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