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A proposed injector for the LCLS linac

Description: The Linac Coherent Light Source (LCLS) will use the last portion of the SLAC accelerator as a driver for a short wavelength FEL. The injector must produce 1-nC, 3-ps rms electron bunches at a repetition rate of up to 120 Hz with a normalized rms emittance of about 1 mm-mrad. The injector design takes advantage of the photocathode rf gun technology developed since its conception in the mid 1980`s, in particular the S-band rf gun developed by the SLAC/BNL/UCLA collaboration, and emittance compensation techniques developed in the last decade. The injector beamline has been designed using the SUPERFISH, POISSON, PARMELA, and TRANSPORT codes in a consistent way to simulate the beam from the gun up to the entrance of the main accelerator linac where the beam energy is 150 MeV. PARMELA simulations indicate that at 150 MeV, space charge effects are negligible.
Date: November 1, 1996
Creator: Yeremian, A.D.; Bharadwaj, V.K.; Emma, P.; Miller, R.H.; Palmer, D.T. & Woodley, M.D.
Partner: UNT Libraries Government Documents Department

Summary of emittance control in the SLC linac

Description: The SLC electron-positron collider requires micron size beams at the collision point in order to make maximum luminosity, which requires small beam emittances. These small emittances must be produced in the damping rings and accelerated down the linac without significant enlargement. The design (invariant) emittances {gamma}{var epsilon} at the exit of the damping rings are 1.7 {times} 10{sup {minus}5} radian meters (r-m) both horizontally (x) and vertically (y). The allowed emittance at the exit of the linac is 3 {times} 10{sup {minus}5} r-m. This report describes measurements of the beam emittance at various locations along the beam's trajectory and the techniques used to diagnose and correct errors. 6 refs., 9 figs.
Date: May 1, 1991
Creator: Seeman, J.T.; Adolphsen, C.; Bane, K.L.F.; Emma, P.; Decker, F.J.; Hsu, I. et al.
Partner: UNT Libraries Government Documents Department

Dispersion and betatron matching into the linac

Description: In high energy linear colliders, the low emittance beam from a damping ring has to be preserved all the way to the linac, in the linac and to the interaction point. In particular, the Ring-To-Linac (RTL) section of the SLAC Linear Collider (SLC) should provide an exact betatron and dispersion match from the damping ring to the linac. A beam with a non-zero dispersion shows up immediately as an increased emittance, while with a betatron mismatch the beam filaments in the linac. Experimental tests and tuning procedures have shown that the linearized beta matching algorithms are insufficient if the actual transport line has some unknown errors not included in the model. Also, adjusting quadrupole strengths steers the beam if it is offset in the quadrupole magnets. These and other effects have lead to a lengthy tuning process, which in the end improves the matching, but is not optimal. Different ideas will be discussed which should improve this matching procedure and make it a more reliable, faster and simpler process. 5 refs., 2 figs.
Date: May 1, 1991
Creator: Decker, F.J.; Adolphsen, C.; Corbett, W.J.; Emma, P.; Hsu, I.; Moshammer, H. et al.
Partner: UNT Libraries Government Documents Department

Using a fast-gated camera for measurements of transverse beam distributions and damping times

Description: With a fast-gated camera, synchrotron light was used for studying the transverse beam distributions and damping times in the Stanford Linear Collider (SLC) damping rings. By digitizing the image in the camera signal, the turn-by-turn time evolution of the transverse beam distribution was monitored and analyzed. The projections of the digitized image were fit with Gaussian functions to determine the moments of the distribution. Practical applications include the determination of injection matching parameters and the transverse damping times. In this report we describe a synchrotron light monitor and present experimental data obtained in the SLC damping rings.
Date: November 1, 1992
Creator: Minty, M.; Brown, R.; Decker, F. J.; Emma, P.; Krejcik, P.; Limberg, T. 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

LCLS Injector Drive Laser

Description: 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.
Date: November 2, 2007
Creator: Dowell, D.H.; Castro, J.; Emma, P.; Frisch, J.; Gilevich, A.; Hays, G. et al.
Partner: UNT Libraries Government Documents Department

