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Accelerator Issues and Design Paul Emma, SLAC Dec. 12, 2003

LCLS. Accelerator Issues and Design Paul Emma, SLAC Dec. 12, 2003. Design of Compression and Acceleration Systems Technical Challenges Full System Simulations. For an FEL…. l u. l r. N l r  0.5 m m. ‘. ‘Slice’ versus ‘Projected’ Emittance. For a collider….

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Accelerator Issues and Design Paul Emma, SLAC Dec. 12, 2003

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  1. LCLS Accelerator Issues and DesignPaul Emma, SLACDec. 12, 2003 • Design of Compression and Acceleration Systems • Technical Challenges • Full System Simulations Paul Emma, SLAC

  2. For an FEL… lu lr Nlr 0.5 mm ‘ ‘Slice’ versus ‘Projected’ Emittance For a collider… collision integrates over bunch length — ‘projected’ emittance is important e- slips back w.r.t. photons by lr (= 1.5 Å) per period …FEL integrates over slippage length: ‘slice’ emittance is important Paul Emma, SLAC

  3. eN = 1.2 mm P 10 GW eN = 2.1 mm P 0.1 GW SASE X-ray FEL is very sensitive to electron ‘slice’ emittance lr = 1.5 Å courtesy S. Reiche Instead of mild luminosity loss, power nearly switches OFF. However, longer wavelength, such as 15 Å (4.5 GeV), is much easier (eN 6 mm). Paul Emma, SLAC

  4. Nominal System Design 1.5-Å SASE FEL Linac: • Requirements • Acceleration to 14.1 GeV (~3 GeV min.) • Bunch compression to 3.4 kA • Emittance preservation (<20% ‘slice’ of 1-mm-mrad) • Final energy spread (0.01% ‘slice’, <0.1% ‘projected’) • Minimal sensitivity to system ‘jitter’ (charge, phase, voltage, ...) • Diagnostics integrated into optics • Flexible operations (1.5 Å →15 Å, low-charge, chirp, etc.) use 2 compressors, 3 linacs Paul Emma, SLAC

  5. 1 km Nominal System Design • Constraints • Use existing SLAC linac compatible with PEP-II operation • Undulator located beyond research yard Paul Emma, SLAC

  6. LCLS versus SLC • LCLS Advantages • Shorter linac (1 km < 3 km) • Shorter bunch in linac (1 mm → 0.2 mm → 0.02 mm) • Lower charge (1 nC < 7 nC) • ‘Slice’ emittance important, not projected • No positrons, no sextupoles, no rolls, no DR’s, no RTL’s, no arcs • Round beams (no x-y coupling issues) • Disadvantages • Lower initial linac energy (135 MeV < 1.2 GeV) • Smaller emittance (1/1 mm < 4/40 mm) • Emittance more critical (>2 mm kills FEL power) • Tighter RF, charge, & timing jitter tol’s (~0.1 deg) • CSR is new issue • RF gun less stable platform than damping ring Paul Emma, SLAC

  7. Design Strategy • Design longitudinal optics first • Set proper compression in two stages • Minimize final energy spread • Minimize Ipk and Efsensitivity to gun charge and timing jitter • Design transverse optics second • Minimize transverse wakefields, CSR, and chromatic effects • Build in emittance, energy spread, bunch-length diagnostics • Track entire system • Iterate design Paul Emma, SLAC

  8. Nominal LCLS Linac Parameters for 1.5-Å FEL Single bunch, 1-nC charge, 1.2-mm slice emittance, 120-Hz repetition rate… 250 MeV z  0.19 mm   1.6 % 4.54 GeV z  0.022 mm   0.71 % 14.1 GeV z  0.022 mm   0.01 % 6 MeV z  0.83 mm   0.05 % 135 MeV z  0.83 mm   0.10 % Linac-X L =0.6 m rf= -160 rf gun Linac-1 L 9 m rf  -25° Linac-2 L 330 m rf  -41° Linac-3 L 550 m rf  -10° new Linac-0 L =6 m undulator L =125 m 21-1b 21-1d 21-3b 24-6d 25-1a 30-8c X ...existing linac BC-1 L 6 m R56 -39 mm BC-2 L 22 m R56 -25 mm DL-1 L 12 m R56 0 LTU L =275 m R56  0 research yard SLAC linac tunnel (RF phase: frf= 0 at accelerating crest) Paul Emma, SLAC

