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RF Design of the LCLS Gun

Description: Final dimensions for the LCLS RF gun are described. This gun, referred to as the LCLS gun, is a modified version of the UCLA/BNL/SLAC 1.6 cell S-Band RF gun [1], referred to as the prototype gun. The changes include a larger mode separation (15 MHz for the LCLS gun vs. 3.5 MHz for the prototype gun), a larger radius at the iris between the 2 cells, a reduced surface field on the curvature of the iris between the two cells, Z power coupling, increased cooling channels for operation at 120 Hz, dual rf feed, deformation tuning of the full cell, and field probes in both cells. Temporal shaping of the klystron pulse, to reduce the average power dissipated in the gun, has also been adopted. By increasing the mode separation, the amplitude of the 0-mode electric field on the cathode decreases from 10% of the peak on axis field for the prototype gun to less than 3% for the LCLS gun for the steady state fields. Beam performance is improved as shown by the PARMELA simulations. The gun should be designed to accept a future load lock system. Modifications follow the recommendations of our RF review committee [2]. Files and reference documents are compiled in Section IV.
Date: December 13, 2010
Creator: Limborg-Deprey, C
Partner: UNT Libraries Government Documents Department

LCLS Injector Straight-Ahead Spectrometer

Description: The spectrometer design was modified to allow the measurement of uncorrelated energy spread for the nominal lattice. One bunch from every 120 each second would be sent to the straight ahead spectrometer while the transverse cavity is on. The implementation of this 'stealing mode' will not be available for the LCLS commissioning and the early stage of operation. However, the spectrometer was redesigned to retain that option. The energy feedback relies independently on the beam position of the beam in the dispersive section of dogleg 1 (DL1). The main modification of the spectrometer design is the Pole face rotation of 7.5 degrees on both entrance and exit faces. The location and range of operation of the 3 quadrupoles remains unchanged relative to those of the earlier design.
Date: December 10, 2010
Creator: Limborg-Deprey , C.
Partner: UNT Libraries Government Documents Department

Wakefield Calculations for Radiation Stopper 1 (RST1)

Description: The main result of this note is that no wakefield mitigation is required for the Radiation Stopper (RST1) in the LCLS injector. The RST1 geometry is not symmetric in the vertical direction, and we derive a slight modification to the diffraction model wake for a cylindrically symmetric (2D) cavity that can be used for this problem. Performing a full 3D MAFIA calculation for the nominal 1 mm (rms) long bunch, we show that the modified diffraction model well describes the wakefields generated in RST1. The results imply an on-axis emittance growth of 0.0075%, well below the 0.5% tolerance threshold. To reach the 0.5% threshold the beam would need to be mis-steered by a large amount - 7 mm - from the axis. One reason that the effect is small is that the beta functions at the RST1 are small.
Date: December 13, 2010
Creator: Limborg-Deprey, C.
Partner: UNT Libraries Government Documents Department

Wakefield Computations for the Injector (Part I)

Description: In this document, we report on basic wakefield computations used to establish the impedance budget for the LCLS injector. Systematic comparisons between analytic formulae and results from ABCI are done. Finally, a comparison between 2D and 3D wakefield calculations are given for a cross. The three parts of the document are presented as follows: (1) ABCI computations for a few structures (Flange, Bellows...); (2) Comparison analytic with ABCI runs; and (3) Comparison Cross and Cavity using MAFIA.
Date: December 13, 2010
Creator: Limborg-Deprey, C.
Partner: UNT Libraries Government Documents Department

Maximizing Brightness in Photoinjectors

Description: If the laser pulse driving photoinjectors could be arbitrarily shaped, the emittance growth induced by space charge effects could be totally compensated for. In particular, for RF guns the photo-electron distribution leaving the cathode should have a 3D-ellipsoidal shape. The emittance at the end of the injector could be as small as the cathode emittance. We explore how the emittance and the brightness can be optimized for photoinjector based on RF gun depending on the peak current requirements. Techniques available to produce those ideal laser pulse shapes are also discussed.
Date: December 16, 2005
Creator: Limborg-Deprey, C.
Partner: UNT Libraries Government Documents Department