Precision Measurement of the Undulator K Parameter using Spontaneous Radiation

Description: Obtaining precise values of the undulator parameter, K, is critical for producing high-gain FEL radiation. At the LCLS [1], where the FEL wavelength reaches down to 1.5 {angstrom}, the relative precision of K must satisfy ({Delta}K/K){sub rms} {approx}< 0.015% over the full length of the undulator. Transverse misalignments, construction errors, radiation damage, and temperature variations all contribute to errors in the mean K values among the undulator segments. It is therefore important to develop some means to measure relative K values, after installation and alignment. We propose a method using the angle-integrated spontaneous radiation spectrum of two nearby undulator segments, and the natural shot-to-shot energy jitter of the electron beam. Simulation of this scheme is presented using both ideal and measured undulator fields. By ''leap-frogging'' to different pairs of segments with extended separations we hope to confirm or correct the values of K, including proper tapering, over the entire 130-m long LCLS undulator.
Date: April 17, 2007
Creator: Welch, J.J.; Arthur, J.; Emma, P.; Hastings, J.B.; Huang, Z.; Nuhn, H.D. et al.
Partner: UNT Libraries Government Documents Department

Picosecond Bunch length and Energy-z correlation measurements at SLAC's A-Line and End Station A

Description: We report on measurements of picosecond bunch lengths and the energy-z correlation of the bunch with a high energy electron test beam to the A-line and End Station A (ESA) facilities at SLAC. The bunch length and the energy-z correlation of the bunch are measured at the end of the linac using a synchrotron light monitor diagnostic at a high dispersion point in the A-line and a transverse RF deflecting cavity at the end of the linac. Measurements of the bunch length in ESA were made using high frequency diodes (up to 100 GHz) and pyroelectric detectors at a ceramic gap in the beamline. Modeling of the beam's longitudinal phase space through the linac and A-line to ESA is done using the 2-dimensional tracking program LiTrack, and LiTrack simulation results are compared with data. High frequency diode and pyroelectric detectors are planned to be used as part of a bunch length feedback system for the LCLS FEL at SLAC. The LCLS also plans precise bunch length and energy-z correlation measurements using transverse RF deflecting cavities.
Date: June 27, 2007
Creator: Molloy, Stephen; Emma, P.; Frisch, J.C.; Iverson, R.H.; Ross, M.; McCormick, D.J. et al.
Partner: UNT Libraries Government Documents Department

Longitudinal Bunch Shape Diagnostics With Coherent Radiation And a Transverse Deflecting Cavity at TTF2

Description: At the DESY TTF2 linear accelerator three special techniques to characterize the longitudinal charge distribution of the electron bunches that drive the free-electron laser are currently under study: electro-optical sampling, far-infrared spectral analysis of coherent radiation and the use of a transverse deflecting cavity to streak the bunch. The principles and implementations of the latter two are described in this paper. Details on electro-optical sampling can be found in [1].
Date: August 4, 2005
Creator: Grimm, O.; Frohlich, L.; Klose, K.; Nagl, M.; Peters, O.; Rossbach, J. et al.
Partner: UNT Libraries Government Documents Department

Database Applications to Integrate Beam Line Optics Changes with the Engineering Databases

Description: The LCLS project databases provide key nomenclature information while integrating many engineering and physics processes in the building of an accelerator. Starting with the elements existing in the beam line optics files, the engineers add non-beam-line elements, and controls engineers assign ''Formal Device Names'' to these elements. Inventory, power supplies, racks, crates and cable plants are databases that are being integrated into the project database. This approach replaces individual spreadsheets and/or integrates standalone existing institutional databases.
Date: July 6, 2007
Creator: Chan, A.; Bellomo, P.; Crane, G.R.; Emma, P.; Grunhaus, E.; Luchini, K. et al.
Partner: UNT Libraries Government Documents Department

Measurements of Compression and Emittance Growth after the First LCLS Bunch Compressor Chicane

