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Start-Up of FEL Oscillator from Shot Noise

Description: In free-electron laser (FEL) oscillators, as inself-amplified spontaneous emission (SASE) FELs, the buildup of cavitypower starts from shot noise resulting from the discreteness ofelectronic charge. It is important to do the start-up analysis for thebuild-up of cavity power in order to fix the macropulse width from theelectron accelerator such that the system reaches saturation. In thispaper, we use the time-dependent simulation code GINGER [1]toperformthis analysis. We present results of this analysis for theparameters of the Compact Ultrafast TErahertz FEL (CUTE-FEL) [2]beingbuilt atRRCAT.
Date: January 25, 2007
Creator: Kumar, V.; Krishnagopal, S. & Fawley, W.M.
Partner: UNT Libraries Government Documents Department

Life on the edge: squirrel-cage fringe fields and their effects in the MBE-4 combiner experiment

Description: The MBE-4 combiner experiment employs an electrostatic combined-function focusing/bending element, the so-called ``squirrel-cage`` just before the actual merging region. There has been concern that non-linear fields, primarily in the fringe regions at the beginning and end of the cage, may be strong enough to lead to significant emittance degradation. This note present the results of numerical calculations which determined the anharmonic, non-linear components of the 3D fields in the cage and the resultant, orbit-integrated effects upon the MBE-4 beamlets. We find that while the anharmonic effects are small compared to the dipole deflection, the resultant transverse emittance growth is significant when compared to the expected value of the initial emittance of the individual beamlets.
Date: February 1, 1996
Creator: Fawley, W.M.
Partner: UNT Libraries Government Documents Department

{open_quotes}Optical Guiding{close_quotes} limits on extraction efficiencies of single-pass, tapered wiggler amplifiers

Description: Single-pass, tapered wiggler amplifiers have an attractive feature of being able, in theory at least, of extracting a large portion of the electron beam energy into light. In circumstances where an optical FEL wiggler length is significantly longer than the Rayleigh length Z{sub R} corresponding to the electron beam radius, diffraction losses must be controlled via the phenomenon of optical guiding. Since the strength of the guiding depends upon the effective refractive index n exceeding one, and since (n-1) is inversely proportional to the optical electric field, there is a natural limiting mechanism to the on-axis field strength and thus the rate at which energy may be extracted from the electron beam. In particular, the extraction efficiency for a prebunched beam asymptotically grows linearly with z rather than quadratically. We present analytical and numerical simulation results concerning this behavior and discuss its applicability to various FEL designs including oscillator/amplifier-radiator configurations.
Date: August 1, 1995
Creator: Fawley, W.M.
Partner: UNT Libraries Government Documents Department

Optical modeling of induction-linac driven free-electron lasers

Description: The free-electron laser (FEL) simulation code FRED, developed at Lawrence Livermore National Laboratory (LLNL) primarily to model single-pass FEL amplifiers driven by induction linear accelerators, is described. The main emphasis is on the modeling of optical propagation in the laser and on the differences between the requirements for modeling rf-linac-driven vs. induction-linac-driven FELs. Examples of optical guiding and mode cleanup are presented for a 50 ..mu..m FEL.
Date: March 31, 1986
Creator: Scharlemann, E.T. & Fawley, W.M.
Partner: UNT Libraries Government Documents Department

Modeling of beam focusing and kink instability for colliding relativistic electron and positron beams

Description: A simulation code has been developed and exercised to study the time evolution and resultant liminosity of intersecting pulses of electrons and positrons. Under the extreme conditions of high current and small radius recently proposed for the SLAC facility, the pulses mutually pinch to a smaller mean radius than that achieved by free flight alone. The effective luminosity is enhanced by a factor of approx. 3 in the best case, corresponding to zero initial emittance and pulse length equal 1/4 the mean betatron wavelength. Gaussian profiles in the longitudinal and transverse coordinates are preferred over flat profiles in order to minimize disruptive oscillations in radius which reduce luminosity. A second potential source of disruption is the kink instability. This is always present to some degree during the interaction of opposed pulses. However, the maximum growth rate is only one half the angular betatron frequency and therefore mode growth is limited to a very low level if the pulses are short compared with a betatron wavelength.
Date: February 8, 1980
Creator: Fawley, W.M. & Lee, E.P.
Partner: UNT Libraries Government Documents Department

