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Instabilities during and at the end of the betatron squeeze

Instabilities during and at the end of the betatron squeeze. X. Buffat , L. R. Carver, R. De Maria, K. Li, E. H . Maclean, E. Métral , M. Schenk. 2 nd Internal LHC Instability Review 29. November 2016. Acknowledgements Many colleagues across ABP, OP. Remarks and outline.

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Instabilities during and at the end of the betatron squeeze

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  1. Instabilities during and at the end of the betatron squeeze X. Buffat, L. R. Carver, R. De Maria, K. Li, E. H. Maclean, E. Métral, M. Schenk 2nd Internal LHC Instability Review 29. November 2016 Acknowledgements Many colleagues across ABP, OP

  2. Remarks and outline Questions to be addressed General remarks • Betatron squeeze is a very dynamical part of the machine cycle (optics). • Chromaticity and linear coupling are the main relevant parameters for collective instabilities during the squeeze. Questions to be addressed • Did we observe instabilities during LHC Run II? Can we explain them? • Squeeze-instability in B2 V (2015) • Instabilities linked to linear coupling (2016) • Why is the beam more stable than predicted at end of squeeze (EoS) in 2016 (β* = 40 cm)? • Comparison with situation in 2015 at EoS (β* = 80 cm)Effect of Q" and other non-linearitieson beam stability • Do we have margin? Future studies? M. Schenk et al.

  3. (I) Squeeze-instability in B2 V (2015)

  4. (I) Squeeze-instability in B2 V (2015) • Emittance blow-up in beam 2 (V) routinely observed at β* ≈ 9 m. • About 30 bunches of the first batch are affected. • Cure: increase Q’ from 10 to 15 units (for 25 ns beam). • Likely cause of the instability is the strong variation of Q’ along the squeeze [2], combined with e-cloud effects (Q’ dependence, blow-up pattern). • Instability seen with a similar signature for BCMS beam even at Q’ = 15. • Not observed in 2016 (Q’ = 15), and not fully understood – possible reasons: BCMS ‘only’ 2 x 48 b. per batch, different ADT settings, scrubbing, … ? 25ns beam – chromaticity at 10 units 7 units BCMS beam – chromaticity at 15 units M. Schenk et al.

  5. (II) Instabilities linked to linear coupling (2016)

  6. (II) Instabilities linked to linear coupling (2016) Overview • Linear coupling can lead to loss of Landau damping (see Lee’s first talk) • Observations during squeeze in 2016: • Single bunch, 16.04.16 (see backup) - ADT on, Q’ ≈ 9 / 9, LOF = 285 A (4 times higher than required without coupling). - Increase in BBQ |C-| for β* < 300 cm, instability in B2 (H) & emittance blow-up. • Full beams (2076 b.), 18.07.16(see backup) - Similar signature in BBQ |C-| (at β* ≈ 120 cm). • Observations before and after coupling correction, 25 & 26.09.16Part I (before correction): - 600 b., ADT on, Q’ ≈ 15 / 15, LOF = 470 A. - Again similar signature in BBQ |C-| during squeeze (β* ≤ 45 cm). - Instabilities & emittance blow-up in B1 (H and V). Part II: - Coupling measurements (AC dipole) and few corrections of |C-| applied by OMC. - BBQ |C-| can be misleading as it measures local coupling. Part III (after correction):- Following fills with resp. 1200 b. and 2220 b. did not show emittance blow-up anymore during squeeze. M. Schenk et al.

  7. Instabilities linked to linear coupling Beam 1 Beam 2 Before coupling correction (25. & 26.09.16) Fill 5332 (600 b.) • Losses and emittance blow-up in beam 1 right after squeeze. • Similar picture from BBQ, showing activity in H & V. • Increase in BBQ |C-| for β* ≤ 45 cm. M. Schenk et al.

  8. Instabilities linked to linear coupling Beam 1 Beam 2 Beam 1 Beam 2 After coupling correction (25. & 26.09.16) Fill 5338 (1200 b.) Fill 5339 (2220 b.) • Fills with 1200 b. and 2220 b. resp. do not show any additional emittance blow-up during squeeze anymore. M. Schenk et al.

  9. (III) Why is the beam more stableat EoS in 2016?

  10. (III) Why is the beam more stable at EoS in 2016? Comparison with situation in 2015 MD751 (28.08.15) • EoS (β* = 80 cm) • Q’ ≈ 11 (H/V) • Single bunch threshold is LOF≈ 80 A (norm.). • Consistent with measurements at flat top. MD1751 (02.08.16) • EoS (β* = 40 cm) • Q’ ≈ 13 / 16 (H/V) • 2076 b. nom. BCMS as well as 964 non-coll. b. stablew. LO off(no beam-beam). • Emittance blow-up in H (LOF ≈ 80 A), but no losses. Fill 4804 (16.04.16) • Flat top(β* = 300 cm) • Q’ ≈ 9 / 8 (H/V) • Single bunch threshold is LOF ≈ 63 A (norm.). • Head-tail mode (0, 2). • Consistent w. former MDs in 2015 (346, 751) and model predictions. • Two likely explanations • Non-linearities in interaction regions at EoSAdditional betatron detuning with transverse amplitude (Landau damping). Model (direct terms): < 10 % MO at β* = 80 cm, ≈ 4 times larger at β* = 40 cm. • Q’’ contribution from lattice at EoSAdditional betatron detuning with longitudinal amplitude (Landau damping) and additional chromaticity. • Q’’ along the squeeze & MD1831: Single Bunch Instabilities with Q'' and Non-Linear Corrections M. Schenk et al.