  9. RMS Bunch Length and Energy Spread sector-21 sector-25 sector-30 FFTB++ sd sz Paul Emma, SLAC

  10. after L2 after DL1 after L1 after BC2 after X-RF after L3 after BC1 at und. time profile energy profile phase space sz = 830 mm sz = 190 mm sz = 830 mm sz = 23 mm sz = 830 mm sz = 23 mm FINAL sz = 190 mm sz = 23 mm Paul Emma, SLAC

  11. X-band RF at decelerating phase corrects 2nd-order and allows unchanged z-distribution lx = ls/4 Slope linearized avoid! 1  -40° x = p 0.6-m section, 19 MV available at SLAC (200-mm alignment) X-band RF used to Linearize Compression (f = 11.424 GHz) S-band RF curvature and 2nd-order momentum compaction cause sharp peak current spike Paul Emma, SLAC

  12. Transverse Wakefields and Component Misalignments Choose b-phase adv/cell for each linac to minimize emittance dilution: L2 phase adv/cell optimized L3 phase adv/cell optimized sz = 22 mm sz = 195 mm x also misaligned quads/BPMs generate dispersion  De wakes on wakes off wakes on wakes off Paul Emma, SLAC

  13. Transverse Optics from Cathode to e- Dump LCLS MAD Deck  Cathode to e- Dump (2200 elements) Dyx,y 30º Dyx,y 75º Thanks to M. Woodley Paul Emma, SLAC

  14. 4.0 mm (BC1) 2.6 mm (BC2) RMS Transverse Beam Sizes from Cathode to e- Dump 1 mm 100 mm undulator 10 mm Paul Emma, SLAC

  15. Alignment and Roll Tolerances (most > 1 mm, > 1 deg) Paul Emma, SLAC

  16. Linac RF Section Modifications If modulators on 20-6, -7, and -8 used for injector, lose another 670 MeV (1.56 GeV total) Paul Emma, SLAC

  17. Injector to Linac Interface courtesy L. Bentson “Linac” Responsibility Starts Here (21-1b) Paul Emma, SLAC

  18. Linac-1 Through BC1 21-1b 21-1c 21-1d 21-3b Paul Emma, SLAC

  19. BC2 Area 24-6d 25-1a Paul Emma, SLAC

  20. 3 cm contraction collimator BPM screen Moveable Chicanes (BC1 shown) BPM critical for energy feedback (20 mm resolution) offset: 17 to 30 cm (24 cm nominal) collimator BPM quadrupole screen Paul Emma, SLAC

  21. 80 mm 45 mm 12 mm x Field quality requirement too tight with fixed chicane... • Also needed: • BPM res. 20 mm • BPM linearity • profile monitor • collimator • requires: |b2/b0| < 0.002% @ r = 2 cm (moveable chicane requires 0.070%) SPPS dipoles: |b2/b0| < 0.010% @ 2 cm (just barely met) Paul Emma, SLAC

  22. Future Multiple Undulators +4º +2º N S -2º Paul Emma, SLAC

  23. Linac-To-Undulator (LTU) vertical bends energy centroid & spread meas. (310-5 & 10-4) + collimation • vertical bend 4.7 mrad • horizontal jog 1.25 m • energy diagnostics • emittance diagnostics • collimators • CSR cancellation • branch points for future undulators • spontaneous undulator possible 4 e-wires, 6 collimators Paul Emma, SLAC

  24. Collimation for Undulator Protection 2.5 mm well shadowed in x, y, and E Paul Emma, SLAC

  25. hy by Electron Dump x-rays→ quads soft bend e-→ permanent vert. bends powered vert. bends e-→ quad screen (sE/E = 10-5 5 mm) dump Paul Emma, SLAC

  26. Specification Sheets on Every New Magnet • BX01 DL1 dipole: • z-location • field • current • trim info. • alignment tol.’s • length • max/min strength • etc... Paul Emma, SLAC

  27. LCLS Technical Challenges • Coherent Synchrotron Radiation in Bends • projected emittance growth • micro-bunching instability (+ LSC — see Z. Huang talk) • Emittance Preservation in Linacs • transverse wakefields • misalignments & chromaticity • Machine Stability • gun and rf system jitter • energy and bunch length feedback Paul Emma, SLAC

