MD 2490 Measurement of the TMCI Threshold at

MD 2490: Measurement of the TMCI Threshold at Flat-Top D. Amorim, S. Antipov, N. Biancacci, X. Buffat, L. Carver, E. Métral, A. Mereghetti, B. Salvant, D. Valuch With inputs from R. Bruce, R. Tomas Garcia 2017 -07 -11 LSWG, presentation of MD 2490 1

Tune shift vs. intensity High priority • Check the LHC impedance model for different collimator settings – Reaching the TMCI might be difficult, and the actual intensity threshold might be lower than simulated – The TMCI threshold can be extrapolated from a tune shift vs. intensity measurement – Measurement with both beams to investigate discrepancies between B 1 and B 2 Single bunch intensity / 1011 ppb – Measurement repeated for different collimator settings to get more data Test on 20/05/2017 (with Q’=15) – Would allow to better understand instabilities observed in B 1 H 2017 -07 -11 LSWG, presentation of MD 2490 2

Tune shift vs. intensity High priority Check the LHC impedance model for different collimator settings – Reaching the TMCI might be difficult, and the actual intensity threshold might be lower than simulated – The TMCI threshold can be extrapolated from a tune shift vs. intensity measurement – Measurement with both beams to investigate discrepancies between B 1 and B 2 – Measurement repeated for different collimator settings to get more data – Would allow to better understand instabilities observed in B 1 H • Single bunch intensity / 1011 ppb Possible intensity points 2017 -07 -11 LSWG, presentation of MD 2490 3

MD 2479: Transverse Beam Stability in the HL-LHC era D. Amorim, S. Antipov, N. Biancacci, X. Buffat, L. Carver, E. Métral, A. Mereghetti, B. Salvant, D. Valuch With inputs from R. Bruce, R. Tomas Garcia 2017 -07 -11 LSWG, presentation of MD 2479 4

Stability margin at flat-top • Reproduce the HL-LHC impedance and check the stability margins – HL-LHC impedance budget is lower than the LHC, but beam brightness is higher – Reproduce the HL-LHC impedance budget assuming Molybdenum collimators by retracting the gaps of some collimators (to be defined later) – Use a HL-LHC nominal bunch to assess the octupole current needed to stabilize the beam • Measurement plan (8 h) – 2 bunches per beam of different intensities (ex: 0. 5· 1011 and 2. 3· 1011), stay below 3· 1011 total intensity – Ramp with relaxed collimators settings and increased octupoles to keep the high intensity bunch stable – Measure emittance and chromaticity – Decrease the octupole current until the beam gets unstable – Perform the measurement on both beam and planes – Repeat the measurement in two other ramps with different chromaticity settings (ex: Q’=10 and Q’=5) 2017 -07 -11 LSWG, presentation of MD 2479 5

Backup 2017 -07 -11 MD 2490: Measurement of the TMCI Threshold at Flat-Top 6

Observation of TMCI • At low chromaticities, Transverse Mode Coupling Instability could become an intensity limitation – Measurement of the TMCI threshold was recommended by CMAC 11 – TMCI could be a limitation in the FCC-hh – At low chromaticities, mode coupling might be involved in two effects: • • With the damper on, an instability could arise for bunch intensities lower than the TMCI threshold The stability diagram theory predicting the octupole current thresholds doesn’t include mode coupling: higher octupole current might be needed for stabilization – Tune shift measurements for nominal collimators setup in 2017 place the TMCI threshold between Simulated TMCI threshold 2017 -07 -11 MD 2490: Measurement of the TMCI Threshold at Flat-Top 7

Destabilizing effect of the transverse damper Nominal collimator settings, 100 turns damper Intensity region where mode coupling could induce an instability 2017 -07 -11 MD 2490: Measurement of the TMCI Threshold at Flat-Top 8

Destabilizing effect of the transverse damper Intensity region where mode coupling could induce an instability 2017 -07 -11 MD 2490: Measurement of the TMCI Threshold at Flat-Top 9

Destabilizing effect of the transverse damper Intensity region where mode coupling could induce an instability 2017 -07 -11 MD 2490: Measurement of the TMCI Threshold at Flat-Top 10

Destabilizing effect of the transverse damper Intensity region where mode coupling could induce an instability 2017 -07 -11 MD 2490: Measurement of the TMCI Threshold at Flat-Top 11

Lower TMCI threshold with tighter collimator settings Tighter collimator settings in IR 7, no damper Intensity threshold for TMCI 2017 -07 -11 MD 2490: Measurement of the TMCI Threshold at Flat-Top 12

Lower TMCI threshold with tighter collimator settings Tighter collimator settings in IR 7, no damper Simulated Head-Tail monitor signal Intensity threshold for TMCI 2017 -07 -11 MD 2490: Measurement of the TMCI Threshold at Flat-Top 13