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North RTL grid scan'' studies

Description: This study was made in response to screen measurements which indicated an emittance growth of nearly a factor of two within the North RTL or linac girder-1. Betatron oscillations are induced at the beginning of the North RTL to search for gross geometric aberrations arising within the RTL or sector-2 of the linac. The oscillations are induced horizontally and vertically with two X or two Y dipole correctors stepped in a nested loop fashion. In both cases the full set of RTL and first girder sector-2 linac beam position monitors (BPMs) are sampled in X and Y for each corrector setting. Horizontal (or vertical) data from pairs of BPMs are then transformed to phase space coordinates by the linear transformation constructed assuming the transport optics between the BPMs is known. A second transformation is then made to normalized phase space coordinates by using Twiss parameters consistent with the assumed transport optics. By careful choice of initial Twiss parameters the initial grid can be made square for convenience in graphical interpretation. A linear grid'' is then fitted to the transformed data points for each pair of BPMs. The area of each grid is calculated and linearity qualitatively evaluated. Furthermore, although not the focus of this study, the beta match at each BPM can be quantified. 6 figs.
Date: October 17, 1990
Creator: Emma, P.
Partner: UNT Libraries Government Documents Department

Linear fitting of BPM orbits and lattice parameters

Description: Linear least xi/sup 2/-fits will in general give wrong results to the data since the errors are not properly handled. In this note we discuss a solution for this problem. This paper covers briefly the standard X/sup 2/-fitting of trajectory parameters and discusses the case of linear correlations between independent measurements which are all subject to stochastic noise. An analytic solution for unbiased fits including the error analysis is derived. Examples are presented. The mathematical analysis is involved and possibly not of a general interest. The fit methods are available as general software packages on the MCC/SLC-VAX. Similar packages were produced for the HP 28 S pocket calculator for testing purposes. The methods were successfully applied for online monitoring of dispersion functions and transfer-matrix elements. 3 figs.
Date: February 1, 1989
Creator: Lohse, T. & Emma, P.
Partner: UNT Libraries Government Documents Department

Difference orbits and BPM-resolutions

Description: In the interaction region of the SLC, the information of beam position monitors (BPM) is used to get crucial information on the beam parameters, like the position, the inclination, or the deflection of the beams at the interaction point. The information is usually extracted from the BPM readings by fitting orbits, using the known twiss parameters at the BPM positions. The quality of these fits depends, however, on the assumed resolutions of the BPMs. This is a particularly serious problem in the interaction region, since BPMs with quite different resolutions and very different values for the beta-function have to be combined. This note presents a method which allows empirical extraction of BPM-resolutions from the orbit-fits. The method has been originally developed in reference. It has been tested by Monte Carlo simulations and successfully exploited for the measurement of spatial resolutions of a time projection chamber. 2 refs., 3 figs., 5 tabs.
Date: June 20, 1988
Creator: Emma, P. & Lohse, T.
Partner: UNT Libraries Government Documents Department

Linac Sector 21-30 'QE' Quadrupole Magnet Excitation in LCLS Operation

Description: The existing SLAC linac quadrupole magnets, described as the 'QE' quadrupoles, are designed for excitation currents up to 200 A with length-integrated gradients up to 100 kG needed to focus up to 50-GeV electron or positron beams. The eight 'QE' magnets within a linac sector are powered in series on a single 200-A bulk power supply, and each magnet in sectors 21-29 also includes a 20-A booster supply (adds 0-20 A to each magnet) for gradient tapering within the sector. Sector 30 is different again with individual 200-A power supplies per magnet. The LCLS will use these same quadrupole magnets, mostly in their existing locations and with their present power supplies, but for an electron energy range of 0.25-14 GeV. This much lower beam energy will require good power supply regulation ({le} 0.05% rms for f > 0.5 Hz) [1] and accurate field excitation at much lower currents, as low as a few amperes. The present (pre-2006) gradient-to-current polynomials (IVBU) in the SLC database are based on magnetic measurements at excitation currents above 20 A, and are not accurate in extrapolation below about 10 A. For this reason the IVBU polynomials for all 'QE' quadrupoles in sectors 21-30 will be updated in 2007 for LCLS operation. Magnets beyond Q21601 will be updated with a new generic polynomial (with polarity considerations) based on magnetic measurements from zero current all the way up to 200 A, with additional data taken in the 0-20 A range for accuracy in LCLS mode (see Fig. 1). In addition, the first five LCLS 'QE' magnets (Q21201, Q21301, Q21401, Q21501, and Q21601), most of which run at the lowest currents of <3 A, have been individually measured and fitted with a specific polynomial to meet their special settings in LCLS mode. All changes will continue to fully ...
Date: November 24, 2010
Creator: Emma, P
Partner: UNT Libraries Government Documents Department