Alternate Tunings for the Linac Coherent Light Source Photoinjector

Description: The Linac Coherent Light Source (LCLS) is an x-ray free-electron laser (FEL) project based on the SLAC linac. The LCLS Photoinjector beamline has been designed to deliver 10-ps long electron bunches of 1 nC with a normalized projected transverse emittance smaller than 1.2 mm-mrad at 135 MeV. Tolerances and regulation requirements are tight for this tuning. Half of the total emittance at the end of the injector comes from the ''cathode emittance'' which is 0.7 mm-mrad for our nominal 1nC tuning. As the ''cathode emittance'' scales linearly with laser spot radius, the emittance will be dramatically reduced for smaller radius, but this is only possible at lower charge. In particular, for a 0.2 nC charge, we believe we can achieve an emittance closer to 0.4 mm-mrad. This working point will be easier to tune and the beam quality should be much easier to maintain than for the 1 nC case. In the second half of this paper, we discuss optimum laser pulse shapes. We demonstrate that the benefits of the ellipsoidal shapes seem to be important enough so that serious investigations should be carried out in the production of such pulses.
Date: March 17, 2006
Creator: Limborg-Deprey, C. & Emma, P.
Partner: UNT Libraries Government Documents Department

Optimum Electron Distributions for Space Charge Dominated Beams in Photoinjectors

Description: The optimum photo-electron distribution from the cathode of an RF photoinjector producing a space charge dominated beam is a uniform distribution contained in an ellipsoid. For such a bunch distribution, the space charge forces are linear and the emittance growth induced by those forces is totally reversible and consequently can be compensated. With the appropriate tuning of the emittance compensation optics, the emittance, at the end of photoinjector beamline, for an ellipsoidal laser pulse, would only have two contributions, the cathode emittance and the RF emittance. For the peak currents of 50A and 100 A required from the SBand and L-Band RF gun photoinjectors discussed here, the RF emittance contribution is negligible. If such an ellipsoidal photo-electron distribution were available, the emittance at the end of the beamline could be reduced to the cathode emittance. Its value would be reduced by more than 40% from that obtained using cylindrical shape laser pulses. This potentially dramatic improvement warrants review of the challenges associated with the production of ellipsoidal photo-electrons. We assume the photo-electrons emission time to be short enough that the ellipsoidal electron pulse shape will come directly from the laser pulse. We shift the challenge to ellipsoidal laser pulse shaping. To expose limiting technical issues, we consider the generation of ellipsoidal laser pulse shape in terms of three different concepts.
Date: June 15, 2006
Creator: Limborg-Deprey, C. & Bolton, P. R.
Partner: UNT Libraries Government Documents Department

Diagnostics for the LCLS Photoinjector Beamline

Description: 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 cavity and with the quadrupole scan emittance station. The designs of these two spectrometers have been supported by simulations from MAD and PARMELA.
Date: March 17, 2006
Creator: Limborg-Deprey, C.; Dowell, D.; Schmerge, J.F. & /SLAC
Partner: UNT Libraries Government Documents Department

Commissioning the SPEAR3 Diagnostic Beamlines

Description: SPEAR3 has two diagnostic beam lines: an x-ray pinhole camera and a visible/UV beam line. The pinhole camera images {approx}8 keV dipole synchrotron radiation (SR) incident on a phosphor screen. The visible beam line delivers conventional optical radiation to an array of cameras and diagnostic elements on a 1x3m optical bench. This paper briefly reviews the pinhole camera system while concentrating more on visible beam line power transmission calculations and streak camera data. Impedance estimates in the nominal optics and bunch length measurements in low-alpha optics are presented.
Date: June 5, 2007
Creator: Corbett, W.J.; Limborg-Deprey, C.; Mok, W.Y.; Ringwall, A. & /SLAC
Partner: UNT Libraries Government Documents Department

The SPEAR 3 Diagnostic Beamlines

Description: SPEAR 3 has two diagnostic beam lines: an x-ray pinhole camera and a visible/UV laboratory. The pinhole camera images {approx}8 keV dipole radiation on a phosphor screen with a remote computer to capture digital images and a parallel video signal to the control room. The visible/UV beam line features an 8 mm high GlidCop ''cold finger'' to remove the x-ray core of the beam. The remaining light is deflected horizontally onto an optical bench where it is focused via reflective (Cassegrain) or refractive optics. The visible beam can be split into branch lines for a variety of experimental applications. This paper describes the experimental set up and projected use of both systems.
Date: March 14, 2006
Creator: Corbett, W.J.; Limborg-Deprey, C.; Mok, W.Y.; Ringwall, A. & /SLAC
Partner: UNT Libraries Government Documents Department

