LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

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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 The permanent magnet material must therefore be protected from direct hits by the electron beam. The material is a powder-metallurgical product and is quite brittle, i.e., has very low ductility, and exposure to one pulse might fracture it. Independent of structural concerns, temperature changes of this magnitude would cause permanent changes in the remanent magnetic field, which are not acceptable. Since the magnetic material has atomic properties very similar to copper, neither copper nor materials with equal or higher atomic number are suitable as primary collimator materials. The primary material must be protected by a low-Z material like titanium. 8.8.1.3 Undulator Vacuum Chamber There are two distinctly different beam exposure scenarios for the undulator vacuum chamber. The first is direct e - -beam exposure at the entrance to the undulator with the momentum vector approximately parallel to the undulator and vacuum chamber axis (this assumes that no collimator is in place). The second exposure scenario results from excessive beam deflection inside the undulator resulting in the beam impinging at shallow angles onto the vacuum chamber. Selection of an appropriate material for the vacuum chamber involves tradeoffs between physics performance, survival during direct primary beam exposure, and ease of manufacture, and thus economics. Physics performance dictates a chamber material of low electrical resistivity, at least on the inside surface, to keep the resistive wall wake function at acceptable levels. Materials like copper and aluminum are good choices. Long-term survival against direct hits by the e - -beam requires a low-Z material with good strength and endurance characteristics. Titanium and some of its alloys, as well as some aluminum alloys, are good choices. Since the undulator and its vacuum chamber are ~100 m long, the chamber needs to be built in segments (anticipated modular length ~3.4 m) and joined by vacuum flanges and bellows. The materials mentioned above are technically feasible to use, but they also present fabrication, installation, and economic challenges. Copper, aluminum, titanium, and stainless steel were evaluated for possible use as vacuum chamber material. Stainless steel is the final choice, and an analysis of its response to the two exposure scenarios is presented below. Cost effective manufacturing, ease of installation, and maintenance for ultra-high vacuum make stainless steel a first choice, but at the expense of high electrical resistivity. This handicap can be compensated by surface coating with a low resistivity material. 8.8.1.4 Beam Strikes at the Entrance to the Vacuum Chamber Using the minimum ionization loss and no shower multiplicity ( = 1), the power deposition at normal incidence to the chamber is using dE/dx for iron to approximate stainless steel. 8-42 ♦ U N D U L A T O R P dE dx N PRR -19 ' = (- ρ / ) × 1.6× 10 = ( − ) Π e 6 10 −19 11.6× 10 × 0.625× 10 × 1.6× 10 × 120 = 1.4 W/cm (8.34)
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 Next, again assuming a Gaussian beam intensity distribution with σ ~32 µm, and also assuming uniform intensity within the radial interval 0 < r ≤ σ, the heat source term becomes S ~17 kW/cm 3 . The temperature rise per single rf-bunch follows as ∆T = 17 × 10 3 /(4 × 120) ~35°C. The resulting thermal stresses are σth ~ 8.63 × 10 7 N/m 2 (12,500 psi) for a fully restrained body. Thin-walled tubing and beam exposure near the surface (inside or outside) will remove some of these restraints and thereby reduce the magnitude of these stresses at the expense of increased elastic strain. The endurance limit for the type of stainless steel used for vacuum chamber tubing (300 Series) is σEnd ~ 1.73 × 10 8 N/m 2 (25,000 psi), and there would be no problem for this level of beam exposure. The yield strength σy ~σth is ~ 2.07 × 10 8 N/m 2 (30,000 psi) is also significantly above the exposure stress, and no plastic, permanent deformations would occur. 8.8.1.5 Beam Strikes Inside the Undulator Once inside the undulator, the electron beam can experience additional deflections. Based on alignment considerations for both the undulator and the quadrupoles, it is desirable to have a dynamic range of ± 500 µm at each magnet mover. For an assumed maximum quadrupole gradient of 105 T/m and an effective magnetic length of leff = 5 cm, the maximum kick angle becomes 50 µrad. The present design value of the center-to-center module length is 3.4 m. The deflection at the end of one modular section is therefore 170 µm. The vacuum chamber inside diameter is 5.0 mm. It can readily be shown that five consecutive maximum kicks will amount to a deflection of ~2.6 mm and the beam could strike the chamber wall near the end of undulator segment 5 with a maximum angle of θ ~300 µrad (see Figure 8.26). The shortest longitudinal distance, li, over which the 2σ core of the incident Gaussian distribution could strike the vacuum chamber is then li = 2σ/θ = 21.3 cm ≡ 13 Xo for stainless steel with 1 Xo ~1.66 cm. Similarly, the shortest distance of the momentum vector traversing the vacuum chamber wall is lt = t/θ = 167 cm ≡ 100 Xo. This means that every conceivable e - energy envisioned for the undulator reaches the peak of the electromagnetic cascade inside the vacuum chamber wall, and also, with the exception of particles scattered out of the wall in the transverse direction, the chamber wall is almost a complete absorber of the cascade. Examining the region of shower maximum where the normalized power deposition varies little (dP/dT ~ 0) and using the longitudinal interval of Tmax ± 1 Xo ≡ 4.5 to 6.5 Xo ≡ 3.3 cm, Monte Carlo simulations using the EGS code show that ~0.23 Pav is absorbed in this region. The volume element defined by 2σ and ± 1 Xo is a “skewed” ellipsoid, and after folding in a double convoluted Gaussian, the expected power deposition is P ~ 0.23 CPav = 0.23 × 0.4 × 1.8 × 10 3 4.5 - 6.5 Xo ~ 165 W. Let ∆σ be the average transverse increase in σ at the depth location of the ellipsoid; then the two axes are (li + 2∆σ ~ li ~21.5 cm) and (2σ + 2∆σ ~ 0.27 cm). The effective volume of the ellipsoid is then V = Aeff h ~ [(li + 2∆σ) (2σ + 2∆σ) π/4] 2Xo ~ [21.5 × 0.27π/4] 2 × 1.66 × 300 × 10 -6 ~4.54 × 10 -3 cm 3 . Before arriving at a heat source term S, allowance has to be made for transverse leakage of shower particles out of the chamber wall. Monte Carlo calculations of a beam impinging in the center of a thin-walled stainless steel tube of similar wall thickness (1.27 mm) with the momentum vector parallel to the tube axis have been made. These resulted in volumetric power deposition values approximately a factor of 3.5 lower than those found for a semi-infinite U N D U L A T O R ♦ 8-43
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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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5 TECHNICAL SYNOPSIS FEL Parameters
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and the total peak beam power L C L
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5.4.2.2 Case II - High Charge Limit
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∆P ∆I sat pl / P / I sat pk L C
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5.9 Summary L C L S C O N C E P T U
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6 Injector TECHNICAL SYNOPSIS The i
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εn,x (µm) 4 3 2 1 4-2001 8560A95
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Quantum Yield (electrons/photon) 10
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Bz (T) 0.3 0.2 0.1 1-2002 8560A92 L
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Master Clock 9.917 MHz x8 x6 x6 Df
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1-2002 8560A198 Linac Center Line L
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B z (kG) 3 2 1 4-2001 8560A91 L C L
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γε x,y (µm) 3 2 1 0 3-2001 8560A
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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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Page 294 and 295: L C L S C O N C E P T U A L D E S I
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Page 304 and 305: 8.1 Overview 8.1.1 Introduction 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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