North RTL 'Grid Scan' Studies

Description: This study was made in response to screen measurements which indicated an emittance growth of nearly a factor of two within the North RTL or linac girder-1. Betatron oscillations are induced at the beginning of the North RTL to search for gross geometric aberrations arising within the RTL or sector-2 of the linac. The oscillations are induced horizontally and vertically with two X or two Y dipole correctors stepped in a nested loop fashion. In both cases the full set of RTL and first girder sector-2 linac beam position monitors (BPMs) are sampled in X and Y for each corrector setting. Horizontal (or vertical) data from pairs of BPMs are then transformed to phase space coordinates by the linear transformation constructed assuming the transport optics between the BPMs is known. A second transformation is then made to normalized phase space coordinates by using Twiss parameters consistent with the assumed transport optics. By careful choice of initial Twiss parameters the initial grid can be made square for convenience in graphical interpretation. A linear ''grid'' is then fitted to the transformed data points for each pair of BPMs. The area of each grid is calculated and linearity qualitatively evaluated. Furthermore, although not the focus of this study, the beta match at each BPM can be quantified.
Date: February 26, 2007
Creator: Emma, P.
Partner: UNT Libraries Government Documents Department

The Stanford Linear Collider

Description: The Stanford Linear Collider (SLC) is the first and only high-energy e{sup +}e{sup {minus}} linear collider in the world. Its most remarkable features are high intensity, submicron sized, polarized (e{sup {minus}}) beams at a single interaction point. The main challenges posed by these unique characteristics include machine-wide emittance preservation, consistent high intensity operation, polarized electron production and transport, and the achievement of a high degree of beam stability on all time scales. In addition to serving as an important machine for the study of Z{sup 0} boson production and decay using polarized beams, the SLC is also an indispensable source of hands-on experience for future linear colliders. Each new year of operation has been highlighted with a marked improvement in performance. The most significant improvements for the 1994-95 run include new low impedance vacuum chambers for the damping rings, an upgrade to the optics and diagnostics of the final focus systems, and a higher degree of polarization from the electron source. As a result, the average luminosity has nearly doubled over the previous year with peaks approaching 10{sup 30} cm{sup {minus}2}s{sup {minus}1} and an 80% electron polarization at the interaction point. These developments as well as the remaining identifiable performance limitations will be discussed.
Date: June 1, 1995
Creator: Emma, P.
Partner: UNT Libraries Government Documents Department

Beam based alignment of sector-1 of the SLC linac

Description: A technique is described which uses the beam to simultaneously measure quadrupole magnet and beam position monitor (BPM) transverse misalignments. The technique is applied to sector-1 of the SLC linac where simultaneous acceleration of electron and positron beams with minimal steering elements and BPMs makes quadrupole alignment critical for high transmission of the large transverse emittance positron beam. Simulation results as well as measurements are presented.
Date: March 1, 1992
Creator: Emma, P.
Partner: UNT Libraries Government Documents Department

