Free-electron lasers Juergen Pfingstner, University of Oslo, October 2015,

Outline A.Introduction to FELs 1.Photon science 2.X-ray light sources 3.FEL basics B.FEL Theory 1.Overview 2.Low-gain FEL theory 3.High-gain FEL theory C.Additional FEL topics 1.Seeding schemes 2.Schemes for increased output power 3.Ultra-short X-ray pulses 4.Creation of unusual X-rays

References [1] A. Wolksi, A Short Introduction to Free Electron Lasers, (CERN Accelerator School, Granada, Spain, 2012). Gives a short introduction to the topic. [2] P. Schmüser, M. Dohlus, J. Rossbach, Ch. Behrens, Free-Electron Lasers in the Ultraviolet and X-Ray Regime, (Springer International Publishing Switzerland 2014). Very valuable reference. Also accessible for beginners. Main resource for this lecture: much material is used in this course. [3] E.L. Saldin, E.A. Schneidmiller, M.V. Yurkov, The Physics of Free Electron Lasers, (Springer, Berlin, Heidelberg, 2000). High mathematical level. Not so much for beginners.

A. Introduction to FELs

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

Interaction of different particles with matter Electron scattering: Interaction mainly with shell electrons of probe. Determination of electric structure. Interaction is very strong (short de Broglie wavelength) and therefore mainly at the surface. Example: electron microscopy. Photon scattering: Also interacts with shell electrons. But scattering is 1000 times weaker then for electrons, and hence photons penetrate further into probes. Often better for thicker probes (avoids multiple- scattering) and objects in solution (water window). Example: X-ray light sources. Neutron scattering: Magnetic scattering, mainly with atom cores. Determination of magnetic structure. Complementary information. Example: European spallation source (ESS).

Photon interaction with matter Wave length [m] Photon energy [eV] Radiation name Excited processes High power sources Laser Synchr. light sources FEL

X-ray interaction processes Soft X-rays Hard X-rays 5Å 0.1Å 100Å 1Å Ionisation processes of electrons Excitation of nucleus Elastic scattering of photons and electrons Elastic scattering: no energy change of photons. Main application: diffraction imaging reveals geometric structure. Inelastic scattering: photons change energy. Main application: spectroscopy reveals electronic structure.

Method 1: Spectroscopy For us, the hot light source is an accelerator driven X- ray source. No continuous spectrum, but scan over different wave lengths. No prism necessary.

Example for spectroscopy (at FELs) Ph. Wernet et al., “Real-Time Evolution of the Valence Electronic Structure in a Dissociating Molecule” PRL 103, (2009). Excitation of Br 2 molecule with pump (optical laser) to dissociating state. Measure spectra with probe (here VUV laser) at different time delays. Change of spectra contains information about bond breaking dynamics. This pump and probe technique is very recent development.

Method 2: Diffraction imaging Photon beam: Coherent light has wave fronts that can interfere. Wavelength in the order of the probe. Probe: Photons scatter from electron cloud. Scattered light is a spherical wave starting at the interaction point. Detector: Photons from different scattering point have different phases, and create interference pattern. Image is the Fourier transform of probe. Reconstruction: Inverse Fourier transform But no phase information (phase problem )

Motivation for protein imaging: e.g. pharmacology Pharmacological development are nowadays still based to a good extent on trial and error. The action of Viagra was understood only The drug was created for the first time in Tamiflu (anti-flu) was the first medicament that was specifically tailored. Knowledge about the atomic structure of the virus was used (Synchrotron Light Source). This helps to make drug research more systematic and efficient.

Example for diffraction imaging M. Suga et al. “Native structure of photosystem II at 1.95 A resolution viewed by femtosecond X-ray pulses”, Nature Letters. Motivation: Photo-synthesis converts light from the sun very effective into chemical energy that triggers the conversion of CO 2 to O 2. If Photo-synthesis would be fully understood then it could be maybe used as an alternative source of energy. The involved proteins have been studied in synchrotron light sources. Problem: long measurement times could change structure of protein. Measurements with FEL (SACLA) are single shot! The results give slightly different results of distances between atoms. The mechanism is understood now better and could help to make synthetic catalysts.

Demanded X-ray properties X-ray spectral bandwidth Δω/ω 0 : Spectroscopy: exact shape of the spectra contains information. X-rays with large bandwidth smear fine structure of the spectra (energy resolution). If possible monochromatic X-rays. E [keV] Absorption X-ray brightness B: The smaller the observed objects, the higher the photon density has to be. The proper measure is the brightness, which takes into account the spectral purity and the photon angle: At higher B, the less averaging is necessary in the experiment (dream of single shot measurement). Averaging modifies the structure of the probe and changes outcome. X-ray wavelength λ: Depends on experiment (see slides before).... photon flux per second and relative bandwidth. … standard deviation of x.