6 Design and Principles of Synchrotrons and Circular Colliders
229
6.3.7 Continuous Transfer Extraction
A frequent requirement in an accelerator complex is to fill a large circumference
machine with the contents of a smaller machine. One way of doing this is boxcar
stacking; another technique is continuous transfer [30], where the beam in the first
machine is extracted over a number of turns, like peeling the skin from an orange
in a continuous strip. The machine tune is brought near to the appropriate integer n,
where the beam will be extracted in n+1 turns. A fast closed bump is then applied
to the circulating beam with kickers to move the beam partly across a septum,
such that a fraction of the beam is cut and extracted. The machine tune rotates the
beam in phase space such that subsequent slices are extracted—when the n th turn
is extracted, the bump amplitude is increased to extract the remaining central part.
This process is of use where the injector can service other machines or experiments
while the receiving machine is accelerating the beam, since it minimises the time
spent filling. The disadvantage of the technique is that large beam losses occur at
the septum, with the transfer efficiency typically 85%. The transfer can be made
with a bunched beam, leaving space for the kicker rise time, but this means that the
receiving machine will need to capture a beam with strong intensity modulation.
Another feature of this extraction is that the extracted slices all have different
emittances, as the slices in phase space are all different.
6.3.8 Resonant Continuous Transfer Extraction
To reduce the beam losses from continuous transfer, a hybrid technique has
been developed and deployed called Multi-Turn Extraction [31] where non-linear
resonances are excited which define stable areas in phase space. These are populated
by the controlled crossing of a resonance, and the islands are then separated by
varying the multipole strength to provide a physical separation at the septum, to
reduce or avoid transverse losses. The beam needs to be bunched with a gap to avoid
losses during the kicker rise time. In addition to the lower losses, another advantage
of this technique is that the extracted islands all have the same emittance.
6.3.9 Other Injection and Extraction Techniques
More exotic injection and extraction techniques also exist as working systems or
concepts. These include radio-frequency stacking [32], pion-decay injection into
muon storage rings [33] and combined cooling and stacking [34]. Charge exchange
extraction [35] is used in cyclotrons, with a stripping foil, to convert for example
H − to p+, or H
+
2 to H
2+
2 so that the beam is then deflected out of the accelerator.
Finally, very high energy particle extraction can be envisaged with a bent crystal
replacing the septum [36].
229
6.3.7 Continuous Transfer Extraction
A frequent requirement in an accelerator complex is to fill a large circumference
machine with the contents of a smaller machine. One way of doing this is boxcar
stacking; another technique is continuous transfer [30], where the beam in the first
machine is extracted over a number of turns, like peeling the skin from an orange
in a continuous strip. The machine tune is brought near to the appropriate integer n,
where the beam will be extracted in n+1 turns. A fast closed bump is then applied
to the circulating beam with kickers to move the beam partly across a septum,
such that a fraction of the beam is cut and extracted. The machine tune rotates the
beam in phase space such that subsequent slices are extracted—when the n th turn
is extracted, the bump amplitude is increased to extract the remaining central part.
This process is of use where the injector can service other machines or experiments
while the receiving machine is accelerating the beam, since it minimises the time
spent filling. The disadvantage of the technique is that large beam losses occur at
the septum, with the transfer efficiency typically 85%. The transfer can be made
with a bunched beam, leaving space for the kicker rise time, but this means that the
receiving machine will need to capture a beam with strong intensity modulation.
Another feature of this extraction is that the extracted slices all have different
emittances, as the slices in phase space are all different.
6.3.8 Resonant Continuous Transfer Extraction
To reduce the beam losses from continuous transfer, a hybrid technique has
been developed and deployed called Multi-Turn Extraction [31] where non-linear
resonances are excited which define stable areas in phase space. These are populated
by the controlled crossing of a resonance, and the islands are then separated by
varying the multipole strength to provide a physical separation at the septum, to
reduce or avoid transverse losses. The beam needs to be bunched with a gap to avoid
losses during the kicker rise time. In addition to the lower losses, another advantage
of this technique is that the extracted islands all have the same emittance.
6.3.9 Other Injection and Extraction Techniques
More exotic injection and extraction techniques also exist as working systems or
concepts. These include radio-frequency stacking [32], pion-decay injection into
muon storage rings [33] and combined cooling and stacking [34]. Charge exchange
extraction [35] is used in cyclotrons, with a stripping foil, to convert for example
H − to p+, or H
+
2 to H
2+
2 so that the beam is then deflected out of the accelerator.
Finally, very high energy particle extraction can be envisaged with a bent crystal
replacing the septum [36].