mitigating this problem, one must change the full cell geometry significantly. We have investigated a method for accomplishing this, in which we stretch the gun structure to tune the resonant frequency up by over 2 MHz. We constructed a device to perform this stretching and tested the modified photoinjector in an RF test bed. We succeeded in putting approximately 8.4 MW of RF power into the gun, an improvement over the 4 MW routinely achieved with a similar gun using conventional tuning methods installed at the UCLA Neptune laboratory. Recent results in testing this gun with a magnesium cathode insert are reported as well. |
| keywords: | pbpl |
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| title: | QUINDI- A Code to Simulate Coherent Emission from Bending Systems |
| format: | conference procceeding |
| conference: | 2007 Particle Accelerator Conference |
| year: | 2007 |
| 3 authors: | | | |
| abstract: | We present a newly developed code, QUINDI, to address the numerical challenge of calculating the radiation spectra from electron bunches in bending magnet systems. This code provides a better tool for designing diagnostic systems such as bunch length monitors in magnetic chicanes. The program calculates emission on a first principle basis, combining the dominant emission processes in a bending magnet system -- edge and synchrotron radiation. The core algorithm is based on the Lienard-Wiechert potential and utilizes parallel computer architecture to cover complete electron beam distributions with a high resolution spatial grid. The program models the coherence level of the emitted radiation from the electron bunch, focusing on long frequency components. |
| keywords: | software pac quindi pbpl |
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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. |
| keywords: | pbpl |
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| title: | MITIGATION OF ELECTRIC BREAKDOWN IN AN RF PHOTOINJECTOR BY REMOVAL OF TUNING RODS IN HIGH-FIELD REGIONS |
| format: | preprint |
| year: | 2007 |
| 5 authors: | | | | | |
| abstract: | The pi-mode resonant frequency of the 1.6 cell SLAC/BNL/UCLA style RF photoinjector electron gun is conventionally tuned using cylindrical copper tuning pieces that extend into the full-cell cavity through holes in the side of the gun. This design begins to fail in many versions of this popular gun design at higher voltage levels, when the cavity undergoes electric breakdown in the vicinity of the tuners. In order to remove the tuners from the region of high electric field, mitigating this problem, the full cell geometry must be changed significantly. We report on a method of accomplishing this, in which we use a mechanical device of custom design to stretch the cavity structure of an existing photoinjector in order to tune the resonant frequency up by over 2 MHz. We present results of testing the modified photoinjector in an RF test bed with both copper and magnesium cathodes, succeeding in putting approximately 8 - 10 MW of RF power into the gun. This is an improvement over the 4 MW routinely achieved in a similar gun using conventional tuning methods installed at the UCLA Neptune laboratory. |
| keywords: | pbpl |
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| title: | Strong Focusing for Planar Undulators |
| format: | conference procceeding |
| conference: | 3rd Advanced Accelerator Concepts Workshop |
| year: | |
| 2 authors: | | |
| abstract: | The range of parameters in which the free electron laser (FEL) is used has greatly expanded in recent years. The present trend towards short wavelength operation with long undulators places tight requirements on the electron beam quality. In particular, the need to maintain a well focused beam is critical to successful operation of such an FEL. This paper examines the use of alternating gradient (AG) sextupole focusing in planar undulators. After a brief review of various undulator focusing schemes, the equations of motion for an electron in an undulator field with a strong sextupole component are examined. It is shown that the mean electron longitudinal velocity can be kept constant through each fosusing and defocusing section. Thus, the betatron velocity modulation associated with transverse focusing can be limited. Analytic as well as smooth approximation solutions are provided for AG sextupole focusing. Examples using the proposed SLAC 4 nm FEL as well as th UCLA 10.6 µm FEL parameters are also discussed. |
| keywords: | pbpl |
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| title: | A Coherent Compton Backscattering High Gain FEL Using an X-Band Microwave Undulator |
| format: | conference procceeding |
| conference: | Proceedings of the 27th International Free Electron Laser Conference |
| year: | 2005 |
| 6 authors: | | | | | | |
| abstract: | High power microwave sources at X-Band, delivering 400 to 500 of megawatts for about 400 ns, have been recently developed. These sources can power a microwave undulator with short period and large gap, and can be used in short wavelength FELs reaching the nm region at a beam energy of about 1 GeV. We present here an experiment designed to demonstrate that microwave undulators have the field quality needed for high gain FELs. compensate RF power losses, it might be convenient to have an open waveguide geometry, as will be discussed later in the paper. However the initial FEL calculations are based on a circular cross-section waveguide. The undulator is powered by two transverse electric modes, shifted in the time phase by |
| keywords: | pbpl |
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| title: | Investigation of X-Ray Harmonics in the Polarized Nonlinear Inverse Compton Scattering Experiment at UCLA |
| format: | conference procceeding |
| conference: | Proceedings of the 27th International Free Electron Laser Conference |
| 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 |
| keywords: | pbpl |
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| title: | Simulations, Diagnostics and Recent Results of the VISA II Experiment |
| format: | conference procceeding |
| conference: | Proceedings of the 27th International Free Electron Laser Conference |
| year: | August 2005 |
| 17 authors: | | | | | | | | | | | | | | | | | |
| abstract: | The VISA II experiment entails the use of a chirped electron beam to drive a high gain SASE FEL. Sextupoles are implemented to correct the longitudinal aberrations affecting the high energy spread chirped beam during transport to the undulator. The output radiation is diagnosed with a modified frequency resolved optical gating (FROG) technique. The double differential energy spectrum is measured with a pair of slits and a set of gratings. In this paper, we report on start-to-end simulations, radiation diagnostics, as well as initial experimental results. Technical considerations for future experimental methods are also addressed. |
| keywords: | pbpl |
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| title: | Ultra-High Density Electron Beams for Beam Radiation and Beam Plasma Interaction |
| format: | conference procceeding |
| conference: | 2005 Particle Accelerator Conference |
| year: | |
| 11 authors: | | | | | | | | | | | |
