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34 papers
| title: | Generation and Measurement of Relativistic Electron Bunches Characterizzed by a Linearly Ramped Current Profile |
| format: | journal article |
| year: | May 28, 2008 |
| 3 authors: | | | |
| abstract: | We report the first successful attempt to generate ultrashort (1–10 ps) relativistic electron bunches characterized by a ramped longitudinal current profile that rises linearly from head to tail and then falls sharply to zero. Bunches with this type of longitudinal shape may be applied to plasma-based accelerator schemes as an optimized drive beam, and to free-electron lasers as a means of reducing asymmetry in microbunching due to slippage. The scheme used to generate the ramped bunches employs an aniso-chronous dogleg beam line with nonlinear correction elements to compress a beam having an initial positive time-energy chirp. The beam current profile is measured using a deflecting mode cavity, and a pseudoreconstruction of the beam’s longitudinal phase space distribution is obtained by using this diagnostic with a residual horizontal dispersion after the dogleg. |
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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: | Longitudinal Shaping of Relativistic Bunches of Electrons Generated by an RF Photoinjector |
| format: | thesis |
| year: | Oct 2007 |
| 1 author: | |
| abstract: | We propose a mechanism for the generation of ultrashort (sub-ps to a few ps) relativistic electron bunches having a ramp-shaped current profile that rises gradually from the head to the tail, followed by a sharp cutoff. This type of current distribution approximates the idealized profile predicted by linear 1D and nonlinear 2D calculations to be optimal for driving large-amplitude wakefields in a plasma wakefield accelerator (PWFA). The proposed scheme utilizes a dogleg beamline (or dispersionless translating section) as a bunch compressor to impart a linear negative longitudinal dispersion transformation on the longitudinal phase space of a beam that is initially chirped in energy versus longitudinal position within the bunch. A theory to describe this mechanism is derived using first and second-order transport matrix theory. The theoretical results, combined with simulations using the particle transport codes ELEGANT and PARMELA, indicate that sextupole corrector magnets are required in order to cancel out nonlinear chromatic effects which tend to otherwise disrupt the ramped shape of the current distribution. To provide a venue for a proof-of-principle experiment to test these predictions, a dogleg beamline was designed and built at the UCLA Neptune accelerator laboratory. A standing wave deflecting cavity was designed and built as a temporal diagnostic for measuring the current profiles of the electron bunches after passing through the dogleg. Second-order horizontal dispersion measurements and coherent transition radiation bunch length measurements of the electron beam after passing through the dogleg show good agreement with the predictions of theory and simulation. Deflecting cavity measurements were conducted to directly measure the current profiles of both compressed and uncompressed electron bunches, with and without sextupole correction. The uncompressed bunch is found to have an asymmetrical (non-Gaussian) shape, presumably inherited from the pulse shape of the photoinjector drive laser. Results for an initially chirped beam show that ramp-shaped bunches can be produced by the proposed method, although due to the asymmetric initial (i.e. pre-compression) current profile of the electron bunches produced by the Neptune photoinjector, it is found to be necessary to overcompensate with the sextupole magnets in order to achieve a ramp-shaped profile. |
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| title: | Optinum Beam Creation In Photoinjectors Using Space-Charge Expansion |
| format: | conference procceeding |
| conference: | |
| year: | 2006 |
| 16 authors: | | | | | | | | | | | | | | | | |
| abstract: | It has recently been shown that by illuminating a photocathode with an ultra-short laser pulse of appropriate transverse 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 compatible, 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 Frascati 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 experiment, as well as plans for future experiments to resolve the ellipsoidal bunch shape at low energy. Future measurements at high energy based on fs resolution RF sweepers are discussed. |
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| title: | Experiment to Measure Ramped Electron Bunches at the UCLA Neptune Laboratory Using a Transverse Deflecting Cavity |
