6 Design and Principles of Synchrotrons and Circular Colliders
227
Fig. 6.16 Schematic of fast
extraction system with kicker,
septum and orbit bumpers.
For higher energy machines,
protection devices to intercept
and dilute any mis-kicked
extracted beam are placed in
front of the septum and
downstream QF quadrupole
Extracted beam
Bumper
magnet
Bumper
magnet
Septum magnet Kicker magnet
QF
QD
QF
QD
QF
Bumped
circulaƟng beam
system design feature. Closed orbit bumps can be used to move the beam closer to
the septum, to reduce the required kick strength, Fig. 6.16.
6.3.6 Resonant Extraction
Many rate-limited applications such as physics experiments, test beams or medical
treatment beams require a slow flux of particles with as uniform a time structure as
possible. Resonant extraction using the third integer is the most common method of
providing such uniform spills. In this ‘slow’ extraction [24], a triangular stable area
in phase space (usually horizontal) is defined by exciting sextupole elements, and
by moving the machine tune close to the third integer resonance. Before the start of
the extraction process, particles remain stable if their single-particle emittances are
smaller than the area of the stable triangle.
The beam is extracted by driving some particles unstable in a controlled way.
The unstable particle amplitudes increase rapidly, following the outward-going
separatrix every three turns, and the particles eventually move into the high-field
region of a very thin electrostatic septum and are extracted, Fig. 6.17. The rate of
extraction is controlled either by modulating the excitation process or by controlling
the stable area. Several techniques for driving the particles unstable are possible:
i) the stable area can be reduced by increasing the resonance (sextupole) strength
or by moving the tune closer to the third integer. Increasing the resonance
strength reduces the stable area, but the smallest amplitude particles cannot
be extracted, and changing the resonance affects the machine optics. Crossing
the resonant tune offers the advantage that all of the beam can be extracted;
however, the optics is still perturbed and in addition the position of the extracted
beam in phase space changes as particles are extracted;
227
Fig. 6.16 Schematic of fast
extraction system with kicker,
septum and orbit bumpers.
For higher energy machines,
protection devices to intercept
and dilute any mis-kicked
extracted beam are placed in
front of the septum and
downstream QF quadrupole
Extracted beam
Bumper
magnet
Bumper
magnet
Septum magnet Kicker magnet
QF
QD
QF
QD
QF
Bumped
circulaƟng beam
system design feature. Closed orbit bumps can be used to move the beam closer to
the septum, to reduce the required kick strength, Fig. 6.16.
6.3.6 Resonant Extraction
Many rate-limited applications such as physics experiments, test beams or medical
treatment beams require a slow flux of particles with as uniform a time structure as
possible. Resonant extraction using the third integer is the most common method of
providing such uniform spills. In this ‘slow’ extraction [24], a triangular stable area
in phase space (usually horizontal) is defined by exciting sextupole elements, and
by moving the machine tune close to the third integer resonance. Before the start of
the extraction process, particles remain stable if their single-particle emittances are
smaller than the area of the stable triangle.
The beam is extracted by driving some particles unstable in a controlled way.
The unstable particle amplitudes increase rapidly, following the outward-going
separatrix every three turns, and the particles eventually move into the high-field
region of a very thin electrostatic septum and are extracted, Fig. 6.17. The rate of
extraction is controlled either by modulating the excitation process or by controlling
the stable area. Several techniques for driving the particles unstable are possible:
i) the stable area can be reduced by increasing the resonance (sextupole) strength
or by moving the tune closer to the third integer. Increasing the resonance
strength reduces the stable area, but the smallest amplitude particles cannot
be extracted, and changing the resonance affects the machine optics. Crossing
the resonant tune offers the advantage that all of the beam can be extracted;
however, the optics is still perturbed and in addition the position of the extracted
beam in phase space changes as particles are extracted;