| abstract: | Current and future applications of high brightness electron beams, which include advanced accelerators such as the plasma wake-field accelerator (PWFA) and beam radiation interactions such as inverse-Compton scattering (ICS), require both transverse and longitudinal beam sizes on the order of tens of microns. Ultra-high density beams may be produced at moderate energy (50 MeV) by compression and subsequent strong focusing of low emittance, photoinjector sources. We describe the implementation of this method used at the PLEIADES ICS x-ray source in which the photoinjector-generated beam has been compressed to 300 fsec duration using the velocity bunching technique and focused to 20 um rms size using an extremely high gradient, permanent magnet quadrupole (PMQ) focusing system. |
| keywords: | pbpl |
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| title: | Experiments on laser driven beatwave acceleration in a ponderomotively formed plasma channel |
| format: | journal article |
| year: | 2004 |
| 10 authors: | | | | | | | | | | |
| abstract: | A 10 ps long beam of 12 MeV electrons is externally injected into a ~3-cm long plasma beatwave excited in a laser ionized hydrogen gas. The electrons have been accelerated to 50 MeV with a gradient of ~1.3 GeV/m. It is shown that when the effective plasma wave amplitude-length product is limited by ionization-induced defocusing (IID), acceleration of electrons is significantly enhanced by using a laser pulse with a duration longer than the time required for ions to move across the laser spot size. Both experiments and two-dimensional simulations reveal that, in this case, self-guiding of the laser pulse in a ponderomotively formed plasma channel occurs. This compensates for IID and drives the beatwave over the longer length compared to when such a channel is not present. |
| keywords: | pbpl |
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| title: | Very High Energy Gain at the Neptune Inverse Free Electron Laser Experiment |
| format: | journal article |
| year: | Dec 2004 |
| 16 authors: | | | | | | | | | | | | | | | | |
| abstract: | We report the observation of energy gain in excess of 20 MeV at the Inverse Free Electron Laser Accelerator experiment at the Neptune Laboratory at UCLA. A 14.5 MeV electron beam is injected in an undulator strongly tapered in period and field amplitude. The IFEL driver is a CO2 10.6 µm laser with power larger than 400 GW. The Rayleigh range of the laser, ~ 1.8 cm, is much shorter than the undulator length so that the interaction is diffraction dominated. A few per cent of the injected particles are trapped in a stable accelerating bucket. Electrons with energies up to 35 MeV are measured by a magnetic spectrometer. Three-dimensional simulations, in good agreement with the measured electron energy spectrum, indicate that most of the acceleration occurs in the first 25 cm of the undulator, corresponding to an energy gradient larger than 70 MeV/m. The measured energy spectrum also indicates that higher harmonic Inverse Free Electron Laser interaction takes place in the second section of the undulator. |
| keywords: | pbpl |
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| title: | Conceptual Design for a 1-GeV IFEL Accelerator |
| format: | journal article |
| year: | Dec 2004 |
| 5 authors: | | | | | |
| abstract: | A conceptual design for a multistaged 1-GeV IFEL laser-driven accelerator (laser linac) was developed using the Staged Electron Laser Acceleration (STELLA) inverse free electron laser (IFEL) model created at STI Optronics. A comparison with the UCLA TREDI model yields good agreement with the STELLA model. The 1-GeV IFEL laser linac consists of an IFEL buncher for forming microbunches and four IFEL acceleration stages. Electrons enter the laser linac from a conventional microwave-driven linac (51 MeV). The acceleration stages are driven by 10-TW laser beams at 1.06-µm. It is found good trapping occurs as the electrons are accelerated; however, refocusing of the e-beam between acceleration stages is needed to control detrapping effects. The energy spread of the trapped electrons is also small. This design exercise was in support of the task placed upon the EM Structure-Based Accelerators Working Group at the 2004 Advanced Accelerator Concepts Workshop. It demonstrates that a 1-GeV IFEL laser linac is feasible with present technology. ©2004 American Institute of Physics |
| keywords: | pbpl |
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| title: | UCLA Neptune Ramped Electron Bunch Experiment |
| format: | journal article |
| year: | 2004 |
| 5 authors: | | | | | |
| abstract: | A ramped electron bunch (i.e. one having a current density which rises linearly from the head to the tail and then drops sharply to zero) has been predicted to be an ideal drive beam for the plasma wake field accelerator due to the large transformer ratio it is capable of generating. A scheme was recently proposed for the creation of a relativistic electron bunch that approximates a ramped current profile [England, et al., AIP Conf. Proc. 647, p.884 (2002)], using a dogleg or dispersionless translating section as a bunch compressor. An experiment is underway at the Neptune laboratory using this scheme to create such a beam. The diagnostic being developed for measuring the temporal profile of the beam is an X-Band transverse deflecting mode cavity. ©2004 American Institute of Physics |
| keywords: | pbpl |
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| title: | Study of X-ray Harmonics of the Polarized Inverse Compton Scattering Experiment at UCLA |
| format: | journal article |
| 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 |
| keywords: | pbpl |
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| title: | Study of a THz IFEL prebuncher for laser-plasma accelerators |
| format: | journal article |
| year: | 2004 |
| 7 authors: | | | | | | | |
| abstract: | For monoenergetic acceleration of electrons, the injected particles need to be bunched with the same periodicity as the accelerating structure. In a laser-driven plasma beatwave accelerator, the accelerating structure (plasma wave) is phase-locked to the CO2 beat-wave used to drive it. Using the same beat-wave to generate high power FIR radiation via difference frequency mixing in GaAs ensures that the radiation has the same phase relationship as the plasma wave before it saturates and detunes from the pump. Therefore, this radiation can be used to prebunch an existing electron beam based on an Inverse Free Electron Laser (IFEL) concept. Here we report the progress on the proposed THz microbunching experiment in the Neptune laboratory. A 50 cm long prebuncher is optimized using simulation codes for minimum FIR power required. The injected 5ps long electron beam is expected to form a series of 45 µm long microbunches containing over 40% of the injected current after 1.6 m drift space following the undulator. Preliminary experimental results on THz generation are also presented. |
| keywords: | pbpl |
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| title: | Acceleration of electrons in a diffraction-dominated IFEL |
| format: | journal article |
| year: | 2004 |
| 16 authors: | | | | | | | | | | | | | | | | |
