1. 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
LSWG, preliminary results of MD2490

2. 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
LSWG, preliminary results of MD2490

3. 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 𝑄 𝑠 to 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
LSWG, preliminary results of MD2490

4. 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
LSWG, preliminary results of MD2490

5. LSWG, preliminary results of MD2490
Backup
LSWG, preliminary results of MD2490

6. 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 6
4.5
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
6.5 6
B1H tunes for the second fill
Spread on the values is of the order of 3·10-5
LSWG, preliminary results of MD2490

7. 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
LSWG, preliminary results of MD2490

8. 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: and ), stay below 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
LSWG, preliminary results of MD2490

9. 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
LSWG, preliminary results of MD2490

10. 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
LSWG, preliminary results of MD2490

11. 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
LSWG, preliminary results of MD2490

12. 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
LSWG, preliminary results of MD2490

13. Lower TMCI threshold with tighter collimator settings
Tighter collimator settings in IR7, no damper
Intensity threshold for TMCI
LSWG, preliminary results of MD2490

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