Operational aspects of experimental accelerator physics Page: 9 of 28
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greater than or equal to the resolution. Modern
accelerator BPM systems have resolutions approaching 5 to
10 microns and accuracies better than 150 microns.
2.2 Orbit Correction
2.2.1 Reference Orbit Determination
One of the most common procedures performed in a
high energy accelerator control room is orbit correction,
or flattening. Shortly after commissioning an accelerator,
a "reference orbit" is established. This reference orbit
is simply a set of BPM readings deemed desirable for
various reasons. Usually, the reference orbit is believed
to be the orbit that agrees most closely with the magnetic
centerlines of the storage ring quadrupoles and sextupoles,
thus yielding the most linear optics. This is commonly the
case in high energy colliders. For a synchrotron radiation
source, on the other hand, the reference orbit is usually
the orbit where all operational photon beamlines are
adequately illuminated. Ideally, this orbit passes close
to the quadrupole and sextupole magnetic centerlines, as in
HEP machines. Determination of the reference orbit can be
quite involved, and typically must be repeated many times
over the life of an accelerator, simply because things
move.
The magnetic centerline of a quadrupole can be
found using the beam, by taking advantage of the fact that
a quadrupole will not steer unless the beam is off center.
The technique requires that a BPM be located adjacent to
the quad in question, and that the quad be independently
controllable (as compared to a string of quads wired in
series). In addition, at least one other BPM elsewhere in
the ring is required, if not a complete orbit. If a single
BPM is used, it should be placed an appropriate fraction of
betatron wavelengths from the quad in question and it, as
well as its associated electronics, must be reliable.
Finally, a means of moving the beam at the quad with high
resolution is needed. This can be done using a local bump
(see Section 2.2.3). A global orbit distortion can be
used, but is less desirable.
Suppose the magnetic centerline of quad Q1 is to
be determined in the vertical plane. Define the vertical
position of the beam as measured by the BPM adjacent to Ql
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Decker, G.A. Operational aspects of experimental accelerator physics, report, July 1, 1995; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc792469/m1/9/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.