| abstract: | We report on the observation of energy gain in excess of 20 MeV at the Inverse Free Electron Laser Accelerator ex- periment at the Neptune Laboratory at UCLA. A 14.5 MeV electron beam is injected in a 50 cm long undulator strongly tapered both in period and field amplitude. A CO2 10.6 μm laser with power > 300 GW is used as the IFEL driver. The Rayleigh range of the laser ( ∼ 1.8 cm) is shorter than the undulator length so that the interaction is diffraction dom- inated. Few per cent of the injected particles are trapped in stable accelerating buckets and electrons with energies up to 35 MeV are detected on the magnetic spectrometer. Three dimensional simulations are in good agreement with the electron energy spectrums observed in the experiment and indicate that substantial energy exchange between laser and electron beam only occurs in the first 25-30 cm of the undulator. An energy gradient of > 70 MeV is inferred. In the second section of the undulator higher harmonic IFEL interaction is observed. |
| keywords: | pbpl |
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| title: | DIELECTRIC WAKEFIELD ACCELERATOR EXPERIMENTS AT THE |
| format: | journal article |
| year: | |
| 17 authors: | | | | | | | | | | | | | | | | | |
| abstract: | Electron bunches with the unparalleled combination of high charge, low emittances, and short time duration, as first produced at the SLAC Final Focus Test Beam (FFTB), are foreseen to be produced at the SABER facility. These types of bunches have enabled wakefield driven accelerating schemes of multi-GV/m in plasmas. In the context of the Dielectric Wakefield Accelerators (DWA) such beams, having rms bunch length as short as 20 um, have been used to drive 100 um and 200 um ID hollow tubes above 20 GV/m surface fields. These FFTB tests enabled the measurement of a breakdown threshold in fused silica (with full data analysis still ongoing) [1]. With the construction and commissioning of the SABER facility at SLAC, new experiments would be made possible to test further aspects of DWAs including materials, tube geometrical variations, direct measurements of the Cerenkov fields, and proof of acceleration in tubes >10 cm in length. This collaboration will investigate breakdown thresholds and accelerating fields in new materials including CVD diamond. Here we describe the experimental plans, beam parameters, simulations, and progress to date as well as future prospects for machines based of DWA structures. |
| keywords: | pbpl |
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| title: | Chicane Radiation Measurements with a Compressed Electron Beam at the BNL ATF |
| format: | journal article |
| year: | 2007 |
| 12 authors: | | | | | | | | | | | | |
| abstract: | |
| keywords: | pbpl |
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| title: | Status of the SPARC-X Project |
| format: | journal article |
| year: | 2007 |
| 97 authors: | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| abstract: | SPARC-X is a two branch project consisting in the SPARC test facility dedicated to the development and test of critical subsystems such as high brightness photoinjector and a modular expandable undulator for SASE-FEL experiments at 500 nm with seeding, and the SPARX facility aiming at generation of high brilliance coherent radiation in the 1.5-13 nm range, based on the achieved expertise. The projects are supported by MIUR (Research Department of Italian Government) and Regione Lazio. SPARC has completed the commissioning phase of the photoinjector in November 2006. The achieved experimental results are here summarized together with the status of the second phase commissioning plans. The SPARX project is based on the generation of ultra high peak brightness electron beams at the energy of 1 and 2 GeV generating radiation in the 1.5-13 nm range. The construction is at the moment planned in two steps starting with a 1 GeV Linac. The project layout including both RF-compression and magnetic chicane techniques has been studied. |
| keywords: | pbpl |
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| title: | The New Photoinjector for the Fermi Project |
| format: | journal article |
| year: | 2007 |
| 14 authors: | | | | | | | | | | | | | | |
| abstract: | FERMI@elettra is a single-pass FEL user facility covering the spectral range 100-10 nm. It will be located near the Italian third generation Synchrotron Light Source facility ELETTRA and will make use of the existing 1.0 GeV normal conducting Linac [1]. To obtain the high beam brightness required by the project, the present Linac electron source will be substituted with a photocathode RF gun now under development in the framework of a collaboration between Sincrotrone Trieste (ST) and Particle Beam Physics Laboratory (PBPL) at UCLA. The new gun will use an improved design of the 1.6 cell accelerating structure already developed at PBPL, scaled to 2998 MHz. We expect that the new gun design will allow a beam brightness increase by a factor 3-4 over the older version of the device. Some technical choices of the new design, including the enhancement of the mode separation, removal of the RF tuners, full cell symmetrization to limit the dipole and quadrupole RF field as well as an improved solenoid yoke design for multipole field corrections, will be discussed. |
| keywords: | pbpl |
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| title: | THE UCLA HELICAL PERMANENT-MAGNET INVERSE FREE |
| format: | journal article |
| year: | 2007 |
| 7 authors: | | | | | | | |
| abstract: | The Inverse Free Electron Laser (IFEL) is capable, in principle, of reaching accelerating gradients of up to 1 GV/m making it a prospective accelerator scheme for linear colliders. The Neptune IFEL at UCLA utilizes a 15 MeV Photoinjector-generated electron beam of 0.5 nC and a CO2 laser with peak energy of up to 100 J, and will be able to accelerate electrons to 100 MeV over an 80 cm long, novel helical permanent-magnet undulator. Past IFELs have been limited in their average accelerating gradient due to the Gouy phase shift caused by tight focusing of the drive laser. Here, laser guiding is implemented via an innovative Open Iris-Loaded Waveguide Structure (OILS) scheme which ensures that the laser mode size and wave front are conserved through the undulator. The results of the first phase of the experiment are discussed in this paper, including the design and construction of a short micro-bunching undulator, testing of the OILS waveresults of corresponding simulations.waveguide to test the coupling by observing a micro- bunching. Then, as a second step, we will build a long waveguide and a tapered undulator (80 cm). |
| keywords: | pbpl |
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| title: | OPTIMUM ELECTRON BUNCH CREATION IN A PHOTOINJECTOR |
| format: | journal article |
| year: | 2007 |
| 16 authors: | | | | | | | | | | | | | | | | |
| abstract: | Recent studies have shown that by illuminating a pho- tocathode with an ultra-short laser pulse of appropriate transverse profile, a uniform density, ellipsoidally shaped electron bunch can be dynamically formed. Linear space- charge fields then exist in all dimensions inside of the bunch, which minimizes emittance growth. Here we study this process, and its marriage to the standard emittance compensation scenario that is implemented in most modern photoinjectors. We show that the two processes are com- patible, with simulations indicating that a very high bright- ness beam can be obtained. An initial time-resolved experi- ment has been performed at the SPARC injector in Frascati, involving Cerenkov radiation produced at an aerogel. We discuss the results of this preliminary experiment, as well as plans for future experiments at the UCLA Pegasus labo- ratory to resolve the ellipsoidal bunch shape at low energy. Future measurements at high energy based on fs resolution RF sweepers are discussed. |
