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title: Creation, Manipulation, and Diagnosis of Intense, Relativistic Picosecond Photo-electron Beams
format: thesis
year: 2002
1 author: S. Anderson
abstract: The radio frequency photoinjector is the pre-eminent source for advanced electron beam applications that require extremely high phase space density (high brightness) beams. Because of their high phase space density, the collective fields generated by photoinjector beams dominate their behavior. These space-charge fields influence every aspect of the beam's handling, including its acceleration, measurement, and transport. The effects of spece-charge must be carefully considered in all of these beam handling procedures in order to deliver the highest brightness beams possible. This dissertation investigates the space-charge dominated physical processes involved in the acceleration and propagation, emittance measurement, and magnetic compression fo photoinjector beams. In the analysis of the behavior of these beams, emphasis is placed on the techniques used to compensate for space-charge forces, and maximize beam brightness. The rectilinear motion of a space-charge cominated beam is analyzed, including both linear and nonlinear self forces, in order to determine the evolution of the beam's transverse emittance as it is accelerated and transported through the photoinjector. It is found that the emittance can be made to oscillate by judicious use of external forces, and that this oscillation can be manipulated to minimize the beam's emittance, compensating for the effects of both linear and nonlinear space-charge forces, at a given location of interest. The creation of a high brightness beam in the presence of emittance oscillations is critically dependent on phase space diagnosis. Thus the measurement of emittance of intense beams is investigated experimentally, theoretically, and in simulation, for quadrupole scanning and multi-slit based measurement techniques. The quadrupole scanning method is found to have systematic errors for space-charge dominated beams, and experimental measurements using this technique give consistently higher emittance values than both the slit-based measurements and simulations. Finally, the measurement of emittance growth and transverse phase space distortions induced by magnetic compression of the beam to sub-picosecond lengths is described. A clear bifurcation of the phase space is observed when the beam is strongly compressed. This effect is found to be correlated to the folding of the beam distribution in configuration space.
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title: Plasma Wake-field Acceleration in the Blowout Regime
format: thesis
year: 1998
1 author: N. Barov
abstract: The blowout regime is a limiting case of a nonlinearly excited plasma wave that overcomes problems identified in past linear regime studies of the plasma wake-field accelerator. Here, it is studied with the aid of simulations and by conducting experiments. Fluid model and particle-in-cell simulations have been used to study the propagation of drive pulses short enough to couple to the plasma mode, over distances comparable to the energy depletion length. Evolution of the beam radial dynamics in the simulations, and the associated wake-field phase shifts, outline phenomena important for achieving the maximum energy gain for the accelerated electrons. An experiment to demonstrate long-term drive beam propagation of a short electron pulse in an underdense plasma shows good agreement with simulations, with minor differences thought to originate from the limitation of simulating only a two-dimensional system. An experiment to measure electron acceleration is part of an ongoing effort, and has shown a 2.6 MeV energy gain and a 22 MeV/m acceleration gradient.
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title: Design, Construction and Testing of a Radiofrequency Electron Photoinjector for the Next Generation Linear Collider
format: thesis
year: 1997
1 author: E. Colby
abstract: The design of a high bunch charge (8 nC), low emittance (< 20 nn-mr) radiofrequency electron photoinjector matched to the requirements of the TESLA Test Facility is presented. A 1.625-cell iris-coupled pi-mode structure with high average accelerating gradient is chosen for its high shunt, impedance, simplicity, and ability to accomodate an externally mounted solenoid for simultaneous beam divergence control and emittance compensation. A novel split-solenoid focussing assembly is employed, allowing emittance compensated beam extraction over a wide range of gun gradients. Beam optics are optimized for an overall injector consisting of the electron gun followed by one linac capture section (providing acceleration to 18 MeV), and a dipole chicane for magnetic bunch compression to achieve a bunch length sigma_z of 1 mm. Analytic and simulation work yielding a space charge emittance compensated gun design in a new high charge regime is detailed. Experimental measurements made on a prototype gun and injector at the Argonne Wakefield Acclerator Facility are detailed. Experimental results indicate a beam of 8 nC charge with bunch length ~ 1.1 mm has been produced at emittances of less than 60 x 60 pi mm-mr at an energy of 16 MeV with an energy spread of 240 keV. Experimental results, although in significant disagreement (by a factor of two) with initial simulations (which assume idealized laser properties), are in fair agreement with simulations carried out using the measured performance of the laser.