Description: The Linac Coherent Light Source (LCLS) is a SASE xray free-electron laser project presently under construction at SLAC. The injector section from RF photocathode gun through first bunch compressor chicane was installed during the fall of 2006. The first bunch compressor is located at 250 MeV and nominally compresses a 1-nC electron bunch from an rms length of about 1 mm to 0.2 mm. Transverse phase space and bunch length diagnostics are located immediately after the chicane. We present preliminary measurements and simulations of the longitudinal and transverse phase space after the chicane in various beam conditions, including extreme compression with micron-scale current spikes.
Date: November 2, 2007
Creator: Bane, K.; Ding, Y.; Emma, P.; Frisch, J.; Huang, Z.; Loos, H. et al.
Partner: UNT Libraries Government Documents Department


Description: The Visible-Infrared SASE Amplifier (VISA) FEL is an experimental device designed to show Self Amplified Spontaneous Emission (SASE) to saturation in the visible light energy range. It will generate a resonant wavelength output from 800--600 nm, so that silicon detectors may be used to characterize the optical properties of the FEL radiation. VISA is the first SASE FEL designed to reach saturation, and its diagnostics will provide important checks of theory. This paper includes a description of the VISA undulator, the magnet measuring and shimming system, and the alignment strategy. VISA will have a 4 m pure permanent magnet undulator comprising four 99 cm segments, each with 55 periods of 18 mm length. The undulator has distributed focusing built into it, to reduce the average beta function of the 70--85 MeV electron beam to about 30 cm. There are four FODO cells per segment. The permanent magnet focusing lattice consists of blocks mounted on either side of the electron beam, in the undulator gap. The most important undulator error parameter for a free electron laser is the trajectory walkoff or lack of overlap of the photon and electron beams. Using pulsed wire magnet measurements and magnet shimming, the authors expect to be able to control trajectory walkoff to less than {+-}50 pm per field gain length.
Date: August 16, 1998
Partner: UNT Libraries Government Documents Department

Recent improvements in the SLC positron system performance

Description: The positron system is very specific to the SLC in that the positrons are accelerated in the same linac as the electrons that produce them and the electrons with which they collide. Some of the difficulties in tuning this system to peak performance are thus unlikely to be encountered in future linear colliders, but many of the lessons learned in beam matching are useful for future machines. The design and commissioning of this system has been previously reported so we only briefly describe the major subsystems before detailing the tuning and diagnostics involved in optimizing the performance of the overall system.
Date: March 1, 1992
Creator: Krejcik, P.; Corbett, J.; Ecklund, S.; Emma, P.; Fieguth, T.; Helm, R. et al.
Partner: UNT Libraries Government Documents Department

Measured emittance versus store time in the SLC damping ring

Description: Emittance studies at the SLC North Damping Ring led to precise measurements of the damping time using three independent methods. These measurements were done at three different locations: (1) in the ring using a fast gated video camera which allows the acquisition of the image of the synchrotron light from a single turn, (2) using the extracted beam and a single wire scanner in the ring-to-linac transport line, and (3) in the linac using four wire scanners. In addition the extracted beam emittance was studied as a function of various parameters. A significant dependence on the tune was observed.
Date: March 1, 1992
Creator: Decker, F.J.; Emma, P.; Krejcik, P.; Limberg, T.; Minty, M.; Moshammer, H. et al.
Partner: UNT Libraries Government Documents Department

First Results of the LCLS Laser-Heater System

Description: The Linac Coherent Light Source (LCLS) is an x-ray Free-Electron Laser (FEL) project that has just achieved its first lasing at 1.5 {angstrom} radiation wavelength. The very bright electron beam required to drive this FEL is susceptible to a microbunching instability in the magnetic bunch compressors that may increase the slice energy spread beyond the FEL tolerance. To control the slice energy spread and to suppress the microbunching instability, a laser heater (LH) system is installed in the LCLS injector area at 135 MeV, right before the RF deflector that is used for the time-resolved electron diagnostics. This unique component is used to add a small level of intrinsic energy spread to the electron beam in order to Landau damp the microbunching instability before it potentially breaks up the high brightness electron beam. The system was fully installed and tested in the fall of 2008, and effects of heating on the electron beam and the x-ray FEL were studied during the 2009 commissioning period. The laser heater system is composed of a 4-dipole chicane; a 9-period, planar, permanent-magnet, adjustable-gap undulator at the center of the chicane; one OTR screen on each side of the undulator for electron/laser spatial alignment; and an IR laser (up to 15-MW power) which co-propagates with the electron beam inside the undulator generating a 758-nm energy modulation along the bunch. The final two dipoles of the 4-dipole chicane time-smear this modulation leaving only a thermal-like intrinsic energy spread within the bunch. Table 1 lists the main parameters for this system. The very bright electron beam required for an x-ray free-electron laser (FEL), such as the LCLS, is susceptible to a microbunching instability in the magnetic bunch compressors, prior to the FEL undulator. The uncorrelated electron energy spread in the LCLS can be increased by an order ...
Date: December 16, 2011
Creator: Emma, P; Boyce, R.F.; Brachmann, A.; Carr, R.; Decker, F.-J.; Ding, Y. et al.
Partner: UNT Libraries Government Documents Department

