SLAC-PUB-16451 Overview of Light Sources Kent Wootton SLAC National Accelerator Laboratory US Particle Accelerator School Fundamentals of Accelerator Physics 02/02/2016 University of Texas Austin, TX This work was supported in part by the Department of Energy contract DE-AC02-76SF00515. Key references • D. A. Edwards and M. J. Syphers, An Introduction to the Physics of High Energy Accelerators, Wiley, Weinheim, Germany (1993). • DOI: 10.1002/9783527617272 • H. Wiedemann, Particle Accelerator Physics, 4th ed., Springer, Heidelberg, Germany (2015). • DOI: 10.1007/978-3-319-18317-6 • E. J. N. Wilson, An Introduction to Particle Accelerators, Oxford University Press, Oxford, UK (2001). • DOI: 10.1093/acprof:oso/9780198508298.001.0001 • A. Wolski, Beam Dynamics in High Energy Particle Accelerators, Imperial College Press, London, UK (2014). • DOI: 10.1142/p899 2 Third generation storage ring light sources 1. Electron gun 2. Linac 3. Booster synchrotron 4. Storage ring • bending magnets • insertion devices 5. Beamlines 6. Endstations 3 Why use synchrotron radiation? • Wavelength-tunable • 10 eV → 100 keV • High intensity • Spatial coherence • Polarised • Pulsed h Source: lightsources.org 4 Who uses synchrotron radiation as a light source? • Touches every aspect of science • Benefits mostly outside physics • Users predominantly working in universities, national laboratories Source: Advanced Photon Source Annual Report 2014 5 Who uses SR? Imaging Absorption contrast imaging Phase contrast imaging 6 Who uses SR? Imaging X-ray fluorescence mapping 7 Who uses SR? Diffraction Protein crystallography FFT 8 The arms race – light source brilliance Brilliance/brightness = 퐹 2 4th ℬ 4휋 휀푥휀푦 3rd 1st & 2nd K. Wille, The Physics of Particle Accelerators: An Introduction, Oxford University Press, Oxford, UK (2000). J. B. Parise and G. E. Brown, Jr., Elements, 2, 37-42 (2006). 9 Light sources of the world Source: Advanced Photon Source Annual Report 2014 10 Outline – Overview of Light Sources • User requirements of synchrotron radiation • Storage ring light sources • First and second generation storage rings • FODO lattices • Insertion devices – wigglers and undulators • Third generation storage rings • Achromat lattices • Diffraction-Limited Storage Rings • Free-Electron Lasers 11 Generations of storage ring light sources 12 First generation storage ring light sources • Parasitic use of synchrotrons, storage ring colliders • Bending magnet radiation (incoherent, broadband) − 푒 + 푒 S. Doniach, et al., J. Synchrotron Radiat., 4, 380-395 (1997). 13 Particle physics discovery! • Early 1960’s – Anello di Accumulazione (AdA) collider • E = 1.5 GeV • Significant for accelerator physics, but no particle discovery • Early 1970’s – SPEAR-I • E = 4.5 GeV (maximum) • 1974 – / ( ) discovery kept machines at E = 1.55 GeV 퐽 휓 푐푐̅ Source: Brookhaven National Laboratory C. Bernardini, Phys. Perspect., 6, 156-183 (2004). S. Williams, CERN Courier, 1 Jun, 2003 (2003). 14 Parasitic users of bending magnet radiation • Critical photon energy 3 퐸 • Colliders typically want휖 maximum푐 ∝ 휌 (minimise SR power) • Bending magnet light sources want minimum (maximise SR) 휌 휌 For a given current, independent / Source: Australian Synchrotron of beam energy! 