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Published byAngel Tebbetts Modified about 1 year ago

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Q20: The X-band collider has much tighter requirements for the alignment of the beam orbit with the structure axis, yet the basic instrumental precision for alignment is the same as for the L-band collider. The SLC had great difficulty reaching its design luminosity in part because of the difficulty in controlling the beam orbit. How can it be demonstrated that the necessary control of the orbit can be obtained for the GLC/NLC? (cf. Q7) 6/29/2004 K. ITRP CALTECH

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How do they do it? BNS autophasing Orbit feedback Initial alignment Beam-based alignment using –Movers for quads and rf girders –BPMs at quads and structures Solving issues at SLC

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BNS Autophasing Cancels the head-tail wake force by variation of the quadrupole focusing force with an energy slope within a bunch. Valid for incoming orbit error/jitter. The necessary energy correlation for X-band is 1/5 of SLC, same as FFTB.

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Orbit Feedback SLC had 50 orbit/beam feedbacks controlling 250 parameters. “These systems have been studied extensively in simulations and various performance limitations seen at SLC are now well understood. ” “The systems planned for the X-band collider have improvements in functionality and connectivity to correct these limitations and should give excellent performance.”

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Initial Alignment Only necessary to set components within the capture range of the beam-based alignment. 50 m is possible for all components ( m is adequate). 25 m is possible for structure to rf-girder (4structures/girder) by Coordinate Measuring Machines. 100 m is possible for rf-girders.

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Beam-based Alignment Movers at all quads(x,y), rf-girders(x,y, ). BPM: –1/quad, 0.25 m resolution. –2/structure (=8/girder), 1 m resolution. Correction algorithms are verified by simulations, resulting 40% vertical emittance growth through the linac.

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No SLC problems any more! No vibration of quads by a better support. “What about quads in the cryostat for the Cold (no measurement yet)?” No long-range wakefield by the DDS. No microwave instability in the damping ring, by carefull impedance estimation. “The Cold damping ring has less margin for the impedance budget.”

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X-band orbit/emittance tuning scheme

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Movers/BPM failures The X-band linac can preserve the emittance within allowable level if –1,200 quad BPMs –12,000 structure BPMs –1,200 quad movers –3,000 girder movers work flawlessly. How much tolerance do they have for malfunction of these devices? –An estimation has been made for mover failures (T. Himmel et al.) - No serious effects. –BPM malfunctioning will be also OK (by PT). –The model dependence of the correction may be OK (by PT).

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Mover Failures - T. Himel et al, SLAC-TN Simulation studies of accelerator alignment as a function of time suggest that the Technology Options Report is probably far too pessimistic in estimating the luminosity impact of failed movers. Although the luminosity will gradually degrade as more movers become stuck and as the rest of the beam line diffuses further and further away from the stuck elements, the time required for even a 1% luminosity reduction appears to be much larger than the anticipated period between repair opportunities. This is the case even assuming a relatively large diffusive ground motion component, such as what has been measured at KEK. We conclude that mover failures will cause negligible luminosity loss in the NLC.

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BPM errors The emittance growth time scales with A and with the number of BPMs. Given a linac with 600 BPMs and your KEKB ratio (3.3% failed BPMs) this would yield an emittance growth time of about 6 months -- that is, it would be necessary to do something other than standard mover steering after 6 months of operating like this (either tune emittance bumps or fix the BPMs). - P. Tenenbaum (private communications) KEKB: 30 out of 900 BPMs are regularly in “inconsistent” state.

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Model dependence of the correction algorithm The errors in quad strengths, local energy is as high as 1% at SLC (sometimes 10% at KEK), while the field measurements are always better than 0.1%. we have two correction algorithms (DFS and simple moversteering). They are both model-dependent, but they are also both nulling techniques which permit iteration to improve their solutions: in the caseof DFS you expect to reach a point at which when you turn RF stations off the orbit does not change; in the case of simple mover steering you continue to move the magnets until the BPM orbit matches what you want(typically, move them until measured orbit = gold orbit). This means that if the errors are larger than expected, they will still converge but mayrequire more iterations. - PT (private comm.)

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More issues Multibunch effects: bunch-to-bunch difference/jitter in –initial orbit/emittance –energy/phase –bunch length/energy spread –Residual effect by the remaining long range wake Robustness of the structure BPM –BPM center = center of the short range wake? –Effects under the high power environment? My impression: Nothing fatal has been found for the tuning of the X-band linac, but it still needs more attention than the cold machine.

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