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Photoelectric injector design code

Description: We will describe a computer code based on an analysis for an emittance growth mechanism for electron beams in photoelectric injectors. The analysis leads to a generic injector design with a single external solenoid used for both focusing the beam and reducing the correlated emittance. The position of the solenoid is given by a complicated integral expression, depending on the accelerating gradient and rf focusing. The computer code described here integrates this expression and calculates the best solenoid lens position for a given phasing and field amplitudes of the accelerating cavities. 5 refs., 2 figs.
Date: January 1, 1989
Creator: Carlsten, B.E.
Partner: UNT Libraries Government Documents Department

Emittance growth caused by bends in the Los Alamos free-electron laser energy recovery experiment

Description: Experimentally transporting the beam from the wiggler to the decelerators in the energy recovery experiment (ERX) at the Los Alamos National Laboratory free-electron laser was more difficult than expected because of the large initial emittance in the beam. This emittance was apparently caused in an early 60/sup 0/ achromatic bend. To get this beam through subsequent bends without wall interception, the quadrupole focusing had to be changed from the design amount; as a result, the emittance grew further. This paper discusses various mechanisms for this emittance growth in the 60/sup 0/ bend, including effects caused by path changes in the bend resulting from wake-field-induced energy changes of particles in the beam and examines emittance filters, ranging from a simple aperture near a beam crossover to more complicated telescope schemes designed to regain the original emittance before the 60/sup 0/ bend.
Date: January 1, 1987
Creator: Carlsten, B.E.
Partner: UNT Libraries Government Documents Department

New photoelectric injector design for the Los Alamos National Laboratory XUV FEL accelerator

Description: The injector for the Los Alamos National Laboratory XUV FEL accelerator has been redesigned to provide more charge to the wiggler. The new design can deliver 8nC of charge within 20 ps with a normalized 90% emittance of <25 /Pi//center dot/mm/center dot/mrad to the wiggler at an energy of 200 MeV. In addition to the new design of the injector, we analyze the emittance growth and subsequent reduction through the injector, including different mechanisms for emittance growth and the methods used to eliminate the correlated emittance. 7 refs., 5 figs.
Date: January 1, 1988
Creator: Carlsten, B.E.
Partner: UNT Libraries Government Documents Department

Emittance growth of an nonequilibrium intense electron beam in a transport channel with discrete focusing

Description: The author analyzes the emittance growth mechanisms for a continuous, intense electron beam in a focusing transport channel, over distances short enough that the beam does not reach equilibrium. The emittance grows from the effect of nonlinear forces arising from (1) current density nonuniformities, (2) energy variations leading to nonlinearities in the space-charge force even if the current density is uniform, (3) axial variations in the radial vector potential, (4) an axial velocity shear along the beam, and (5) an energy redistribution of the beam as the beam compresses or expands. The emittance growth is studied analytically and numerically for the cases of balanced flow, tight focusing, and slight beam scalloping, and is additionally studied numerically for an existing 6-MeV induction linear accelerator. Rules for minimizing the emittance along a beamline are established. Some emittance growth will always occur, both from current density nonuniformities that arise along the transport and from beam radius changes along the transport.
Date: February 1, 1997
Creator: Carlsten, B.E.
Partner: UNT Libraries Government Documents Department

Self-consistent klystron simulations

Description: A numerical analysis of large-signal klystron behavior based on general wave-particle interaction theory is presented. The computer code presented is tailored for the minimum amount of complexity needed in klystron simulation. The code includes self-consistent electron motion, space-charge fields, and intermediate and output fields. It also includes use of time periodicity to simplify the problem, accurate representation of the space-charge fields, accurate representation of the cavity standing-wave fields, and a sophisticated particle-pushing routine. In the paper, examples are given that show the effects of cavity detunings, of varying the magnetic field profile, of electron beam asymmetries from the gun, and of variations in external load impedance. 4 refs., 7 figs.
Date: January 1, 1985
Creator: Carlsten, B.E. & Tallerico, P.J.
Partner: UNT Libraries Government Documents Department

Preliminary injector, accelerator, and beamline design for rf-linac-driven XUV free-electron lasers