Particle in cell simulations of disruption

Description: An improved numerical particle simulation code has been developed to study the electrodynamical interaction of colliding electron and positron beams. Contrary to previously reported work (which was in error due to a coding mistake), it is found that the maximum enhancement in luminosity due to the mutual pinch effect can exceed a factor of 6 for beams with Gaussian radial profiles and approaches 5 for uniform profiles. These new results are in much better agreement with those originally found by R. Hollebeek.
Date: June 1, 1987
Creator: Fawley, W.M. & Lee, E.P.
Partner: UNT Libraries Government Documents Department

Output characteristics of SASE-driven short wavelength FEL`s

Description: This paper investigates various properties of the ``microspikes`` associated with self-amplified spontaneous emission (SASE) in a short wavelength free-electron laser (FEL). Using results from the 2-D numerical simulation code GINGER, we confirm theoretical predictions such as the convective group velocity in the exponential gain regime. In the saturated gain regime beyond the initial saturation, we find that the average radiation power continues to grow with an approximately linearly dependence upon undulator length. Moreover, the spectrum significantly broadens and shifts in wavelength to the redward direction, with{ital P(w)} approaching a constant, asymptotic value. This is in marked contrast to the exponential gain regime where the spectrum steadily narrows, {ital P(w)} grows, and the central wavelength remains constant with {ital z}. Via use of a spectrogram diagnostic {ital S(w,t)}, it appears that the radiation pattern in the saturated gain regime is composed of an ensemble of distinct ``sinews`` whose widths AA remain approximately constant but whose central wavelengths can ``chirp`` by varying a small extent with {ital t}.
Date: February 1, 1997
Creator: Fawley, W.M.
Partner: UNT Libraries Government Documents Department

Multidimensional simulation studies of the SELENE FEL oscillator/buncher followed by a radiator/amplifier output scheme

Description: We analyze and present numerical simulations of the so-called electron output scheme [G. I. Erg et al., 15th Int. FEL Conf., The Hague, The Netherlands, 1993, Book of Abstracts p. 50; Preprint Budker INP 93-75] applied to the SELENE proposal of using a high power FEL to illuminate satellite solar cells. In this scheme, a first stage FEL oscillator bunches the electron beam while a second stage ``radiator`` extracts high power radiation. Our analysis suggests only in the case where the radiator employs a long, tapered undulator will the electron output scheme produce a significant increase in extraction efficiency over what is obtainable from a simple, single-stage oscillator. 1- and 2-D numerical simulations of a 1.7{mu}m FEL employing the electron output scheme show reasonably large bunching fractions ({approximately} 0.3--0.4) at the output of the oscillator stage but only {le}2% extraction efficiency from the radiator stage.
Date: February 1, 1995
Creator: Hahn, S.J. & Fawley, W.M.
Partner: UNT Libraries Government Documents Department

A pre- and post-processor for the ICOOL muon transport code

Description: ICOOL[1] is a Fortran77 macroparticle transport code widely used by researchers to study the front end of a neutrino factory/muon collider[2]. In part due to the desire that ICOOL be usable over multiple computer platforms and operating systems, the code uses simple text files for input/output services. This choice together with user-driven requests for greater and greater choice of lattice element type and configuration has led to ICOOL input decks becoming rather difficult to compose and modify easily. Moreover, the lack of a standard graphical post-processor has prevented many ICOOL users from extracting all but the most simple results from the output files. Here I present two attempts to improve this situation: First, a simple but quite general graphical pre-processor (NIME) written in the Tcl/TK[3] to permit users to write and maintain ASCII-formatted input files by use of simple macro definitions and expansions. Second, an interactive post-processor written in Fortran90 and NCAR graphics, which allows users to define, extract, and then examine the behavior of various particle subsets. In this paper I show some examples of use of both the pre- and post-processor for a standard ICOOL run.
Date: May 30, 2001
Creator: Fawley, W.M.
Partner: UNT Libraries Government Documents Department

Radially resolved simulation of a high-gain free electron laser amplifier

Description: The results of a two-dimensional simulation of a high-gain free electron laser (FEL) amplifier is presented. The simulation solves the inhomogeneous paraxial wave equation. The source term is radially resolved and is obtained by tracking the interaction of the laser field with localized macroparticles.
Date: June 27, 1983
Creator: Fawley, W.M.; Prosnitz, D.; Doss, S. & Gelinas, R.
Partner: UNT Libraries Government Documents Department