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title: Observation of High Gain and Intensity Fluctuations in Self-Amplified Spntaneous Emission free-Electron Lasers
format: thesis
year: 1998
1 author: M. Hogan
abstract: This thesis presents the results of two recent free electron laser (FEL) experiments operating in the self amplified spontaneous emission (SASE) mode. An X-ray laser would offer a unique way to explore the structure of matter at the atomic and molecular scale. Among the various schemes proposed to reach this wavelength region, the free electron laser, operating without mirrors in a self amplified spontaneous emission mode offers a favorable scaling law. It has also been shown that utilizing state of the art linear accelerators and electron sources it is possible to build an X-ray SASE FEL, and this has lead to two major proposals to build a SASE X-ray FEL, one at SLAC, the other at DESY. The theory on which the SASE X-ray FEL is based, has been developed over many years, but the experimental data to support it are few and incomplete. Very large gain in SASE has so fr been observed in the centimeter to millimeter waves, and in the medium infrared (IR) at Los Alamos; recently gain in the near IR has been observed at Orsay and at Brookhaven. The intensity distribution function has been previously measured only for spontaneous undulator radiation, with no amplification, and long bunches. This theseis analyzes two recent experiments designed to verify the models of high gain FELs. High gain FEL theory is reviewed with a emphasis on the characteristics of SASE measurable by these experiments. The accelerator, beamline components and diagnostics are described with an emphasis on the measurements. The FEL undulators and optical diagnostics are also described, again with an emphasis on the measurements. The experimental data are compared to analytic models, where applicable, and to computer simulation codes.
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title: Experimental Characterization of the Saturating, Near Infrared, Self-Amplified Spontaneous Emission Free Electron Laser: Anayasis of Radiation Properties and Electron beam Dynamics
format: thesis
year: 2002
1 author: A. Murokh
abstract: In this work, the main results of the VISA experiment (Visible to Infrared SASE Amplifier) are presented and analyzed. The purpose of the experiment was to build a state-of-the-art single pass self-amplified spontaneous emission (SAS) free electron laser (FEL) based on a high brightness electron beam, and characterize its operation, including saturation, in the near infrared spectral region. This experiment was hosted by Accelerator Test Facility (ATF) at Brookhaven National laboratory, which is a users facility that provides high brightness relativistic electron beams generated with the photoinjector. During the experiment, SASE FEL performance was studied in two regimes: a long bunch, lower gain operation; and a short bunch high gain regime. The transition between the two conditions was possible due to a novel bunch compressionmechanism, which was discovered in teh course of the experiment. This compression allowed the variation fo peak current in teh electron beam before it was launched into the 4-m VISA undulator. In the long bunch regime, a SASE FEL power gain length of 29 cm was obtained, and the generated radiation spectral and statistical properties were characterized. In the short bunch regime, a power gain length of under 18 cm was achieved at 842 nm, which is at least a factor of two shorter than ever previously achieved in this spectral range. Further, FEL saturation was obtained before the undulator exit. The FEL system's performance was measured along the length of the VISA undulator, and in the final state. Statistical, spectral and angular properties of the short bunch SASE radiation have been measured int eh exponential gain regime, and at saturation. One of the most important aspects of the data analysis presented in this thesis was the development and use of start-to-end numerical simulations of the experiment. The dynamics of the ATF electron beam was modeled starting from teh photocathode, through acceleration, transport, and inside the VISA undulator. The model allowed simulation SASE process for different beam conditions, including the effects of the novel bunch compression mechanism on the electron beam 6-D phase space distribution. The numberical simulations displayed an excellent agreement with the experimental data, and became key to understanding cimplex dynamics of the SASE FEL process at VISA.
