Orbit and trajectory correction 73
In linacs or transport lines, the BPMs report beam positions on each beam
pass. In storage rings, the repetition rate of bunches is typically on the order
of a few hundred MHz. Modern BPM electronics on most of the new rings
can resolve the beam position on turn-by-turn basis. The turn-by-turn BPM
readings report the average position of all bunches in the ring on the same
pass. Turn-by-turn BPMs can monitor the beam motion of bunches moving in
phase, such as in the typical case of betatron oscillations when the bunches in
the beam are launched with common position and angle coordinates, or when
the beam is kicked by a pinger. The turn-by-turn beam positions measured by
the BPMs for a beam undergoing betatron oscillations contains information
of the linear optics of the machine.
The BPMs also report the average beam position over many turns. This is
the typical mode of operation for BPMs during beam delivery to users. The
average positions can be reported at a high frequency (e.g., 10 kHz), which can
be used for fast orbit correction. When the beam is not in an excited state
(synchrotron motion, betatron motion, or instability), the orbit is typically
dominated by low-frequency motion. Because the beam in betatron motion
oscillates around the closed orbit in high frequency, the time averaged position
is a good measurement of the closed orbit as the oscillation is cancelled out.
The average beam position have a high precision. It is common for the position
readings to have a standard deviation (noise sigma) at or below 1 µm.
In an accelerator, BPMs are distributed throughout the beam path. BPMs
are required at certain critical locations in the machine, for example, near
the interaction points in a collider, or at the ends of straight sections that
host undulators in synchrotron light sources. In other areas, BPMs are placed
to sufficiently sample the orbit errors in order to prevent undetected orbit
drifts. Orbit responses or betatron oscillations measured by BPMs can also
be used for linear optics calibration. It is advisable to have a few BPMs in
each betatron period. In a periodic lattice, the BPMs are usually located at
identical positions in each cell.
3.1.2 Orbit correctors
Orbit correctors are used to steer the beam orbit toward the desired target
orbit or trajectory. The steering is achieved by generating or changing a dipole
field on the path of the beam. The dipole field deflects the direction of the
propagation of the beam. The deflection is called a kick. The trajectory downstream of the kick is thus modified. In a circular accelerator, a kick changes
the closed orbit all around the ring.
An orbit corrector can be a standalone magnet, a set of wires on a multipurpose magnet, or trim coils on a main dipole magnet. Orbit correction can
also be achieved by modifying the setpoint of the main dipoles. Trim coils
or main dipoles have slow responses to changes due to the large magnetic
inductance of the main dipole magnets. It could take seconds for the magnets to settle on the new setpoints. Therefore they cannot be used to correct
In linacs or transport lines, the BPMs report beam positions on each beam
pass. In storage rings, the repetition rate of bunches is typically on the order
of a few hundred MHz. Modern BPM electronics on most of the new rings
can resolve the beam position on turn-by-turn basis. The turn-by-turn BPM
readings report the average position of all bunches in the ring on the same
pass. Turn-by-turn BPMs can monitor the beam motion of bunches moving in
phase, such as in the typical case of betatron oscillations when the bunches in
the beam are launched with common position and angle coordinates, or when
the beam is kicked by a pinger. The turn-by-turn beam positions measured by
the BPMs for a beam undergoing betatron oscillations contains information
of the linear optics of the machine.
The BPMs also report the average beam position over many turns. This is
the typical mode of operation for BPMs during beam delivery to users. The
average positions can be reported at a high frequency (e.g., 10 kHz), which can
be used for fast orbit correction. When the beam is not in an excited state
(synchrotron motion, betatron motion, or instability), the orbit is typically
dominated by low-frequency motion. Because the beam in betatron motion
oscillates around the closed orbit in high frequency, the time averaged position
is a good measurement of the closed orbit as the oscillation is cancelled out.
The average beam position have a high precision. It is common for the position
readings to have a standard deviation (noise sigma) at or below 1 µm.
In an accelerator, BPMs are distributed throughout the beam path. BPMs
are required at certain critical locations in the machine, for example, near
the interaction points in a collider, or at the ends of straight sections that
host undulators in synchrotron light sources. In other areas, BPMs are placed
to sufficiently sample the orbit errors in order to prevent undetected orbit
drifts. Orbit responses or betatron oscillations measured by BPMs can also
be used for linear optics calibration. It is advisable to have a few BPMs in
each betatron period. In a periodic lattice, the BPMs are usually located at
identical positions in each cell.
3.1.2 Orbit correctors
Orbit correctors are used to steer the beam orbit toward the desired target
orbit or trajectory. The steering is achieved by generating or changing a dipole
field on the path of the beam. The dipole field deflects the direction of the
propagation of the beam. The deflection is called a kick. The trajectory downstream of the kick is thus modified. In a circular accelerator, a kick changes
the closed orbit all around the ring.
An orbit corrector can be a standalone magnet, a set of wires on a multipurpose magnet, or trim coils on a main dipole magnet. Orbit correction can
also be achieved by modifying the setpoint of the main dipoles. Trim coils
or main dipoles have slow responses to changes due to the large magnetic
inductance of the main dipole magnets. It could take seconds for the magnets to settle on the new setpoints. Therefore they cannot be used to correct