Use of the Lorentz-Boosted Frame Transformation to Simulate Free-Electron Laser Amplifier Physics

Description: Recently [1]it has been pointed out that numerical simulation of some systems containing charged particles with highly relativistic directed motion can by speeded up by orders of magnitude by choice of the proper Lorentz boosted frame. A particularly good example is that of short wavelength free-electron lasers (FELs) in which a high energy (E0>_ 250 MeV) electron beam interacts with a static magnetic undulator. In the optimal boost frame with Lorentz factor gamma F, the red-shifted FEL radiation and blue shifted undulator have identical wavelengths and the number of required time-steps (presuming the Courant condition applies) decreases by a factor of g2 F for fullyelectromagnetic simulation. We have adapted the WARP code [2]to apply this method to several FEL problems including coherent spontaneous emission (CSE) from pre-bunched e-beams, and strong exponential gain in a single pass amplifier configuration. We discuss our results and compare with those from the"standard" FEL simulation approach which adopts the eikonal approximation for propagation ofthe radiation field.
Date: July 27, 2008
Creator: Fawley, W.M. & Vay, J.-L.
Partner: UNT Libraries Government Documents Department

THE SECOND STAGE OF FERMI@ELETTRA: A SEEDED FEL IN THE SOFT X-RAY SPECTRAL RANGE

Description: The second stage of the FERMI FEL, named FEL-2, is based on the principle of high-gain harmonic generation and relies on a double-seeded cascade. Recent developments stimulated a revision of the original setup, which was designed to cover the spectral range between 40 and 10 nm. The numerical simulations we present here show that the nominal (expected) electron-beam performance allows extension of the FEL spectral range down to 4 nm. A significant amount of third harmonic power can be also expected. We also show that the proposed setup is flexible enough for exploiting future developments of new seed sources, e.g., high harmonic generation in gases.
Date: August 14, 2009
Creator: Allaria, E.; DeNinno, G. & Fawley, W. M.
Partner: UNT Libraries Government Documents Department

Heavy-ion inertial fusion: influence of target gain on accelerator parameters for vacuum-propagation regimes in reaction chambers

Description: Target physics imposes requirements on the design of inertial fusion drivers. The influence of beam propagation in near vacuum fusion reaction chambers is evaluated for the relation between target gain and the phase-space requirements of heavy-ion accelerators. Initial results suggest that neutralization of the ion beam has a much greater positive effect than the deleterious one of beam stripping provided that the fusion chamber pressure is < 10/sup -3/ torr (of Li vapor or equivalent).
Date: March 4, 1982
Creator: Mark, J.W.K.; Bangerter, R.O.; Barletta, W.A.; Fawley, W.M. & Judd, D.L.
Partner: UNT Libraries Government Documents Department

Heavy-ion inertial fusion: comments on achievable temperatures for disk-heating experiments using proposed accelerator

Description: Calculations suggest that experiments relating to beam deposition, focusing and transport can be performed within the context of current test-bed accelerator design proposals. Since the test beds have lower ion kinetic energy and beam pulse power as compared to reactor drivers, we achieve high-beam intensities at the focal spot by using short focal distance and properly designed beam optics. In this regard, the extremely low beam emittance of suggested multi-beam designs are very useful. Preliminary results suggest that intensities of greater than 100 TW/cm/sup 2/ are achievable. Given these intensities, deposition experiments with heating of disks to greater than 100 eV are expected. We could also expect as much as 1 to 3 kA of incident ion current on these disks with beam intensities almost comparable to that of reactor targets. Thus, if any anomalous plasma effects on deposition emerge, the conditions should be available for testing some of them. On the other hand, these deposition experiments have low ion kinetic energy per nucleon. About 4 to 5 MeV/nucleon is achievable if lighter ions such as sodium were used. But for lighter ions, plasma effects in deposition might be more severe because heavy-ion beams are more stiff.
Date: April 7, 1981
Creator: Mark, J.W.K.; Bangerter, R.O.; Fawley, W.M.; Yu, S.; Garren, A. & Krafft, G.
Partner: UNT Libraries Government Documents Department