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title: Coherent Radiation Diagnosis of Self Amplified Spontaneous Emission free Electron Laser-Derived Electron beam Microbunching
format: thesis
year: 1999
1 author: A. Tremaine
abstract: This thesis presents an experiment in which the longitudinal profile of an electron beam was measured by studying the coherrent transition radiation (CTR) emitted when the beam strikes a thin conducting foil. A high gain Self Amplified Spontaneous Emission (SASE) Free Electron laser (FEL) was implemented and the source fo the longitudinal beam profile modulation. Diagnostics measuring very short periodic electron beam modulation will be necessary for future experiments in which the modulating wavelength will be several microns. Up to this point, there have been reliable tools used for such longitudinal beam profile measurements. However, the limits of resolution in these devices are being approached and new and less expensive methods are needed. Transition radiation from an electron beam striking a metallic surface is an easily emplementable and inexpensive diagnostic and is shown to be a reliable diagnostic for the future. This thesis presents the theoretical calculation of the expected CTR photon spectrums and compares the analysis with an experiment recently done on an electron beam which has been longitudinally modulated by SASE FEL. The accelerator beamline and its parameters important to the experiment are described. Also, discussed, are the requirements on the system needed for the best CTR emission possible and the importance in choosing a good metallic radiating foil. Results from the data are compared with computer simulation in which these issues are taken into account. Also, the experimental results point out approximations used in traditional transition radiation modeling that will not be valid in future CTR experiemnts and more rigorous theorectical analysis will be needed.
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title: Numerical Studies for a Single Pass High Gain Free-Electron Laser
format: thesis
year: 1999
1 author: Sven Reiche
abstract: The impo rtance of single pass high gain Free-Electron Lasers (FEL) is due to its the unique char acteristic of the short, high intense radiati on pulse at a well defined, tunable frequency. Extending the radiation wavelength to the ultra violet or X-ray region is an on-going research project and will open new branches of experiments based on this radiation. Several projects are proposed or currently unde rc onstruction such as the integrated X-ray FEL at the TESLA linear collider and the VUV FEL at the TESLA Test Facility. For all of these projects a detailed study is of importance, including all effects which might influence the FEL performan ce. In particular, for an FEL operating at X-ray wavelengths, the amplification process is more sensitive to any kind of disturbance. Some of these perturbations influence the FEL per formance in a complex way. Because existing codes are not capable to cover these proble ms the simulation cod eGEN ESIS 1.3 h as b een developed. The new features of GENESIS 1.3 incl udes the discretization of the radiation field on a Cartesian grid and the input of arbitrary profiles of the undulator field and the electron beam in longitudinal direction. The code is capable of covering aspects such as beam halos, wake fields a nd non-periodic focusing structures to name a few. Using GENESIS 1.3 special aspects of the TTF-FEL and TESLA FEL have been studied. The simulation cove rs the transverse motion of the electron beam, the impact of wake fields as well as the increase of the energy spread due to the qua ntum fluctuation of the incoherently emitted radiation.
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title: Experimental Requirements for a Self Amplified Spontaneous Emission Test Syste: Design, Construction, Simulation and Analysis of the UCLA High Gain Free Electron Laser
format: thesis
year: 1995
1 author: G. Travish
abstract: This thesis presents the UCLA high gain free electron laser (FEL). FELs have long been proposed as sources of radiation in regimes diffuclt to obtain with conventional lasers. High average power, microwave, far-IR, UV and X-ray are regimes and charactericstics difficult for conventional lasers to achieve. Free Electron Lasers, in principle, do not suffer from the same limitations (atomic transitions, heat dissimpation, thermal lensing, etc.) as atomic and molecular lasers. However, oscillator FELs are still impeded by the need for suitable optics. A high gain FEL, on the other had, requires no oscillator, and can operate in regimes where high quality optics are unavailable. A high gain FEL which requires no input signal, and amplifies the spontaneous emission produced by its own beam is said to operating in the Self Amplified Spontaneous Emission (SASE) mode. A SASE FEL can operate at wavelenghts where no conventional coherent sources are available (i.e., x-rays). High gain FEL experimental work has been very limited, with only a few experiments performed at "optical" wavelengths. No SASE FEL has been operated outside the microwave regime. This work describes an experiment designed to verify the models of high gain FELs, and operate an infrared SASE FEL. High gain FEL theory is reviewed. An analysis of the PBPL FEL is made using analytic as well as numerical models. Experimental effects such as the limited accuracy of beam diagnostics are taken into account. It is show that there are great experimental problems to over come in attempting to determine the preformance and effective start-up level of the PBPL FEL. Some of these difficulties are expected to be shared by future high gain FELs. The PBPL experiment is described with an exphasis on operational problems significant to the FEL. The accelerator, beamline components and diagnostics are described in detail along with design issues and performance parameters. The FEL undulator and optical diagnostics are also described and test data is given. This thesis shows the complexities associated with a high gain FEL, and attempts to determine what can be learned from such an experiment.