Description: The proposed Los Alamos National Laboratory XUV free-electron laser (FEL) facility requires exceptional beam quality at high peak currents. Although the beam quality needed for a demonstration machine lasing at 50 nm is not far from what can be expected with extensions of present linacs to higher energy, conventional injector technology will not meet the requirements needed for lasing at 12 or 4 nm. We have conceived a preliminary injector and accelerator design that will meet these requirements. Using the Los Alamos photoelectric injector, it appears that normalized 90% emittances of 24 ..pi.. dot mm dot mrad can be attained in a relatively straightforward manner, and emittances down to 4 ..pi.. doe mm dot mrad are possible. Beamline simulations have been performed with the particle-pushing code PARMELA, using particle-dump inputs from the particle-in-cell code ISIS. The latter models the photoelectric gun up to the range between 0.75 and 1 MeV. Designs including electron guns with Pierce geometries have also been studied. Using an injector with a large planar-cathode Pierce gun seems to satisfy the 50-nm lasing requirements. In addition, other beamline questions have been studied. Beamline bends have been designed that are achromatic and nearly isochronous. The threshold for cumulative beam breakup and the emittance growth caused by transverse resistive-wall beam instability have been calculated. Finally, we discuss the advantages and disadvantages of building a straight-line machine versus a recycling machine, including recycling instability current levels. 11 refs., 16 figs.
Date: January 1, 1987
Creator: Carlsten, B.E. & Chan, K.C.D.
Partner: UNT Libraries Government Documents Department

Space-charged-induced emittance growth in the transport of high-brightness electron beams

Description: The emittance induced by space charge in a drifting beam of finite length has been investigated, and a scaling law has been obtained from simple considerations of the different rates of expansion of different portions of the beam. The scaling law predicts the initial rate of emittance growth, before the beam shape has distorted significantly, and thus represents an upper bound on the rate of emittance increase. This scaling law has been substantiated by particle-in-cell simulation and the dependence on geometric factors evaluated for specific choices of the beam profile. For long, axially nonuniform beams, the geometric factors have been evaluated explicitly for Gaussian profiles, and other shapes.
Date: March 1, 1987
Creator: Jones, M.E. & Carlsten, B.E.
Partner: UNT Libraries Government Documents Department

Photoelectric injector design considerations

Description: We will present an analysis for different emittance growth mechanisms for electron beams in photoelectric injectors. The mechanisms will be broken up into three groups: space-charge forces due to self-similar expansion, space-charge forces due to non-self-similar expansion (including divergences and convergences of the beam), and rf forces. We will show that some of the emittance can be eliminated downstream, particularly that of the first group. General design considerations will become clear from this analysis and a generic design will be presented. In addition, a photoelectric injector design for both the Los Alamos National Laboratory XUV FEL and a compact free-electron laser (FEL) will be used to show a numerical agreement with the analysis. 5 refs., 2 figs.
Date: January 1, 1988
Creator: Carlsten, B.E. & Sheffield, R.L.
Partner: UNT Libraries Government Documents Department

Computer modeling of the klystron

Description: Although the klystron was invented over 40 years ago, it still is quite a scientific challenge to analyze this seemingly simple device. Accurate calculation of space-charge density and particle motions are required, but there are dozens of parameters that must be optimized. Thus the analysis must be inexpensive to run, because over 100 runs typically are required to optimize a particular design. A self-consistent ring model of the klystron interaction is being developed for use as a design tool, with the initial goal of improving the LAMPF klystrons to conserve electrical energy. A mathematical model that includes large-signal effects, relativity, harmonic and fundamental cavities, extended cavity fields, and self-consistent calculations of the induced currents is being developed. The model is self-consistent because the gap fields are calculated interactively, because the currents that drive the cavities are not known until after the beam transverses the cavities. Several test cases have been run, and the basic theory and results to date are discussed.
Date: January 1, 1983
Creator: Tallerico, P.J. & Carlsten, B.E.
Partner: UNT Libraries Government Documents Department

Emittance growth of a short electron bunch in circular motion

Description: A short electron bunch undergoing circular motion produces space-charge forces that do not decrease with increasing bunch energy, unlike those induced by straight-line motion. These energy-independent forces can be separated into a noninertial space-charge force and a coherent synchrotron radiation force. These forces result in an energy spread in the bunch, and can lead to a potentially large emittance growth. These effects can take place in both (1) bunch compression systems used to increase the peak current and (2) the wiggler itself Numerical estimates of the emittance growth in a wiggler for a 1-ps long, 1-mm radius, 1-nC electron bunch can be as large as 0.1 {pi} mm mrad per wiggle period; the energy spread can grow as much as 30 keV per wiggle period. These types of beam quality degradation may become significant for future, short-wavelength free-electron lasers requiring high-brightness electron beams, especially for self-amplified spontaneous emission operation.
Date: September 1996
Creator: Carlsten, B. E. & Goldstein, J. C.
Partner: UNT Libraries Government Documents Department

INEX (integrated numerical experiment) simulations of the Los Alamos HIBAF (high-brightness accelerator free-electron laser) free-electron laser MOPA (master oscillator power amplifier) experiment

Description: We present results of Integrated Numerical Experiment (INEX) simulations of the performance of a 1-m untapered wiggler FEL oscillator driving a 2-m wiggler FEL amplifier for the new HIBAF (High-Brightness Accelerator Free-Electron Laser) facility at Los Alamos. INEX simulations utilize a numerically-generated electron micropulse, from ISIS/PARMELA calculations of the photoinjector/linac/beam transport system, in the 3-D FEL simulation code FELEX. 13 refs., 10 figs., 1 tab.
Date: January 1, 1989
Creator: Goldstein, J.C.; Carlsten, B.E. & McVey, B.D.
Partner: UNT Libraries Government Documents Department