| format: | conference procceeding |
| conference: | 12th Advanced Accelerator Concepts Workshop |
| year: | 2006 |
| 5 authors: | | | | | |
| abstract: | A proof of principle experiment is underway at the UCLA Neptune laboratory to test the concept of generating linearly ramped relativistic electron bunches (rising in density from head to tail followed by a sharp cutoff) by using a sextupole-corrected dogleg section as a bunch compressor. Bunches with this structure have been predicted to be ideal for use as a plasma wake-field drive beam. The diagnostic being developed to measure the time profile of the beam is an X-Band (9.6 GHz) deflecting cavity. The recently completed cavity is a 9-cell standing wave structure operating in a TM110-like mode, designed to measure the temporal structure of the 2 to 10 ps, 14 MeV electron bunches generated by the Neptune S-band photoinjector and plane-wave transformer (PWT) accelerator beamline, with 50 fs resolution. We discuss the experimental plan for the ramped bunch experiment and present preliminary data related to the tuning and operation of the deflecting cavity. |
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| title: | Dielectric Wakefield Accelerating Structure as a Source of Terahertz Coherent Cerenkov Radiation |
| format: | conference procceeding |
| conference: | |
| year: | |
| 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: | EXPERIMENTAL TESTING OF DYNAMICALLY OPTIMIZED PHOTOELECTRON BEAMS |
| format: | conference procceeding |
| conference: | Erice 2005 |
| year: | 2006 |
| 16 authors: | | | | | | | | | | | | | | | | |
| abstract: | We discuss the design of and initial results from an experiment in space-charge dominated beam dynamics which explores a new regime of high-brightness electron beam generation at the SPARC (located at INFN-LNF, Frascati) photoinjector. The scheme under study employs the natural tendency in intense electron beams to configure themselves to produce a uniform density, giving a nearly ideal beam from the viewpoint of space charge-induced emittance. The experiments are aimed at testing the marriage of this idea with a related concept, emittance compensation, We show that the existing infrastructure at SPARC is nearly ideal for the proposed tests, and that this new regime of operating photoinjector may be the preferred method of obtaining highest brightness beams with lower energy spread. We discuss the design of the experiment, including developing of a novel time-dependent, aerogel-based imaging system. This system has been installed at SPARC, and first evidence for nearly uniformly filled ellipsoidal charge distributions recorded. |
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| title: | Neptune Second Harmonic Generation Autocorrelator |
| format: | tech note |
| year: | 2005 |
| 1 author: | |
| abstract: | This document provides detailed information on the basic theoretical knowledge required to understand the operational principles of the Second Harmonic Generation Autocorrelator used for measuring the pulse length of the infrared drive laser at the Neptune Laboratory. This autocorrelator was originally assembled by Kari Sanders and Sergei Tochitsky ca. 2000-2001. Some information about its operation may be found in Kari Sanders' master's thesis. In short, the autocorrelator operates by splitting the infrared (1064nm) pulse produced by the Neptune regenerative amplifier and grating compressor and then crossing the two split pulses inside of a nonlinear KDP crystal, via a Type I "ooe" interaction, to produce noncollinear second harmonic frequency upconversion to green (532 nm). The intensity of the green as a function of the delay between the two split pulses produces the autocorrelation of the pulse, which can be used to extract the pulse length and (to some extent) the time structure of the pulse. The theory of autocorrelation will not be discussed in this document. |
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| title: | New Features in UCLA-PARMELA v 2.1 |
| format: | tech note |
| year: | 2005 |
| 1 author: | |
| abstract: | This document describes modifications to the UCLA-PARMELA source code version 2.0. The modifications were made by R. J. England during the period from 2003 to 2005. The changes to the code consist of: (1) the implementation of the "SEXT" card for sextupoles, and (2) changes to the "TRWAVE" card which permit the fields to be printed out to a data file. These changes are detailed in the following subsections. In addition, (3) I document the input structure for the CELL card in versions 2.0 and 2.1 since this was found to differ from the description given in the UCLA-PARMELA manual. The modified code version number has been increased to v 2.1. |