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title: Transverse Beam Break-Up in Linear Electron Accelerators
format: thesis
year: 1989
1 author: G. Travish
abstract: Extensive work has been done involving beam break-up (BBU) in linear accelerators. There is a wealth of analytic and experimental data available. Recently, a comprehensive computer code was developed at LBL to study beam break-up due to wakefields. This thesis will review the basic physics of beam break-up in linear accelerators. A survey of BBU analytic work and a re-derivation of a few significant results will be given at a level better suited to advanced undergraduate and graduate students. Numerical work will center on two devices: the Advanced Test Accelerator at LLNL; and the Two Beam Accelerator, a proposed high gradient accelerator. Additionally, experimental data will be reviewed (for the ATA) to establish agreement between analytic, numerical and experimental work. Although the numerical simulations allow for extremely complex cases to be quickly analyzed, the simpler cases addressed by analytic work (theory) will be heavily emphasized. It will be an aim of this thesis to be at a level which is comprehendible to a student.
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title: Plasma Density Transition Trapping of Electrons in Plasma Wakefield Accelerators
format: thesis
year: 2004
1 author: M.C. Thompson
abstract: Plasma based electron beam sources, which are now under development, will produce beams with much higher particle densities than are currently available. Plasma sources can create beams with brightness (the measure of achievable beam density) orders of magnitude greater than radio frequency photoinjectors, the current state-of-the-art. Plasma density transition trapping is one example of the many plasma electron beam source schemes under development. Plasma density transition trapping is a recently proposed self-injection mechanism for plasma wake field accelerators. The technique uses a sharp downward plasma density transition to trap and accelerate background plasma electrons in a plasma wake field. This dissertation examines the different regimes in which plasma density transition trapping can operate and the quality of the electron beams captured in terms of emittance, energy spread, and brightness. This is accomplished using two-dimensional Particle-In-Cell (PIC) simulations which show that the captured beam parameters can be optimized by manipulating the overall plasma density, as well as the density profile. A general set of scaling laws is developed that predicts how the brightness of transition trapping beams, or the beams produced by any plasma system, scales with the plasma density. These scaling laws predict that beam brightness increases linearly with the plasma density of the source. The design and execution of the first plasma density transition trapping experiment is also documented in this dissertation. Plasma with density on the order of 10E-13 / cm^-3 was used in the experiment. Plasma density transitions steep enough to produce trapping at this density were created and measured. Interaction between the plasma transition and a driving electron beam pulse did not, however, produce trapped electrons. Detailed measurements of the drive beam parameters revealed that it did not meet the trapping experiment design criteria. Simulations using the measured plasma and beam parameters predict zero captured charge in agreement with observations.