Modifications of the LCLS Photoinjector Beamline

Description: The LCLS Photoinjector beamline is now in the Design and Engineering stage. The fabrication and installation of this beamline is scheduled for the summer 2006. The Photoinjector will deliver 10 ps long electron bunches of 1nC with a normalized transverse emittance of less than 1 mm.mrad for 80% of the slices constituting the core of the bunch at 135 MeV. The calculations done to finalize the specifications of the photoinjector beamline components are described. Modifications include a new exit energy, additional focusing between the two linac modules, the insertion of a ''laser heater'', and a new geometry for the coupling cells of the RF structures. We also discuss two interesting tunings, one for the nominal charge of 1nC but using a longer laser pulse and the second one for a lower charge of 0.2nC. Sensitivity to field errors and misalignment for those two new configurations is compared to that of the nominal tuning.
Date: May 9, 2005
Creator: Dowell, D.; Gierman, S.M.; Limborg-Deprey, C. & /SLAC
Partner: UNT Libraries Government Documents Department

Beam Dynamics Study of X-Band Linac Driven X-Ray FELS

Description: Several linac driven X-ray Free Electron Lasers (XFELs) are being developed to provide high brightness photon beams with very short, tunable wavelengths. In this paper, three XFEL configurations are proposed that achieve LCLS-like performance using X-band linac drivers. These linacs are more versatile, efficient and compact than ones using S-band or C-band rf technology. For each of the designs, the overall accelerator layout and the shaping of the bunch longitudinal phase space are described briefly. During the last 40 years, the photon wavelengths from linac driven FELs have been pushed shorter by increasing the electron beam energy and adopting shorter period undulators. Recently, the wavelengths have reached the X-ray range, with FLASH (Free-Electron Laser in Hamburg) and LCLS (Linac Coherent Light Source) successfully providing users with soft and hard X-rays, respectively. FLASH uses a 1.2 GeV L-band (1.3 GHz) superconducting linac driver and can deliver 10-70 fs FWHM long photon pulses in a wavelength range of 44 nm to 4.1 nm. LCLS uses the last third of the SLAC 3 km S-band (2.856 GHz) normal-conducting linac to produce 3.5 GeV to 15 GeV bunches to generate soft and hard X-rays with good spatial coherence at wavelengths from 2.2 nm to 0.12 nm. Newer XFELs (at Spring8 and PSI) use C-band (5.7 GHz) normal-conducting linac drivers, which can sustain higher acceleration gradients, and hence shorten the linac length, and are more efficient at converting rf energy to bunch energy. The X-band (11.4 GHz) rf technology developed for NLC/GLC offers even higher gradients and efficiencies, and the shorter rf wavelength allows more versatility in longitudinal bunch phase space compression and manipulation. In the following sections, three different configurations of X-band linac driven XFELs are described that operate from 6 to 14 GeV. The first (LOW CHARGE DESIGN) has an electron bunch charge ...
Date: December 13, 2011
Creator: Adolphsen, C.; Limborg-Deprey, C.; Raubenheimer, T.O.; Wu, J.; /SLAC; Sun, Y. et al.
Partner: UNT Libraries Government Documents Department

Computation of the Longitudinal Space Charge Effect in Photoinjectors

Description: The LCLS Photoinjector produces a 100A, 10 ps long electron bunch which is later compressed down to 230 fs to produce the peak current required for generating SASE radiation. SASE saturation will be reached in the LCLS only if the emittance and uncorrelated energy spread remain respectively below 1.2 mm.mrad and 5.10{sup -4}. This high beam quality will not be met if the Longitudinal Space Charge (LSC) instability develops in the injector and gets amplified in the compressors. The LSC instability originates in the injector beamline, from an initial modulation on top of the photoelectron pulse leaving the cathode. Numerical computations, performed with Multiparticle Space Charge tracking codes, showing the evolution of the longitudinal phase space along the LCLS injector beamline, are discussed. Their results are compared with those deduced from theoretical models in different regimes of energy and acceleration and for different modulation wavelengths. This study justifies the necessity to insert a ''laser heater'' in the LCLS Photoinjector beamline.
Date: May 9, 2005
Creator: Emma, P.; Huang, Z.; Limborg-Deprey, C.; Welch, J.J.; Wu, J. & /SLAC
Partner: UNT Libraries Government Documents Department

Linac Coherent Light Source Electron Beam Collimation

Description: This paper describes the design and simulation of the electron beam collimation system in the Linac Coherent Light Source (LCLS). Dark current is expected from the gun and some of the accelerating cavities. Particle tracking of the expected dark current through the entire LCLS linac, from gun through FEL undulator, is used to estimate final particle extent in the undulator as well as expected beam loss at each collimator or aperture restriction. A table of collimators and aperture restrictions is listed along with halo particle loss results, which includes an estimate of average continuous beam power lost. In addition, the transverse wakefield alignment tolerances are calculated for each collimator.
Date: April 27, 2007
Creator: Wu, J.; Dowell, D.; Emma, P.; Limborg-Deprey, C.; Schmerge, J.F. & /SLAC
Partner: UNT Libraries Government Documents Department