| keywords: | pbpl |
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| title: | Observation of Multi-GeV Breakdown Thresholds in Dielectric Wakefield Structures |
| format: | journal article |
| year: | 2007 |
| 12 authors: | | | | | | | | | | | | |
| abstract: | An experiment designed to test the breakdown threshold of a dielectric subjected to the GV/m-scale electric-fields of an intense electron-beam has been completed. In this experiment at the Final Focus Test Beam (FFTB) facility, the 30 GeV SLAC electron beam was focused down and propagated through short fused-silica capillary-tubes with internal diameters of as little as 100 μm. The electric field at the inner surface of the tubes was varied from about 1 GV/m to 22 GV/m by adjusting the longitudinal compres- sion of the electron bunch. We observed a sharp increase in optical emissions from the capillaries in the middle part of this surface field range which we believe indicates the transition between sustainable field levels and breakdown. If this initial interpretation is correct, the multi-GV/m sur- faced fields that were sustained equate to on axis accelerat- ing field of several GV/m. |
| keywords: | pbpl |
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| title: | OBSERVATIONS OF UNDERDENSE PLASMA LENS FOCUSING OF |
| format: | journal article |
| year: | 2007 |
| 10 authors: | | | | | | | | | | |
| abstract: | Focusing of a 15 MeV, 19 nC electron bunch by an un- derdense plasma lens operated just beyond the threshold of the underdense condition has been demonstrated in ex- periments at the Fermilab NICADD Photoinjector Labora- tory (FNPL). The strong 1.9 cm focal-length plasma-lens focused both transverse directions simultaneously and re- duced the minimum area of the beam spot by a factor of 23. Analysis of the beam-envelope evolution observed near the beam waist shows that the spherical aberrations of this un- derdense lens are lower than those of an overdense plasma lens, as predicted by theory. Correlations between the beam charge and the properties of the beam focus corroborate this conclusion. |
| keywords: | pbpl |
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| title: | Characterization of Orbital Angular Momentum Modes in FEL Radiation |
| format: | journal article |
| year: | 2007 |
| 5 authors: | | | | | |
| abstract: | It is well known that Gaussian modes of paraxial light can possess orbital angular momentum (OAM) as a result of a exp( −ilφ) azimuthal angular dependence[1]. Recent experiments on the VISA free-electron laser operating as self-amplified spontaneous amplifier (SASE) at λ=840 nm have produced transverse electromagnetic radiation inten- sity patterns that suggest the presence of one or more OAM modes. Though the intensity profiles from VISA are com- pelling, work remains to discern the phase structure since OAM modes are revealed by a characteristic helical phase dependence, with a pathological singularity on axis. Here we discuss current efforts to characterize the mode content of the VISA FEL through experiments, theoretical mod- els, and simulations. Possible methods to generate OAM modes in situ are discussed. |
| keywords: | pbpl |
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| title: | The FINDER Photoinjector |
| format: | journal article |
| year: | 2007 |
| 6 authors: | | | | | | |
| abstract: | The FINDER photoinjector was developed for the inverse Compton scattering experiment under UCLA- LLNL collaboration. The improvements of this gun from SPARC gun [1, 2] at INFN-LNF (Frascati) and previous UCLA versions of the 1.6 cell S-band photoinjector are detailed here. The gun is designed to have large mode separation to suppress 0 mode excitation which may be a cause of the emittance degradation. In an effort to reduce the RF quadrupole effect the full cell tuners are replaced by vacuum ports. The laser ports are also omitted. Two openings of the solenoid shield are added to top and bottom of it where there were only two openings on the sides. S11 of the cavity was presented. The on-axis electric field was measured by the bead drop method to show the good field balance. The magnetic field in the emittance compensation solenoid was measured and the quadrupole components were derived. |
| keywords: | pbpl |
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| title: | Seeded VISA: A 1064 nm Laser-Seeded FEL Amplifier at the BNL ATF |
| format: | journal article |
| year: | 2007 |
| 7 authors: | | | | | | | |
| abstract: | An experimental study of a seeded Free Electron Laser (FEL) using the VISA undulator and a Nd:YAG seed laser will be performed at the Accelerator Test Facility (ATF) at Brookhaven National Laboratory. The study is motivated by the demand for a short Rayleigh length FEL amplifier at 1 micron to allow for high power transmission with min- imal damage of transport optics. Planned measurements include transverse and longitudinal coherence, angular dis- tribution, and wavelength spectrum of the FEL radiation. The effects of detuning the electron beam energy will be studied, with an emphasis on control of the radiation emis- sion angles and increase of the amplifier efficiency. Results of start-to-end simulations are also presented. |
| keywords: | pbpl |
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| title: | Commissioning of a High-Brightness Photoinjector for Compton Scattering X-Ray Sources |
| format: | journal article |
| year: | 2007 |
| 13 authors: | | | | | | | | | | | | | |
| abstract: | Compton scattering of intense laser pulses with ultra- relativistic electron beams has proven to be an attractive source of high-brightness x-rays with keV to MeV ener- gies. This type of x-ray source requires the electron beam brightness to be comparable with that used in x-ray free- electron lasers and laser and plasma based advanced accel- erators. We describe the development and commissioning of a 1.6 cell RF photoinjector for use in Compton scatter- ing experiments at LLNL. Injector development issues such as RF cavity design, beam dynamics simulations, emit- tance diagnostic development, results of sputtered magne- sium photo-cathode experiments, and UV laser pulse shap- ing are discussed. Initial operation of the photoinjector is described. |
| keywords: | pbpl |
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| title: | Coherent Cherenkov Radiation as a Temporal Diagnostic for Microbunched Beams |
| format: | journal article |
| year: | 2007 |
| 4 authors: | | | | |
| abstract: | Cherenkov radiation of a relativistic e-beam traversing a thin section of aerogel is analized, putting the stress on the coherent contribution due to the intra-beam, transverse and longitudinal structure. The use of this tool as a temporal diagnostic for micro-bunched beams makes possible to improve the amount of collected power at the microbunching frequency several orders of magnitude more respect to the uncoherent Cherenkov contribution. The non-idealities of a real beam are taken in account, and some techniques aimed on enhancing the coherent part of radiation are proposed and analized analitically. |
| keywords: | pbpl |
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| title: | Experimental Characterization of the Transverse Phase Space of a 60-MeV Electron Beam through a Compressor Chicane |