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| title: | X-Band Dipole Mode Deflecting Cavity for the UCLA Neptune Beamline |
| format: | conference procceeding |
| conference: | 2005 Particle Accelerator Conference |
| year: | 2005 |
| 5 authors: | | | | | |
| abstract: | We report progress on the design and construction of a nine-cell cavity operating in a TM110-like dipole mode for use as a temporal diagnostic of the 14 MeV, 300 pC electron bunches generated at the UCLA Neptune Laboratory linear accelerator, with an anticipated temporal resolution of 50 fs at a peak input power of 50 kW. The cavity is a center-fed standing-wave pi-mode structure, operating at 9.6 GHz, and incorporating a knife-edge and gasket assembly which minimizes the need for brazing or welding. Results of initial RF testing of the prototype cavity are discussed and compared with simulation results obtained using the commercial code HFSS. |
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| title: | High Energy Gain of Trapped Electrons in a Tapered, Diffraction-Dominated Inverse-Free-Electron Laser |
| format: | preprint |
| year: | 2005 |
| 17 authors: | | | | | | | | | | | | | | | | | |
| abstract: | Energy gain of trapped electrons in excess of 20 MeV has been demonstrated in an Inverse-Free- Electron-Laser (IFEL) accelerator experiment. A 14.5 MeV electron beam is copropagated with a 400 GW CO2 laser beam in a 50 cm long undulator strongly tapered in period and ¯eld amplitude. The Rayleigh range of the laser, » 1.8 cm, is much shorter than the undulator length yielding a di®raction-dominated interaction. Experimental results on the dependence of the acceleration on injection energy, laser focus position, and laser power are discussed. Simulations, in good agreement with the experimental data, show that most of the energy gain occurs in the ¯rst half of the undulator at a gradient of 70 MeV/m and that the structure in the measured energy spectrum arises because of higher harmonic IFEL interaction in the second half of the undulator. |
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| title: | Velocity bunching of high-brightness electron beams |
| format: | journal article |
| year: | 2005 |
| 11 authors: | | | | | | | | | | | |
| abstract: | Velocity bunching has been recently proposed as a tool for compressing electron beam pulses in modernhigh brightness photoinjector sources. This tool is familiar from earlier schemes implemented for bunching dc electron sources, but presents peculiar challenges when applied to high current, low emittance beams from photoinjectors. The main difficulty foreseen is control of emittance oscillations in the beam in this scheme, which can be naturally considered as an extension of the emittance compensation process at moderate energies. This paper presents two scenarios in which velocity bunching, combined with emittance control, is to play a role in nascent projects. The first is termed ballistic bunching, where the changing of relative particle velocities and positions occur in distinct regions, a short high gradient linac, and a drift length. This scenario is discussed in the context of the proposed ORION photoinjector. Simulations are used to explore the relationship between the degree of bunching, and the emittance compensation process. Experimental measurements performed at the UCLA Neptune Laboratory of the surprisingly robust bunching process, as well as accompanying deleterious transverse effects, are presented. An unanticipated mechanism for emittance growth in bends for highly momentum chirped beam was identified and studied in these experiments. The second scenario may be designated as phase space rotation, and corresponds closely to the recent proposal of Ferrario and Serafini. Its implementation for the compression of the electron beam pulse length in the PLEIADES inverse Compton scattering (ICS) experiment at LLNL is discussed. It is shown in simulations that optimum compression may be obtained by manipulation of the phases in low gradient traveling wave accelerator sections. Measurements of the bunching and emittance control achieved in such an implementation at PLEIADES, as well as aspects of the use of velocity-bunched beam directly in ICS experiments, are presented. |
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| title: | Sextupole Correction of the Longitudinal Transport of Relativistic Beams in Dispersionless Translating Sections |
| format: | journal article |
| year: | |
| 6 authors: | | | | | | |
