SPS resistive-wall coupled-bunch instabilities – measurements and first simulations

N. Mounet and E. Métral - BE/ABP/ICE - SPSU meeting 17/03/2011

SPS resistive-wall coupled-bunch instabilities – measurements and first

simulations

N. Mounet and E. Métral

Acknowledgements: T. Bohl, W. Hofle, L. Jensen, G. Rumolo, B. Salvant

!! Still preliminary results !!


Introduction• Several MDs performed (15-16 Sept. 2009, 3 Nov. 2009 & 22 July

2010) to measure the transverse coupled-bunch instability rise times with the 25ns LHC beam (one or several batches). The goal is to compare with multibunch HEADTAIL simulations (new code) using the SPS impedance model.

• Idea: after injection oscillations are damped, switch off the transverse damper for typically 10 ms and observe (with the LHC-BPM) the growth of an instability. Try for several values of the chromaticity.

• Here we present results on the last of these MD, where conditions were thought to be optimum: chromaticity and bunch length were measured on the same beam the same day, and LHC-BPM was fully functional.

• Transverse dampers timing:

• Chromaticity:


Timing of the MDs

t (ms)0 (inj) t1d

t2d

-5

v1 on

v2 on

v1 off

v2 off

v1 md on

v2 md on

v1 md off

v2 md off damper (vertical here)

magnets cycle

Initial chromaticity (x or y) (at inj. energy, before magnet ramp)

Chromaticity profile after change

t (ms)t2

c t3c

t1c t4

c

Measurement between t1d

and t2d, which are between

t2c and t3

c


Foreword: uncertainties• The instability rise times measured might well include

some electron cloud contribution. A way to prevent this would be to use the CNGS beam (5ns spacing, almost full machine filled), but it is more difficult to simulate.

• The rise times could also include contribution from single-bunch instabilities, in particular at negative chromaticities.

• The horizontal chromaticity measured on the MD of 22/07/2011 (the MD analyzed in the following slides) is subject to caution: if we believe it, only strongly negative chromaticity allow single batch instability, whereas in 2009 we observed instabilities even with slightly positive chromaticity.


Parameters (MD 22/07/2011)• Qx=26.1289, Qy=26.1641.

• Bunch length (total): between 2.5 and 3ns (depending on the bunch).

• RF voltage: 3 MV.

• Supercycle: LHC_4Inj_FB10860_FT835_Ext19415_2010_v1

• 1 batch of 72 bunches, 1.2 1011 p+/bunch.

• Timing: t1d=2000ms, t2

d=2010ms (dampers), and

t1c=1970ms, t2

c=2000ms, t3c=2100ms, t4

c=2150ms (chromaticity trim).

• LHC-BPM delay set to 2016 or 2017 ms (additional delay of ~16ms is software related), 1000 turns acquired (usually).

• FBCT around the time when feedback is off:


Multibunch instabilities: horizontal

• LHC BPM vs. number of turns, for successive bunches:


Example of instabilities: horizontal

• LHC BPM along the bunch train, for successive turns:


Example of instabilities: horizontal

• LHC BPM vs. number of turns, for successive bunches :


Example of instabilities: vertical (but triggered by horizontal plane)

Example of instabilities: vertical (but triggered by horizontal plane)



Tunes along the bunch train


Quite significant tune shift (up to 0.01), smaller with higher chromaticity

Instability rise times vs. chromaticity

• Rise time measurements (on several sets of data) compared to first Headtail multibunch simulations (with beam pipe resistive-wall as single source of impedance):


→ Order of magnitude “ok” but dependence in chromaticity totally out.

→ Problem of chromaticity offset in measurement ? Or trapped modes change completely the impedance picture ? Or something else (e.g. bug in code, e-cloud or single-bunch in meas.) ?

Instability rise times vs. chromaticity in horizontal• Rise time measurements (on several sets of data) compared to first Headtail

multibunch simulations, along the bunch train:


→ Measurements show a clear decrease of rise time when going toward the tail of the train.

→ On the contrary, simulations give flat rise times (first bunch as fast unstable as last bunch).


Preliminary conclusions for horizontal instability

The instability seems to be coupled-bunch.

Positive tune shift is seen along the bunch train → could it be due to electron cloud ? Or to the negative quadrupolar impedance in x ?

First Headtail multibunch simulations with the beam pipe resistive-wall impedance give correct order of magnitude for the rise times but wrong chromaticity dependence and wrong behaviour along the bunch train.

Wrong offset in chromaticity in the measurements ?

• FBCT around the time when feedback is off:


Multibunch instabilities: vertical

• LHC BPM vs. number of turns, for successive bunches:


Example of instabilities: vertical


Example of instabilities: vertical


Vertical tune along the bunch train


Tune shift along the train is not clear (mainly noise, poor resolution)

Instability rise times vs. chromaticity in vertical

• Rise time measurements (on several sets of data) compared to first Headtail multibunch simulations (with beam pipe resistive-wall as single source of impedance):


→ Order of magnitude “ok” but dependence in chromaticity totally out.

→ Trapped modes change completely the impedance picture ? Or something else (e.g. bug in code, e-cloud or single-bunch in meas.) ?

Instability rise times vs. chromaticity in vertical• Rise time measurements (on several sets of data) compared to first Headtail

multibunch simulations, along the bunch train:


→ Measurements show a clear decrease of rise time when going toward the tail of the train.

→ Not so clear in simulations.


Preliminary conclusions for vertical instability

The instability seems to be coupled-bunch.

No clear positive tune shift is seen along the bunch train.

First Headtail multibunch simulations with the beam pipe resistive-wall impedance give correct order of magnitude for the rise times, but wrong chromaticity dependence and maybe wrong behaviour along the bunch train.

in vertical it is less likely that there is a problem of offset in chromaticity in the measurements (comparing to other MDs)…


Future actions

Analyze in detail the rest of the data (same MD with three bunch trains + the two MDs of 2009).

Benchmark the new multibunch Headtail code with analytic formulae (e.g. Sacherer or Laclare) in simple cases, with dipolar impedances only.

Compare the role of single-bunch vs. coupled-bunch instability with the code, in particular for negative chromaticity.

Add other contributions to the impedance model (trapped modes, C magnets) and take into account all the “single-bunch oriented” impedance model in ZBASE (thanks to B. Salvant).

Maybe do other MDs (e.g. with CNGS beam).

SPS resistive-wall coupled-bunch instabilities – measurements and first simulations

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