| format: | journal article |
| year: | 2007 |
| 8 authors: | | | | | | | | |
| abstract: | Space charge and coherent synchrotron radiation may deteriorate electron beam quality when the beam passes through a magnetic bunch compressor. This paper presents the transverse phase-space tomographic measurements for a compressed beam at 60MeV, around which energy the first stage of magnetic bunch compression takes place in most advanced linacs. Transverse phase-space bifurcation of a compressed beam is observed at that energy, but the degree of the space charge-induced bifurcation is appreciably lower than the one observed at 12MeV. |
| keywords: | pbpl |
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| title: | Optimum Beam Creation in Photoinjectors Using Space-Charge Expansion |
| format: | journal article |
| year: | 2006 |
| 4 authors: | | | | |
| abstract: | It has recently been shown that by illuminating a photo- cathode with an ultra-short laser pulse of appropriate trans- verse profile, a uniform density, ellipsoidally shaped bunch is dynamically formed, which then has linear space-charge fields in all dimensions inside of the bunch. We study here this process, and its marriage to the standard emittance compensation scenario that is implemented in most modern photoinjectors. It is seen that the two processes are compat- ible, with simulations indicating that a very high brightness beam can be obtained. The scheme has produced stimulus for a series of experiments at the SPARC injector at Fras- cati in 2006-2007. An initial time-resolved experiment has been performed involving Cerenkov radiation produced at an aerogel. We discuss the results of this preliminary ex- periment, as well as plans for future experiments to resolve the ellipsoidal bunch shape at low energy. Future measure- ments at high energy based on fs resolution RF sweepers are discussed. |
| keywords: | pbpl |
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| title: | Future FEL Studies at the VISA Experiment in the SASE and Seeded Modes |
| format: | journal article |
| year: | 2006 |
| 9 authors: | | | | | | | | | |
| abstract: | The VISA (Visible to Infrared SASE Amplifier) experiment at BNL (Brookhaven National Laboratory) has previously demonstrated saturation at 840 nm in 2001. Further SASE studies, in 2003, have demonstrated an anomalously large bandwidth spread of the FEL spectrum due to off-angle emissions. This paper disseminates the current and future program of the VISA program at BNL. This includes a study of a seeded FEL, using a 1 micron YAG laser as a seed, and the accompanying diagnostics to characterize the radiation. Diagnostics include the double differential spectrometer, a mode converter to investigate the orbital angular momentum of light in the FEL, and an optical pepper-pot for coherence measurements. As usual, start-to-end simulations are presented. |
| keywords: | pbpl |
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| title: | Nuclear Photo-Science And Applications with Thomson-Radiated Extreme X-Ray (T-Rex) Sources |
| format: | journal article |
| year: | 2006 |
| 15 authors: | | | | | | | | | | | | | | | |
| abstract: | Recent advances in high brightness rf gun technology, coupled with novel laser systems and architecture have enabled the development of a new class of compact, tunable, monochromatic light sources capable of producing MeV photons with unprecedented brightness. Such new sources rely on Thomson scattering of incident photons produced by a TW-class laser off a bright relativistic electron beam to generate Doppler-upshifted photons in a highly collimated beam. Scaling laws [1] show that a frequency-doubled, 532 nm wavelength, 1 J, 10 ps Fourier transform-limited drive laser pulse interacting with a 250 MeV, 1 nC, 10 ps, 1 mm.mrad normalized emittance, with 0.1% relative energy spread, can yield a 2.24 MeV γ-ray flash with a peak brightness exceeding 1023 photons/[mm2 x mrad2 x s x 0.1% bandwidth]. This number is > 15 orders of magnitude beyond the output of a third-generation synchrotron at the same photon energy. Above ~ 100 keV, the photons can interact with nuclei, and nuclear applications become viable. In this paper, we present a technical overview of T- REX sources and their capabilities, and give a few examples of potential applications of interest. |
| keywords: | pbpl |
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| title: | LASER BEAT-WAVE MICROBUNCHING OF RELATIVISTIC ELECTRON BEAM IN THE THZ RANGE |
| format: | journal article |
| year: | 2006 |
| 6 authors: | | | | | | |
| abstract: | Laser-driven plasma accelerators have recently demonstrated a ~1GeV energy gain of self-trapped electrons in a several-centimeter-long plasma channel. Potential staging of such devices will require the external injection of an electron beam prebunched on the scale of 1-10 THz into a plasma accelerating structure or plasma LINAC. Seeded FEL/IFEL techniques can be used for modulation of the electron beam longitudinally on the radiation wavelength scale. However, a seed source in this spectral range is not available. At the UCLA Neptune Laboratory a Laser Beat-Wave (LBW) microbunching experiment has begun. The interaction of the electron beam and the LBW results in ponderomotive acceleration and energy modulation on the THz scale. This stage is followed by a ballistic drift of the electrons, where the gained energy modulation is transferred to the beam current modulation. Then the beam is sent into a 33-cm long undulator, where a coherent start-up of THz radiation takes place, and the THz pulse is used for a bunching analysis. The performance of LBW bunching is simulated and analyzed using a 3D FEL code for the parameters of an existing photoinjector and two-wavelength TW CO2 laser system. |
| keywords: | pbpl |
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| title: | Status of the SPARC Photoinjector |
| format: | journal article |
| year: | 2006 |
| 69 authors: | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| abstract: | The SPARC Project is starting the commissioning of its photo-injector. RF gun, RF sources, RF network and control, power supplies, emittance meter, beam diagnostics and control to measure the RF gun beam have been installed. The photocathode drive laser has been characterized in terms of pulse shape and quality. We will report also about first tests made on RF gun and on the emittance meter device. Additional R&D on X-band and S-band structures for velocity bunching are in progress, as well as studies on new photocathode materials. We will also discuss studies on solenoid field defects, beam based alignments and exotic electron bunch production via blow-out of short laser pulses. |
| keywords: | pbpl |
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| title: | Experimental characterization of the transverse phase space of a 60-MeV electron beam through a compressor chicane |
| format: | journal article |
| year: | 2006 |
| 8 authors: | | | | | | | | |
| abstract: | Space charge and coherent synchrotron radiation may deteriorate electron beam quality when the beam passes through a magnetic bunch compressor. This paper presents the transverse phase-space tomographic measurements for a compressed beam at 60 MeV, around which energy the first stage of magnetic bunch compression takes place in most advanced linacs. Transverse phase-space bifurcation of a compressed beam is observed at that energy, but the degree of the space charge-induced bifurcation is appreciably lower than the one observed at 12 MeV. |
