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ABSTRACT
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ABSTRACT
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The optical cavity of the Boeing visible free electron laser was reconfigured from a concentric cavity to a glancing incidence ring resonator in late 1989 and was operated until December 1990. the crucial requirement for the optical... more
The optical cavity of the Boeing visible free electron laser was reconfigured from a concentric cavity to a glancing incidence ring resonator in late 1989 and was operated until December 1990. the crucial requirement for the optical cavity of an FEL is to provide an optical mode which is spatially and temporally matched to the electron beam as it moves through the wiggler. Several new optical diagnostics were developed to determine when the above requirement was satisfied. This paper will discuss those diagnostics which achieved and maintained the alignment of the ring resonator within tolerance to lase and measured the quality of lasing. The new diagnostics included measurements of the focus position and Rayleigh range of the ring resonator optics to determine the spatial match of the optical mode through the wiggler, and a measurement of the position of the optical axis for multiple passes around the ring resonator to determine the stability of the resonator alignment. Accelerator performance was determined by measuring the electron beam pulse width and charge, which indicated electron beam brightness, and by measuring the width of the spontaneous emission spectrum, which gave an indication of the alignment between the electron beam and the optical axis. Temporal overlap of electron and optical pulses was assured by measuring the optical cavity length. In addition, several other diagnostics which indicated FEL performance will be described: optical energy per micropulse, small signal gain, ringdown loss, laser pulse width, laser wavelength, and time resolved spectroscopy.
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This paper describes pulse compression and energy recovery experiments using an RF photocathode gun and injector accelerator at a fundamental RF of 433 MHz (1f) and a third-harmonic RF accelerator section operating at 1300 MHz (3f). For... more
This paper describes pulse compression and energy recovery experiments using an RF photocathode gun and injector accelerator at a fundamental RF of 433 MHz (1f) and a third-harmonic RF accelerator section operating at 1300 MHz (3f). For pulse compression, the 3f section is used both to program the energy slew and correct for RF-induced curvature in the electron beam micropulse
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The π mode or accelerating mode in a 1.6 cell rf gun is normally the only mode considered in rf gun simulations. However, due to the finite Q there is a small but measurable 0 mode present even at steady state. The π mode by definition... more
The π mode or accelerating mode in a 1.6 cell rf gun is normally the only mode considered in rf gun simulations. However, due to the finite Q there is a small but measurable 0 mode present even at steady state. The π mode by definition has a 180° phase shift between cells but this phase shift for the total field is several degrees different. This results in a correlated energy spread exiting the gun. A comparison of simulation and experiment will be shown.
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... QE X: 4.438 Y: 7.016e-006 X: 4.299 Y: 5.053e-006 4 Comparison of Expt. and Theory for the Intrinsic Emittance CP Hauri et al., PRL 104,234802(2010) QE of Copper vs. Effective Work Function 2 3mc eff x n ϕ ω ... lN Q 2 2 0 16 π ρ = =... more
... QE X: 4.438 Y: 7.016e-006 X: 4.299 Y: 5.053e-006 4 Comparison of Expt. and Theory for the Intrinsic Emittance CP Hauri et al., PRL 104,234802(2010) QE of Copper vs. Effective Work Function 2 3mc eff x n ϕ ω ... lN Q 2 2 0 16 π ρ = = Leap of faith: Assume 4 2 2 r x p p ≈ mc p x ...
We have measured the angular asymmetry in the (γ, n) reaction on natural lead, cadmium, and calcium in the energy region of the isovector quadrupole resonance (20–40 MeV). The asymmetry increases from low values, about 0.2, to high... more
We have measured the angular asymmetry in the (γ, n) reaction on natural lead, cadmium, and calcium in the energy region of the isovector quadrupole resonance (20–40 MeV). The asymmetry increases from low values, about 0.2, to high values, 0.6–0.8, where this resonance is expected. This increasing asymmetry is interpreted as resulting from the interference between the E1 and E2 isovector amplitudes. We have fitted direct−semidirect calculations for lead and cadmium and have obtained resonance energies of 23.5 ± 1.5 and 26.5 ± 1.5 MeV for these elements, respectively. For calcium, preliminary results indicate a resonance energy of 34 ± 2 MeV. The widths are found to be considerably larger than those of the E1 resonance.
