Nonlinear Field Quality Checks

1. Nonlinear Field Quality Checks Frank Zimmermann LHCCWG 12.07.2006 Based on presentation in Chamonix 2003 including references therein (O. Bruning, S. Fartoukh, M. Hayes, B. Jeanneret, J.-P. Koutchouk, R. Ostojic, Q. Qin, F. Schmidt, S. Weisz, and others)

2. LHC optics in a nutshell 8 arcs with 27 FODO cells each (23 regular cells, 2x2 cells for dispersion suppressor) phase advance/cell ~ 90 degrees 8 straight sections, 4 of which low-b insertions

3. 3 types of corrector circuits • spool pieces at dipole ends for b3, b4 and b5; powered differently per arc and per beam; total number of circuits 48 • lattice correctors for a2, b2, a3, b3, and b4; total number of circuits 168 • correction coils for triplet field errors a3, b3, a4, b4, & b6; total number 40 total # of independent correction circuits: 256!

4. #arc correction circuits & #elements/circuit

5. nonlinear optics tolerances numbers include considerations on dynamic aperture, tune footprint, and off-momentum measurements [O. Bruning, S. Fartoukh, LHC PR 501]

6. general nonlinear measurement scheme measure change on phase advance or tune resulting from an off-center orbit in a nonlinear field; orbit can be shifted either by closed-orbit bumps or using dispersion [study by Jean-Pierre Koutchouk for RHIC low-b insertions] orbit bump yields: dispersion bump yields: MAD design: differences are of the order of 10%! 10% variation in b3 or b5 corrector strengths can change dynamic aperture by >1s [Q. Qin, S. Weisz, LHC PN 42, 1996]

7. normal sextupole b3 Local correction (arc by arc) to within 50% needed [M. Hayes, LHC PR 522] Proposed procedure: • pre-set spool pieces to values determined from magnet measurements • adjust Q’ to ~2 units with 2 (pairs of) families of lattice sextupoles • check local correction with local dispersion bumps or groups of 7p bumps across individual arcs, octant by octant • complementarily or additionally, measure off-momentum (chromatic) phase advance

8. local b3 correction: bumps across one arc scheme by M. Hayes [LHC PR 522] & O. Bruning [LHC PR 473] orbit with 7p bump across one arc; peak value of 3 mm tune difference vs. quality of correction DQ~0.01 for 20% mispowering of b3 spool pieces; → we can adjust b3 to within 10% (much better than 50% target)

9. local b3 correction: chromatic phase advance [Chamonix03] simulated Df for 1s kick for dp/p=10-3 and dp/p=0; 3 cases: (1) no spool piece mispowered, (2) sextupole circuit KCS45 missing (BPMs 194 to 257), (3) decapole circuit KCD45 missing we can detect missing b3 circuits, but not missing b5!

10. example: chromatic phase-advance measurement in the SPS at 26 GeV measured Df in the SPS from averaging over four 5-10 mm (2-4 s) kicks for Dp/p=5x10-3 and Dp/p=0 [R Tomas]; large discrepancies remain – at the level of the expected missing b3 signature for LHC

11. skew sextupole a3 J.-P. Koutchouk [LHC PN 113]: generation of 2nd o. chromaticity where , and , tolerance on this effect Proposed procedure: likely no correction needed; check with off-momentum closest tune approach tolerance to meet:

12. normal octupole b4 global correction to within 30% needed [M. Hayes, LHC PR 522] Proposed procedure: • leave lattice octupoles switched off • separate tunes (to reduce contribution from a3) & minimize Q’’ • verify detuning with amplitude If Qy’’ and dQx/dey are zeroed, dQx/dex, dQy/dey and Qx’’ are corrected to about 10% uncor.: DQ~10-3 at 3s, cor.: DQ<10-4 at 3s

13. normal decapole b5 Local correction (arc by arc) to within 50% needed [M. Hayes, LHC PR 522] Proposed procedure: • first minimize Q’’’ (global) • measure off-momentum detuning with amplitude; without any correction expect DQ~10-3 at 3s and d=10-3 (8x smaller for single arc) • complementarily, measure chromatic phase advance (effect could be difficult to detect) • measure resonant driving terms (option) • varying kicks to obtain “frequency map’’ (option)

14. nonlinear chromaticity simulated nonlinear chromaticities in the LHC; optics with all correctors active, and with missing b3 or b5 spool piece circuit though fitted 3rd order coefficient changes, also here identification of missing b5 circuit looks challenging!

15. SPS as testbed for off-momentum studies? SPS at 26 GeV as nonlinear as the LHC (3.5 times LHC tolerance in Q’’’)

16. conclusions • huge number of LHC corrector circuits; only spool pieces considered • if effect of corrector important, we can adjust it with beam-based methods • several alternative diagnostics schemes available for each type of error • methods function well in simulations including BPM noise (except b5) • reality may be different (SPS example) • verify as many procedures as possible in the SPS (or PS,…)