Effects of Tapered Betafunction in the LCLS Undulators

Description: The Linac Coherent Light Source (LCLS) is an X-ray free-electron laser (FEL) project based on the SLAC linac [1]. With its nominal set of electron beam, focusing and undulator parameters, it is designed to achieve SASE saturation at an undulator length of about 100 m with an average power of 10 GW. In order to keep the electron beam focused in the undulators, a FODO lattice is integrated along the entire length of the undulators. Nominally, the quadrupole strengths are chosen to produce nearly constant betafunction and beam size along the undulator, optimized for the FEL interaction in the exponential growth regime. Since these quadrupoles are electromagnetic, it is possible to adjust the individual quadrupole strength to vary the betafunction and the beam size along the undulator, tailoring the FEL interaction in the start-up and the saturation regimes. In this paper, we present simulation studies of the tapered betafunction in the LCLS undulator and discuss the generated X-ray properties.
Date: September 30, 2005
Creator: Goldammer, K.; Emma, P. & Huang, Z.
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

Peak Current Optimization for LCLS Bunch Compressor 2

Description: In this study, we calculate the effects of Coherent Synchrotron Radiation (CSR) in the LCLS bunch compression section BC2[3] on the resulting FEL performance, considering a realistic, strongly non-gaussian longitudinal charge distribution. The longitudinal chirping required for the bunch compression process leads to a non-linear, non-monotonous {delta}(z) functional dependence (Fig. 1 shows the current distribution and the energy offset along the bunch). We model this functional dependence by matching it to a cubic polynomial {delta} {approx} c{sub 0} + c{sub 1}z + c{sub 2}z{sup 2} + c{sub 3}z{sup 3}. During compression, the charge distribution in the z-{delta} plane will ''fold over'', as shown in fig. 2. This leads to a cusp at each end of the current distribution I(z), as shown in figure 3. High |l'(z)| values will lead to high longitudinal CSR fields, with possible detrimental effects on the transverse projected and slice emittance as well as energy spread, possibly affecting FEL performance.
Date: May 9, 2005
Creator: Emma, P. & Kabel, A. C.
Partner: UNT Libraries Government Documents Department

Linac Coherent Light Source (LCLS) Bunch-Length Monitor using Coherent Radiation

Description: The Linac Coherent Light Source (LCLS) is a SASE x-ray Free-Electron Laser (FEL) based on the final kilometer of the Stanford Linear Accelerator. One of the most critical diagnostic devices is the bunch length monitor (BLM), which is to be installed right after each compressor utilizing coherent radiation from the last bending magnet. We describe the components and the optical layout of such a BLM. Based on the setup geometry, we discuss some issues about the coherent radiation signal.
Date: March 21, 2007
Creator: Wu, Juhao & Emma, P.
Partner: UNT Libraries Government Documents Department

The Linac Cooherent Light Source (LCLS) Accelerator

Description: The Linac Coherent Light Source (LCLS) is a SASE x-ray Free-Electron Laser (FEL) based on the final kilometer of the Stanford Linear Accelerator. Such an FEL requires a high energy, high brightness electron beam to drive the FEL instability to saturation. When fed by an RF-photocathode gun, and modified to include two bunch compressor chicanes, the SLAC linac will provide such a high quality beam at 14 GeV and 1-{micro}m normalized emittance. In this paper, we report on recent linac studies, including beam stability and tolerances, longitudinal and transverse feedback systems, conventional and time-resolved diagnostics, and beam collimation systems. Construction and installation of the injector through first bunch compressor will be completed by December 2006, and electron commissioning is scheduled to begin in January of 2007.
Date: March 21, 2007
Creator: Wu, Juhao & Emma, P.
Partner: UNT Libraries Government Documents Department

Trajectory Stability Modeling And Tolerances in the LCLS

Description: To maintain stable performance of the Linac Coherent Light Source (LCLS) x-ray free-electron laser, one must control the electron trajectory stability through the undulator to a small fraction of the beam size. BPM-based feedback loops running at 120 Hz will be effective in controlling jitter at low frequencies less than a few Hz. On the other hand, linac and injector stability tolerances must be chosen to limit jitter at higher frequencies. In this paper we study possible sources of high frequency jitter, including: (1) steering coil current regulation; (2) quadrupole magnet transverse vibrations; (3) quadrupole current regulation with transverse misalignments; (4) charge variations coupled to jitter through transverse wakefields of misaligned RF structures; and (5) bunch length variations coupled through coherent synchrotron radiation in the bunch compressor chicanes. Based on this study, we set component tolerances and estimate expected trajectory stability in the LCLS.
Date: April 27, 2007
Creator: Wu, J. & Emma, P.
Partner: UNT Libraries Government Documents Department