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Several accelerators, including the SLC, JLAB, Mainz, Bates/MIT, and Bonn have successfully operated for medium and high energy physics experiments using polarized electron beams generated by dc-biased guns employing GaAs photocathodes.... more
Several accelerators, including the SLC, JLAB, Mainz, Bates/MIT, and Bonn have successfully operated for medium and high energy physics experiments using polarized electron beams generated by dc-biased guns employing GaAs photocathodes. Since these guns have all used a bias on the order of 100 kV, the longitudinal emittance of the extracted bunch is rather poor. Downstream rf bunching systems increase the transverse emittance. An rf gun with a GaAs photocathode would eliminate the need for separate rf bunchers, resulting in a simpler injection system. In addition, the thermal emittance of GaAs-type cathodes is significantly lower than for other photocathode materials. The environmental requirements for operating activated GaAs photocathodes cannot be met by rf guns as currently designed and operated. These requirements, including limits on vacuum and electron back bombardment, are discussed in some detail. Modifications to actual and proposed rf gun designs that would allow these re...
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The first of six 1.3-GHz, 18-cell, constant impedance, traveling wave linac structures has been placed under RF power and used to accelerate an electron beam. The design of the structures and low power measurements of insertion loss and... more
The first of six 1.3-GHz, 18-cell, constant impedance, traveling wave linac structures has been placed under RF power and used to accelerate an electron beam. The design of the structures and low power measurements of insertion loss and group delay are described. The observed voltage gain under high power is compared with predictions based on low power measurements and URMEL calculations for the accelerator mode in a single cell. 1
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Requirements for the LCLS injector drive laser present significant challenges to the design of the system. While progress has been demonstrated in spatial shape, temporal shape, UV generation and rep-rate, a laser that meets all of the... more
Requirements for the LCLS injector drive laser present significant challenges to the design of the system. While progress has been demonstrated in spatial shape, temporal shape, UV generation and rep-rate, a laser that meets all of the LCLS specifications simultaneously has yet to be demonstrated. These challenges are compounded by the stability and reliability requirements. The drive laser and transport system has been installed and tested. We will report on the current operational state of the laser and plans for future improvements.
The SSRL Gun Test Facility (GTF) was built to develop a high brightness electron injector for the LCLS and has been operational since 1996. A total of five different metal cathodes (4 Cu and 1 Mg) have been installed on the GTF gun. The... more
The SSRL Gun Test Facility (GTF) was built to develop a high brightness electron injector for the LCLS and has been operational since 1996. A total of five different metal cathodes (4 Cu and 1 Mg) have been installed on the GTF gun. The rf processing history with the different cathodes will be presented including peak field achieved at the cathode. The LCLS gun is intended to operate at 120 MV/m and fields up to 140 MV/m have been achieved in the GTF gun. After installing a new cathode the number of rf pulses required to reach 120 MV/m is approximately 5-10 million. Total emitted dark current and Fowler Nordheim plots are also shown over the life of the cathode.
An 18-MeV, 433-MHz linac capable of operating at 25% RF duty factor (DF) is being commissioned for FEL applications. Comprising a two-cell RF photocathode injector[1] followed by four new multicell cavities[2], the linac is an extension... more
An 18-MeV, 433-MHz linac capable of operating at 25% RF duty factor (DF) is being commissioned for FEL applications. Comprising a two-cell RF photocathode injector[1] followed by four new multicell cavities[2], the linac is an extension of the photoinjector which previously delivered 5 nC at 27 MHz micropulse repetition frequency, and 25% DF. The system is constructed using equipment from the Ground Based Laser and the Average Power Laser Experiment (APLE). The linac can serve as the driver for an infrared FEL or as the preaccelerator for a higher energy linac driving a visible FEL [3]. Introduction The High Duty Factor Electron Linac comprises a photocathode injector, accelerator, beamline, associated controls, vacuum, alignment and beam diagnostics. The present layout of the accelerator and beamline is shown in Fig. 1. At this writing, the accelerator and beamline have been aligned and are under vacuum. The accelerator cavities are ready to accept high power RF and only a few cont...