Three-Dimensional Quasistatic Model for High Brightness Beam Dynamics Simulation

Description: In this paper, we present a three-dimensional quasistatic model for high brightness beam dynamics simulation in rf/dc photoinjectors, rf linacs, and similar devices on parallel computers. In this model, electrostatic space-charge forces within a charged particle beam are calculated self-consistently at each time step by solving the three-dimensional Poisson equation in the beam frame and then transforming back to the laboratory frame. When the beam has a large energy spread, it is divided into a number of energy bins or slices so that the space-charge forces are calculated from the contribution of each bin and summed together. Image-charge effects from conducting photocathode are also included efficiently using a shifted-Green function method. For a beam with large aspect ratio, e.g., during emission, an integrated Green function method is used to solve the three-dimensional Poisson equation. Using this model, we studied beam transport in one Linac Coherent Light Sources photoinjector design through the first traveling wave linac with initial misalignment with respect to the accelerating axis.
Date: June 19, 2006
Creator: Qiang, Ji; Lidia, S.; Ryne, R.D.; /LBL, Berkeley; Limborg-Deprey, C. & /SLAC
Partner: UNT Libraries Government Documents Department

Measurement and Analysis of Field Emission Electrons in the LCLS Gun

Description: The field emission was measured during the high-power testing of the LCLS photocathode RF gun. A careful study and analysis of the field emission electrons, or dark current is important in assessing the gun's internal surface quality in actual operation, especially those surfaces with high fields. The first indication of a good RF gun design and fabrication is short processing time to the required fields and low electron emission at high fields. The charge per 2 microsecond long RF pulse (the dark charge) was measured as a function of the peak cathode field for the 1.6 cell, 2.856GHz LCLS RF gun. Faraday cup data was taken for cathode peak RF fields up to 120MV/m producing a maximum of 0.6nC/RF pulse for a diamond-turned polycrystalline copper cathode installed in the gun. Digitized images of the dark charge were taken using a 100 micron thick YAG crystal for a range of solenoid fields to determine the location and angular distribution of the field emitters. The FN plots and emitter image analysis will be described in this paper.
Date: November 2, 2007
Creator: Dowell, D.H.; Jongewaard, E.; Limborg-Deprey, C.; Schmerge, J.F.; Vlieks, A. & /SLAC
Partner: UNT Libraries Government Documents Department

Uncorrelated Energy Spread and Longitudinal Emittance of a Photoinjector Beam

Description: Longitudinal phase space properties of a photoinjector beam are important in many areas of high-brightness beam applications such as bunch compression, transverse-to-longitudinal emittance exchange, and high-gain free-electron lasers. In this paper, we discuss both the rf and the space charge contributions to the uncorrelated energy spread of the beam generated from a laser-driven rf gun. We compare analytical expressions for the uncorrelated energy spread and the longitudinal emittance with numerical simulations and recent experimental results.
Date: May 25, 2005
Creator: Huang, Z; Dowell, D.; Emma, P.; Limborg-Deprey, C.; Stupakov, G.; Wu, J. et al.
Partner: UNT Libraries Government Documents Department

Modifications on RF Components in the LCLS Injector

Description: 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 {pi} 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.
Date: March 3, 2006
Creator: Limborg-Deprey, C.; Dowell, D.; Li, Z.; Schmerge, J.F.; Xiao, L. & /SLAC
Partner: UNT Libraries Government Documents Department

Dual Feed RF Gun Design for the LCLS

Description: In order to remove the dipole field introduced by the coupler in existing S-band BNL/SLAC/UCLA 1.6 cell RF gun, a dual feed design for the LCLS RF gun is proposed together with several significant changes. The improvements include adopting z-coupling instead of {theta}-coupling, modifying the iris dimensions and profile to increase 0- and {pi}-mode separation from 3.4 to 15MHz and reduce the surface field on the iris, incorporating racetrack cavity shape to minimize the quadrupole field, increasing cooling for operation at 120Hz and other small changes to improve performance and diagnostic capabilities. The 3D gun structure had been modeled with the parallel finite element complex eigensolver Omega3p to provide the desired RF parameters and to generate the gun cavity dimensions needed for fabrication. In this paper the RF gun design will be presented.
Date: May 23, 2005
Creator: Xiao, L.; Boyce, R.F.; Dowell, D.H.; Li, Z.; Limborg-Deprey, C.; Schmerge, J.F. et al.
Partner: UNT Libraries Government Documents Department