Second and Third Harmonic Measurements at the Linac Coherent Light Source

Description: The Linac Coherent Light Source (LCLS) started user commissioning in October of 2009, producing Free Electron Laser (FEL) radiation between 800 eV and 8 keV [1]. The fundamental wavelength of the FEL dominates radiation in the beamlines, but the beam also produces nonnegligible levels of radiation at higher harmonics. The harmonics may be desirable as a source of harder X-rays, but may also contribute backgrounds to user experiments. In this paper we present preliminary measurements of the second and third harmonic content in the FEL. We also measure the photon energy cutoff of the soft X-ray mirrors to determine the extent to which higher harmonics reach the experimental stations. We present preliminary second and third harmonic measurements for LCLS. At low energies (below 1 keV fundamental) we measure less than 0.1% second harmonic content. The second harmonic will be present in the soft X-ray beam line for fundamental photon energies below approximately 1.1 keV. At low and high energies, we measure third harmonic content ranging from 0.5% to 3%, which is consistent with expectations. For both second and third harmonics, experimental work is ongoing. More rigorous analysis of the data will be completed soon.
Date: January 3, 2011
Creator: Ratner, D.; /Stanford U., Phys. Dept.; Brachmann, A.; Decker, F.J.; Ding, Y.; Dowell, D. et al.
Partner: UNT Libraries Government Documents Department

Photon Beamlines and Diagnostics at LCLS

Description: The Linac Coherent Light Source (LCLS) is the first hard-x-ray free electron laser in operation. The turn-on of LCLS was rapid and operation has been reliable. Performance has exceeded the design parameters in several areas. The photon energy output covers a range from 480 eV to over 9 keV; the pulse energy is typically 2-3 mJ, with a maximum of 4 mJ at 2 keV. Electron pulse lengths can be varied from 500 fs to shorter than 10 fs. A low-charge option at 20 pC is being explored, which delivers pulses shorter than 10 fs with a reduced pulse energy, typically around 0.2 mJ. On-demand, single-shot and multi-shot modes up to 60 Hz (planned is 120 Hz) can be made available. The photon diagnostics built for LCLS have been commissioned and provide measurements of various properties of the FEL beam, such as pulse energy, beam size and position, wavelength, and allows for intensity attenuation over the entire wavelength range. The two soft x-ray instruments, the Atomic Molecular and Optics (AMO) and Soft X-ray Material Science (SXR) stations, are fully operational and completed their second user run in mid September 2010. The third user run is scheduled from October to December 2010, and will include the first hard x-ray instrument X-ray Pump-and-Probe (XPP). Three additional hard x-ray stations will follow: CXI (Coherent X-ray Imaging) is planned to start commissioning in December 2010, the XCS (X-ray correlation spectroscopy) instrument will start in June 2011, and the station for Matter in Extreme Conditions (MEC) in 2012. A list of past and future milestones for LCLS commission and operations is shown in table 1. The LCLS hard x-ray Free Electron Laser at SLAC reported first lasing in April of 2009. Since then two successful user runs have been completed at the two soft x-ray ...
Date: February 7, 2011
Creator: Moeller, S.; Arthur, J.; Brachmann, A.; Coffee, R.; Decker, F.-J.; Edstrom, S. et al.
Partner: UNT Libraries Government Documents Department