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title: Design, Construction, Simulation and Implementation of a Magnetic Electron Bunch Compressor
format: thesis
year: 2004
1 author: R. Agustsson
abstract: Throughout the last half century, electron beam brightness has been the quantitative benchmark for measuring the advancement of light sources. The generation of these high brightness beams often require compaction techniques to decrease the longitudinal extent of the beam, consequently increasing the current. Presently, one of the popular methods for obtaining the amplification of current in electron beams is by magnetic compression. Magnetic bunch compression is a fairly common tool utilized by the accelerator community, however it can still be considered a stand alone instrument since the details of the physics that affect this type of electron beam manipulation are not categorically well understood. Of particular interest is the coupling between the longitudinal and transverse phase space planes and the forces responsible for the effects that contribute to the dilution of the transverse emittance in the bend plane of the beam. This thesis details the design, construction, simulation, testing and implementation of a magnetic bunch compressor at the Accelerator Test Facility (ATF), a leading photoinjector users' facility located within the Brookhaven National Laboratory (BNL). Simulations show that the magnetic chicane bunch compressor increases the peak current of the ATF electron beam by a significant factor while only sustaining a nominal increase in the bend plane emittance. Additionally, some interesting effects from the inclusion of coherent synchrotron radiation in the simulations such as linearization of the lower energy portion of the beam and smearing of the longitudinal distribution are demonstrated.
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title: Experimental and Analytical Study of a High Gain Self Amplified Spontaneous Emission Free Electron Laser Operating in a Large Spectral Bandwidth Regime
format: thesis
year: 2005
1 author: Gerard Andonian
abstract: The drive to create and measure ultra-short pulses in the x-ray regime advances the ongoing development of free electron lasers (FEL). Several proposed schemes, to shorten the pulse length of the radiation, involve driving the FEL with a chirped (linear longitudinal phase space correlation) electron beam in the self amplified spontaneous emission (SASE) mode. This dissertation examines the experiments conducted under such conditions, canvassing analytical and numerical studies of beam dynamics and radiation properties, experimental observations, and descriptions of the development of novel diagnostics. The VISA (Visible-Infrared SASE Amplifier) program has achieved saturation at 840 nm within a 4 m long undulator. A novel bunch compression mechanism during transport was discovered and ultimately responsible for the high peak current required to drive the FEL. Start-to-end simulations, detailing the dynamics from electron beam inception at the photocathode to the FEL radiation properties at the undulator, were successfully benchmarked to observable data.
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title: Successful beam-beam tuneshift compensation
format: thesis
year: 2005
1 author: Kip Bishofberger
abstract: The performance of synchrotron colliders has been limited by the beam-beam limit, a maximum tuneshift that colliding bunches could sustain. Due to bunch-to-bunch tune variation and intra-bunch tune spread, larger tuneshifts produce severe emittance growth. Breaking through this constraint has been viewed as impossible for several decades. A device known as the Tevatron Electron Lens (TEL) has been designed, constructed, and tested in the Tevatron at Fermilab. This device produces a pulsed beam of electrons which interact with the antiproton bunches in the Tevatron. The peak beam current is typically 2 A, and the beam energy can range from 4 keV to 12 keV. The bunches interact with the beam
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title: An adjustable short-focal length, high-gradient PMQ electron-beam final-focus system for the PLEIADES ultra-fast x-ray Thomson source
format: thesis
year: 2006
1 author: Jae Lim
abstract: In the span of a 100 year since the discovery of Ørst x-rays by Roentgen that won him the Ørst Nobel prize in physics, several types of radiation sources have been developed. Currently, radiations at extremely short wavelengths have only been accessed at synchrotron radiation sources. However, the current 3rd gener- ation synchrotron sources can only produce x-rays of energy up to 60 keV and pulse lengths of several picoseconds long. But needs for shorter wavelength and shorter pulse duration radiations demanded by scientists to understand the nature of matter at atomic/molecular scale initiated the new scientiØc research for the production of sub-picosecond, hard x-rays. At the Lawrence Livermore National Laboratory, a Thomson x-ray source in the backscattering mode { a head-on col- lision between a high intensity Ti:Sapphire Chirped Pulse AmpliØcation laser and a relativistic electron beam { called the PLEIADES (Picosecond Laser-Electron Inter-Action for the Dynamical Evaluation of Structures) laboratory has been developed. Early works demonstrated the production of quasi-monochromatic, femto-second long, hard x-rays. Initially reported x-ray ∞ux was in the low range xx of 10 5 photons per shot. During the early stage of PLEIADES experiments, 15 T/m electromagnet Ønal focusing quadrupoles (in a triplet lattice conØguration) were employed to focus the beam to a 40-50 π m spot-size. A larger focal spot-size beam has a low- density of electron particles available at the interaction with incident photons, which leads to a low scattering probability. The current dissertation shows that by employing a 560 T/m PMQ (Permanent-Magnet Quadrupole) Ønal focus system, an electron beam as small as 10-20 π m can be achieved. The implementation of this Ønal focus system demonstrated the improvement of the total x-ray ∞ux by two orders of magnitude. The PMQ Ønal focus system also produced small electron beams consistently over 30-100 MeV electron beam energy, which enabled the production of x-ray energy over 40-140 keV. In this dissertation, the PLEIADES Thomson x-ray facility will be described in detail includes the 100 MeV linac and the FALCON laser system. Later, we will discuss the design, construction and implementation of the PMQ Ønal focus system in the beamline. The measurement of electron beam parameters before and after the Ønal focus system will be presented. The beam measurements at the interaction region were accomplished with the use of both OTR (Optical Transition Radiation) imaged by a CCD camera and the fast streak camera for respective spatial and temporal alignments. The theoretical analysis in \real beam" eÆects and spacetime beam jitter eÆects will be given to help understand the observations. A 3D simulation tool developed for x-ray data analysis was used to provide direct comparisons with the x-ray ∞ux, spectrum distribution and transverse x-ray profile.