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The Linac Coherent Light Source (LCLS) is a SASE free electron laser using the last 1/3 of the SLAC two mile linac to produce 1.5 to 15 angstrom x-rays in a 100 meter long undulator for use in a variety of x-ray science experiments. In... more
The Linac Coherent Light Source (LCLS) is a SASE free electron laser using the last 1/3 of the SLAC two mile linac to produce 1.5 to 15 angstrom x-rays in a 100 meter long undulator for use in a variety of x-ray science experiments. In order to satisfy the demanding electron beam requirements, a new 135 MeV photo-injector will be built in an existing, off-axis vault at the 2/3 point of the main linac. The injector accelerator consists of a BNL/SLAC/UCLA 1.6 cell S-band gun followed by two 3-meter long SLAC accelerator sections. The 6MeV beam from the gun is matched into the first accelerator section and accelerated to 135 MeV before injection onto the main linac axis with a 35 degree bend. Several modifications to the rf gun, linac and beamline as well as the inclusion of several diagnostics have been incorporated into the injector design to achieve the required 1.2 micron projected emittance at a charge of 1 nC. In addition, an inverse free electron laser, the laser heater, will be...
Design of the first generation LCLS injector is nearing completion. Fabrication has begun and component installation is planned for 2006. We discuss the last modifications made on both the 1.6 cell S-Band RF gun and the SLAC S-Band... more
Design of the first generation LCLS injector is nearing completion. Fabrication has begun and component installation is planned for 2006. We discuss the last modifications made on both the 1.6 cell S-Band RF gun and the SLAC S-Band accelerating structures to minimize irreversible emittance growth. The mode separation between the 0 and π modes was increased from 3.4 MHz to 15 MHz. Dual feed and racetrack shapes have been incorporated in the full cell of the new gun. The linac sections were also modified to accommodate dual feeds and racetrack shapes in their input cells. PARMELA simulations indicating the need for these modifications are presented.
In this paper we resurrect an idea originally proposed by Serafini[1] in 1992 for an RF photocathode gun capable of operating simultaneously at the fundamental frequency and a higher frequency harmonic. Driving the gun at two frequencies... more
In this paper we resurrect an idea originally proposed by Serafini[1] in 1992 for an RF photocathode gun capable of operating simultaneously at the fundamental frequency and a higher frequency harmonic. Driving the gun at two frequencies with the proper field ratio and relative phase produces a beam with essentially no rf emittance and a linear longitudinal phase space distribution. Such a gun allows a completely new range of operating parameters for controlling space charge emittance growth. In addition, the linear longitudinal phase space distribution aids in bunch compression. This paper will compare results of simulations for the two-frequency gun with the standard rf gun, and the unique properties of the two-frequency gun will be discussed. PACS codes: 29.25.Bx,29.27.Ac,41.60.Cr,41.85.Ar
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The quantum efficiency from a metal cathode is strongly dependent on the field at the cathode due to the Schottky effect. Since the field is time dependent the quantum efficiency is also time dependent. Thus the laser pulse shape used to... more
The quantum efficiency from a metal cathode is strongly dependent on the field at the cathode due to the Schottky effect. Since the field is time dependent the quantum efficiency is also time dependent. Thus the laser pulse shape used to generate electrons in a photocathode rf gun is not the same as the electron bunch shape. In addition since the thermal emittance and quantum efficiency are related, the thermal emittance is also time dependent. Contributed to the workshop on
For the Linac Coherent Light Source II (LCLS-II) project at SLAC, a 1.3 GHz superconducting rf (SRF) linac is being constructed that will generate 4 GeV electron bunches at a high repetition rate to drive x-ray free electron lasers. The... more
For the Linac Coherent Light Source II (LCLS-II) project at SLAC, a 1.3 GHz superconducting rf (SRF) linac is being constructed that will generate 4 GeV electron bunches at a high repetition rate to drive x-ray free electron lasers. The LCLS-II electron source, which comprises the first three meters of the electron injector, includes two normal-conducting, continuous-wave rf cavities: a one-cell, 185.7 MHz gun and a two-cell, 1.3 GHz buncher. It also includes a gun load-lock system that allows photocathodes to be changed under vacuum. The components in this beam-line section were designed and built by Lawrence Berkeley National Laboratory based on experience from their advanced photoinjector experiment program. In combination with the SLAC UV laser system, the electron source is designed to produce beam rates up to 1 MHz with average currents up to $30\text{ }\text{ }\ensuremath{\mu}\mathrm{A}$ initially. The source was installed in mid-2018, well in advance of the SRF linac, which ...
