LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

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6.1 Introduction L C L S C O N C E P T U A L D E S I G N R E P O R T Unlike light sources based on storage rings for which the beam properties of the stored beam due to synchrotron radiation are almost entirely decoupled from the properties of the injector beam, the performance of a linac-based Free Electron Laser (FEL) is directly dependent on the quality of the electron drive beam as produced by the injector and preserved by the main linac. As discussed in Chapter 4, FEL Physics, the optimum phase-space matching between the FEL electrons and photons is achieved if the electron beam emittance is
L C L S C O N C E P T U A L D E S I G N R E P O R T correspond to those predicted for an optimized S-band photoinjector using emittance compensation and assuming an electron bunch with initial uniform spatial and temporal charge distributions with σr and σz of 0.71 mm rms and 2.9 ps rms respectively. The photoinjector is actually designed to allow optimized performance with bunch charge as low as 0.2 nC. Given an optimized injector design for 1 nC, as the charge is lowered the emittance will remain optimized if the charge density is held constant by scaling each dimension of the bunch as Q 13 . Thus bunches with σz as short as 2 ps may be needed. Then, if the effects of thermal emittance are ignored, the optimized integrated emittance is expected to scale as Q 23 [5]. A low energy spread at 150 MeV is also required. An acceptable value of σ γ γ o ≤ 0.1% will be achieved for the integrated bunch by adjusting rf phases in the booster linac. An electron bunch can be analyzed in terms of axial slices. Unless otherwise specified, the thickness of a slice is some minor fraction of the total longitudinal width of the bunch. While the slice energy spread is important, it cannot be readily adjusted. A value of σ γ slice γ o ≤ 0.02% at 150 MeV is desired. In addition to low emittance and energy spread, there are two additional challenging requirements that are derived from the sensitivity of the current and energy jitters in the undulator to the charge and timing jitters at the gun. The first of these challenges concerns charge jitter at the gun. Simulations indicate that the rms peak current jitter in the undulator⎯where rms values are measured over a few seconds with the <strong>LCLS</strong> operating at 120 Hz⎯will be 12% for an rms charge jitter at the gun of 6%. (See Table 7.4.) However, when all sources of current jitter are taken into account, the rms charge jitter at the gun must be reduced to the order of about 2% to maintain the 12% rms peak current jitter at the undulator. (See Table 7.5.) Since the goal is to keep the rms peak current jitter in the undulator under 12%, a criterion of ≤2% has been adopted for the rms charge jitter for the <strong>LCLS</strong> photoinjector gun. Section 6.4, Laser System, describes how this tolerance will be achieved. The second challenge concerns the timing jitter of the electron bunch with respect to the rf driving the gun and linac. The timing jitter affects both the peak current and energy in the undulator. To keep the rms peak current jitter in the undulator below 12%, the rms timing jitter need only be less than 4.0 ps. However, the rms timing jitter must be less than 1.4 ps to maintain the rms energy jitter in the undulator below the desired 0.1% (Table 7.4). But again, when the various sources of energy jitter are considered together, the rms timing jitter at the gun must be reduced to the order of 0.9 ps (Table 7.5). In Section 6.4, Laser System, it is shown that a value of ≤0.5 ps for the rms timing jitter of the electron bunch with respect to the rf driving the gun and linac should be achievable. Since accomplishing this goal will slightly relieve the charge jitter criterion at the gun, this lower value of timing jitter has been adopted as the criterion for the injector laser system. These beam requirements are summarized in Table 6.1 below. 6-3 ♦ I NJECTOR
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Linac Coherent Light Source (LCLS)
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L C L S C O N C E P T U A L D E S I
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Preface This Conceptual Design Repo
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Table of Contents 1 Executive Summa
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6. Two experiment halls L C L S C O
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2.7.1.7 Conventional Facilities L C
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3 TECHNICAL SYNOPSIS Scientific Bas
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γεx,y (µm) yn 10 0 -10 -10 0 10
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σ γ /γ 0 (percent) ∆N/N 0.02 0
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5 0 -5 5 0 -5 5 0 -5 L C L S C O N
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Table 6.5 PARMELA Output Parameters
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gama x (micro.m) 3 2 1 0 0 L C L S
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∆E/E 0 (%) ∆N/N 0 -1 -2 0.02 0
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〈∆E/E 0 〉 /% I pk /I pk0 0.1
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β (m) 35 30 25 20 15 10 5 0 K21_1B
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β (m) 60 50 40 30 20 10 0 L C L S
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economic reasons. L C L S C O N C E
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Phase [deg. S-band] L C L S C O N C
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New Solid-State Sub-Booster and Ele
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7.8.2 Bunch Length Diagnostics L C
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Magnet String Location Existing, Ne
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8.1 Overview 8.1.1 Introduction L C
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Mu in the pole, for mu
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Gasload (Torr-l/sec-cm) (x10 -12 )
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8.10.4 Conclusion L C L S C O N C E
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x θ j > 0 L C L S C O N C E P T U
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Quad ∆X/µm Quad ∆Y/µm −500
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∆X/µm ∆Y/µm L C L S C O N C E
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8.13.2 Undulator Cell Structure L C
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9.2 Layout and Optics 9.2.1 Experim
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Fast Valve L C L S C O N C E P T U
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0.01 10 -4 10 -6 10 -8 10 -10 10 -1
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Table 9.5 Mirror requirements L C L
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Refractive Focusing System L C L S
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Pulse Split/Delay L C L S C O N C E
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Figure 9.20 LCLS Ion Chamber L C L
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9.4.3.2 Pulse Length L C L S C O N
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9.4.3.6 Divergence L C L S C O N C
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10 Conventional Facilities TECHNICA
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1-2002 8560A198 Linac Center Line L
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Figure 10.6 Near hall floor plan 10
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11 Controls TECHNICAL SYNOPSIS The
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11.5.3 Motion Controls L C L S C O
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12.1 Procedural Overview L C L S C
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13 Environment, Safety and Health a
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Table 13-1 Hazard Identification an
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13.1 Ionizing Radiation The design
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Table 13-2 Minimum Training Require
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13.10 SLAC References L C L S C O N
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L C L S D E S I G N S T U D Y R E P
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15 TECHNICAL SYNOPSIS Work Breakdow
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A A.1 FEL-Physics A.1.1 Performance
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A.2 Photo-Injector A.2.1 Gun-Laser
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A.2.3.2 Electron Beam L C L S C O N
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A.2.4.7 Vacuum L C L S C O N C E P
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A.3.8 DL-2 A.3.8.1 Subsystem L C L
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A.4 Undulator A.4.1 Undulator A.4.1
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B Control Points B.1 Injector Contr
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C Glossary ACO Anneaux Collisions O
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