1. A. Introduction to FELs A.1 Photon Science A.2 X-ray light sources A.2.1 First and second generation A.2.2 Third generation A.2.3 Fourth generation: FELs A.3 FEL basics A.3.1 Low- and high-gain FELs A.3.2 High-gain FEL facilities

3. Synchrotron radiation (SR) Electromagnetic radiation from a relativistic charged particle und a circular trajectory. – Circular motion is an accelerated motion. – SR is created from particles in circular accelerators. Context – 1873 Maxwell’s equation – 1887 Hertz: electromagnetic waves – 1898Liénard: General radiation from accelerated charge (non-relativistic). – 1900Wiechert: General radiation from relativistic particles. – 1908Schott: Solution for electrons on circular orbit. – 1946Blewett: First indirect observation of SR (energy loss of particles). – 1947First direct observation of synchrotron radiation, GE lab. – 1949Schwinger: Nice form of theory. Angular properties of SR – SR is concentrated in a forward cone. – Opening angle of cone is θ=1/γ. No non-relativistic nNo relativistic n Total power of SR: – The relativistic factor γ is 2000 times higher for electrons compared to protons. – Electrons radiate much more than protons and are hence used in all accelerator-driven light source. ρ … bend radius α … fine structure constant

4. Spectral properties of SR Synchrotron light comes in series of flashes with synchrotron revolution period T 0. Spectrum consists of harmonics of: Since flashes are very short, harmonics reach to high frequencies: Example (Swiss Light Source): Shape of spectrum is given by The power spectrum is then given by where ω c contains the energy dependence

5. 2 nd generation: Strongly growing interest in SR. Dedicated storage rings. Radiation from bend magnets. Brightness 10 12 ( X-ray tube 10 7 ) First 2G LS, Tantalus 1968, SRC in Wisconsin. 1 st and 2 nd generation light sources 1 st generation: Parasitic use of SR from electron storage rings. Radiation from bend magnets is extracted from holes in the beam pipe. First direct observation of predicted SR, GE Synchrotron, NY state, 1947.

6. Synchrotron principle (revisited) Tantalus, 1 st generation light source Magnets: Particle beam is kept on a “quasi”-circular trajectory by dipole magnets. Particles are focused with quadrupole magnets. Injection and extraction via kicker and septum magnets. RF cavity: Acceleration with RF cavities. If beam is accelerated, dipole fields have to be increased synchronously (synchrotron). RF also focuses the beam longitudinally.

7. A. Introduction to FELs A.1 Photon Science A.2 X-ray light sources A.2.1 First and second generation A.2.2 Third generation A.2.3 Fourth generation: FELs A.3 FEL basics A.3.1 Low- and high-gain FELs A.3.2 High-gain FEL facilities

8. Wigglers and undulators (insertion devices) Sequence of dipoles of changing polarity. Bending in both directions creates wiggle motion of beam. Continuous bending causes strong radiation. Due to bending, cone of SR with angle Since beam changes direction, SR cone points in different directions Wiggler radiation Undulator radiation

9. Wiggler and undulator radiation Wiggler radiation: Light still arrives in small flashes on a far detector. Properties are very similar to SR form a bending magnet, but stronger Undulator radiation: Emitted dipole SR radiation overlaps. Filter effect due to constructive and destructive interference. Perfect interference condition only for one wavelength and its harmonics (derivation later).

10. 3 rd generation light sources Specially designed synchrotrons that use wigglers and undulators to produce SR of high brightness. Brightness in the order of 10 20 (10 7 X-ray tubes). First machines started in early 1990’s. About 10-20 machines world wide. Many straight sections for undulator and wiggler insertion. Many photon experiments can be operated simultaneously. Soleil in Paris: Anlage und schematischer Aufbau

11. Experimental setup Courtesy R. Bartolini

12. Reaching highest brightness Brightness B: Nowadays Σ is dominated by σ e and Hence, 3G light sources aim for tiny ε. … photon flux … combined size of electron and photon beam in x At some point, photon beam starts to dominate Σ. The ultimate limit is called diffraction limit This corresponds to a transversally fully coherent photon beam. Also large current to increase Φ.

13. Lattice influence in beam emittance In an electron ring the emittance ε is not determined by the injected ε, but the lattice. The emittance shrinks due to radiation damping until this reaches an due to the randomness of the photon emission (quantum excitation). It can be shown that the equilibrium emittance is given by The damping partitioning number jx ≈ 1 (no quadrupole field in bends) and I 2 and I 5 are the synchrotron radiation integrals given by The bending radius ρ us usually fixed and so ε 0 mainly depends on the dispersion in the dipoles. It can be shown that the ideal FODO lattice has an phase advance of 137 degree and An ideal lattice would have ε TME =ε FOTO /(12√15). Real lattice try to approach this value.

