Various continuous-wave (CW) electron gun technologies are reviewed, including DC, superconducting radio frequency RF (SRF), hybrid DC-SRF and normal-conducting RF. Also, the SLAC Linac Coherent Light Source II (LCLS-II) normal-conducting RF gun and injector are described, and the performance to date, including the bunch emittance achieved and the dark current observed, is presented.

The injector is the low energy part of a linac, where space charge and non relativistic kinematic effects may affect the electron beam quality significantly, and in the case of single pass systems determines the brightness in the downstream components. Following the increasing demand for high repetition rate user facilities, a normal conducting, high repetition rate (1 MHz) RF gun operating at 186 MHz has been constructed at LBNL and is under operation. In the current paper, we report on the status of the beam dynamics studies. For this, a multi-objected approach is used, where both the transverse and the longitudinal phase space quality is optimized, as quantified by the transverse emittance and the bunch length and energy spread respectively. We also report on different bunch charge operating modes, as well as the effect of different gun gradients.

Type T thermocouples are commonly used in industrial measurement applications due to their accuracy relative to other thermocouple types, low cost, and the ready availability of measurement equipment. Type T thermocouples are very effective when used in differential measurements, as there is no cold junction compensation necessary for the connections to the measurement equipment. Type T's published accuracy specifications result in its frequent use in low temperature applications. An examination of over 328 samples from a number of manufacturers has been completed for this investigation. Samples were compared to a Standard Platinum Resistance Thermometer (SPRT) at the LN2 boiling point along with four other standardized measurement points using a characterized ice point reference, low-thermal EMF scanner and an 8.5 digit multimeter, and the data compiled and analyzed. The test points were approximately -196 degrees C, -75 degrees C, 0 degrees C, +100 degrees C, and +200 degrees C. These data show an anomaly in the conformance to the reference functions where the reference functions meet at 0 degrees C. Additionally, in the temperature region between -100 degrees C to -200 degrees C, a positive offset of up to 5.4 degrees C exists between the reference function equations published in the ASTM E230-06 for the nitrogen point and the measured response of the actual wire. This paper will examine the historical and technological reasons for this anomaly in the both the ASTM and IEC reference functions.

The LCLS accelerator is presently in a commissioning phase[1] and produces a 14GeV beam with normalized emittances on the order of one mm-mr, and peak current exceeding 3000 Amps. The design of the beam measurement system relies heavily on optical transition radiation profile monitors, in conjunction with transverse RF cavities, and conventional energy spectrometers. It has been found that the high peak currents and small longitudinal phase space of the beam generate strong coherent optical emission that limits the quantitative use of OTR and other prompt optical diagnostics, requiring the use of wire scanners or fluorescent screen based measurements. We present the results of beam measurements, measurements of the coherent optical effects, and future plans for the diagnostics.

The LCLS accelerator is presently in a commissioning phase(1) and produces a 14GeV beam with normalized emittances on the order of one mm-mr, and peak current exceeding 3000 Amps. The design of the beam measurement system relies heavily on optical transition radiation profile monitors, in conjunction with transverse RF cavities, and conventional energy spectrometers. It has been found that the high peak currents and small longitudinal phase space of the beam generate strong coherent optical emission that limits the quantitative use of OTR and other prompt optical diagnostics, requiring the use of wire scanners or fluorescent screen based measurements. We present the results of beam measurements, measurements of the coherent optical effects, and future plans for the diagnostics.

The LCLS hard X-ray Free Electron Laser at SLAC reported first lasing in April 2009. Since then two successful user runs have been completed at the two soft X-ray stations. The first hard X-ray station has started commissioning in July 2010. Beam diagnostics play an essential role for tuning the machine and delivering the requested beam properties to the users. An overview of the LCLS photon diagnostics will be presented including some selected commissioning results. Plans for future improvements and upgrades will be briefly discussed.

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 of magnitude to provide strong Landau damping against the instability without degrading the FEL performance. To this end, a 'laser-heater' system has been installed in the LCLS injector, which modulates the energy of a 135-MeV electron bunch with an IR laser beam in a short undulator, enclosed within a four-dipole chicane. The last half of the chicane time-smears the energy modulation leaving an effective thermal energy spread increase. We present the first commissioning results of this system, its operational issues, its impact on the microbunching instability, and finally its effect on the FEL performance.

The recently commissioned Linac Coherent Light Source is an X-ray free-electron laser at the SLAC National Accelerator Laboratory. It produces coherent soft and hard X-rays with peak brightness nearly ten orders of magnitude beyond conventional synchrotron sources and a range of pulse durations from 500 to <10 fs (10 −15 s). With these beam characteristics this light source is capable of imaging the structure and dynamics of matter at atomic size and timescales. The facility is now operating at X-ray wavelengths from 22 to 1.2 Å and is presently delivering this high-brilliance beam to a growing array of scientific researchers. We describe the operation and performance of this new ‘fourth-generation light source’.

