MD2490: Measurement of the TMCI Threshold at Flat-Top D.Amorim, S.Antipov, S.Arsenyev, L.Barraud, N.Biancacci, X.Buffat, L.Carver, T.Levens, A.Mereghetti, E.Métral, D.Valuch, Many thanks to LHC and Injectors OP 2017-09-26 LSWG, preliminary results of MD2490

Goal of the MD and execution Goal: 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 possible discrepancies between B1 and B2 Measurement repeated for different collimator settings to get more data MD overview (two ramps, fills 6210 and 6212) First ramp with 3 bunches per beam: 0.6·1011, 1.1·1011 and 1.3·1011 Maximum octupoles (564A), chromaticity reduced to 𝑄′≃5 Tune measurement with series of ADT excitation Secondary and primary collimators closed in steps of 0.5𝜎 to increase the machine impedance Repeat the ADT excitation At the end of the fill, the damper was switched off: medium and high intensity bunches became unstable Second ramp with 2 bunches per beam: 0.8·1011 and 1.9·1011 Repeat the measurement procedure applied during the first ramp At the end of the second fill, an impedance measurement of the TCSPM on B2 was programmed Dumped on a quench, the TCSPM measurement was done at the end of MD2191 on Sunday night 2017-09-26 LSWG, preliminary results of MD2490

Tune shift vs. intensity Clear tune shifts observed From the first fill: the tune-shift is linear with bunch intensity as expected Different tune-shifts are observed for the different collimators settings From the second fill: large tune-shifts are observed for the high intensity bunch (1.9·1011) For this bunch the tune-shifts are of the order of 1 4 𝑄 𝑠 to 1 2 𝑄 𝑠 (0.5·10-3 to 1·10-3) Mode shifts (in units of 𝑄 𝑠 ) Fill 6210 Fill 6212 Single bunch intensity / 1011 ppb Remark: spread on the values is of the order of 5·10-5 2017-09-26 LSWG, preliminary results of MD2490

LSWG, preliminary results of MD2490 Conclusion First results show a good agreement between the impedance model prediction and the measurements for B1H with nominal collimator settings More analysis to be performed: B2, V plane and tighter collimator gaps With nominal collimators settings, the TMCI threshold seems far enough (more than 3·1011 ppb) The effect of the damper needs to be further investigated Despite the octupoles at full current, the high intensity bunches became unstable when the damper gain was reduced/removed 2017-09-26 LSWG, preliminary results of MD2490

LSWG, preliminary results of MD2490 Backup 2017-09-26 LSWG, preliminary results of MD2490

Tune-shifts for B1H B1H tunes for the first fill TCP7 TCSG7 Bunch 1 (0.6·1011) Bunch 2 (1.1·1011) Bunch 3 (1.3·1011) 5 6.5 0.26745 0.26720 0.26710 6 0.26726 0.26700 0.26690 4.5 0.26718 0.26680 B1H tunes for the first fill Spread on the values is of the order of 5·10-5 TCP7 TCSG7 Bunch 1 (0.8·1011) Bunch 2 (1.9·1011) 5 20 0.26773 0.26745 6.5 0.26751 0.26667 6 0.26738 0.26581 B1H tunes for the second fill Spread on the values is of the order of 3·10-5 2017-09-26 LSWG, preliminary results of MD2490

First analysis of B1H instabilities during the first fill During the first ramp the high intensity bunch became unstable in B1H when reducing the damper gain by half (from ~60turns to ~120turns) Octupoles were at maximum current (564A) and chromaticity was 𝑄′≃5 The instability observed had a rise-time of ~6s Simulations predict that ~100A of octupoles should stabilize a single bunch of 1.3·1011 Simulations predict a rise time of ~7s without octupoles At the end of the fill, the damper was switched off: medium and high intensity bunches became unstable Octupoles were still at maximum current (564A) and chromaticity was 𝑄′≃5 The instability observed had a rise-time of ~0.6s Simulations predict that ~280A of octupoles should stabilize a single bunch of 1.3·1011 Simulations predict a rise time of ~0.9s without octupoles A high damper gain seems necessary to stabilize the beam despite the octupoles being at maximum current 2017-09-26 LSWG, preliminary results of MD2490

LSWG, preliminary results of MD2490 Observation of TMCI At low chromaticities, Transverse Mode Coupling Instability could become an intensity limitation Measurement of the TMCI threshold was recommended by CMAC11 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 Measurement plan 2 bunches per beam of different intensities (ex: 0.8 1011 and 2.0 1011), stay below 3 1011 total intensity Ramp with increased damper and octupoles Reduce chromaticity (𝑄′≳0) Reduce the damper gain to have a clear TMCI measurement Close TCP7 and TCSG7 in steps of 0.5𝜎 until the high intensity bunch become unstable The measurement could be performed at the end of the tune shift vs. intensity measurement 2017-09-26 LSWG, preliminary results of MD2490

LSWG, preliminary results of MD2490 Destabilizing effect of the transverse damper Nominal collimator settings, 100 turns damper Intensity region where mode coupling could induce an instability 2017-09-26 LSWG, preliminary results of MD2490

LSWG, preliminary results of MD2490 Destabilizing effect of the transverse damper TCP7 at 5𝜎, TCSG7 at 6𝜎, 100 turns damper Intensity region where mode coupling could induce an instability 2017-09-26 LSWG, preliminary results of MD2490

LSWG, preliminary results of MD2490 Destabilizing effect of the transverse damper TCP7 at 4.5𝜎, TCSG7 at 5.5𝜎, 100 turns damper Intensity region where mode coupling could induce an instability 2017-09-26 LSWG, preliminary results of MD2490

LSWG, preliminary results of MD2490 Destabilizing effect of the transverse damper TCP7 at 4𝜎, TCSG7 at 5𝜎, 100 turns damper Intensity region where mode coupling could induce an instability 2017-09-26 LSWG, preliminary results of MD2490

Lower TMCI threshold with tighter collimator settings Tighter collimator settings in IR7, no damper Intensity threshold for TMCI 2017-09-26 LSWG, preliminary results of MD2490

Lower TMCI threshold with tighter collimator settings Tighter collimator settings in IR7, no damper Simulated Head-Tail monitor signal Intensity threshold for TMCI 2017-09-26 LSWG, preliminary results of MD2490