퐽 휓 H. Winick, ‘Properties of Synchrotron Radiation’, in Synchrotron Radiation Research, Springer, New York, (1980), p. 17 15 Second generation storage ring light sources • Dedicated electron storage ring light sources • Bending magnet radiation • Predominantly separated- function FODO lattices • Typically VUV, soft X-ray photon energies • Some hard X-ray rings E. Rowe and F. Mills, Part. Accel., 4, 211-227 (1973). 16 Second generation light source – beginning and end National Synchrotron Light Source – I (Brookhaven) 17 Wigglers • Electron storage ring • = 3. • For a given circumference 퐵휌 3356퐸 of bending magnets, bending radius fixed (need to bend beam by RF ) • For a given beam energy, 2휋 no flexibility in magnetic field of bending magnets E. Rowe and F. Mills, Part. Accel., 4, 211-227 (1973). 18 Wigglers ⊙ ⊗ ⊙ ⊗ ⊙ ⊗ ⊙ 19 Wigglers • Incoherent superposition of many bending magnets Source: Australian Synchrotron • Flux scales linearly with number of periods • Critical photon energy linear with magnetic field • Same as bending magnet 20 Undulators ⊙ ⊗ ⊙ ⊗ ⊙ ⊗ ⊙ ⊗ ⊙ ⊗ ⊙ 21 Undulators 휆푢 휆 = 1 + + , 2 22 휆푢 퐾 2 2 휆 2 훾 휃 = 훾 93.4 m T 푞푒 퐾 휆푢퐵 ≈ 휆푢 퐵 22 2휋휋푚푒푐 Undulator radiation • Bright odd harmonics, null even harmonics • Even harmonics don’t perfectly cancel • Non-zero emittance 2 3 4 5 6 휆 휆 휆 휆 휆 휆 • What if you want a different photon energy? 23 Tuning an undulator – deflection parameter = 1 + + 2 22 휆푢 퐾 2 2 • Assume휆 2 fixed, vary훾 휃 훾 = 93.4 휆푢 • Magnetic field or energy Envelopes 퐾 휆푢퐵 • Use permanent magnets, 퐵 훾 vary by varying gap S N S N S N S N S N S N 퐵 S N S N S N S N S N S N 24 A real undulator 25 Difference between undulators and wigglers • Fundamentally, there is no difference! = 퐾 Wiggler,휃푤 if 1 훾 퐾 ≫ ⊙ ⊗ ⊙ ⊗ ⊙ ⊗ ⊙ Undulator, if 1 퐾 ≤ ⊙ ⊗ ⊙ ⊗ ⊙ ⊗ ⊙ 26 Comparison of radiation sources Bending magnet Opening angle 1 Wiggler 휃 ≈ 훾 퐾 Undulator 휃 ≈ 훾 1 After D. Attwood, ‘Soft X-Rays and Extreme휃 ≈ Ultraviolet Radiation’, Lecture Notes, UC Berkeley (2009). 27 Center for X-Ray Optics and Advanced Light Source,푁훾 ‘X-Ray Data Booklet’, LBNL/PUB-490 Rev. 3 (2009). Third generation storage ring light sources • Dedicated storage rings designed specifically for wiggler and undulator light sources (insertion devices) 28 Lattices for light sources • Optimisation? Brilliance/brightness = 4 퐹 • Vertical emittance ideallyℬ zero2 휋 휀푥휀푦 • Practically, 0.01 • Arising from uncorrected betatron coupling with horizontal 휀푦 ≈ 휀푥 plane • Minimising equilibrium emittance is key 휀푥 29 Lattices for light sources • Equilibrium emittance scaling law / d = 2 1/ 3d 2 훾 ∮ ℋ 휌 푠 퐶푞훾 3 푥 퐶푞 2 휀 55푥 ≈ ℱ 푥 휃 = 풥 ∮ 휌= 3푠.84 × 10풥 m 32 3 ℏ푐 −13 • Scale factor퐶푞 (not flux2 ) 푚푐 ℱ 퐹 M. Sommer, Optimization of the Emittance of Electrons (Positrons) Storage Rings, Laboratoire de l'Accélérateur Linéaire, LAL/RT/83-15 (1983). 30 Lattice scale factor 퓕 2 퐶푞훾 3 Name Unit Cell 푥 Ref. 휀 ≈ ℱ 푥 휃 풥 Wiedemann (1980) FODO 1.2 (minimum)퓕 Wolski (2014) 1 TME 0.0215 Wolski (2014) 12 15 1 ≈ TAL 0.2582 Ropert (1993) 15 1 ≈ DBA 0.0646 Sommer (1981) 4 15 7 ≈ TBA 0.0502 Ropert (1993) 36 15 1 ≈ + 1 MBA Wolski (2014) 12 15 1 푀 31 푀 − What is the difference? Curly-H function = + 2 + • Minimising horizontal 2dispersion function′ ′2and its ℋ 훾푥퐷푥 훼푥퐷푥퐷푥 휋푥퐷푥 derivative in the bending magnets SPEAR-2 (FODO) SPEAR-3 (DBA) 32 Example: SPEAR-2 to SPEAR-3 2 퐶푞훾 3 • Let’s assume휀푥 ≈ ℱ both 휃rings Real machines? 