14. Beam physics aspects in 3 rd generation light sources Special low emittance lattices are used -Double bend achromat (DBA). -Triple bend achromat (TBA). -Multi bend achromat (DBA). Emittance in vertical direction mainly given by coupling from the horizontal plane. -Coupling and optics corrections. These lattices need strong quadrupole and sextupole magnets, which reduced the dynamic aperture. -Optimization of dynamic aperture. -Important to guaranty injection efficiency. Also collective effects are an important topic due to high beam current: -Wakefields, IBS, … Double bend achromat

15. A. Introduction to FELs A.1 Photon Science A.2 X-ray light sources A.2.1 First and second generation A.2.2 Third generation A.2.3 Fourth generation: FELs A.3 FEL basics A.3.1 Low- and high-gain FELs A.3.2 High-gain FEL facilities

16. ISR and CSR Incoherent SR (ISR): Radiation from a bunch of N electrons. If bunch length σ z >> λ l then the individual light waves interfere randomly (random phase) The radiation power P ISR grows linear with N Coherent SR (CSR): If σ z < λ l then electrons radiate in phase And the power P CSR grows quadratic with N

17. FEL principle: CSR via micro-bunching Free electron laser create CSR instead of ISR (synchrotron light source). This increases brightness dramatically: B ≈ 10 33 compared to 10 20 -10 25 in 3G light source. For CSR, σ z < λ l ≈ 1Å = 0.33 attosec. There is no technical solution to create such beams. Instead, one makes use of the micro- bunching instability. Micro-bunching Light of the correct wavelength co-propagates with the e- beam in long undulator. Light can be created via ISR or via an external seed laser. Energy exchange of light and e - leads to a micro-bunching of the e - beam.

18. Comparison FELs and 3 rd generation light sources Advantages of FELs: Factor 10 10 higher peak brilliance. Transversally fully coherent laser light (diffraction limited). Possibility of ultra-short pulses. Advantage of 3G LS: Many beam lines can be supplied at the same time (up to 40). Higher repetition rate (but still no higher average brilliance).

19. Existing hard X-ray FELs LCLS SACLA At SLAC (Stanford, USA). First hard X-ray FEL, 2009. Powerful linac: 15 GeV Total length: about 1500m At RIKEN (East of Japan). Second hard XFEL, 2011. Reduced length due to advanced undulator technology. Total length: 700m

20. Future XFEL facilities Operational soft XFELs: – FLASH, DESY, Hamburg – ELETTRA, Trieste, Italy European XFEL: – Hamburg, Germany – In construction phase, 3.3 km long – Will be the most powerful FEL facility – Supraconducting linac for higher average brilliance PAL-FEL – Pohang, Korea – In construction phase SwissFEL – PSI, Switzerland – In construction phase Several other soft and hard XFEL facilities envisioned or already funded. EuXFEL tunnel PAL-FEL

21. New photon science at FELs: protein nano-crystallography Very important topic in medicine, pharmacology and biology. In 3D LS, brightness not high enough for single shot meas.: Crystallography (proteins, viruses, …). Averaging over time. Disadvantages Difficult to manufacture crystals. Change of crystal due to energy deposition. In FELs, brightness still not high enough for single shot but Crystals can be much smaller: nano-crystals. Much easier to produce. Photon intensity destroys nano-crystal in single shot. Averaging over many crystals to get protein. No change of the probe due to averaging. Dream of single shot measurement: maybe soon. Protein crystal Protein nano-crystal

22. Experimental setup for (nano-)crystallography 3 rd generation light source: Macroscopic crystal, mm scale. Mounted on goniometer to rotate it during radiation. Averaging over longer time. Free-electron laser: Nano-crystals in solution. Liquid jet focused to um level. X-ray destroy crystal if hit. Optimal X-ray rep. rate is when every volume of liquid jet is covered.

23. New photon science at FELs: Time-resolved measurements in fs scale 3G light source: Averaging over time to get enough intensity. No time resolved measurement possible. Free electron laser: Due to higher brightness, time resolved measurements are possible. Time resolution of fs allows to start molecular dynamics. Pump - probe experiments: Process of interest in sample is started with pump pulse: THz, laser, X-ray. Then after time τ sample is probed with FEL X-ray. Time difference is varied to get time evolution of process. Many samples are necessary to scan over τ.