3 Quantum Optical Phenomena in Nuclear Resonant Scattering
159
3.9.1 Diffraction-Limited Storage Rings
A new generation of accelerator-based x-ray sources is presently emerging that relies
on a novel type of electron optics for the storage-ring, the so-called multi-bend
achromat (MBA) [143–145]. The new concept allows for a significant reduction
of the horizontal emittance x compared to existing facilities. The emittance is a
property of a radiation source and is given by the product of beam diameter and beam
divergence. The smaller the emittance, the higher is the degree of spatial coherence
of the source, which is the most desirable property of a radiation source for focusing
or imaging applications. The emittance cannot be arbitrarily decreased, but is limited
by the fundamental effect of diffraction of the beam by the shape of its own beam
cross section. For that reason there are worldwide great efforts undertaken to reduce
the emittance of synchrotron radiation sources as much as possible to reach the
diffraction limit. The synchrotron radiation source MAX IV in Lund, Sweden, is the
first light source that was successfully commissioned with this new lattice type [146],
and SIRIUS in Campinas, Brazil, is currently under construction. Amongst the highenergy synchrotron radiation sources (electron energy equal or larger than 6 GeV), the
European Synchrotron Radiation Source (ESRF) in Grenoble, France, will undergo
an upgrade starting in winter 2018 to the fourth generation light source ESRF-EBS
with an emittance as low as x ≈ 130 picometer-radian (pmrad) at an electron beam
energy of 6 GeV [147]. Many other sources worldwide have upgrade plans along
these lines, among which are also the high-energy storage-ring sources Advanced
Photon Source (APS) at Argonne National Laboratory (USA) [148], SPring-8 in
Harima (Japan), and PETRA IV at DESY in Hamburg (Germany) [149]. The latter
facility will be the first source reaching the diffraction limit for hard x-rays at 10 keV
photon energy. The implementation of the new electron optics in the storage ring
allows for an increase of the spectral brightness by one to two orders of magnitude.
This will dramatically change the landscape of synchrotron radiation facilities in the
next decade, see Fig. 3.25.
The benefit of diffraction-limited storage rings for the field of nuclear quantum
optics will not be an increase in the photon degeneracy parameter η, but primarily the
concentration of more resonant photons in a given scattering volume as compared to
present-day sources. This is enabled by the high degree of lateral coherence which
facilitates a very efficient collimation and focusing of x-rays, leading to an enhanced
coupling of x-rays to nuclear ensembles, e.g., in cavities and photonic nanostructures.
3.9.2 X-Ray Free-Electron Lasers: SASE-XFEL and XFELO
A tremendous increase of the photon degeneracy η will be reached by x-ray sources
that rely on a fundamentally different mechanism of radiation generation. This is the
principle of self-amplified spontaneous emission (SASE) which forms the basis for
free-electron lasers (FEL). The SASE principle leads to a increase in the brilliance
159
3.9.1 Diffraction-Limited Storage Rings
A new generation of accelerator-based x-ray sources is presently emerging that relies
on a novel type of electron optics for the storage-ring, the so-called multi-bend
achromat (MBA) [143–145]. The new concept allows for a significant reduction
of the horizontal emittance x compared to existing facilities. The emittance is a
property of a radiation source and is given by the product of beam diameter and beam
divergence. The smaller the emittance, the higher is the degree of spatial coherence
of the source, which is the most desirable property of a radiation source for focusing
or imaging applications. The emittance cannot be arbitrarily decreased, but is limited
by the fundamental effect of diffraction of the beam by the shape of its own beam
cross section. For that reason there are worldwide great efforts undertaken to reduce
the emittance of synchrotron radiation sources as much as possible to reach the
diffraction limit. The synchrotron radiation source MAX IV in Lund, Sweden, is the
first light source that was successfully commissioned with this new lattice type [146],
and SIRIUS in Campinas, Brazil, is currently under construction. Amongst the highenergy synchrotron radiation sources (electron energy equal or larger than 6 GeV), the
European Synchrotron Radiation Source (ESRF) in Grenoble, France, will undergo
an upgrade starting in winter 2018 to the fourth generation light source ESRF-EBS
with an emittance as low as x ≈ 130 picometer-radian (pmrad) at an electron beam
energy of 6 GeV [147]. Many other sources worldwide have upgrade plans along
these lines, among which are also the high-energy storage-ring sources Advanced
Photon Source (APS) at Argonne National Laboratory (USA) [148], SPring-8 in
Harima (Japan), and PETRA IV at DESY in Hamburg (Germany) [149]. The latter
facility will be the first source reaching the diffraction limit for hard x-rays at 10 keV
photon energy. The implementation of the new electron optics in the storage ring
allows for an increase of the spectral brightness by one to two orders of magnitude.
This will dramatically change the landscape of synchrotron radiation facilities in the
next decade, see Fig. 3.25.
The benefit of diffraction-limited storage rings for the field of nuclear quantum
optics will not be an increase in the photon degeneracy parameter η, but primarily the
concentration of more resonant photons in a given scattering volume as compared to
present-day sources. This is enabled by the high degree of lateral coherence which
facilitates a very efficient collimation and focusing of x-rays, leading to an enhanced
coupling of x-rays to nuclear ensembles, e.g., in cavities and photonic nanostructures.
3.9.2 X-Ray Free-Electron Lasers: SASE-XFEL and XFELO
A tremendous increase of the photon degeneracy η will be reached by x-ray sources
that rely on a fundamentally different mechanism of radiation generation. This is the
principle of self-amplified spontaneous emission (SASE) which forms the basis for
free-electron lasers (FEL). The SASE principle leads to a increase in the brilliance