ALCW at SLAC, January 7, 2004J. Rogers, Novel Schemes for Damping Rings1 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, 2004J. Rogers, Novel Schemes for Damping Rings2 Alternating bends (P. Raimondi, A. Wolski) Problem to be solved: limited dynamic aperture of conventional damping rings. Conventional rings have small dispersion and  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, 2004J. Rogers, Novel Schemes for Damping Rings3 Alternating bends Arc cell— uses combined function dipoles.

ALCW at SLAC, January 7, 2004J. Rogers, Novel Schemes for Damping Rings4 Alternating bends Chromaticity correction section— dispersion is generated by weak bends, pairs of sextupoles are separated by  in phase to cancel the sextupole nonlinearity.

ALCW at SLAC, January 7, 2004J. Rogers, Novel Schemes for Damping Rings5 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, 2004J. Rogers, Novel Schemes for Damping Rings6 Fourier series kicker (G. Gollin) See George Gollin’s talk this session

ALCW at SLAC, January 7, 2004J. Rogers, Novel Schemes for Damping Rings7 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. Bunch spacing can be made smaller than the kicker rise/fall time (by a factor of 4), allowing for a smaller ring. RF section kicker

ALCW at SLAC, January 7, 2004J. Rogers, Novel Schemes for Damping Rings8 Preliminary optics design of the separation has been done. The circumference must be made equal for all channels. The “on- momentum” channel must be longer than the two “off-momentum” channels.

ALCW at SLAC, January 7, 2004J. Rogers, Novel Schemes for Damping Rings9 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, 2004J. Rogers, Novel Schemes for Damping Rings10 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, 2004J. Rogers, Novel Schemes for Damping Rings11 Bunches are ejected from the large ring to the main linac at bucket passages Bunches are injected into the small ring at bucket passages Trains are transferred from the small to large ring starting at bucket passages

ALCW at SLAC, January 7, 2004J. Rogers, Novel Schemes for Damping Rings12 Injection/extraction from trailing edge of a train damping in large ring extraction to bunch compressor & linac refill large ring circumference of small ring circumference of large ring time extraction from large ring injection to small ring transfer from small to large ring Simplified timing example: 3 trains of 3 bunches

ALCW at SLAC, January 7, 2004J. Rogers, Novel Schemes for Damping Rings13 Design example (timing)

ALCW at SLAC, January 7, 2004J. Rogers, Novel Schemes for Damping Rings14

ALCW at SLAC, January 7, 2004J. Rogers, Novel Schemes for Damping Rings15

ALCW at SLAC, January 7, 2004J. Rogers, Novel Schemes for Damping Rings16

ALCW at SLAC, January 7, 2004J. Rogers, Novel Schemes for Damping Rings17 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.