1 X-Ray Sources at Large-Scale Facilities
3
Sections 1.4 and 1.5 outline the most pertinent features of the latest generation of
storage rings, so-called diffraction-limited storage rings (DLSRs), and X-ray freeelectron lasers (XFELs), respectively.
1.2 A Brief Description of Synchrotrons
1.2.1 Introduction
A synchrotron consists of a ring-shaped evacuated vessel (the storage ring, having
a circumference measured typically in a few hundreds of metres, Fig. 1.2) in which
high-energy electrons circulate at highly relativistic velocities, and so-called ‘beamlines’, that extract and use the radiation emitted by the electrons tangentially to their
orbital path, at positions defined by components known as bending magnets (BMs)
and insertion devices (IDs).
The electron energy E at synchrotrons is typically of the order of a few GeV. The
emitted photons, on the other hand, have energies measured anywhere between a few
eV (just above the visible) and several hundred keV, in the ultrahard X-ray regime.
1
Note, however, that even the photon energies of the latter are still some four orders
of magnitude smaller than the electrons’ kinetic energy E in the storage ring.
The electrons are forced into a closed path by bending magnets, which exert a
centripetal Lorentz force on them. It is here, and in straight sections in which the
insertion devices are installed (see Sect. 1.3), that they emit electromagnetic (EM)
radiation.
The electrons lose energy due to their emitting EM radiation. This must be replenished and is achieved via axial acceleration through one or more radio-frequency (RF)
cavities installed in the storage ring.
1.2.2 The Lorentz Factor
Before we proceed further, the dimensionless parameter γ is introduced. This socalled ‘Lorentz factor’ expresses the ratio of the electron energy E to the rest mass
energy of the electrons m e c
2
= 511 keV (m e = 9.109 × 10
−31 kg is the electron rest
mass and c = 2.9979 × 10
8 m s
−1 the speed of light), that is
γ =
E
m e c 2 .
(1.1)
1 Some facilities host beamlines that extend down into the far-infrared regime; these relatively
uncommon sources are not discussed here.
3
Sections 1.4 and 1.5 outline the most pertinent features of the latest generation of
storage rings, so-called diffraction-limited storage rings (DLSRs), and X-ray freeelectron lasers (XFELs), respectively.
1.2 A Brief Description of Synchrotrons
1.2.1 Introduction
A synchrotron consists of a ring-shaped evacuated vessel (the storage ring, having
a circumference measured typically in a few hundreds of metres, Fig. 1.2) in which
high-energy electrons circulate at highly relativistic velocities, and so-called ‘beamlines’, that extract and use the radiation emitted by the electrons tangentially to their
orbital path, at positions defined by components known as bending magnets (BMs)
and insertion devices (IDs).
The electron energy E at synchrotrons is typically of the order of a few GeV. The
emitted photons, on the other hand, have energies measured anywhere between a few
eV (just above the visible) and several hundred keV, in the ultrahard X-ray regime.
1
Note, however, that even the photon energies of the latter are still some four orders
of magnitude smaller than the electrons’ kinetic energy E in the storage ring.
The electrons are forced into a closed path by bending magnets, which exert a
centripetal Lorentz force on them. It is here, and in straight sections in which the
insertion devices are installed (see Sect. 1.3), that they emit electromagnetic (EM)
radiation.
The electrons lose energy due to their emitting EM radiation. This must be replenished and is achieved via axial acceleration through one or more radio-frequency (RF)
cavities installed in the storage ring.
1.2.2 The Lorentz Factor
Before we proceed further, the dimensionless parameter γ is introduced. This socalled ‘Lorentz factor’ expresses the ratio of the electron energy E to the rest mass
energy of the electrons m e c
2
= 511 keV (m e = 9.109 × 10
−31 kg is the electron rest
mass and c = 2.9979 × 10
8 m s
−1 the speed of light), that is
γ =
E
m e c 2 .
(1.1)
1 Some facilities host beamlines that extend down into the far-infrared regime; these relatively
uncommon sources are not discussed here.