풥푥 • = 1 • = 3.0 GeV ( = 5871) SPEAR-2 풥푥 • SPEAR-2, FODO lattice = 160 nm rad 퐸 훾 • FODO, 1.2 • 32 bending magnets 휀푥 = 11.ℱ25°≈ 0.196 rad SPEAR-3 • 120 nm rad = 6 nm rad 휃 ≡ • SPEAR-3, DBA lattice 휀푥 ≈ 푥 • DBA, 0.0646 Real machines휀 don’t run • 36 bending magnets at the theoretical limit! = 10°ℱ ≈ 0.175 rad • 4.5 nm rad 휃 ≡ 푥 R. Hettel,휀 ≈ et al., Design of the SPEAR-3 Light Source, SLAC-PUB-9721 (2003) 33 Theoretical emittance limit and real machines • Operating a lattice close to the theoretical limit requires strong quadrupole fields • Leads to large negative natural 퐵푦 chromaticity • Lattice has very small dispersion (deliberately) 푥 • Chromaticity compensation requires strong sextupole fields • Leads to strong third order resonances in tune space • Non-linear dynamics become the problem • Dynamic aperture becomes very small Source: Wolski, CERN-2010-004.1 (2011). • Difficult to inject off-axis and store electron beams 34 Light sources of the world Source: Advanced Photon Source Annual Report 2014 35 Future light sources • How much can the average brightness be usefully increased? • ‘Continuous’ sources (e.g. storage rings) • How much can the peak brightness be usefully increased? • Pulsed sources (e.g. linacs) 36 Diffraction-Limited Storage Rings • Optimisation of average brightness • Use insertion devices (undulators, wigglers) as light sources (not bending magnets) • Diffraction limited source emittance < 휆 • Soft X-rays, 1 nm 휀 푥 <4휋80 pm rad • Strategy is to maximise number of bending magnets 휆 ≈ → 휀푥 (separated by quadrupoles) 37 MAX-IV – a diffraction-limited storage ring 38 MAX-IV – a diffraction-limited storage ring • MBA (seven- bend achromat) • 20 unit cells • = 340 pm rad • Spatially 휀푥 coherent for > 4 nm 휆 P. Tavares, et al., J. Synchrotron Radiat., 21, 862-877 (2014). 39 3rd generation storage ring magnets, MAX-IV magnets 40 Free-electron lasers • Storage ring light sources are spontaneous sources • Photon emission is random, uncorrelated in phase • Photon emission can be stimulated Source: flash.desy.de 41 Free-electron lasers Source: Los Alamos National Accelerator Laboratory (2010). 42 Free-electron laser (SACLA, Japan) 43 Energy Recovery Linacs • The ‘hybrid car’ of accelerators • Wasteful to accelerate a beam only once and lose that energy • Accelerate a beam to full energy, use it once, and then decelerate it to recover Source: Bilderback, et al., J. the energy Phys. B: At. Mol. Opt. Phys., 38, S773-S797 (2005). • Technology test facilities • Infrared THz sources 44 Coherent synchrotron radiation • Bunch length shorter than SR wavelength? Coherent • Coherent, for wavelengths longer than Incoherent bunch • ~1 ps bunch • Far infrared “THz” source Source: http://www.lns.cornell.edu/~ib38/research.html J. Schwinger, “On Radiation by Electrons in a Betatron”, A Quantum Legacy: Seminal Papers of Julian Schwinger, World Scientific, 307-331, (2000) (LBNL-39088). 45 Summary • Desirable properties of synchrotron radiation as a light source • Historically, synchrotron radiation users parasitic • Technologies such as insertion devices developed as a result • Present day, dedicated synchrotron radiation laboratories • Light source technology developing in two main directions • Average brightness (Diffraction-Limited Storage Rings) • Peak brightness (Free-Electron Lasers) This work was supported in part by the Department of Energy contract DE-AC02-76SF00515.