| keywords: | pbpl |
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| title: | Seeded free-electron and inverse free-electron laser techniques for radiation |
| format: | journal article |
| year: | 2006 |
| 6 authors: | | | | | | |
| abstract: | A comprehensive analysis is presented that describes amplification of a seed THz pulse in a single-pass free-electron laser (FEL) driven by a photoinjector. The dynamics of the radiation pulse and the modulated electron beam are modeled using the time-dependent FEL code, GENESIS 1.3. A 10-ps (FWHM) electron beam with a peak current of 50 –100 A allows amplification of a |
| keywords: | pbpl |
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| title: | Nb-Pb Superconducting RF-Gun |
| format: | journal article |
| year: | 2006 |
| 18 authors: | | | | | | | | | | | | | | | | | | |
| abstract: | We report on the status of an electron RF-gun made of two superconductors: niobium and lead. The presented design combines the advantages of the RF performance of bulk niobium superconducting cavities and the reasonably high quantum efficiency of lead, as compared to other superconducting metals. The concept, mentioned in a previous paper, follows the attractive approach of all niobium superconducting RF-gun as it has been proposed by the BNL group. Measured values of quantum efficiency for lead at various photon energies, analysis of recombination time of photon-broken Copreliminary cold test results are discussed in this paper. |
| keywords: | pbpl |
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| title: | STUDY OF THE EFFECT OF MULTIPOLAR COMPONENTS IN THE. SPARC EMITTANCE COMPENSATION GUN SOLENOID |
| format: | journal article |
| year: | 2006 |
| 11 authors: | | | | | | | | | | | |
| abstract: | The SPARC photoinjector rf gun requires a solenoid immediately downstream for emittance compensation. The analysis of the measured solenoid magnetic maps shows the existence of multipolar components added to the pure solenoid field. The effect of these added fields on beam dynamics and possible correction schemes have been studied from the theoretical point of view and by numerical calculations based on PARMELA/TREDI codes. An accurate 3D numerical modelization by using CST EM Studio has been done, in order to investigate the source of these multipolar components and to suggest some design modifications aimed to reduce their magnitude. The results of this study are presented here. |
| keywords: | pbpl |
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| title: | Electron Beam-laser Interaction near the Cathode in a High Brightness Photoinjector |
| format: | journal article |
| year: | 2006 |
| 4 authors: | | | | |
| abstract: | The production of high charge short bunches in a high brightness photoinjector requires laser pulses driving the cathode with GW range peak power on a mm spot size. The resulting transverse electric field experienced by the electron beam near the cathode is of the order of 200-500 MV/m, well in excess of a typical RF accelerating field of 50-100 MV/m. We present here a preliminary study of the resultant beam dynamics. Simulations including the electron beam-laser interaction have been performed with the code HOMDYN taking into account the superposition of incident and reflected laser pulses as well as space charge fields. Under this conditions the emittance degradation is negligible, as predicted by analytical methods, but a longitudinal charge modulation occurs on the scale of the laser wavelength, in case of oblique incidence, driven by the longitudinal component of the laser field. Charge modulation is transformed into energy modulation via the space charge field, which may produce enhanced microbunching effects when the beam is further compressed in a magnetic chicane. |
| keywords: | pbpl |
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| title: | Status of the SPARX FEL Project |
| format: | journal article |
| year: | 2006 |
| 67 authors: | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| abstract: | The SPARX project consists in an X-ray-FEL facility jointly supported by MIUR (Research Department of Italian Government), Regione Lazio, CNR, ENEA, INFN and Rome University Tor Vergata. It is the natural extension of the ongoing activities of the SPARC collaboration. The aim is the generation of electron beams characterized by ultra-high peak brightness at the energy of 1 and 2 GeV, for the first and the second phase respectively. The beam is expected to drive a single pass FEL experiment in the range of 13.5÷6 nm and 6÷1.5 nm, at 1 GeV and 2 GeV respectively, both in SASE and SEEDED FEL configurations. A hybrid scheme of RF and magnetic compression will be adopted, based on the expertise achieved at the SPARC high brightness photoinjector presently under commissioning at Frascati INFN-LNF Laboratories [1,2]. The use of superconducting and exotic undulator sections will be also exploited. In this paper we report the progress of the collaboration together with start to end simulation results based on a combined scheme of RF compression techniques. |
| keywords: | pbpl |
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| title: | The Design of a Hybrid Photoinjector for High Brightness Beam Applications |
| format: | journal article |
| year: | 2006 |
| 10 authors: | | | | | | | | | | |
| abstract: | In this paper, we illustrate the electromagnetic and beam dynamics design procedure of a new class of photoinjector, a hybrid standing/traveling wave structure. In this device a standing wave RF gun section is integrated with a downstream traveling wave structure through a coupling cell that feeds simultaneously the two sections. We discuss the advantages in RF and beam performance of the hybrid photoinjector compared to conventional systems. The electromagnetic design has been performed using the 3D electromagnetic code HFSS. Results of beam dynamics simulations in different operating conditions are also discussed. |
| keywords: | pbpl |
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| title: | Development of a Waveguide FEL Seeded in the 1-3 THz Range for Microbunching Experiment at the Neptune Laboratory |
| format: | journal article |
| year: | 2006 |
| 8 authors: | | | | | | | | |
| abstract: | IFEL and FEL techniques can be used to modulate an electron beam on the scale of the radiation wavelength. However, the lack of a high power radiation source in the 100-300 μm range hinders the progress on THz IFEL microbunching. In this paper, we discuss microbunching of an electron beam using a single-pass FEL seeded with a low power THz pulse generated by frequency mixing of CO2 laser lines in a GaAs nonlinear crystal. A narrowband THz seed source is pumped by a dual beam TEA CO2 laser and can be tuned in the 1-3 THz range. The THz radiation is guided through a hollow waveguide inside the planar FEL undulator driven by a photoinjector. By using a time-dependent FEL code GENESIS 1.3, we optimized the undulator parameters and analyzed the dynamics of the modulated electron beam. By using a ~ 8 MeV electron beam with a peak current of 40 A and a ~1kW THz seed with wavelength 200 μm, the energy modulation up to 1.3% can be achieved in a ~1.8-m long undulator with a constant period of 2.7 cm. At present, the THz seed source is built and fully characterized. The results of transmission measurements for THz waveguides are also discussed. |