| abstract: | We examine the use of sextupole magnets to correct nonlinearities in the longitudinal phase space transformation of a relativistic beam of charged particles in a dispersionless translating section, or dogleg. Through heuristic analytical arguments and examples derived from recent experimental endeavors and augmented by simulations using the particle tracking codes PARMELA and ELEGANT, sextupole corrections are found to be potentially useful in facilitating the use of such structures for beam compression or for shaping the current profile of the beam, by manipulation of the second-order longitudinal dispersion (or T566). Recent experimental evidence of the use of sextupoles to manipulate second-order horizontal and longitudinal dispersion of the beam is presented. The theoretical and experimental results indicate that these manipulations could be used to create an electron bunch with a current profile having a long ramp followed by a sharp cut-off, which is optimal for driving large amplitude wake fields in a plasma wake field accelerator. |
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| title: | UCLA Neptune CTR Interferometer |
| format: | tech note |
| year: | 2005 |
| 1 author: | |
| abstract: | The purpose of this document is to outline the basic operating theory behind the coherent transition radiation (CTR) interferometry technique employed at the UCLA Neptune Laboratory for diagnosing the bunch length of the electron beam produced by the Neptune 1.6-cell gun and PWT accelerating structure. General theoretical results are presented and then applied specifically to the Neptune interferometer developed by Uwe Happek, which we refer to henceforth as the Uwefarometer. Particular attention is paid to understanding the theory of the wire-grid polarizers which are used in the Uwefarometer to split and recombine the terahertz CTR radiation. |
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| title: | Beam Shaping and Compression Scheme for the UCLA Neptune Laboratory |
| format: | conference procceeding |
| conference: | 2003 Particle Accelerator Conference |
| year: | 2003 |
| 4 authors: | | | | |
| abstract: | We have recently added a dispersionless translating section to the UCLA Neptune linear accelerator beamline. This new section of beamline will serve as a venue for beam shaping and compression experiments using the 14MeV electron beam produced by the UCLA Neptune PWT linac and newly installed photoinjector. An examination of the first and second-order optics indicates that when certain nonlinear effects are minimized through the use of sextupole magnets, the longitudinal dispersion is dominated by a negative R56 which, for an appropriately chirped initial beam, can be used to create a ramped beam of a few picosecond duration that would be ideal for driving large amplitude wake fields in a plasma and producing high transformer ratios. The beamline is now in operation. Preliminary data indicate that the beamline optics are well-predicted by simulation and that sextupoles can be used successfully to eliminate nonlinear horizontal dispersion. Future experiments are planned for measuring beam compression (using CTR autocorrelation) and doing longitudinal phase space tomography (using a transverse deflecting cavity). |
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| title: | Plasma electron fluid motion and wave breaking near a density transition |
| format: | journal article |
| year: | 2002 |
| 3 authors: | | | |
| abstract: | Recently, Suk, Barov, and Rosenzweig [Phys. Rev. Lett. 86, 1011 (2001)] proposed a scheme for trapping background electrons in a plasma wake field using a sudden downward transition in the background ion density, where the density transition length is small compared to the plasma skin depth. In the present paper we present a fluid dynamical description of this mechanism that is self-consistent up to the point of wave breaking. A one-dimensional nonlinear relativistic second-order differential equation is derived for the electron fluid velocity in Lagrangian coordinates. Numerical integrations of this equation are used to map out the regions of parameter space in which wave breaking occurs and to determine the extent of the downstream region of plasma involved in wave breaking. Comparisons with one-dimensional particle-in-cell (PIC) simulations show that the onset of trapping occurs at the parameter values where wave breaking begins in the fluid analysis, but that the downstream extent of plasma involved in wave breaking is not a reliable predictor of the number of trapped particles. The PIC simulations also reveal that particles initially located on the upstream side of the density transition may become trapped, although these particles do not participate in wave breaking in the fluid description. |
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| title: | Longitudinal Beam Shaping and Compression Scheme for the UCLA Neptune Laboratory |
| format: | conference procceeding |
