70 Beam-based Correction and Optimization for Accelerators
Orbit or trajectory correction needs several prerequisites. First, there must
be diagnostics to measure the beam orbit. This is usually done with beam position monitors (BPMs). It is possible to use beam screens, scanning wires,
cameras, or other diagnostics. Second, there must exist steering magnets that
can effectively alter the beam orbit. The steering magnets can be dedicated
short dipole magnets, correction windings on the main bending magnets, or
the main bending magnets themselves. The steering magnets are the actuators
or knobs for orbit and trajectory correction. Third, the target orbit or trajectory must be determined. This could be done empirically. Finally, and very
importantly, there need to be effective algorithms to calculate the required
changes on the actuators for the orbit to move toward the target.
Orbit correction for circular accelerators in the early days was based on the
harmonic analysis of the orbit errors (HARMON) or by iteratively applying
the most effective knobs (MICADO) [5]. These methods target the global distribution of orbit errors. Orbit stability for photon beams in synchrotron light
sources was initially achieved with local orbit bumps [46]. Real-time global
orbit correction in light sources was first implemented with the harmonics
correction approach [127, 126]. Singular value decomposition (SVD) [41, 42]
was introduced for global orbit correction later [21, 23]. With a large number of precise and fast BPMs and correctors distributed throughout the beam
line, orbit stability can be achieved with high precision in modern accelerators
using the SVD-based method.
In this chapter we first briefly discuss orbit measurement, steering magnets, and the determination of orbit target. The main focus is the SVD-based
method for orbit and trajectory correction.
3.1 ACCELERATOR COMPONENTS FOR ORBIT CORRECTION
3.1.1 Beam position monitors
Beam positions in the vacuum chamber of accelerators are typically measured
with BPMs. Position measurement with BPMs is non-invasive to the beam.
A BPM uses electromagnetic pick-ups attached to the vacuum chamber to
detect the image current of the beam. The strengths of the image current
signals depend on the proximity of the beam to the pick-ups. When the pickups are arranged symmetrically about chamber center, any deviation of the
beam position from the center will be reflected on the signal strengths of the
pick-ups. The signal difference normalized by the total signal strength can
thus serve as a measurement of the beam position.
The pick-ups for BPMs can be button shaped or strip-lines. Button BPMs
are common in electron accelerators. A button BPM may consist of four electrodes diagonally arranged on the vacuum chamber. The buttons are typically
rotated by 45
◦ from the horizontal mid-plane in electron storage rings to avoid
synchrotron radiation damage. This is illustrated in Figure 3.1 for a round
vacuum chamber.
Orbit or trajectory correction needs several prerequisites. First, there must
be diagnostics to measure the beam orbit. This is usually done with beam position monitors (BPMs). It is possible to use beam screens, scanning wires,
cameras, or other diagnostics. Second, there must exist steering magnets that
can effectively alter the beam orbit. The steering magnets can be dedicated
short dipole magnets, correction windings on the main bending magnets, or
the main bending magnets themselves. The steering magnets are the actuators
or knobs for orbit and trajectory correction. Third, the target orbit or trajectory must be determined. This could be done empirically. Finally, and very
importantly, there need to be effective algorithms to calculate the required
changes on the actuators for the orbit to move toward the target.
Orbit correction for circular accelerators in the early days was based on the
harmonic analysis of the orbit errors (HARMON) or by iteratively applying
the most effective knobs (MICADO) [5]. These methods target the global distribution of orbit errors. Orbit stability for photon beams in synchrotron light
sources was initially achieved with local orbit bumps [46]. Real-time global
orbit correction in light sources was first implemented with the harmonics
correction approach [127, 126]. Singular value decomposition (SVD) [41, 42]
was introduced for global orbit correction later [21, 23]. With a large number of precise and fast BPMs and correctors distributed throughout the beam
line, orbit stability can be achieved with high precision in modern accelerators
using the SVD-based method.
In this chapter we first briefly discuss orbit measurement, steering magnets, and the determination of orbit target. The main focus is the SVD-based
method for orbit and trajectory correction.
3.1 ACCELERATOR COMPONENTS FOR ORBIT CORRECTION
3.1.1 Beam position monitors
Beam positions in the vacuum chamber of accelerators are typically measured
with BPMs. Position measurement with BPMs is non-invasive to the beam.
A BPM uses electromagnetic pick-ups attached to the vacuum chamber to
detect the image current of the beam. The strengths of the image current
signals depend on the proximity of the beam to the pick-ups. When the pickups are arranged symmetrically about chamber center, any deviation of the
beam position from the center will be reflected on the signal strengths of the
pick-ups. The signal difference normalized by the total signal strength can
thus serve as a measurement of the beam position.
The pick-ups for BPMs can be button shaped or strip-lines. Button BPMs
are common in electron accelerators. A button BPM may consist of four electrodes diagonally arranged on the vacuum chamber. The buttons are typically
rotated by 45
◦ from the horizontal mid-plane in electron storage rings to avoid
synchrotron radiation damage. This is illustrated in Figure 3.1 for a round
vacuum chamber.