GINGER simulations of short-pulse effects in the LEUTL FEL

Description: While the long-pulse, coasting beam model is often used in analysis and simulation of self-amplified spontaneous emission (SASE) free-electron lasers (FELs), many current SASE demonstration experiments employ relatively short electron bunches whose pulse length is on the order of the radiation slippage length. In particular, the low-energy undulator test line (LEUTL) FEL at the Advanced Photon Source has recently lased and nominally saturated in both visible and near-ultraviolet wavelength regions with a sub-ps pulse length that is somewhat shorter than the total slippage length in the 22-m undulator system. In this paper we explore several characteristics of the short pulse regime for SASE FELs with the multidimensional, time-dependent simulation code GINGER, concentrating on making a direct comparison with the experimental results from LEUTL. Items of interest include the radiation gain length, pulse energy, saturation position, and spectral bandwidth. We address the importance of short-pulse effects when scaling the LEUTL results to proposed x-ray FELs and also briefly discuss the possible importance of coherent spontaneous emission at startup.
Date: July 1, 2001
Creator: Huang, Z. & Fawley, W.M.
Partner: UNT Libraries Government Documents Department

Design and construction of a large aperture quadrupole electromagnet for ILSE

Description: We are currently constructing a prototype quadrupole electromagnet for the proposed Induction Linac Systems Experiment (ILSE) at LBNL. ILSE will address many physics and engineering issues relevant to the design of a heavy-ion fusion driver accelerator. The pulsed electromagnet has two layers of current windings and will produce a field gradient of 28 T/m, wi a usable aperture of 6 cm. It operates at a repetition rate of 1 Hz, steady-state. In this paper, we discuss how the interaction of various concerns such as maximum dynamic aperture, short lattice period, field quality, iron yoke weight, heat transfer, and voltage standoff have led to our particular design choices. We also present 2- and 3-D numerical calculations concerning field topography and the results of transport simulations of space-charge dominated ion beams with ILSE parameters.
Date: August 1, 1995
Creator: Fawley, W.M.; Vella, M.C.; Peters, C.; Stuart, M. & Faltens, A.
Partner: UNT Libraries Government Documents Department

Merits of a sub-harmonic approach to a single-pass, 1.5-{Angstrom} FEL

Description: SLAC/SSRL and collaborators elsewhere are studying th physics of a single-pass, FEL amplifier operating in th 1 -- 2 {Angstrom}, wavelength region based on electron beams from the SLAC linac at {approximately} 15 GeV energy. Hoping to reduce the total wiggler length needed to reach saturation when starting from shot noise, we have examined the benefits of making the first part of the wiggler resonant at a subharmonic wavelength (e.g. 4.5 {Angstrom}) at which the gain length can be significantly shorter. This leads to bunching of the electron beam at both the subharmonic and fundaments wavelengths, thus providing a strong coherent ``seed`` for exponential growth of radiation at the fundamental in the second part of the wiggler. Using both multi-harmonic and multi-frequency 2D FEL simulation codes, we have examined the predicted performance of such devices and the sensitivity to electron beam parameters such as current, emittance, and instantaneous energy spread.
Date: March 1, 1995
Creator: Fawley, W.M.; Nuhn, H.D.; Bonifacio, R. & Scharlemann, E.T.
Partner: UNT Libraries Government Documents Department

Design and construction of a large aperture, quadrupole electromagnet prototype for ILSE

Description: We are currently constructing a prototype quadrupole electromagnet for the proposed Induction Linac Systems Experiment (ILSE) at LBL. ILSE will address many physi and engineering issues relevant to the design of a heavy-ion fusion driver accelerator. The pulsed electromagnet has two layers of current windings and will produce a field gradient exceeding 25 T/m at a repetition rate of 1 Hz steady-state. In this paper, we discuss how the interaction of various concerns such as maximum dynamic aperture, short lattice period, field quality, iron yoke weight, heat transfer, and voltage standoff have led to our particular design choices. We also present 2- and 3-D numerical calculations concerning field topography and the results of transport simulations of space-charge dominated ion beams with ILSE parameters.
Date: April 1, 1995
Creator: Stuart, M.; Faltens, A.; Fawley, W.M.; Peters, C. & Vella, M.C.
Partner: UNT Libraries Government Documents Department