Two spectrometers have been added to the LCLS photoinjector beamline. The first one will be located close to the exit of the Photoinjector RF gun. With this diagnostic, we will measure beam energy, energy spread (correlated and... more
Two spectrometers have been added to the LCLS photoinjector beamline. The first one will be located close to the exit of the Photoinjector RF gun. With this diagnostic, we will measure beam energy, energy spread (correlated and uncorrelated), possibly deleterious structure in the longitudinal phase space induced by longitudinal space charge force, and slice thermal emittance … This extensive characterization of the 5MeV electron bunch will be made possible by combining this spectrometer with other diagnostics (YAG screens and Cerenkov Radiator). A second spectrometer located at the end of the beamline has been designed to characterize the 6 dimensional phase space of the 135MeV beam to be injected in the main accelerator. At that second spectrometer station, we will measure energy, energy spread (correlated and uncorrelated), longitudinal phase space, slice emittances... Those last two measurements require using this spectrometer in combination with the transverse RF deflecting cavi...
The first of six 1.3-GHz, 18-cell, constant impedance, traveling wave linac structures has been placed under RF power and used to accelerate an electron beam. The design of the structures and low power measurements of insertion loss and... more
The first of six 1.3-GHz, 18-cell, constant impedance, traveling wave linac structures has been placed under RF power and used to accelerate an electron beam. The design of the structures and low power measurements of insertion loss and group delay are described. The observed voltage gain under high power is compared with predictions based on low power measurements and URMEL calculations for the accelerator mode in a single cell.
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The INEX (integrated numerical experiment) numerical model is applied to the 0.6 {mu}m FEL oscillator at Boeing Aerospace and Electronics Company in Seattle, WA. This system consists of a 110 MeV L-band rf linac, a beam transport line... more
The INEX (integrated numerical experiment) numerical model is applied to the 0.6 {mu}m FEL oscillator at Boeing Aerospace and Electronics Company in Seattle, WA. This system consists of a 110 MeV L-band rf linac, a beam transport line from the accelerator to the entrance of the wiggler, the 5.0 meter THUNDER variable taper wiggler, and a near concentric two mirror optical oscillator. Many aspects of the model for the electron beam accelerator and transport line agree with experimental measurements. Predictions for lasing performance are compared with data obtained in May and June 1989 using a mild tapered wiggler. We obtain good agreement with the achieved extraction efficiency, while 1D pulse simulations reproduce the observed sideband instability. 15 refs., 11 figs.
