News on TMCI in the SPS:Injecting high intensity bunches

Benoit for the MD team:

T. Bohl, K. Cornelis, H. Damerau, W. Hofle, E. Metral, G. Rumolo, B. Salvant, PSB, PS and SPS OP teams,

A. Bullit, Rafaello (SLAC), T. Bogey, J. Albertone, C. Boccard, L. Jensen (BI)

Contex: LHC luminosity• Key parameter to assess the performance of a collider: luminosity

Nb(i) = number of protons per bunch in beam i

frev = revolution frequencyN = number of bunchesx,y = transverse beam sizes

High luminosity means high intensity and low transverse beam sizes

• These dense bunches also need to be produced and accelerated in all the injectors.

The performance of the injectors also affects the LHC luminosity

Context: SPS limitationsSPS is able to produce the nominal beam for LHC.

What about the ultimate beam? What are the bottlenecks? Currently, 3 potential bottlenecks are studied in detail:

• Electron cloud

• Available RF power for acceleration

• Transverse single bunch instability (TMCI)

Aim of this study: What is the maximum single bunch intensity we can store in the SPS without losses?

Very preliminary MD results.

Need for discussion and validation!!!!

MD summary• July 13th : 2 hours of measurements (recurring MPS problem)

we tried to inject as much intensity (single bunch) as possible in the SPS results: - “strong” injection losses (TMCI?) - 2.5 1011 p/b rather stable after injection losses.

- intensity after losses pushed to 3.2 1011 p/b if emittance is blown up with a BTV in transfer line- lowering vertical chromaticity after injection leads to strong losses

• July 15th : 3 hours of measurements we tried to observe the threshold at which the injection losses disappear

- injecting 1.5 1011 p/b lead to injection losses (even when increasing chromaticity) due to very small transverse emittance?

• Issues

most limiting issue is the need to disconnect the BPMs (MOPOS) in sextant 1, 2 and 5 for high intensity single bunches. MD is not really parallel any more… very small transverse emittances Headtail monitor was not working (it is working now thanks to hard reboot) issues to read the data from the exponential couplers linked to OASIS difficulties to match the bunch to the bucket at injection need for a better control of the bunch parameters before and after injection.

Basically blind on the transverse shapes

MD Parameters (July 13th)• Capture in RF voltage of 1.6 to 1.8 MV• injected bunch length =4 ns• injected longitudinal emittance ~ 0.38 eVs• Beam lost during transverse emittance measurements in the PS…• LOF and LOD (octupoles) set to 0

Corrected by Giovanni on 15th

MD Parameters (July 15th)

• Capture in RF voltage of 1.6 to 1.8 MV• injected bunch length =3.6 ns• injected longitudinal emittance ~ 0.32 eVs• Transverse emittances (at 2.8 1011 p/b) = 1.3 mm.mrad (normalized 1 sigma)• Played with LOD and LOF to try and stabilize the beam

SPS BCT (injection and end FB)

-5.0E+10

0.0E+00

5.0E+10

1.0E+11

1.5E+11

2.0E+11

2.5E+11

3.0E+11

3.5E+11

4.0E+11

13:26 13:55 14:24 14:52 15:21 15:50

Nb

SPS Injection

SPS dump

PS ejection

LHCINDIV MDPS

Summary of bunch intensity at PS ejection and in the SPS (10 ms after injection and end flat bottom)

Tune correctionDecreasing y on flat bottom

BTV in TT10

Decreasing y on flat bottom

Intensity in the SPS

Without BTV in TT10With BTV in TT10

Losses at injection ~ 20% to 30%

Losses at injection ~ 5% to 10%

Transverse emittance on flat bottom (after losses)

Without BTV in TT10 With BTV in TT10

Horizontal Vertical

H~3 mm.mrad (norm, 1sig)V~4 mm.mrad (norm, 1sig)

Horizontal Vertical

H~6 mm.mrad (norm, 1sig)V~6 mm.mrad (norm, 1sig)

Decreasing chromaticity on flat bottom

Decrease vertical chromaticity from 0.28 to 0.12 fast vertical instability with heavy losses

PS ejection BCT and SPS BCT (injection and end FB)

0.0E+00

5.0E+10

1.0E+11

1.5E+11

2.0E+11

2.5E+11

3.0E+11

3.5E+11

4.0E+11

14:03 14:18 14:32 14:47 15:01 15:15 15:30

Nb

0

0.05

0.1

0.15

0.2

0.25

0.3

Q'x

Nb @ PS ejection

Nb @ SPS Injection

Nb @ SPS dump

Q'x@inj

Q'x@1s

Trying to find the stability threshold…

Not clear yet…With y ~ 0.3

HEADTAIL simulations with the current impedance model would predict stable bunch at 2 1011 p/b

Very preliminary summarywe tried to inject as much intensity (single bunch) as possible in the

SPS - “strong” injection losses (TMCI?) - 2.5 1011 p/b rather stable after injection losses.

- intensity after losses pushed to 3.2 1011 p/b if emittance is blown up with a BTV in transfer line

- lowering vertical chromaticity after injection leads to strong losses

we tried to observe the threshold at which the injection losses disappear

- injecting 1.5 1011 p/b lead to injection losses (even when increasing chromaticity)

due to very small transverse emittance?

Issues for next week• Need to disconnect the MOPOS

only possible if the LHC is not running at injection energy.

need BI manpower (disconnection is 20 minutes per sextant).

• We need transverse bunch shapes measurements (for instability pattern and fast losses)

– Headtail monitor should work now– Directional couplers

• What can we do to increase the transverse emittance before injection on the SPS?

• Large longitudinal oscillations at injection in the SPS. What about transverse? (we are blind since there is no MOPOS…)

• Need to control the bunch parameters