Free-electron lasers
Juergen Pfingstner, University of Oslo, October 2015, juergen.pfingstner@fys.uio.no
Outline
A.
Introduction to FELs
1.
2.
3.
B.
FEL Theory
1.
2.
3.
C.
Photon science
X-ray light sources
FEL basics
Overview
Low-gain FEL theory
High-gain FEL theory
Additional FEL topics
1.
2.
3.
4.
Seeding schemes
Schemes for increased
output power
Ultra-short X-ray pulses
Creation of unusual X-rays
References
[1]
A. Wolksi, A Short Introduction to Free Electron Lasers, (CERN Accelerator School,
Granada, Spain, 2012).
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[2]
P. Schmüser, M. Dohlus, J. Rossbach, Ch. Behrens, Free-Electron Lasers in the Ultraviolet
and X-Ray Regime, (Springer International Publishing Switzerland 2014).
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[3]
Gives a short introduction to the topic.
Very valuable reference.
Also accessible for beginners.
Main resource for this lecture: much material is used in this course.
E.L. Saldin, E.A. Schneidmiller, M.V. Yurkov, The Physics of Free Electron Lasers,
(Springer, Berlin, Heidelberg, 2000).
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High mathematical level.
Not so much for beginners.
A. Introduction to FELs
A.1 Photon Science
Interaction of different particles with matter
Electron scattering:
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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:
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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 multiplescattering) and objects in solution (water window).
Example: X-ray light sources.
Neutron scattering:
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Magnetic scattering, mainly with atom cores.
Determination of magnetic structure.
Complementary information.
Example: European spallation source (ESS).
Photon interaction with matter
Radiation name
Wave length [m]
Photon energy [eV]
Excited
processes
High power
sources
Laser
FEL
Synchr. light sources
X-ray interaction processes
Soft X-rays
100Å
Hard X-rays
5Å
1Å
0.1Å
Elastic scattering of photons and electrons
Ionisation processes of electrons
Excitation of
nucleus
• 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
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For us, the hot light
source is an
accelerator driven Xray source.
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No continuous
spectrum, but scan
over different wave
lengths.
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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).
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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.
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The action of Viagra was
understood only 2003.
The drug was created for the first
time in 1989.
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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
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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 wavelength λ:
X-ray brightness B:
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Depends on experiment (see slides
before).
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X-ray spectral bandwidth Δω/ω0:
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...
…
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Absorption
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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.
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E [keV]
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:
photon flux per second and
relative bandwidth.
standard deviation of x.
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.