Transcript News on TMCI in the SPS

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 frequency
N = number of bunches
x,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 limitations
SPS 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
Basically blind
 Headtail monitor was not working (it is working now thanks to hard reboot) on the
 issues to read the data from the exponential couplers linked to OASIS
transverse shapes
 difficulties to match the bunch to the bucket at injection
 need for a better control of the bunch parameters before and after injection.
MD Parameters (July 13th)
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Capture in RF voltage of 1.6 to 1.8 MV
injected bunch length =4 ns
Corrected by Giovanni on 15th
injected longitudinal emittance ~ 0.38 eVs
Beam lost during transverse emittance measurements in the PS…
LOF and LOD (octupoles) set to 0
MD Parameters (July 15th)
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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
Summary of bunch intensity at PS ejection
and in the SPS (10 ms after injection and end flat bottom)
SPS BCT (injection and end FB)
Tune correction
4.0E+11
Decreasing y
on flat bottom
Decreasing y
on flat bottom
SPS Injection
SPS dump
PS ejection
3.5E+11
3.0E+11
Nb
2.5E+11
2.0E+11
1.5E+11
1.0E+11
5.0E+10
BTV in TT10
0.0E+00
13:26
-5.0E+10
13:55
LHCINDIV
14:24
14:52
MDPS
15:21
15:50
Intensity in the SPS
With BTV in TT10
Without 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
Horizontal
Vertical
H~3 mm.mrad (norm, 1sig)
V~4 mm.mrad (norm, 1sig)
With BTV in TT10
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)
4.0E+11
0.3
3.5E+11
0.25
3.0E+11
Nb
2.0E+11
0.15
1.5E+11
1.0E+11
5.0E+10
0.0E+00
14:03
0.1
Nb @ PS ejection
Nb @ SPS Injection
Nb @ SPS dump
Q'x@inj
Q'x@1s
14:18
14:32
0.05
0
14:47
15:01
15:15
15:30
Q'x
0.2
2.5E+11
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 summary
we 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?
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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