  11. (III) Why is the beam more stable at EoS in 2016? Lattice-Q’’ during squeeze (without Landau Octupoles) • Measured with pilot beams without LO (19.09.16). • Good agreement with MAD-X. • Contribution to Q’’ from lattice is negligible for β* ≥ 80 cm, but becomes significant atβ* = 40 cm. • One possible reason for better stability at EoS in 2016 compared to 2015. M. Schenk et al.

  12. (III) Why is the beam more stable at EoS in 2016? MD1831, effects of Q’’ and non-linearities • Part I: Flat top: See Lee’s talk “Instabilities at end of ramp and flat top” • Part II: End of squeeze Non-lin. corrections Corr. lattice Q’’ to zero Squeeze Beam 1 Confirm result of MD1751: Beams are stable w/o LO. Beam 2 M. Schenk et al.

  13. -14’312± 6’950 18’129± 4’945 2’549± 5’706 6’746 ± 4’452 (III) Why is the beam more stable at EoS in 2016? MD1831, effects of Q’’ and non-linearities • Part I: Flat top: See Lee’s talk “Instabilities at end of ramp and flat top” • Part II: End of squeeze Non-lin. corrections Corr. lattice Q’’ to zero Squeeze B1V B1H Beam 1 -14’312± 6’950 B2V B2H 2’549± 5’706 Beam 2 -1’263± 6’453 -2’924± 5’831 M. Schenk et al.

  14. (III) Why is the beam more stable at EoS in 2016? MD1831, effects of Q’’ and non-linearities • Part I: Flat top: See Lee’s talk “Instabilities at end of ramp and flat top” • Part II: End of squeeze Non-lin. corrections Corr. lattice Q’’ to zero Squeeze Beam 1 Beams stable at LO 0 A even with Q’’ ≈ 0, likely due to non-linearities in IRs. Beam 2 M. Schenk et al.

  15. (III) Why is the beam more stable at EoS in 2016? MD1831, effects of Q’’ and non-linearities • Part I: Flat top: See Lee’s talk “Instabilities at end of ramp and flat top” • Part II: End of squeeze Non-lin. corrections Corr. lattice Q’’ to zero Squeeze Beam 1 After correcting for b(4) non-linearities: - Instability in B2 V only at LOF ≈ 40 A. To be compared to flat top stabilitythreshold LOF ≈ 100 A. - No instability in B1, even afterreversingsigns of LO. Other non-linearities contributing to amplitude detuning are most likely explanation for stability at LOF << 100 A. Beam 2 M. Schenk et al.

  16. Do we have margin? / Future studies? • Vertical instability in beam 2 during squeeze (2015) • Cause is not fully understood, likely explanation is e-cloud (dependence on chromaticity, blow-up signature). • Cured with Q’ = 15, hence not much margin in Q’ due to the large excursions taken in Q’(V) during squeeze. • Not observed in 2016, but may return with longer batches (BCMS)? • May need further studies (e-cloud). • Instabilities linked to linear coupling (2016) • Coupling needs to be well corrected, i.e. not much margin. • If squeeze will happen at injection tunes in the future, coupling effects may be less of an issue. • Non-linearities / Q’’ • Shown in MD that control over / correction of Q’’ can be achieved. • Non-linearitiesin IRs lead to substantial stabilisation. Even after correction, single bunches remain stable at LOF ≈ 40 A (instead of expected 100 A). M. Schenk et al.

  17. References [1] L.R. Carver, Instabilities and beam induced heating in 2015, Evian 2015, 16.12.15. [2] M. Solfaroli, How precisely can we control our magnets?, HL-LHC-LARP Annual Meeting, 30.10.15. [3] E. Métral, Status of understanding of instabilities: focus on linear coupling, LMC, 05.10.16. [4] L.R. Carver et al., MD346: Summary of single bunch instability threshold measurements, CERN-ACC-NOTE-2016-0002, 08.01.16. [5] L.R. Carver et al., MD751: Train instability threshold, CERN-ACC-NOTE- 2016-0004, 08.01.16. [6] L.R. Carver et al., MD1831: Single Bunch Instabilities with Q'' and Non- Linear Corrections, in preparation, Nov. 2016. [7] E.H. Maclean, Experience with IR-nonlinear correctors, WP2 meeting, 13.09.16. M. Schenk et al.

  18. Backup

  19. Instabilities linked to linear coupling I. Dedicated single bunch measurements (16.04.16) Beam 2, fill 4803 (1 b.) • Instability developed in beam 2 (H) at about β* = 250 cm despite the high LO current of LOF = 285 A. • Head-tail mode (0, 2). Rise time τ ≈ 30 s. • At the same time, bump in BBQ |C-|. • Emittance in beam 2 (H) increased from 2.1 μm to 3.3 μm (wire scans). • 4 times higher LO threshold than for single bunch case without coupling. Beam 2 H M. Schenk et al.

  20. Another instability possibly linked to linear coupling Beam 1 Beam 2 Fill 5102, 2076 b. per beam (18.07.16) Fill 5102 (2076 b.) • ‘Full’ beams with 2076 b. each. • Emittance blow-up in B1 (V) during the squeeze at β*≈ 120 cm. • Observed drop in lifetime and 0.2 % losses in B1. • Similar characteristics like other instabilities linked to linear coupling. M. Schenk et al.

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