A bunch compressor for the Next Linear Collider

Description: A bunch compressor design for the Next Linear Collider (NLC) is described. The compressor reduces the bunch length by a factor of 40 in two stages. The first stage at 2 GeV consists of an rf section and a wiggler. The second stage at 10 GeV is formed by an arc, an rf section, and a chicane. The final bunch phase is insensitive to initial phase errors and to beam loading in the intermediate S-band pre-linac. Residual longitudinal aberrations of the system are partially compensated. The bunch compressor encompasses a solenoid spin-rotator system at 2 GeV that allows complete control over the spin orientation.
Date: June 1, 1995
Creator: Emma, P.; Raubenheimer, T. & Zimmermann, F.
Partner: UNT Libraries Government Documents Department

Grid scans: A transfer map diagnostic

Description: A beam line transfer map diagnostic is described which uses induced betatron oscillations to search for focusing errors and geometric aberrations. A grid is produced graphically in normalized phase space coordinates with the beta match quantified from this grid. Application to the SLC electron damping Ring-To-Linac (RTL) transport line is presented. 1 ref., 4 figs.
Date: May 1, 1991
Creator: Emma, P. & Spence, W.
Partner: UNT Libraries Government Documents Department

Pulse Length Control in an X-Ray FEL by Using Wakefields

Description: For the users of the high-brightness radiation sources of free-electron lasers it is desirable to reduce the FEL pulse length to 10 fs and below for time-resolved pump and probe experiments. Although it can be achieved by conventional compression methods for the electron beam or the chirped FEL pulse, the technical realization is demanding. In this presentation we study the impact of longitudinal wakefields in the undulator and how their properties can be used to reduced the amplifying part of the bunch to the desired length. Methods of actively controlling the wakefields are presented.
Date: March 18, 2008
Creator: Reiche, S.; Pellegrini, Claudio; Emma, P. & /SLAC, /UCLA
Partner: UNT Libraries Government Documents Department

Electron Signal Detection for the Beam-Finder Wire of the Linac Coherent Light Source Undulator

Description: The Linac Coherent Light Source (LCLS) is a SASE x-ray Free-Electron Laser (FEL) based on the final kilometer of the Stanford Linear Accelerator. The tight tolerances for positioning the electron beam close to the undulator axis calls for the introduction of Beam Finder Wire (BFW) device. A BFW device close to the upstream end of the undulator segment and a quadrupole close to the down stream end of the undulator segment will allow a beam-based undulator segment alignment. Based on the scattering of the electrons on the BFW, we can detect the electron signal in the main dump bends after the undulator to find the beam position. We propose to use a threshold Cherenkov counter for this purpose. According to the signal strength at such a Cherenkov counter, we then suggest choice of material and size for such a BFW device in the undulator.
Date: September 21, 2006
Creator: Wu, Juhao; Emma, P.; Field, R.C. & /SLAC
Partner: UNT Libraries Government Documents Department

Analysis of Longitudinal Space Charge Effects With Radial Dependence

Description: Longitudinal space charge (LSC) force can be a main effect driving the microbunching instability in the linac for an x-ray free-electron laser (FEL). In this paper, the LSC-induced beam modulation is studied using an integral equation approach that takes into account the transverse (radial) variation of LSC field for both the coasting beam limit and bunched beam. Changes of beam energy and the transverse beam size can be also incorporated. We discuss the validity of this approach and compare it with other analyses as well as numerical simulations.
Date: September 30, 2005
Creator: Wu, Juhao; Huang, Z.; Emma, P. & /SLAC
Partner: UNT Libraries Government Documents Department