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title: Energy Loss of a High Charge Bunched Electron Beam in Plasma: Analysis
format: thesis
year: 2003
4 authors: N. Barov | J.B. Rosenzweig | M.C. Thompson | R. Yoder
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title: Longitudinal Shaping of Relativistic Bunches of Electrons Generated by an RF Photoinjector
format: thesis
year: Oct 2007
1 author: R. J. England
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: Generation of Narrow-Band Terahertz Coherent Cherenkov Radiation in a Dielectric Wakefield Structure
format: thesis
year: June 2009
1 author: A. M. Cook
abstract: This study explores the use of a dielectric-lined waveguide structure as a means of producing narrow-band terahertz radiation in the f orm of electron- beam-driven coherent Cherenkov radiation wakefields. This concept builds on previously studied scenarios such as the Cherenkov maser an d the Cherenkov free-electron laser. It is distinct in that it relies solely on coherent wakefield ex- citation instead of a microbunching instability gain proce ss, in analogy to the superradiant regime of FEL operation. The narrow bandwidth is due to the single-mode nature of the excitation, enabled by the exclus ion (due to coherence) of discrete waveguide modes with wavelengths shorter than t he driving electron bunch length. This allows an inherently broadband beam curr ent profile to ra- diate power into a single frequency, which is selectable by a ppropriate choice of design parameters. The theoretical component of this dissertation is aimed at m aking predictions for comparison with experimental results. The functional f orm and propagating xviii mode frequencies of the electromagnetic fields in the wavegu ide structure are found by eigenmode solution in the source-free case beginni ng from Maxwell’s equations; the response of the structure to a driving electr on bunch is then found using a wakefield formalism. Predictions for the frequencie s and radiated energy levels obtained from this analysis are corroborated comput ationally using the commercial particle-in-cell simulation code oopic pro . The experiment is designed to be a proof-of-principle demon stration of the effectiveness of this scenario in converting the energy in an electron beam into electromagnetic radiation. We present detailed measureme nts showing a narrow emission spectrum peaked at 367 ± 3 GHz from a 1 cm long fused silica capil- lary tube with sub-mm transverse dimensions, matching the p redicted (analytical and computational) TM 01 mode resonance to within 1% error. This measurement confirms the expected preferential coherent excitation of t he TM 01 mode over the HEM 11 mode, which lies nearby in frequency but still decisively ou tside the error estimate established over multiple measurements. The meas ured 3 dB band- width is on the order of . 10% and is seen to be transform-limited. We observe a 100 GHz shift in the emitted central frequency when the tube wall thickness is changed by 50  m, demonstrating the modular tunability of the source. Cal- ibrated measurements of the radiated energy register up to 1 0  J per 60–80 ps pulse for an incident sub-picosecond electron beam carryin g 200 pC of charge, corresponding to a peak power of approximately 150 kW. A case study consider- ing the implementation of this scenario using a 10-cm-long s tructure with smaller transverse dimensions indicates a possible yield of 50 MW pe ak power at 1.8 THz and 0.1% bandwidth. This dissertation reports the first direct measurements of n arrow-band THz coherent Cherenkov radiation driven by a sub-picosecond el ectron beam in a die- xix lectric wakefield structure, representing a successful ada ptation of the previously proven Cherenkov FEL concept to the realm of ultra-short ele ctron beams such as are available in state-of-the-art user facilities aroun d the world. These results prove the potential of this method to produce tunable, narro w-band, pulse-length- variable, multi-megawatt peak-power radiation at f > 1 THz in existing modern electron accelerators.