Photoelectric injector designs at Los Alamos National Laboratory

Description: We will describe the photoelectric injector design philosophy and capabilities at Los Alamos National Laboratory. We will discuss our simulation ability and general considerations to minimize the space-charge and rf emittance growths. In particular, we will also outline a high frequency design without magnetic bunching and a low frequency design with magnetic bunching.
Date: January 1, 1990
Creator: Carlsten, B.E.; Sheffield, R.L. & McVey, B.D.
Partner: UNT Libraries Government Documents Department

First measurements of electron-beam transit times and micropulse elongation in a photoelectric injector at the High-Brightness Accelerator FEL (HIBAF)

Description: Key aspects of the dynamics of a photoelectric injector (PEI) on the Los Alamos High-Brightness Accelerator FEL (HIBAF) facility have been investigated using a synchroscan streak camera. By phase-locking the streak camera sweep to the reference 108.3 MHz rf signal, the variations of micropulse temporal elongations (30 to 80% over the drive-laser pulse length) and of transit times (25 ps for a 16{degree}-phase change) were observed for the first time. These results were in good agreement with PARMELA simulations. 2 refs., 8 figs.
Date: January 1, 1990
Creator: Lumpkin, A.H.; Carlsten, B.E. & Feldman, R.B.
Partner: UNT Libraries Government Documents Department

Emittance growth of an electron beam in a periodic focusing channel due to transfer of longitudinal energy to transverse energy

Description: Most discussions about emittance growth and halo production for an intense electron beam in a periodic focusing channel assume that the total transverse energy is constant (or, in other words, that the transverse and longitudinal Hamiltonians are separable). Previous analyses that include variations in the total transverse energy are typically based on a transverse-longitudinal coupling that is either from two-dimensional space-charge modes or particle-particle Coulomb collisions. With the space-charge modes, the energy exchange between the transverse and longitudinal directions is periodic, and of constant magnitude. The total energy transfer for the case of the Coulomb collisions is negligible. This limited increase of energy in the transverse direction from these other effects will limit the amount of transverse emittance growth possible. In this paper, the authors investigate a mechanism in which there is a continual transfer of energy from the longitudinal direction to the transverse direction, leading to essentially unlimited potential transverse emittance growth. This mechanism is caused by an asymmetry of the beam`s betatron motion within the periodic focusing elements. This analysis is based on thermodynamic principles. This mechanism exists for both solenoids and quadrupole focusing, although only solenoid focusing is studied here.
Date: December 31, 1998
Creator: Carlsten, B. E.
Partner: UNT Libraries Government Documents Department

Coherent synchrotron radiation experiments for the LCLS

Description: The authors describe a coherent synchrotron radiation experiment planned at Los Alamos to support the design of the Linac Coherent Light Source (LCLS) x-ray FEL. Preliminary simulations of the LCLS compressors show that a clever tuning strategy can be used to minimize the electron`s beam emittance growth due to noninertial space-charge forces by employing a delicate cancellation of these forces. The purpose of the Los Alamos experiment, using a sub-picosecond chicane compressor, is to benchmark these simulations tools. In this paper, the authors present detailed numerical simulations of the experiment, and point out unique signatures of this effect that are measurable. As predicted previously, the largest emittance growths and induced energy spreads result from the nonradiative components of this space-charge force.
Date: December 31, 1998
Creator: Carlsten, B.E. & Russell, S.J.
Partner: UNT Libraries Government Documents Department

Second-order emittance compensation in an RF photoinjector using RF radial focusing

Description: Emittance compensation using the static axial magnetic field from a solenoid surrounding an rf photoinjector has been used to reduce the rms emittance of the electron beam by up to an order of magnitude, for photoinjectors ranging from 433 MHz to 8 GHz. The residual emittance after standard solenoidal compensation depends primarily on how linear the space-charge force is along the electron bunch in terms of an axial Taylor expansion, and is typically a strong function of both solenoid position and focusing strength. In this paper, the authors investigate the concept of second-order emittance compensation using the combination of a solenoid and radial rf focusing. They numerically demonstrate that (1) lower residual emittances are possible if second-order compensation is introduced by adding radially focusing rf forces in the first photoinjector cavity near the cathode, (2) the residual emittance is less sensitive to solenoid position, (3) the residual emittance is less sensitive to solenoid strength, and (4) the optimal solenoid position is further from the photoinjector cathode, leading to less stringent design requirements, especially at high frequencies.
Date: December 31, 1997
Creator: Carlsten, B.E. & Palmer, D.T.
Partner: UNT Libraries Government Documents Department

