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Orbit effect from main dipoles in SPS at high energy

Orbit effect from main dipoles in SPS at high energy. K. Cornelis. Orbit after alignment at 6000ms (FT optics). Plan H RMS= 1.608 Plan V RMS= 1.226. The B-field in SPS dipoles. Magnetising field from windings (depends on geometry of windings ).

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Orbit effect from main dipoles in SPS at high energy

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  1. Orbit effect from main dipoles in SPS at high energy K. Cornelis

  2. Orbit after alignment at 6000ms (FT optics) • Plan H RMS= 1.608 • Plan V RMS= 1.226 Stephane Cettour Cave

  3. The B-field in SPS dipoles Magnetising field from windings (depends on geometry of windings) Field due to polarisation in the yoke (depends on geometry of the yoke, composition of the yoke, and field) Yoke and windings geometry such that best field quality at 300 GeV.

  4. SPS dipoles 300 GeV

  5. Static fields of dipoles • Yoke composition as homogenous as can be (laminations of special melt). • Magnets measured and shimmed to give the correct at 1T (~220GeV). • Different places of the yoke will come to saturation at different magnetising fields, resulting in a change of geometry of the fields. In identical magnets this change should be the same but, e.g. shims have a different effect then the yoke when in saturation. • From about 2004 until about 2010, uncorrected dipoles have been installed. Recent beam based measurements with uncorrected C- coated magnets showed errors from 10 to 25 mrad

  6. Dynamic effects from induced currents in vacuum chamber dB/dt B B ~ -dB/dt x

  7. Chromaticity effect from Eddy Currents

  8. Dipole error from decaying eddy currents Radial position Time in cycle Amplitude and decay time depend on shape of vacuum chamber, thickness and specific resistance of material. Effect : 6 mrad/magnet at 26 GeV 0.35 mrad/magnet at 450 GeV If magnet differences are at the % level, the orbit kick is less than .05mrad/magnet at 450 GeV

  9. Possible sources of “local” errors Weird vacuum chambers

  10. Possible sources of “local” errors: Earth loops

  11. Earth loops around magnets can enhance the field suppression

  12. Short between spires in the coil gives a strong field supression proportional to dB/dt Normally the short circuit gets worse rapidly and the orbit becomes so bad that the beam is destroyed and the magnet has to be changed

  13. In position 3.26 a temperature dependent dB/dt kick could be observed Kick at max dB/dt/B Time after restart The problem is there since October but it does not get worse.

  14. Summary and Conclusions • In the horizontal plane, the orbit at high energy is not only due to misalignment, but results also from dipole errors. • During several years the shimming of dipoles was neglected resulting in horizontal orbit kicks of a few tens or micro radians. • The errors due to eddy currents in the vacuum chamber are negligible at 450 GeV. • Earth loops around magnets can make a bigger effect but it is difficult to know how much. We have no total picture of the state of the machine. The bad earth connections will be removed during LS1. • These errors are normally stable in time. • Short circuit between windings can give huge kicks, but it normally kills the magnet in a short time. However, it can not be excluded that very weak shorts stay stable and vary slowly in time.

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