Observation of Coherent Optical Transition Radiation in the LCLS Linac

Description: The beam diagnostics in the linac for the Linac Coherent Light Source (LCLS) X-ray FEL project at SLAC includes optical transition radiation (OTR) screens for measurements of transverse and longitudinal beam properties. We report on observations of coherent light emission from the OTR screens (COTR) at visible wavelengths from the uncompressed and compressed electron beam at various stages in the accelerator.
Date: September 18, 2008
Creator: Loosy, H.; Akre, R.; Brachmann, A.; Decker, F.-J.; Ding, Y.; Dowell, D. et al.
Partner: UNT Libraries Government Documents Department

Positron Production by X Rays Emitted By Betatron Motion in a Plasma Wiggler

Description: Positrons in the energy range of 3-30 MeV, produced by x rays emitted by betatron motion in a plasma wiggler of 28.5 GeV electrons from the SLAC accelerator, have been measured. The extremely high-strength plasma wiggler is an ion column induced by the electron beam as it propagates through and ionizes dense lithium vapor. X rays in the range of 1-50 MeV in a forward cone angle of 0.1 mrad collide with a 1.7 mm thick tungsten target to produce electron-positron pairs. The positron spectra are found to be strongly influenced by the plasma density and length as well as the electron bunch length. By characterizing the beam propagation in the ion column these influences are quantified and result in excellent agreement between the measured and calculated positron spectra.
Date: January 25, 2007
Creator: Johnson, D.K.; Auerbach, D.; Blumenfeld, I.; Barnes, C.D.; Clayton, C.E.; Decker, F.J. et al.
Partner: UNT Libraries Government Documents Department

Ionization-Induced Electron Trapping inUltrarelativistic Plasma Wakes

Description: The onset of trapping of electrons born inside a highly relativistic, 3D beam-driven plasma wake is investigated. Trapping occurs in the transition regions of a Li plasma confined by He gas. Li plasma electrons support the wake, and higher ionization potential He atoms are ionized as the beam is focused by Li ions and can be trapped. As the wake amplitude is increased, the onset of trapping is observed. Some electrons gain up to 7.6 GeV in a 30.5 cm plasma. The experimentally inferred trapping threshold is at a wake amplitude of 36 GV/m, in good agreement with an analytical model and PIC simulations.
Date: April 6, 2007
Creator: Oz, E.; Deng, S.; Katsouleas, T.; Muggli, P.; /UCLA; Barnes, C.D. et al.
Partner: UNT Libraries Government Documents Department


Description: We present experimental studies of the gain length and saturation power level from 1.5 nm to 1.5 Angstroms at the Linac Coherent Light Source (LCLS). By disrupting theFEL process with an orbit kick, we are able to measure the X-ray intensity as a function of undulator length. This kick method is cross-checked with the method of removing undulator sections. We also study the FEL-induced electron energy loss after saturation to determine the optimal taper of the undulator K values. The experimental results are compared to theory and simulations.
Date: August 14, 2009
Creator: Ratner, D.; Fawley, W. M.; Brachmann, A.; Decker, F.J.; Ding, Y.; Dowell, D. et al.
Partner: UNT Libraries Government Documents Department

Determination of Longitudinal Phase Space in SLAC Main Accelerator Beams

Description: In the E164 Experiment at that Stanford Linear Accelerator Center (SLAC), we drive plasma wakes for electron acceleration using 28.5 GeV bunches from the main accelerator. These bunches can now be made with an RMS length of 12 microns, and accurate direct measurement of their lengths is not feasible shot by shot. Instead, we use an indirect technique, measuring the energy spectrum at the end of the linac and comparing with detailed simulations of the entire machine. We simulate with LiTrack, a 2D particle tracking code developed at SLAC. Understanding the longitudinal profile allows a better understanding of acceleration in the plasma wake, as well as investigation of related effects. We discuss the method and validation of our phase space determinations.
Date: June 7, 2005
Creator: Barnes, C.; Decker, F.-J.; Emma, P.; Hogan, M.J.; Iverson, R.; Krejcik, P. et al.
Partner: UNT Libraries Government Documents Department