Transcript Novel Schemes for Damping Rings J. Rogers Cornell University • Improving dynamic aperture

Novel Schemes for Damping Rings
J. Rogers
Cornell University
• Improving dynamic aperture
• Reducing the circumference of the TESLA damping rings
ALCW at SLAC, January 7, 2004
J. Rogers, Novel Schemes for Damping Rings
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Alternating bends (P. Raimondi, A. Wolski)
Problem to be solved: limited dynamic aperture of conventional damping
rings.
• Conventional rings have small dispersion and b functions, so
chromaticity correction sextupoles must be strong.
• Conventional rings also use wiggler magnets, which are nonlinear.
Approach: separate damping arcs from chromaticity correction sections.
• Chromaticity correction sections are optimized to minimize aberration
from the sextupoles.
• Arcs use high-field dipole magnets with reverse bends to achieve
damping, instead of wigglers.
ALCW at SLAC, January 7, 2004
J. Rogers, Novel Schemes for Damping Rings
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Alternating bends
Arc cell— uses combined function dipoles.
ALCW at SLAC, January 7, 2004
J. Rogers, Novel Schemes for Damping Rings
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Alternating bends
Chromaticity correction section— dispersion is generated by weak bends, pairs
of sextupoles are separated by p in phase to cancel the sextupole nonlinearity.
ALCW at SLAC, January 7, 2004
J. Rogers, Novel Schemes for Damping Rings
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Responses to the very large TESLA damping rings
Problem to be solved: large circumference of the TESLA damping rings.
TESLA uses 2820 bunches per train, with a 20 ns spacing (limited by kicker
rise/fall time).
• Cost
• Share tunnel with main linacs
• Large space-charge tune spread necessitates coupling bumps
Approaches:
• Stacked rings
• Fourier series kicker
• RF separation at injection/extraction points
• Injection/extraction from trailing edge of a train
Disadvantage:
• Average current is higher in some approaches, making multibunch
instabilities (e.g., electron cloud) more troublesome.
ALCW at SLAC, January 7, 2004
J. Rogers, Novel Schemes for Damping Rings
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Fourier series kicker (G. Gollin)
kicker rf cavities
injection/extraction
deflecting magnet
pT
injection path
See George Gollin’s talk
this session
injection/extraction
deflecting magnet
extraction path
injection path
extraction path
kicker rf cavities
fhigh
ALCW at SLAC, January 7, 2004
fhigh +
3 MHz
fhigh +
6 MHz
...
J. Rogers, Novel Schemes for Damping Rings
fhigh +
(N-1)3 MHz
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RF separation at injection/extraction points (R. Helms, D. Rubin)
• A secondary RF system with a different frequency is used to separate the
beam dispersively, bunch by bunch, into different channels.
• One such channel contains the injection/extraction kicker.
kicker
RF section
RF section
• Bunch spacing can be made smaller than the kicker rise/fall time (by a factor
of 4), allowing for a smaller ring.
ALCW at SLAC, January 7, 2004
J. Rogers, Novel Schemes for Damping Rings
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• Preliminary optics
design of the separation
has been done.
• The circumference
must be made equal for
all channels. The “onmomentum” channel
must be longer than the
two “off-momentum”
channels.
ALCW at SLAC, January 7, 2004
J. Rogers, Novel Schemes for Damping Rings
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Injection/extraction from trailing edge of a train (J. Rogers)
Advantages:
• Bunches are always extracted and injected at the end of a bunch train, so the
injection/extraction kickers need only have a fast rise time. The damping ring
can be much smaller than the dogbone design.
• Positron bunch production rate is greatly reduced, allowing use of a
conventional positron source.
Disadvantage:
An additional small ring is required.
ALCW at SLAC, January 7, 2004
J. Rogers, Novel Schemes for Damping Rings
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Injection/extraction from trailing edge of a train
• Two rings (one large, one small) share a common RF section.
• As damped bunches are extracted from the large ring to the bunch
compressor and main linacs, bunches are injected into the small ring, avoiding
RF transients.
• When all of the damped bunches are extracted from the large ring, the small
ring is full. A transfer kicker located in the common straight section moves all
of the bunches in the small ring as a train to the large ring. This requires a gap
before the stored train, which does not appear in the train extracted to the
main linac.
ALCW at SLAC, January 7, 2004
J. Rogers, Novel Schemes for Damping Rings
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Bunches are ejected from the large ring to the main linac at bucket passages
k b  gi, i  0,1,2,
Nb 1
Bunches are injected into the small ring at bucket passages
kN b  g  1i, i  0,1, 2,
Nb 1
Trains are transferred from the small to large ring starting at bucket passages
k Nb  1b  g   Nb 1b  g  bi, i  0,1, 2,
ALCW at SLAC, January 7, 2004
J. Rogers, Novel Schemes for Damping Rings
N 1
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Injection/extraction from trailing edge of a train
circumference of large ring
circumference of small ring
Simplified timing example:
3 trains of 3 bunches
damping in large ring
extraction from large ring
injection to small ring
transfer from small to large ring
extraction to bunch compressor & linac
time
refill large ring
ALCW at SLAC, January 7, 2004
J. Rogers, Novel Schemes for Damping Rings
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Design example (timing)
ALCW at SLAC, January 7, 2004
J. Rogers, Novel Schemes for Damping Rings
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Summary
• The baseline damping ring designs for the LC projects should work,
although they are not without some risk (2003 ILC-TRC report).
• Alternatives are possible, and there are a number of promising concepts
being developed.
ALCW at SLAC, January 7, 2004
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