| conference: | 10th Advanced Accelerator Concepts Workshop |
| year: | 2002 |
| 3 authors: | | | |
| abstract: | |
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| title: | Velocity bunching experiment at the Neptune Laboratory |
| format: | conference procceeding |
| conference: | 10th Advanced Accelerator Concepts Workshop |
| year: | 2002 |
| 5 authors: | | | | | |
| abstract: | |
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| title: | Dynamics of a Driver Beam Propagating in an Underdense Plasma with a Downward Density Transition |
| format: | conference procceeding |
| conference: | 2001 Particle Accelerator Conference |
| year: | 2001 |
| 7 authors: | | | | | | | |
| abstract: | When a short electron beam propagates in an underdense plasma (plasma density n_p < beam density n_b) with a downward density transition, it is known that some background plasma electrons are trapped and accelerated by the plasma wakefield[1]. Beam quality of the trapped plasma electrons is severely affected by the wakefield that is generated by the driving electron beam, so dynamics and instabilities of the driver beam are very important. In this paper, we present some simulation results on the self-trapping and driver beam dynamics. |
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| title: | Beam-plasma interaction experiments at the UCLA Neptune Laboratory |
| format: | conference procceeding |
| conference: | 2001 Particle Accelerator Conference |
| year: | 2001 |
| 5 authors: | | | | | |
| abstract: | We are currently planning several beam-plasma interaction experiments using the high quality 16 MeV electron beam produced by the Neptune 1.625-cell photoinjector and PWT linac. The underdense (n_b > n_0) plasma lens experiment will examine the focusing of a 4 nC beam of 30 psec pulse duration as it passes through a thin 2 cm Ar plasma lens. We are also developing a negative R_56 compressor beamline that will allow us to create ramped sub-picosecond beams of a shape well suited for driving large amplitude plasma wake fields and producing high transformer ratios. Simulations made using the 2-1/2 dimensional particle-in-cell code MAGIC indicate that we could use these ramped beams to produce fields of up to 10 GeV/m in a 10(16) cm(-3) plasma. Ramped beams are also suitable for driving plasma density gradient trapping experiments. When such a beam passes through a region where the plasma density drops suddenly the fast variation of the wake fields traps several pC of plasma electrons in the accelerating portion of the wake field. |
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| title: | Commissioning of the Neptune photoinjector |
| format: | conference procceeding |
| conference: | 2001 Particle Accelerator Conference |
| year: | 2001 |
| 9 authors: | | | | | | | | | |
| abstract: | The status of the RF photoinjector in the Neptune advanced accelerator laboratory is discussed. The components of the photoinjector: the RF gun and booster linac, chicane compressor, and beam diagnostic systems are described. Measurement techniques used to diagnose the short pulse length, high brightness beam are detailed and measurements of emittance and pulse compression are given. The effect of the pulse compressor on transverse emittance is explored. (11 References). |
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| title: | Commissioning of the Neptune photoinjector |
| format: | conference procceeding |
| conference: | 1999 Particle Accelerator Conference |
| year: | 1999 |
| 21 authors: | | | | | | | | | | | | | | | | | | | | | |
| abstract: | The status of the commissioning of the RF photoinjector in the Neptune advanced accelerator laboratory is discussed. The component parts of the photoinjector, the RF gun, photocathode drive laser system, booster linac, RF system, chicane compressor, beam diagnostics systems, and control system are described. This injector is designed to produce short pulse length, high brightness electron beams. Experiments planned for the immediate future are described. Initial measurements of various beam parameters are presented. (10 References). |
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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: | 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: | commissioning of the UCLA Neptune X-Band Deflecting Cavity and Applications to Current Profile Measurement of Ramped Electron Bunches |
| format: | journal article |
| year: | 2007 |
| 5 authors: | | | | | |
