free-electron lasers juergen pfingstner, university of oslo, october 2015,...

Free-electron lasers

Juergen Pfingstner, University of Oslo, October 2015, juergen.pfingstner@fys.uio.no

Outline

A. Introduction to FELs1. Photon science2. X-ray light sources3. FEL basics

B. FEL Theory1. Overview2. Low-gain FEL theory3. High-gain FEL theory

C. Additional FEL topics1. Seeding schemes2. Schemes for increased

output power3. Ultra-short X-ray pulses4. 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 FELsA.1 Photon ScienceA.2 X-ray light sources

A.2.1 First and second generation

A.2.2 Third generationA.2.3 Fourth generation: FELs

A.3 FEL basicsA.3.1 Low- and high-gain

FELsA.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 0.1Å100Å 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, 013001 (2009).

• Excitation of Br2 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 2003.

• The drug was created for the first time in 1989.

• 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 CO2 to O2. 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/ω0:

• 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]

Abso

rptio

n

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.