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The Gun Test Facility (GTF) was built to test high-brightness sources for the proposed Linac Coherent Light Source (LCLS) at SLAC. The longitudinal emittance exiting the gun has been determined by measuring the energy spectrum after the... more
The Gun Test Facility (GTF) was built to test high-brightness sources for the proposed Linac Coherent Light Source (LCLS) at SLAC. The longitudinal emittance exiting the gun has been determined by measuring the energy spectrum after the linac as a function of the linac phase. The phase-space parameters defining the beam pulse width, correlated energy spread, and slice energy spread at the linac entrance (~5 MeV beam energy) are fit to the measured energy spectra. A large, linear energy-time correlation is observed for bunch charges from 15 to 300 pC. Possible explanations for this correlated energy spread are discussed. Submitted to 24 International Free Electron Laser Conference, Argonne National Laboratory, Argonne, IL, September 9 13, 2002 *Work supported by Department of Energy contract DE-AC03-76SF00515 Longitudinal Emittance Measurements at the SLAC Gun Test Facility D.H. Dowell, P.R. Bolton, J.E. Clendenin, S.M. Gierman, C.G. Limborg, B.F. Murphy, J.F. Schmerge, T. Shaftan St...
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The design for a potential free electron laser (FEL), driven by a 120-MeV linac capable of generating 0.1-A macropulse average current beams at a duty factor of 0.6%, is described. The accelerator will employ a photo-injected, 18-MeV,... more
The design for a potential free electron laser (FEL), driven by a 120-MeV linac capable of generating 0.1-A macropulse average current beams at a duty factor of 0.6%, is described. The accelerator will employ a photo-injected, 18-MeV, 433-MHz linac as an injector, followed by a 1300-MHz longitudinal phase space “linearizer”, a magnetic buncher, and seven sections of 1300-MHz, pulsed traveling
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Results of commissioning the injector and construction progress of the Boeing 1-kW visible free-electron laser. [Proceedings of SPIE 2988, 158 (1997)]. John L. Adamski, David H. Dowell, Thomas D. Hayward, Patrick E. Johnson ...
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New measurements of the (p,Γ) reaction on natural targets of K and Ca at intermediate proton energies (Ep=11 to 36 MeV) reveal excitation functions that are dominated by excited-state giant dipole resonances (ESGDR’s) built upon... more
New measurements of the (p,Γ) reaction on natural targets of K and Ca at intermediate proton energies (Ep=11 to 36 MeV) reveal excitation functions that are dominated by excited-state giant dipole resonances (ESGDR’s) built upon single-particle states in 40Ca and 41Sc. These ESGDR’s all peak near EΓ=20 MeV and have widths which increase monotonically with the final states’s excitation energy. The integrated, inverse (Γ,p0) cross sections are shown to be proportional to the known single-particle spectroscopic strengths indicating a semi-direct mechanism for the capture process.
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The radiative capture of 12C by 14C into the fused nucleus 26Mg has been investigated for ECM=6.2–8.1 MeV. Structures are observed in the excitation functions for Γ-decay to the bound states up to Ex=5.7 MeV in 26Mg. The lack of any... more
The radiative capture of 12C by 14C into the fused nucleus 26Mg has been investigated for ECM=6.2–8.1 MeV. Structures are observed in the excitation functions for Γ-decay to the bound states up to Ex=5.7 MeV in 26Mg. The lack of any structure in elastic scattering and fusion suggests that these might reflect the electromagnetic strength function built upon the low-lying levels of 26Mg. However, the experiment indicates considerable differences between these strength functions for the different final states.
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ABSTRACT We investigate the potential of using rf‐guns as electron sources for relativistic, ultrafast time‐resolved pump‐probe electron diffraction experiments. We explore the feasibility of the experiment by simulating the electron... more
ABSTRACT We investigate the potential of using rf‐guns as electron sources for relativistic, ultrafast time‐resolved pump‐probe electron diffraction experiments. We explore the feasibility of the experiment by simulating the electron trajectories using instrumental design parameters of the Gun Test Facility (GTF) at the Stanford Linear Accelerator (SLAC), and modeling the scattering event using relativistic differential scattering cross sections. The simulations, done for a 1500 nm thick aluminum foil, suggest that single‐shot diffraction patterns can be obtained with electron pulses containing ∼107 electrons, and that a time resolution approaching hundred femtoseconds is possible. © 2006 American Institute of Physics