| keywords: | pbpl |
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| title: | Ultra-high gradient dielectric wakefield accelerator experiments |
| format: | journal article |
| year: | 2006 |
| 12 authors: | | | | | | | | | | | | |
| abstract: | Ultra-high gradient dielectric wakefield accelerators are a potential option for a linear collider afterburner since they are immune to the ion collapse and electron/positron asymmetry problems implicit in a plasma based afterburner. The first phase of an experiment to study the performance of dielectric Cerenkov wakefield accelerating structures at extremely high gradients in the GV/m range has been completed. The experiment took advantage of the unique SLAC FFTB electron beam and its ultra-short pulse lengths and high currents (e.g., σz = 20 μm at Q = 3 nC). The FFTB electron beam was successfully focused down and sent through short lengths of fused silica capillary tubing (ID = 200 μm / OD = 325 μm). The pulse length of the electron beam was varied to produce a range of electric fields between 2 and 20 GV/m at the inner surface of the dielectric tubes. We observed a sharp increase in optical emissions from the capillaries in the middle part of this surface field range which we believe indicates the transition between sustainable field levels and breakdown. If this initial interpretation is correct, the surfaced fields that were sustained equate to on axis accelerating field of several GV/m. In future experiments being developed for the SLAC SABER and BNL ATF we plan to use the coherent Cerenkov radiation emitted from the capillary tube as a field strength diagnostic and demonstrate GV/m range particle energy gain. |
| keywords: | pbpl |
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| title: | Observation of Coherent Edge Radiation Emitted by a 100 Femtosecond Compressed Electron Beam |
| format: | journal article |
| year: | 2006 |
| 13 authors: | | | | | | | | | | | | | |
| abstract: | A chicane compressor developed by UCLA for the production of ultra-short, 60 MeV electron beams at the Brookhaven National Laboratory Accelerator Test Facility has been commissioned, and initial beam physics experiments have been performed. These measurements have established the compression of electron beams to the 100 femtosecond (1 kA peak current) regime, via coherent transition radiation (CTR) based measurements. Investigations of coherent edge radiation (CER) include signatures that differentiate it from coherent synchrotron radiation (CSR), such as polarization and far-field angular distribution. Additionally, the radiation wavelength spectrum is determined from autocorrelation measurements. Radiation properties are compared to detailed start-to-end simulations derived from PARMELA and QUINDI (a Lienard-Wiechert code developed at UCLA). Plans for future experiments which further explore the observed wavelength spectra are presented. |
| keywords: | pbpl |
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| title: | Production of femtosecond pulses and |
| format: | journal article |
| year: | |
| 9 authors: | | | | | | | | | |
| abstract: | |
| keywords: | pbpl |
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| title: | Feasibility Study of a Laser Beat-Wave Seeded THz FEL at the Neptune Laboratory |
| format: | journal article |
| year: | 2005 |
| 6 authors: | | | | | | |
| abstract: | |
| keywords: | pbpl |
| Details | edit | delete | |
| title: | HIgh Energy, High Brightness X-Rays Produced by Compton Back Scattering at the Livermore PLEIADES facility |
| format: | journal article |
| year: | |
| 11 authors: | | | | | | | | | | | |
| abstract: | PLEIADES (Picosecond Laser Electron Interaction for the Dynamic Evaluation of Structures) produces tunable 30-140 keV x-rays with 0.3-5 ps pulse lengths and up to 10^7 photons/pulse by colliding a high brightness electron beam with a high power laser. The electron beam is created by an rf photo-injector system, accelerated by a 120 MeV linac, and focused to 20 μm with novel permanent magnet quadrupoles. To produce Compton back scattered x-rays, the electron bunch is overlapped with a Ti:Sapphire laser that delivers 500 mJ, 100 fs, pulses to the interaction point. K-edge radiography at 115 keV on Uranium has verified the angle correlated energy spectrum inherent in Compton scattering and high-energy tunability of the Livermore source. Current upgrades to the facility will allow laser pumping of targets synchronized to the x-ray source enabling dynamic diffraction and time-resolved studies of high Z materials. Near future plans include extending the radiation energies to >400 keV, allowing for nuclear fluorescence studies of materials. |
| keywords: | pbpl |
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| title: | Production of Terahertz Seed Radiation for FEL/IFEL Microbunchers for Second Generation Plasma Beatwave Experiments at Neptune |
| format: | journal article |
| year: | 2005 |
| 5 authors: | | | | | |
| abstract: | To achieve phase locked injection of short electron bunches in a plasma beatwave accelerator, the Neptune Laboratory will utilize microbunching in an FEL or IFEL system. These systems require terahertz (THz) seed radiation on the order of 10 kW for the FEL and 10 MW for the IFEL bunchers. We report results of experiments on THz generation using nonlinear frequency mixing of CO2 laser lines in GaAs. A two-wavelength laser beam was split and sent onto a 2.5 cm long GaAs crystal cut for noncollinear phase matching. Low power measurements achieved ~1 W of 340 μm radiation using 200 ns CO2 pump pulses with wavelengths 10.3μm and 10.6μm. We also demonstrated tunability of difference frequency radiation, producing 240μm by mixing two different CO2 laser lines. By going to shorter laser pulses and higher intensities, we were able to increase the conversion efficiency while decreasing the surface damage threshold. Using 200ps pulses we produced ~2 MW of 340 μm radiation. Future studies in this area will focus on developing large diameter Quasi-Phase matched structures for production of high power THz radiation using collinear two frequency radiation. |
| keywords: | pbpl |
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| title: | The UCLA/FNPL Time Resolved Underdense Plasma Lens Experiment |
| format: | journal article |
| year: | 2005 |
| 11 authors: | | | | | | | | | | | |
| abstract: | A gaussian underdense plasma lens with peak density 5 × 1012cm-3and a full width half maximum (FWHM) length of 2.2 cm has been used to focus a relativistic electron beam. This plasma lens has a focusing strength equivalent to a quadrupole magnet with a 150 T/m field gradient. The lens focused a 15 MeV, 16 nC electron beam with initial dimensions σr= 500 μm and σz= 5 mm onto an optical transition radiation (OTR) screen 2 cm downstream of the lens. The average transverse area of the plasma focused electron beam was typically demagnified by a factor of 22. The light from the OTR screen was imaged into a streak camera in order to directly measure the correlation between z and σrwithin the beam. |
| keywords: | pbpl |
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| title: | Experimental Characterization of 4-D Transverse Phase-Space of a Compressed Beam |
