1 X-Ray Sources at Large-Scale Facilities
29
e−beam dump
LEG
x−rays
LINAC 2
undulator
LINAC 1
BC
fs laser
Fig. 1.17 Schematic of XFEL architecture. Electron bunches are emitted from a low-emittance gun
(LEG) irradiated by picosecond laser pulses. They are then accelerated in a short LINAC (LINAC 1),
then compressed using one or more bunch compressor magnet chicanes (BC); they are then further
accelerated using a much longer LINAC (LINAC 2) before entering a long undulator, typically a
few hundred metres in length. The SASE process along the undulator produces extremely intense
X-ray pulses with durations of the order of 50 fs. Reproduced from [3] with permission (Copyright
2019, John Wiley and Sons)
BC
LINAC 1
n
n
e
e
Δ
e
x
Δx
Δx
Δx
Δx
Δx
E
Δ
E
Δ
E
Δ
n
Fig. 1.18 Bunch compression in high-gain XFELs. Before entering the first LINAC, the electron
density n e and spread in kinetic energy are both relatively low. The phase of the LINAC RF-field
relative to the passage of the bunch is so adjusted to induce a larger spread of the electrons’ kinetic
energies, whereby the bunch’s trailing edge is made to be more energetic than the leading edge.
Compression is achieved by allowing the bunch to pass through a four-dipole chicane, where the
faster electrons at the back can catch up with the slower electrons at the front, thanks to the shorter
path that they execute. Reproduced from [3] with permission (Copyright 2019, John Wiley and
Sons)
bunch (a phenomenon referred to as ‘chirping’). However, the electrons’ velocities
are already so close to the speed of light that any differences are far too small to allow
the faster electrons at the back of the bunch to catch up with the slower electrons
positioned further forward and thereby squeeze the bunch length. Instead, the bunch
passes through a magnetic four-dipole chicane. The trailing (high-energy) electrons
execute a shorter path through the chicane because they are less deviated by the
chicane’s magnetic field. This shorter path means that they catch up with the less
energetic leading electrons, which are more deviated by the magnet chicane. This
compression thus shortens the bunch duration to approximately 300 fs (equating to a
29
e−beam dump
LEG
x−rays
LINAC 2
undulator
LINAC 1
BC
fs laser
Fig. 1.17 Schematic of XFEL architecture. Electron bunches are emitted from a low-emittance gun
(LEG) irradiated by picosecond laser pulses. They are then accelerated in a short LINAC (LINAC 1),
then compressed using one or more bunch compressor magnet chicanes (BC); they are then further
accelerated using a much longer LINAC (LINAC 2) before entering a long undulator, typically a
few hundred metres in length. The SASE process along the undulator produces extremely intense
X-ray pulses with durations of the order of 50 fs. Reproduced from [3] with permission (Copyright
2019, John Wiley and Sons)
BC
LINAC 1
n
n
e
e
Δ
e
x
Δx
Δx
Δx
Δx
Δx
E
Δ
E
Δ
E
Δ
n
Fig. 1.18 Bunch compression in high-gain XFELs. Before entering the first LINAC, the electron
density n e and spread in kinetic energy are both relatively low. The phase of the LINAC RF-field
relative to the passage of the bunch is so adjusted to induce a larger spread of the electrons’ kinetic
energies, whereby the bunch’s trailing edge is made to be more energetic than the leading edge.
Compression is achieved by allowing the bunch to pass through a four-dipole chicane, where the
faster electrons at the back can catch up with the slower electrons at the front, thanks to the shorter
path that they execute. Reproduced from [3] with permission (Copyright 2019, John Wiley and
Sons)
bunch (a phenomenon referred to as ‘chirping’). However, the electrons’ velocities
are already so close to the speed of light that any differences are far too small to allow
the faster electrons at the back of the bunch to catch up with the slower electrons
positioned further forward and thereby squeeze the bunch length. Instead, the bunch
passes through a magnetic four-dipole chicane. The trailing (high-energy) electrons
execute a shorter path through the chicane because they are less deviated by the
chicane’s magnetic field. This shorter path means that they catch up with the less
energetic leading electrons, which are more deviated by the magnet chicane. This
compression thus shortens the bunch duration to approximately 300 fs (equating to a