  28. Coherent Synchrotron Radiation coherent power N  6109 ~l-1/3 incoherent power vacuum chamber cutoff sz Paul Emma, SLAC

  29. Coherent Synchrotron Radiation (CSR) • Powerful radiation generates energy spread in bends • Induced energy spread breaks achromatic system • Causes bend-plane emittance growth (short bunch is worse) coherent radiation forl > sz bend-plane emittance growth sz l L0 DE/E = 0 s Dx e– R DE/E < 0  overtaking length: L0  (24szR2)1/3 Dx = R16(s)DE/E Paul Emma, SLAC

  30. Coherent Synchrotron Radiation (CSR) in SPPS Chicane ON Chicane OFF gex = 34.2  0.7 mm gex = 27.6  0.6 mm Paul Emma, SLAC

  31. Coherent Synchrotron Radiation (CSR) in SPPS Bend-plane emittance is consistent with calculations and sets upper limit on CSR effect Paul Emma, SLAC

  32. 230 fsec Add slice energy spread to Landau damp instability. ‘Laser-Heater’ see Z. Huang talk energy spread damps bunching sd 310-5 S. Heifets, S. Krinsky, G. Stupakov, SLAC-PUB-9165, March 2002 CSR Micro-bunching* CSR amplifies small modulations on bunch current  Successive bend-systems cause micro-bunching  Growth of slice-energy spread & emittance. without heater sd 310-6 avoid! * First observed by M. Borland (ANL) in LCLS Elegant tracking Paul Emma, SLAC

  33. Misalignments, Steering, and Emittance Correction trajectory after steering BPM, quad, and RF misalignments: (each at 300 mm rms)... then steered in Elegant gex 5 mm gey 2 mm Paul Emma, SLAC

  34. Emittance Correction with Trajectory ‘Bumps’ 100 seeds steering coils De/e15% gex 1.02 mm gey 1.09 mm Thanks to M. Borland (ANL/APS) Paul Emma, SLAC

  35. X- X-band Jitter Budget (<1 minute time-scale) measured RF performance klystron phase rms  0.07° (20 sec) klystron ampl. rms  0.06% (60 sec) Paul Emma, SLAC

  36. LCLS Start-to-End Tracking Simulations • Track entire machine to evaluate beam brightness & FEL • Track machine many times with jitter to test stability budget (M. Borland, ANL) Parmela Elegant Genesis space-charge compression, wakes, CSR, … SASE FEL with wakes Paul Emma, SLAC

  37. mismatch amplitude variation slice 4D centroid osc. amplitude Sliced e- Beam to Evaluate FEL (Dz 0.7 mm) After full system tracking (also studied by S. Reiche) gex gey zx zy Lg < 4 m Paul Emma, SLAC

  38. Ipk Lg gex Pout Machine Stability Simulations • Track LCLS 230 times with Parmela Elegant Genesis • Include wakes, CSR, etc. + jitter budget (M. Borland, ANL) Paul Emma, SLAC

  39. 3 prof. mon.’s (Dyx,y = 60°) rf gun gex,y gex,y gex,y gex,y L0 gex,y L1 L2 L3 X ...existing linac Emittance and Energy Spread Diagnostics* • 5 energy spread meas. stations (optimized for small bx) • 5 emittance meas. stations designed into optics (Dyx,y) • slice measurements possible with transverse RF (L0 & L3) sE sE sE sE sE * see also P. Krejcik talk Paul Emma, SLAC

  40. long. phase space 230 fsec V0 = 0 LCLS simulation RF ‘streak’ V(t) sx e- S-band Built & used at SLAC in 1960’s sz y = kt [mm] V0 = 20 MV * see P. Krejcik talk x = hDE/E [mm] meas. bunch length & slice emittance meas. longitudinal phase space Transverse RF deflector as diagnostic* Paul Emma, SLAC

  41. LCLS Summary • Linac design optimized for nominal 1.5-Å operation • Design is flexible to accommodate 15-Å, low-charge, & chirp • CSR growth of projected emittance – not slice • Much experience on SLAC linac with wakefield control • Beam diagnostics built into design • Full system tracking to… • Evaluate e- brightness preservation, • Calculate SASE gain, • Simulate pulse-to-pulse stability. Full tracking with errors shows FEL saturation at 1.5 Å, but a very challenging machine! Paul Emma, SLAC

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