BBU and Corkscrew Growth Predictions for the Darht Second Axis Accelerator

Description: The second axis accelerator of the Dual Axis Radiographic Hydrodynamic Test (DARHT-II) facility will produce a 2-kA, 20-MeV, 2-{micro}s output electron beam with a design goal of less than 1000 {pi} mm-mrad normalized transverse emittance. In order to meet this goal, both the beam breakup instability (BBJ) and transverse corkscrew motion (due to chromatic phase advance) must be limited in growth. Using data from recent experimental measurements of the transverse impedance of actual DARHT-II accelerator cells by Briggs et al. [2], they have used the LLNL BREAKUP code to predict BBU and corkscrew growth in DARHT-II. The results suggest that BBU growth should not seriously degrade the final achievable spot size at the x-ray converter, presuming the initial excitation level is of the order 100 microns or smaller. For control of corkscrew growth, a major concern is the number of tuning shots needed to utilize effectively the tuning-V algorithm [3]. Presuming that the solenoid magnet alignment falls within spec, they believe that possibly as few as 50-100 shots will be necessary to set the dipole corrector magnet currents. They give some specific examples of tune determination for a hypothetical set of alignment errors.
Date: June 12, 2001
Creator: Chen, Y.J. & Fawley, W.M.
Partner: UNT Libraries Government Documents Department

High order calculation of the multipole content of three dimensional electrostatic geometries

Description: We present an accurate and simple method of 3-D multipole decomposition of the field of arbitrary electrode geometries. The induced charge on the surface is obtained by inverting the capacity matrix. The multipole moment decomposition of the resulting potential is readily accomplished using Differential Algebra methodology. The method is applied to the focussing lattice geometry of the MBE-4 accelerator at LBL. Multipole terms of up to the order 5 are computed, and a numerical accuracy of < 1% is obtained. The effective quadrupole and dodecapole field strength are in good agreement with previous results. 6 refs., 1 fig.
Date: April 1, 1991
Creator: Berz, M.; Fawley, W.M. & Hahn, K.
Partner: UNT Libraries Government Documents Department

Some proposed disk heating, focusing, and beam-transport experiments for heavy-ion inertial-fusion test facilities

Description: Calculations suggest that experiments relating to disk heating, as well as beam deposition, focusing and transport can be performed within the context of current design proposals for accelerator test-facilities. Since the test-facilities have lower ion kinetic energy and beam pulse power as compared to reactor drivers, we achieve high-beam intensities at the focal spot by using short focal distance and properly designed beam optics. In this regard, the low beam emittance of suggested multi-beam designs are very useful. Possibly even higher focal spot brightness could be obtained by plasma lenses which involve external fields on the beam which is stripped to a higher charge state by passing through a plasma cell. Preliminary results suggest that intensities approx. 10/sup 13/ - 10/sup 14/ W/cm/sup 2/ are achievable. Given these intensities, deposition experiments with heating of disks to greater than a million degrees Kelvin (100 eV) are expected.
Date: March 4, 1982
Creator: Mark, J.W.K.; Bangerter, R.O.; Fawley, W.M.; Yu, S. & Judd, D.L.
Partner: UNT Libraries Government Documents Department

Some proposed disk-heating and beam-transport experiments for heavy-ion inertial-fusion test facilities

Description: Calculations suggest that experiments relating to beam deposition, focusing and transport can be performed within the context of current design proposals for accelerator test-facilities. Since the test-facilities have lower ion kinetic energy and beam pulse power as compared to reactor drivers, we achieve high-beam intensities at the focal spot by using short focal distance and properly designed beam optics. In this regard, the low beam emittance of suggested multi-beam designs are very useful. Preliminary results suggest that intensities close to 10/sup 14/ W/cm/sup 2/ are achievable. Given these intensities, deposition experiments with heating of disks to greater than a million degrees Kelvin (100 eV) are expected. We could also expect as much as 1 to 3 kA of incident ion current on these disks with beam intensities almost comparable to that of reactor targets.
Date: June 8, 1981
Creator: Mark, J.W.K.; Bangerter, R.O.; Fawley, W.M.; Yu, S. & Wang, T.S.F.
Partner: UNT Libraries Government Documents Department