In this paper we review the current state of research on energy recovered linacs as drivers for future X-ray sources. For many types of user experiments, such sources may have substantial advantages compared to the workhorse sources of the present: high energy storage rings. Energy recovered linacs need to be improved beyond present experience in both energy and average current to support this application. To build an energy recovered linac based X-ray user facility presents many interesting challenges. We present summaries on the Research and Development (R&D) topics needed for full development of such a source, including the discussion at the Future Light Sources Workshop held in Gaithersburg, Maryland on September 15–17, 2009. A first iteration of an R&D plan is presented that is founded on the notion of building a set of succeedingly larger test accelerators exploring cathode physics, high average current injector physics, and beam recirculation and beam energy recovery at high average current. Our basic conclusion is that a reviewable design of such a source can be developed after an R&D period of five to ten years.

The linac coherent light source (LCLS) is a self-amplified spontaneous emission (SASE) free-electron laser (FEL) operating at fundamental photon energies from 0.5 to 10 keV. Characterization of the higher harmonics present in the FEL beam is important to users, for whom harder x rays can either extend the useful operating wavelength range or increase experimental backgrounds. We present measurements of the power in both the second and third harmonics, and compare the results to expectations from simulations. We also present studies of the transport of harmonics to the users, and the harmonic power as a function of electron beam quality.

The electron beam for the Linac Coherent Light Source (LCLS) at SLAC is accelerated by disk-loaded RF structures over a length of 1 km. The mainly longitudinal field can sometimes exhibit transverse components, which kick the beam in x and/or y. This is normally a stable situation, but when a klystron, which powers some of these structures, has to be switched off and another one switched on, different kicks can lead to quite a different orbit. Some klystrons, configured in an energy and bunch length feedback, caused orbit changes of more than 1 mm, which is about 20 times the sigma beam size. The origins and measurements of these kicks and some efforts (orbit bumps) to reduce them will be discussed. INTRODUCTION Transverse RF kicks had caused some grief during the SLC era of the SLAC Linac, prompting beam studies and a beam-based alignment technique [1,2,3]. Emittance blow up due to an early klystron failure either required a 4 hour retuning or 2 hour repair impacting the program during that time. For LCLS these issues are reduced due to lower currents and shorter bunch length, but some impacts have been observed especially with klystrons 241 and 24-2 which have been used for energy and bunch length feedback and therefore vary constantly. It was possible to flip the phase configuration of say -90°, +30° to +30°, -90°, inducing an orbit oscillation with more than 1 mm amplitude (Fig. 1). Figure 1: RF kick for 360° phase change of klystron 24-2. This caused in some cases charge loss on collimators at the end of the linac before an orbit feedback corrected it. Besides the direct RF kicks, we have to consider dispersive effects too, since the beam energy is changed. These can be linear or even quadratic, as in the case of the BC2 chicane (bunch compressor 2 Fig. 3) [4]. The size of these effects, how they can bias our dispersion tuning procedure, and how we tried to suppress them with dispersive and non-dispersive bumps is discussed. DISPERSIVE OR RF KICKS In general it is tricky to distinguish between an RF kick and a dispersive kick due to an energy change. Two examples are given below. In Figure 2 the difference orbit is shown when a klystron early in the linac (22-8) is switched off. The orbit stays flat for two sectors Li23 and Li24 till it hits the BC2 chicane at Li24 801. There the beam gets a kick mainly in x, since some dispersion from BC2 “leaks” out. One reason for that is that the dispersion tuning procedure (see below) uses the energy feedback to set the energy using klystrons 24-1 and 24-2 (from Fig. 1) generating significant RF kicks. Another example is shown in Fig. 4, where the klystron 25-7 was switched off and the kick occurs right there. If there is no dispersion generated right there, which can be check by switching off another klystron earlier in the linac and observing no additional kick, the induced oscillation can be attributed to an RF kick, in this case 0.4 mm from klystron 25-7. Figure 2: Difference orbit is plotted with the klystron 22-8 off (minus on). An oscillation starts later around Li24 801 indicating a dispersive kick there. ____________________________________________ * Work supported by Department of Energy contract DE-AC03-76SF00515. WE5RFP039 Proceedings of PAC09, Vancouver, BC, Canada 2352 Light Sources and FELs A06 Free Electron Lasers Figure 3: LCLS schematic layout of the Linac sections (L0, L1, L2, and L3) with the two bunch compressors (BC), followed by a dog leg (DL2) and some collimation before reaching the undulator and dump. Figure 4: Difference orbit is plotted with the klystron 25-7 off (minus on). An oscillation starts right there indicating an RF kick. The signed “amplitude” is -0.4 mm in x. The amplitude of the orbits due to some klystron kicks is summarized in Tab. 1. Table 1: Orbit Offset in μm Caused by RF Kicks Klystron x y Klystron x y

The Linac Coherent Light Source (LCLS) X-ray FEL utilizing the last km of the SLAC linac has been operational since April 2009 and finished its first successful user run last December. The various diagnostics for electron beam properties including beam position monitors, wire scanners, beam profile monitors, and bunch length diagnostics are presented as well as diagnostics for the X-ray beam. The low emittance and ultra-short electron beam required for X-ray FEL operation has implications on the transverse and longitudinal diagnostics. The coherence effects of the beam profile monitors and the challenges of measuring fs long bunches are discussed.