| format: | journal article |
| year: | 2005 |
| 8 authors: | | | | | | | | |
| abstract: | |
| keywords: | pbpl |
| Details | edit | delete | |
| title: | Experimental characterization of an ultrafast Thomson scattering x-ray source with three-dimensional time and frequency-domain analysis |
| format: | journal article |
| year: | 2004 |
| 18 authors: | | | | | | | | | | | | | | | | | | |
| abstract: | We present a detailed comparison of the measured characteristics of Thomson backscattered xrays produced at the Picosecond Laser-Electron Interaction for the Dynamic Evaluation of Structures facility at Lawrence Livermore National Laboratory to predicted results from a newly developed, fully three-dimensional time and frequency-domain code. Based on the relativistic differential cross section, this code has the capability to calculate time and space dependent spectra of the x-ray photons produced from linear Thomson scattering for both bandwidth-limited and chirped incident laser pulses. Spectral broadening of the scattered x-ray pulse resulting from the incident laser bandwidth, perpendicular wave vector components in the laser focus, and the transverse and longitudinal phase spaces of the electron beam are included. Electron beam energy, energy spread, and transverse phase space measurements of the electron beam at the interaction point are presented, and the corresponding predicted x-ray characteristics are determined. In addition, time-integrated measurements of the xrays produced from the interaction are presented and shown to agree well with the simulations. |
| keywords: | pbpl |
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| title: | Experimental Generation and Characterization of Uniformly Filled Ellipsoidal Electron-Beam Distributions |
| format: | journal article |
| year: | 2008 |
| 5 authors: | | | | | |
| abstract: | For 40 years, uniformly filled ellipsoidal beam distributions have been studied theoretically, as they hold the promise of generating self-fields linear in the coordinate offset in all three directions. Recently, a scheme for producing such distributions, based on the strong longitudinal expansion of an initially very short beam under its own space-charge forces, has been proposed. In this Letter we present the experimental demonstration of this scheme, obtained by illuminating the cathode in a rf photogun with an ultrashort laser pulse ( |
| keywords: | pbpl prl |
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| title: | A Proposal for an X-Ray Free-Electron Laser Oscillator with an Energy-Recovery Linac |
| format: | journal article |
| year: | 2008 |
| 3 authors: | | | |
| abstract: | We show that a free-electron laser oscillator generating x rays with wavelengths of about 1 A ̊ is feasible using ultralow emittance electron beams of a multi-GeV energy-recovery linac, combined with a low-loss crystal cavity. The device will produce x-ray pulses with 109 photons at a repetition rate of 1–100 MHz. The pulses are temporarily and transversely coherent, with a rms bandwidth of about 2 meV, and rms pulse length of about 1 ps. |
| keywords: | pbpl prl |
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| title: | Virtual dielectric waveguide mode description of a high-gain free-electron laser. II. Modeling and numerical simulations |
| format: | journal article |
| year: | June 20, 2008 |
| 3 authors: | | | |
| abstract: | A high-gain free-electron laser is modeled using an expansion of the radiation field in terms of guided Laguerre-Gaussian modes of a virtual dielectric waveguide E. Hemsing, A. Gover, and J. Rosenzweig, preceding paper, Phys Rev. A 77, 063830 2008 . The radiation profile evolution, power gain, and detuning efficiency characteristics are investigated for seeding with fundamental Gaussian and higher-order Laguerre- Gaussian input modes on a Gaussian e-beam in the collective regime. The full wave evolution solution at different seed radiation injection conditions results in determination of the optimal waist size and waist position of the seed radiation beam for maximum power coupling efficiency. Results for guided mode evolution and power gain are shown to be consistent with simulations performed with the code GENESIS 1.3. The amplification and spontaneous generation of FEL radiation with orbital angular momentum is also considered. |
| keywords: | pbpl pra |
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| title: | Virtual dielectric waveguide mode description of a high-gain free-electron laser. I. Theory |
| format: | journal article |
| year: | June 20, 2008 |
| 3 authors: | | | |
| abstract: | A set of mode-coupled excitation equations for the slowly growing amplitudes of dielectric waveguide eigenmodes is derived as a description of the electromagnetic signal field of a high-gain free-electron laser FEL, including the effects of longitudinal space charge. This approach of describing the field basis set has notable advantages for FEL analysis in providing an efficient characterization of eigenmodes, and in allowing a clear connection to free-space propagation of the input seeding and output radiation. The formulation describes the entire evolution of the radiation wave through the linear gain regime, prior to the onset of saturation, with arbitrary initial conditions. By virtue of the flexibility in the expansion basis, this technique can be used to find the direct coupling and amplification of a particular mode. A simple transformation converts the derived coupled differential excitation equations into a set of coupled algebraic equations and yields a matrix determinant equation for the FEL eigenmodes. A quadratic index medium is used as a model dielectric waveguide to obtain an expression for the predicted spot size of the dominant system eigenmode, in the approximation that it is a single Gaussian mode. |
| keywords: | pbpl pra |
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| title: | Opportunities and Hurdles in Optical Scale Accelerating Structures |
| format: | talk |
| year: | 05/27/2009 |
| 1 author: | |
| abstract: | For some time it has been realized that scaling particle accelerators to optical frequencies presents new opportunities for the nearly ubiquitous use of charged particle beams. Various efforts are underway, albeit far too few, to achieve high gradient particle acceleration in both laser powered and wakefield driven dielectric structures. I will use my own work on the Micro Accelerator Platform (MAP) as an example of the various challenges faced by this class of accelerating structure. The MAP has design parameters similar to other such structures: laser powered; 800 nm period; gap (equivalent to the iris diameter in a conventional linac) of 800 nm; ~1 GV/m gradients; fC-pC charge per bunch; ~10 fs bunch lengths. The MAP and its ilk face hurdles ranging from fabrication technologies to sub-micron beam manipulation. The diagnostics required are demanding, a rich area of work, and overlap well with proposed next generation light source beams. Despite the barriers, these devices hold great promise. In speculating on applications and end uses, which span from colliders to compact light sources to medical devices, I will draw from a number of examples. Finally, I will use this seminar as a bully pulpit to advocate for dielectric accelerators in general. |
| keywords: | pbpl Seminar |
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