Results of the SLAC LCLS Gun High-Power RF Tests

Description: The beam quality and operational requirements for the Linac Coherent Light Source (LCLS) currently being constructed at SLAC are exceptional, requiring the design of a new RF photocathode gun for the electron source. Based on operational experience at SLAC's GTF and SDL and ATF at BNL as well as other laboratories, the 1.6cell s-band (2856MHz) gun was chosen to be the best electron source for the LCLS, however a significant redesign was necessary to achieve the challenging parameters. Detailed 3-D analysis and design was used to produce near-perfect rotationally symmetric rf fields to achieve the emittance requirement. In addition, the thermo-mechanical design allows the gun to operate at 120Hz and a 140MV/m cathode field, or to an average power dissipation of 4kW. Both average and pulsed heating issues are addressed in the LCLS gun design. The first LCLS gun is now fabricated and has been operated with high-power RF. The results of these high-power tests are presented and discussed.
Date: November 2, 2007
Creator: Dowell, D. H.; Jongewaard, E.; Limborg-Deprey, C.; Schmerge, J. F.; Li, Z.; Xiao, L. et al.
Partner: UNT Libraries Government Documents Department

Coupler Design for the LCLS Injector S-Band Structures

Description: The LCLS injector is required to provide a 1-nC, 10-ps bunch with a normalized rms transverse projected emittance of less than 1 micron. The LCLS beam is generated and accelerated in a 1.6-cell S-band RF gun at 120 MV/m up to 6 MeV. The gun is followed by two SLAC 3-m S-band accelerator structures to further accelerate the beam to 135 MeV which moves the beam out of the space-charge dominated regime. In the SLAC S-band structures, the RF power feed is through a single coupling-hole (single-feed coupler) which results in a field asymmetry. The time dependent multipole fields in the coupler induce a transverse kick along the bunch and cause the emittance to increase above the LCLS specification. To meet the stringent emittance requirements for the injector, the single-feed couplers will be replaced by a dual-feed racetrack design to minimize the multipole field effects. We will present detailed studies of the multipole fields in the SLAC linac RF coupler and the improvements with the dual-feed ractrack design using the parallel finite element S-parameter solver S3P.
Date: March 3, 2006
Creator: Li, Z.; Bentson, L.D.; Chan, J.; Dowell, D.H.; Limborg-Deprey, C.; Schmerge, J.F. et al.
Partner: UNT Libraries Government Documents Department

Parallel Finite Element Particle-In-Cell Code for Simulations of Space-charge Dominated Beam-Cavity Interactions

Description: Over the past years, SLAC's Advanced Computations Department (ACD) has developed the parallel finite element (FE) particle-in-cell code Pic3P (Pic2P) for simulations of beam-cavity interactions dominated by space-charge effects. As opposed to standard space-charge dominated beam transport codes, which are based on the electrostatic approximation, Pic3P (Pic2P) includes space-charge, retardation and boundary effects as it self-consistently solves the complete set of Maxwell-Lorentz equations using higher-order FE methods on conformal meshes. Use of efficient, large-scale parallel processing allows for the modeling of photoinjectors with unprecedented accuracy, aiding the design and operation of the next-generation of accelerator facilities. Applications to the Linac Coherent Light Source (LCLS) RF gun are presented.
Date: November 7, 2007
Creator: Candel, A. E.; Kabel, A. C.; Ko, Yong-kyu; Lee, L.; Li, Z.; Limborg-Deprey, C. et al.
Partner: UNT Libraries Government Documents Department

Commissioning the LCLS Injector

Description: The Linac Coherent Light Source (LCLS) is a SASE x-ray Free-Electron Laser (FEL) project presently under construction at SLAC. The injector section, from drive laser and RF photocathode gun through first bunch compressor chicane, was installed in fall 2006. Initial system commissioning with an electron beam was completed in August 2007, with the goal of a 1.2-micron emittance in a 1-nC bunch clearly demonstrated. The second phase of commissioning, including second bunch compressor and full linac, is planned for 2008, with FEL commissioning in 2009. We report experimental results and experience gained in the first phase of commissioning, including the photo-cathode drive laser, RF gun, photocathode, S-band and X-band RF systems, first bunch compressor, and the various beam diagnostics.
Date: November 28, 2007
Creator: Akre, R.; Dowell, D.; Emma, P.; Frisch, J.; Gilevich, S.; Hays, G. et al.
Partner: UNT Libraries Government Documents Department