1 X-Ray Sources at Large-Scale Facilities
27
pulses has been motivated by studies of the dynamics of chemical reactions—this
is one of the most important goals in science, as a biomolecular understanding of
the processes of life, plus most industrial chemical processes, depend intimately
on understanding transient intermediate states. Indeed, the holy grail of physical
chemistry is the direct observation of the making and breaking of chemical bonds
and following reactions through the reconfiguration of the atomic structure. In order
to achieve this, one needs a probe that can spatially resolve the atomic positions,
measured in angstroms, thus necessitating the use of hard X-rays. Moreover, in order
to record a movie of chemical processes, each ‘frame’ must be shorter than the speed
of the process divided by the desired resolution, which we have just stated should
be of the order of an angstrom. Typical velocities associated with atomic motion
are of the order of 1000 m s
−1 , hence each movie frame should be separated by
no more than approximately 100 fs. For example, phonons and molecular rotations
have time scales of approximately 100–1000 fs, while molecular vibrations, crucial
to the process of chemical reactions, are shown to have periodicities of the order of
10–100 fs.
X-ray free-electron lasers (XFELs) were developed to fulfil this goal. They provide
extremely intense and short pulses of X-rays. XFELs and synchrotron facilities differ
substantially in this respect (see Table 1.1). The total average number of photons
per second delivered to a third- or fourth-generation synchrotron beamline after
monochromatization (typically between 10
13 and 10
14 s
−1 ) is comparable to that of
an XFEL beamline, although the latter can vary by orders of magnitude, depending
on the facility repetition rate and whether the beam is monochromatized or not. In
contrast, their time structures are very different: synchrotrons deliver pulses with
the same frequency as the RF supply, typically several hundred million pulses per
second (1//t); each pulse is maybe a few tens of picosecond long (τ ) and contains
the order of 10
4 –10
5 photons. XFELs deliver anything between approximately 100
and several hundreds of thousands of pulses per second, depending on the electron
gun source and the properties of the accelerating LINAC. Most importantly, each
XFEL pulse is only a few tens of femtoseconds long (and can be even shorter,
down to a femtosecond) and contains the order of 10
12 photons. The peak arrival
rate of photons at XFELs is thus approximately 10 billion times higher than that at
synchrotron beamlines.
XFELs thus provide the tools needed to study the dynamics, physical properties
and structure of materials with unsurpassed spatial detail and on a time scale over a
thousand times shorter than that which is possible at synchrotron facilities.
The beam quality in storage rings is limited by the stochastic competing processes
described above in radiation equilibrium. High-gain XFELs are made possible by
a runaway process called self-amplified spontaneous emission (SASE). Because, in
contrast to synchrotrons, the electrons in XFELs are not stored and require only a
few microseconds to traverse the length of the entire XFEL facility, they are far less
perturbed than are the equilibrated electrons in a storage ring, meaning they can
maintain a very low emittance defined by the electron gun, acceleration and bunch
compression mechanism.
27
pulses has been motivated by studies of the dynamics of chemical reactions—this
is one of the most important goals in science, as a biomolecular understanding of
the processes of life, plus most industrial chemical processes, depend intimately
on understanding transient intermediate states. Indeed, the holy grail of physical
chemistry is the direct observation of the making and breaking of chemical bonds
and following reactions through the reconfiguration of the atomic structure. In order
to achieve this, one needs a probe that can spatially resolve the atomic positions,
measured in angstroms, thus necessitating the use of hard X-rays. Moreover, in order
to record a movie of chemical processes, each ‘frame’ must be shorter than the speed
of the process divided by the desired resolution, which we have just stated should
be of the order of an angstrom. Typical velocities associated with atomic motion
are of the order of 1000 m s
−1 , hence each movie frame should be separated by
no more than approximately 100 fs. For example, phonons and molecular rotations
have time scales of approximately 100–1000 fs, while molecular vibrations, crucial
to the process of chemical reactions, are shown to have periodicities of the order of
10–100 fs.
X-ray free-electron lasers (XFELs) were developed to fulfil this goal. They provide
extremely intense and short pulses of X-rays. XFELs and synchrotron facilities differ
substantially in this respect (see Table 1.1). The total average number of photons
per second delivered to a third- or fourth-generation synchrotron beamline after
monochromatization (typically between 10
13 and 10
14 s
−1 ) is comparable to that of
an XFEL beamline, although the latter can vary by orders of magnitude, depending
on the facility repetition rate and whether the beam is monochromatized or not. In
contrast, their time structures are very different: synchrotrons deliver pulses with
the same frequency as the RF supply, typically several hundred million pulses per
second (1//t); each pulse is maybe a few tens of picosecond long (τ ) and contains
the order of 10
4 –10
5 photons. XFELs deliver anything between approximately 100
and several hundreds of thousands of pulses per second, depending on the electron
gun source and the properties of the accelerating LINAC. Most importantly, each
XFEL pulse is only a few tens of femtoseconds long (and can be even shorter,
down to a femtosecond) and contains the order of 10
12 photons. The peak arrival
rate of photons at XFELs is thus approximately 10 billion times higher than that at
synchrotron beamlines.
XFELs thus provide the tools needed to study the dynamics, physical properties
and structure of materials with unsurpassed spatial detail and on a time scale over a
thousand times shorter than that which is possible at synchrotron facilities.
The beam quality in storage rings is limited by the stochastic competing processes
described above in radiation equilibrium. High-gain XFELs are made possible by
a runaway process called self-amplified spontaneous emission (SASE). Because, in
contrast to synchrotrons, the electrons in XFELs are not stored and require only a
few microseconds to traverse the length of the entire XFEL facility, they are far less
perturbed than are the equilibrated electrons in a storage ring, meaning they can
maintain a very low emittance defined by the electron gun, acceleration and bunch
compression mechanism.