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title: Inverse Compton Scattering and Characterization of the Source at the Brookhaven National Laboratory Accelerator Test Facility
format: thesis
year:
1 author: Oliver Williams
abstract: An inverse Compton scattering source at the Accelerator Test Facility has been thoroughly simulated and characterized in preparation for future applications of this unique source of X - ray pho t ons. Simulations were performed using a 3D time and frequency - domain code for various beam parameters achievable at ATF. Calculations show a total flux of 1x10 9 photons per shot is obtainable with photon energies ranging from 5 to 9 keV over the full divergence angle of 8 mrad. The spectral bandwidth is dominated by electron beam angles and energy spread. In experiment, iron and nickel foils were used as an ene rgy diagnostic due to their ix strongly absorbing K - edges near the design central photon energy. Measurements indicate 2x10 6 photons within a 1 mrad emission angle and 4.0±0.8% bandwidth. Simulation is relied upon for information on source size and pulse le ngth and predicts 22 μm and 0.3 - 4 ps, respectively, resulting in a peak brightness of 2.4x10 19 photons/mm 2 /mrad 2 (0.1% bandwidth). A circularly polarized laser resulted in a source energy and angular distribution indicative of circularly polarized x - rays.
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title: Generation and Amplification of Coherent Radiation with Optical Orbital Angular Momentum in a Free-Electron Laser
format: thesis
year: 2011
1 author: Erik Hemsing
abstract: The object of this work is to examine how coherent light that carries orbital angular momentum (OAM) can be generated and amplified in a single pass, high-gain free-electron laser (FEL) at the fundamental operating frequency. This concept unites two rapidly expanding, but at present largely non-overlapping fields of study: high-order OAM light modes, which interact in new ways with matter, and FELs, in which a relativistically energetic electron beam emits coherent, ultra high-brightness, highly frequency-tunable light. The ability to generate OAM light in an FEL enables new regimes of laser interaction physics to be explored at wavelengths down to hard x-rays.
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title: Gigavolt-per-Meter Wakefields in Annular Dielectric Structures
format: thesis
year: 2014
1 author: B.O'Shea
abstract: A wakefield accelerator uses a medium capable of sustaining appropriate electric fields to transfer energy from a drive beam to a witness beam. Examples of such systems include electron beam driven plasma wakefields [1, 2], laser driven plasma wakefields [3] and electron beam driven dielectric wakefield structures [4, 5, 6]. Dielectrics and plasmas are of particular interest because they are capable of maintaining electric fields on the order of GVm −1 and in the case of plasmas upwards of TVm −1 [7]. These systems provide a significant step beyond current radiofrequency accelerating structures capable of peak electric fields on the order of 100 MVm −1 and average effective electric fields of 20 MVm −1 [8]. Furthermore, beam driven dielectric structures produce a wakefield which is phase synchronous with the beam, these structures do not suffer from transit time or dephasing effects [9, 10]. In the case of dielectric wakefield accelerators, the structures under study in this publication, the modes generated by the driving beam are in the terahertz (THz) regime. Thus development of dielectric wakefield accelerators is seen as a path to smaller, more compact accelerating systems and as a means to generate THz radiation [11]. Here we demonstrate a beam-driven iidielectric wakefield accelerating structure that produces sustained fields of 1.35 GVm −1 for many hundreds of thousands of drive beam pulses. In addition we show beam-driven dielectric structures have the potential to produce single mode, high spectral purity THZ radiation of unprecedented energy scale.
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