| abstract: | A 9-cell standing wave deflecting cavity has recently been constructed and installed at the UCLA Neptune Laboratory for use as a temporal diagnostic for the 13 MeV, 300 to 700 pC electron bunches generated by the Neptune photoinjector beamline. The cavity is a center-fed Glid-Cop structure operating in at TM110-like deflecting mode at 9.59616 GHz with a pi phase advance per cell. At the maximum deflecting voltage of 500 kV, the theoretical resolution limit of the device is 50 fs, although with current beam parameters and a spot size of 460 microns RMS the effective resolution is approximately 400 fs. We discuss the operation and testing of the cavity as well as its intended application: measuring the temporal current profile of ramped electron bunches generated using the Neptune dogleg compressor, and we present the first measurements of the electron beam current profile obtained using the deflecting cavity. |
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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: | Commissioning of the UCLA Neptune X-band Deflecting Cavity and Applications to Current Profile Measurement of Ramped Electron Bunches |
| format: | conference procceeding |
| conference: | PAC07 |
| year: | 2007 |
| 5 authors: | | | | | |
| abstract: | A 9-cell standing-wave deflecting cavity has recently been constructed and installed at the UCLA Neptune Lab- oratory for use as a temporal diagnostic for the 13 MeV, 300 to 700 pC electron bunches generated by the Neptune photoinjector beamline. The cavity is a center-fed Glid- Cop structure operating in at TM110-like deflecting mode at 9.59616 GHz with a π phase advance per cell. At the maximum deflecting voltage of 530 kV, the theoretical res- olution limit of the device is 50 fs, although with current beam parameters and a RMS spot size of 460 μm the ef- fective resolution is approximately 400 fs. We discuss the operation and testing of the cavity as well as its intended application of measuring the temporal current profile of ramped electron bunches generated using the Neptune dog- leg compressor, and we present the first measurements of the electron beam current profile obtained using the deflect- ing cavity. |
| keywords: | pbpl_pub |
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| title: | Beam-driven Dielectric Wakefield Accelerating Structure as a Thz Radiation Source |
| format: | conference procceeding |
| conference: | PAC07 |
| 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 μJ 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 μm RMS bunch length, ∼500 pC bunch charge). Preparations underway for installation of the experiment are discussed. |
| keywords: | pbpl_pub |
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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: | 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: | 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: | 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: | High Energy Gain of Trapped Electrons in a Tapered, Diffraction-Dominated Inverse-Free-Electron Laser |
| format: | journal article |
| year: | |
| 17 authors: | | | | | | | | | | | | | | | | | |
| abstract: | Energy gain of trapped electrons in excess of 20 MeV has been demonstrated in an inverse-freeelectron-laser (IFEL) accelerator experiment. A 14.5 MeVelectron beam is copropagated with a 400 GW CO2 laser beam in a 50 cm long undulator strongly tapered in period and field amplitude. The Rayleigh range of the laser, 1:8 cm, is much shorter than the undulator length yielding a diffraction-dominated interaction. Experimental results on the dependence of the acceleration on injection energy, laser focus position, and laser power are discussed. Simulations, in good agreement with the experimental data, show that most of the energy gain occurs in the first half of the undulator at a gradient of 70 MeV=m and that the structure in the measured energy spectrum arises because of higher harmonic IFEL interaction in the second half of the undulator. |
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| title: | High Energy gain IFEL at the UCLA Neptune Laboratory |
| format: | journal article |
| year: | |
| 16 authors: | | | | | | | | | | | | | | | | |
| abstract: | We report on the observation of energy gain in excess of 20MeV at the Inverse Free Electron Laser Accelerator experiment at the Neptune Laboratory at UCLA. A 14.5MeV electron beam is injected in a 50cm longundulator strongly tapered both in period and field amplitude. A CO210.6μm laser with power > 400GW is used as the IFEL driver. The Rayleigh range of the laser(∼1.8cm)is shorter than the undulator length so that the interaction is diffraction dominated. Few per cent of the injected particles are trapped instable 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. Anenergygradientof>70MeVisinferred. In these cond section of the undulator higher harmonic IFEL interaction is observed. |
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