Linac Coherent Light Source Longitudinal Feedback Model

Description: The Linac Coherent Light Source (LCLS) will be the world's first x-ray free-electron laser (FEL). To ensure the vitality of FEL lasing, it is critical to preserve the high quality of the electron beam during acceleration and compression. The peak current and final energy are very sensitive to system jitter. To minimize this sensitivity, a longitudinal feedback system on the bunch length and energy is required, together with other diagnostics and feedback systems (e.g., on transverse phase space). Here, we describe a simulation framework, which includes a realistic jitter model for the LCLS accelerator system, the RF acceleration, structure wakefield, and second order optics. Simulation results show that to meet the tight requirements set by the FEL, such a longitudinal feedback system is mandatory.
Date: June 15, 2005
Creator: Wu, Juhao; Emma, P.; Hendrickson, L. & /SLAC
Partner: UNT Libraries Government Documents Department

SABER Optical Design

Description: SABER, the South Arc Beam Experimental Region, is a proposed new beam line facility designed to replace the Final Focus Test Beam at SLAC. In this paper, we outline the optical design features and beam parameters now envisioned for SABER. A magnetic chicane to compress positron bunches for SABER and a bypass line that could transport electrons or positrons from the two-thirds point of the linac to SABER, bypassing the LCLS systems, are also discussed.
Date: July 7, 2006
Creator: Erickson, R.; Bane, K.; Emma, P.; Nosochkov, y. & /SLAC
Partner: UNT Libraries Government Documents Department

Realization of an X-Band RF System for LCLS

Description: A single X-band (11.424 GHz) accelerating structure is to be incorporated in the LINAC Coherent Light Source (LCLS) design to linearize the energy-time correlation (or gradient) across each bunch; features which originate in the preceding accelerating structures (L0 and L1) [1]. This harmonic RF system will operate near the negative RF crest to decelerate the beam, reducing these non-linear components of the correlation, providing a more efficient compression in the downstream bunch compressor chicanes (BC1 and BC2). These non-linear correlation components, if allowed to grow, would lead to Coherent Synchrotron Radiation (CSR) instabilities in the chicanes, effectively destroying the coherence of the photon radiation in the main LCLS undulator. The many years devoted at SLAC in the development of X-band RF components for the NLC/JLC linear collider project [2], has enabled the technical and financial realization of such an RF system for LCLS. This paper details the requirements for the X-band system and the proposed scheme planned for achieving those requirements.
Date: June 15, 2005
Creator: McIntosh, P.; Akre, R.; Brooks, W.; Emma, P.; Rago, C. & /SLAC
Partner: UNT Libraries Government Documents Department

Coherent Synchrotron Radiation as a Diagnostic Tool for the LCLS Longitudinal Feedback System

Description: The Linac Coherent Light Source (LCLS) will be the world's first x-ray free-electron laser (FEL). To ensure the vitality of FEL lasing, a longitudinal feedback system is required together with other diagnostics. In this paper, we study the possibility of using Coherent Synchrotron Radiation (CSR) from the chicane as a diagnostic tool for bunch length feedback. Studies show that CSR is a good candidate, even for a non-Gaussian, double-horn longitudinal charge distribution as in the LCLS. We further check the possibility for detecting possible microbunching.
Date: June 15, 2005
Creator: Wu, Juhao; Emma, P.; Huang, Z. & /SLAC
Partner: UNT Libraries Government Documents Department

Weak FEL Gain Detection with a Modulated Laser-Based Beam Heater

Description: For an x-ray free-electron laser (FEL) such as the LCLS, the FEL gain signal is accompanied by spontaneous radiation with a significant power level. Detecting the weak FEL gain among the large spontaneous background in the early stage of the exponential growth or for a low quality electron beam is important in commissioning the FEL. In this paper, we describe a simple ''lock-in'' method of weak FEL gain detection, suggested by K. Robinson, accomplished by slowly modulating the laser power of a designated beam heater that controls the local energy spread of the electron beam. We present numerical modeling that shows the effectiveness of this method and discuss its implementation in the LCLS.
Date: May 25, 2005
Creator: Emma, P.; Huang, Z.; Wu, J. & /SLAC
Partner: UNT Libraries Government Documents Department