Penetrations through the LCLS injector shield wall are needed for the alignment of the accelerator, a diagnostic laser beam and utilities, and are shown in figure 1. The 1-inch diameter LCLS injector beam tube is blocked by the PPS stopper when the injector side of the wall is occupied. The two 3-inch diameter penetrations above and to the left of the beam tube are used by Precision Alignment and will be open only during installation of the injector beamline. Additional 3-inch diameter penetrations are for laser beams which will be used for electron beam diagnostics. These will not be plugged when the injector occupied. Other penetrations for the RF waveguide and other utilities are approximately 13-inch from the floor and as such are far from the line-of-sight of any radiation sources. The waveguide and utility penetrations pass only through the thicker wall as shown in the figure. The principal issue is with the two laser penetrations, since these will be open when the linac is operating and people are in the LCLS injector area. A principal concern is radiation streaming through the penetrations due to direct line-of sight of the PEP-2 lines. To answer this, fans of rays were traced through the 3-inch diameter laser penetrations as shown in Figures 2 and 3. Figure 2 gives the top view of the shield walls, the main linac and PEP-2 lines, and the ray-fans. The fans appear to originate between the walls since their angular envelope is defined by the greatest angle possible when rays are just on the 3-inch diameter at the inner most and outermost wall surfaces. The crossovers of all possible rays lie half way between these two surfaces. As the end-on view of Figure 3 clearly shows, there is no direct line-of-sight through the laser penetrations of the PEP-2 or linac beamlines.

The very bright electron beam required for an x-ray free-electron laser (FEL), such as the linac coherent light source (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 of magnitude to provide strong Landau damping against the instability without degrading the FEL performance. To this end, a ``laser-heater'' system has been installed in the LCLS injector, which modulates the energy of a 135-MeV electron bunch with an IR-laser beam in a short undulator, enclosed within a four-dipole chicane. In this paper, we report detailed measurements of laser-heater-induced energy spread, including the unexpected self-heating phenomenon when the laser energy is very low. We discuss the suppression of the microbunching instability with the laser heater and its impact on the x-ray FEL performance. We also present the analysis of these experimental results and develop a three-dimensional longitudinal space charge model to explain the self-heating effect.

The Linac Coherent Light Source is an x-ray FreeElectron Laser (FEL) project being commissioned at SLAC. The very bright electron beam required for the FEL is subjected to various sources of jitter along the accelerator. The peak current, centroid energy, and trajectory of the electron bunch are controlled precisely at the highest repetition rate possible with feedback systems. We report commissioning experience for these systems.

The beam stability specifications for the Linac Coherent Light Source (LCLS) Free-Electron Laser (FEL) at Stanford Linear Accelerator Center are critical for XRay power, pointing, and timing stability. Studies of the transverse, longitudinal, and intensity stability of the electron beam are presented. Some sources are identified, correlated, and quantified.

The Linac Coherent Light Source (LCLS) is an x-ray free-electron laser project presently in a commissioning phase at the SLAC National Accelerator Laboratory. We report here on very low-emittance measurements made at low bunch charge, and a few femtosecond bunch length produced by the LCLS bunch compressors. Start-to-end simulations associated with these beam parameters show the possibilities of generating hundreds of GW at 1.5 A x-ray wavelength and nearly a single longitudinally coherent spike at 1.5 nm with 2-fs duration.

Received 27 October 2009DOI:https://doi.org/10.1103/PhysRevSTAB.12.119901This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

The Linac Coherent Light Source (LCLS) is an x-ray Free-electron Laser (FEL) being commissioned at Stanford Linear Accelerator Center (SLAC). Recent beam measurements have shown that, using the LCLS injector-linac-compressors, the beam emittance is very small at 20 pC. In this paper we perform start-to-end simulations of the entire accelerator including the FEL undulator and study the FEL performance versus the bunch charge. At 20 pC charge, these calculations associated with the measured beam parameters suggest the possibility of generating a longitudinally coherent single x-ray spike with 2-femtosecond (fs) duration at a wavelength of 1.5 nm. At 100 pC charge level, our simulations show an x-ray pulse with 10 femtosecond duration and up to 10{sup 12} photons at a wavelength of 1.5 {angstrom}. These results open exciting possibilities for ultrafast science and single shot molecular imaging.