Rippled-beam free-electron laser

Description: The authors describe a new microwave generation mechanism involving a scalloping annular electron beam. The beam interacts with the axial electric field of a TM{sub 0n} mode in a smooth circular waveguide through the axial free-electron laser interaction, in which the beam ripple period is synchronous with the phase slippage of the rf mode relative to the electron beam. Due to nonlinearities in the orbit equation, the interaction can be made autoresonant, where the phase and amplitude of the gain is independent of the beam energy.
Date: October 1, 1997
Creator: Carlsten, B.E.
Partner: UNT Libraries Government Documents Department

Large-signal klystron simulations using KLSC

Description: The authors describe large-signal klystron simulations using the particle-in-cell code KLSC. This code uses the induced-current model to describe the steady-state cavity modulations and resulting rf fields, and advances the space-charge fields through maxwell`s equations. In this paper, an eight-cavity, high-power S-band klystron simulation is used to highlight various aspects of this simulation technique. In particular, there are specific issues associated with modeling the input cavity, the gain circuit, and the large-signal circuit (including the output cavities), that have to be treated carefully.
Date: October 1, 1997
Creator: Carlsten, B. E. & Ferguson, P.
Partner: UNT Libraries Government Documents Department

Emittance growth of an intense electron beam in a focusing channel

Description: We use single-particle radial equation of motion to identify nonlinear forces which lead to an emittance growth in a focusing channel consisting of solenoids. For a uniform density beam, the two dominant effects are the axial velocity variations within a solenoid due to the particles` azimuthal velocity and changes in the particle`s energy due to radial motion and the radial electric space-charge field. We derive estimates for the emittance growth for a space-charge dominated beam due to these effects, both for the case of a hard focus to a small beam waist and for the case where there is gentle beam scalloping. We also briefly catalog less important emittance growth mechanisms.
Date: August 1, 1997
Creator: Carlsten, B.E.
Partner: UNT Libraries Government Documents Department

Subpicosecond compression experiments at Los Alamos National Laboratory

Description: The authors report on recent experiments using a magnetic chicane compressor at 8 MeV. Electron bunches at both low (0.1 nC) and high (1 nC) charges were compressed from 20 ps to less than 1 ps (FWHM). A transverse deflecting rf cavity was used to measure the bunch length at low charge; the bunch length at high charge was inferred from an induced energy spread of the beam. The longitudinal centrifugal-space charge force is calculated using a point-to-point numerical simulation and is shown not to influence the energy-spread measurement.
Date: September 1, 1995
Creator: Carlsten, B.E.; Russell, S.J. & Kinross-Wright, J.M.
Partner: UNT Libraries Government Documents Department

Subpicosecond, ultra-bright electron injector

Description: We have designed and are building a subpicosecond electron injector. The injector is based on a 7.5 MeV photoinjector, used previously at Los Alamos in the APEX experiment. The nominal design includes magnetically compressing a 20 ps long, 3 nC bunch to a FWHM bunch length of 2/3 ps (peak current in excess of 3 kA) using a four dipole chicane buncher. The geometrial averaged transverse normalized transverse emittance after compression is about 15 ir mm mrad.
Date: July 1, 1995
Creator: Carlsten, B.E.; Milder, M.L. & Kinross-Wright, J.
Partner: UNT Libraries Government Documents Department

Emittance studies at the Los Alamos National Laboratory Free-Electron Laser

Description: Recent emittance studies at the Los Alamos FEL have indicated several areas of concern in the linac and beamline feeding the wiggler. Four emittance growth mechanisms of special importance have been studied. First, a rapid growth of the electron beam's emittance immediately after the spherical gridded Pierce gun resulted, in part, from the long time required for our pulsing electronics to ramp the grid voltage up at the start and down at the end of the pulse, which created a pulse with a cosine-like current distribution as a function of time. The growth was compounded by the extremely small radial beam size (almost a waist) leaving the gun. In addition, we saw evidence of electrostatic charging of the insulators in the gun, reducing the quality of the electron beam further. Second, the action of the solenoidal focusing fields in the low-voltage bunching region was studied, and criteria for a minimum emittance growth were established. Third, maximum misalignment angles and displacements for various elements of the beamline were calculated for the desired low emittance growth. Finally, emittance growth in the horizontal dimension through the nonisochronous bend caused by varying energy depression on the particles due to longitudinal wake fields was both calculated and observed. In addition, we measured energy depressions caused by the wake fields generated by various other elements in the beamline. Strategies were developed to relieve the magnitude of these wake-field effects. 10 refs., 12 figs.
Date: January 1, 1987
Creator: Carlsten, B.E.; Feldman, D.W.; Lumpkin, A.H.; Stein, W.E. & Warren, R.W.
Partner: UNT Libraries Government Documents Department