10
R. Rüffer and A. I. Chumakov
with σ i in mm, σ
i in mrad, and dε/ε in 0.1% energy bandwidth. In Anglo-American
publications very often the brilliance is named “(spectral) brightness”.
1.1.2.2 Time Properties
The energy loss of the particles due to the radiated power (synchrotron radiation)
(see Eq. 1.11) has to be supplied back to keep the particles on their stable orbit. That
is done by rf-transmitters and so-called cavities in the storage ring. The involved
frequencies are in the UHF regime (352 MHz for the ESRF) and define together with
the circumference of the ring the stable positions, called buckets, for the particles.
In case of the ESRF with a circumference of 844 m 992 buckets are available to
be filled with electrons separated by 2.8 ns. In principle any pattern of buckets, out
of the 992, may be filled to allow for the necessary flexibility to the demand at the
experimental stations. Those filled buckets are called bunches and are about 100 ps in
length. Typical filling pattern at the ESRF are currently the multi-bunch mode (7/8
+ 1: meaning that 7/8th of the ring is closely filled with 868 bunches plus a single
bunch just in the center of the remaining gap leaving 176 ns empty space for timing
experiments), 16 bunch mode (16 buckets are filled with a separation of 176 ns each),
and 4 bunch mode (4 buckets are filled with a separation of 704 ns each).
Most of the NRS experiments rely on those two timing modes, 4- and 16-bunch
mode. A special signal (bunch clock) is available for synchronizing this timing with
the experimental needs. An important parameter is the “purity” of the filling. It is
defined as the ratio between the number of photons emitted by electrons in accidentally filled buckets and the number of photons emitted by the electrons of the
nominal bunches. Routinely a purity better than 10
−9 up to 10
−11 is reached by
special cleaning procedures at the ESRF.
1.1.2.3 Insertion Devices
So far we have only considered the bending magnets, responsible for the transverse
acceleration and for keeping the particles on a closed orbit, as a source for synchrotron
radiation.
Third generation SR sources are however characterized by their additional synchrotron radiation sources, so called “insertion devices”, special magnet structures
inserted in dedicated “straight sections” of the storage ring. Their properties may be
tailored to the needs of the experimental stations. The most simple case is a planar
magnet structure of alternating short dipole magnets, see Fig. 1.2. They are arranged
such that the field varies sinusoidally along the particle trajectory
B(x, y = 0, z = 0) = B 0 · cos (
2π
λ u
· x),
(1.19)
R. Rüffer and A. I. Chumakov
with σ i in mm, σ
i in mrad, and dε/ε in 0.1% energy bandwidth. In Anglo-American
publications very often the brilliance is named “(spectral) brightness”.
1.1.2.2 Time Properties
The energy loss of the particles due to the radiated power (synchrotron radiation)
(see Eq. 1.11) has to be supplied back to keep the particles on their stable orbit. That
is done by rf-transmitters and so-called cavities in the storage ring. The involved
frequencies are in the UHF regime (352 MHz for the ESRF) and define together with
the circumference of the ring the stable positions, called buckets, for the particles.
In case of the ESRF with a circumference of 844 m 992 buckets are available to
be filled with electrons separated by 2.8 ns. In principle any pattern of buckets, out
of the 992, may be filled to allow for the necessary flexibility to the demand at the
experimental stations. Those filled buckets are called bunches and are about 100 ps in
length. Typical filling pattern at the ESRF are currently the multi-bunch mode (7/8
+ 1: meaning that 7/8th of the ring is closely filled with 868 bunches plus a single
bunch just in the center of the remaining gap leaving 176 ns empty space for timing
experiments), 16 bunch mode (16 buckets are filled with a separation of 176 ns each),
and 4 bunch mode (4 buckets are filled with a separation of 704 ns each).
Most of the NRS experiments rely on those two timing modes, 4- and 16-bunch
mode. A special signal (bunch clock) is available for synchronizing this timing with
the experimental needs. An important parameter is the “purity” of the filling. It is
defined as the ratio between the number of photons emitted by electrons in accidentally filled buckets and the number of photons emitted by the electrons of the
nominal bunches. Routinely a purity better than 10
−9 up to 10
−11 is reached by
special cleaning procedures at the ESRF.
1.1.2.3 Insertion Devices
So far we have only considered the bending magnets, responsible for the transverse
acceleration and for keeping the particles on a closed orbit, as a source for synchrotron
radiation.
Third generation SR sources are however characterized by their additional synchrotron radiation sources, so called “insertion devices”, special magnet structures
inserted in dedicated “straight sections” of the storage ring. Their properties may be
tailored to the needs of the experimental stations. The most simple case is a planar
magnet structure of alternating short dipole magnets, see Fig. 1.2. They are arranged
such that the field varies sinusoidally along the particle trajectory
B(x, y = 0, z = 0) = B 0 · cos (
2π
λ u
· x),
(1.19)