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Production of bunch doubletsfor scrubbing of the LHC
J. Esteban Muller (simulations), E. Shaposhnikova3 December 2013 LBOC
Thanks to H. Bartosik, T. Bohl, G. Iadarola, W. Hofle, G. Rumolo
For LLRF and ADT compatibility see talks of P. Baudrenghien and G. Kotzian at the last LBOC
Method of bunch splitting into two bunchlets
1. Reduce voltage to minimum possible amplitude (limited by intensity effects) to reduce longitudinal emittance blow-up and uncaptured beam.
2. Jump RF phase by 180 deg (to unstable phase).3. Wait till uncaptured particle will move ½ of RF period and increase
sharply voltage to an amplitude sufficient to recapture particles lost from the bunch center.
Low voltage Jump to unstable phase Wait Voltage increase Buckets are filled
Different options for bunch splitting
1) Bunch splitting in the LHC => 2.5 ns doublets2) Bunch splitting in the SPS => 5.0 ns doublets.
The latter can be done ata) injectionb) flat bottomc) flat topd) intermediate flat portion (~200 GeV)
Main issues for all options
• Acceleration in the SPS– high intensity required for efficient scrubbing
(>1.5x1011/bunch, 25 ns spacing) 200 MHz beam loading => slow cycle, but still limited
– Beam control of this beam structure• Splitting– longitudinal emittance blow-up – particle losses – beam stability: unstable phase, 800 MHz?– no HW available (even for the tests)
Splitting in LHC: preliminary results• Voltage program:
– SPS before extraction: 2 MV– Injection: 3 MV 6 MV
• Emittance blow-up: 0.5-> 1.02 eVs• Particle loss
– During splitting: ~1%– But due to subsequent voltage
reduction during one following injection: ~15%
– 10% total loss measured in SPS at beginning of ramp (1 dip)
=> Voltage program could be optimized to minimize particle loss:- 1st injection: 3 MV 4 MV (~7% losses), 2nd injection: 4MV 5 MV, …=> LLRF probably can reduce voltage only for injecting beam…
Splitting in LHC
Advantages• Issues with doublets only in
LHC, at the last stage• Less problems with
injection into LHC• Sufficient RF voltage/bucket
in LHC
Disadvantages• Doublet spacing 2.5 ns is
much less interesting for scrubbing (Giovanni)
• Losses in LHC during splitting
• Losses during RF manipulations with following SPS injections
Splitting in SPS: at injection• Voltage program (same as in
measurements) :– at injection:
1 MV 3 MV
• Emittance from 0.56 eVs to 0.31 eVs in each bunchlet
• Particle loss during splitting ~1% + 6% to satellites
• More losses should be expected during subsequent injections (3 dips more) due to full bucket
• For splitting at the end of flat bottom results will not be so good
Splitting in SPS: at injection
Advantages• Long bunches from PS ->
very small emittance blow-up
• Doesn’t require RF phase jump (new HW)
• Already tested in the SPS• Losses at lower energy• Doesn’t need additional flat
top in magnetic cycle
Disadvantages• E-cloud in the SPS • Losses during voltage
reduction during the subsequent PS injections(was not observed?)
Splitting in SPS: flat top• Emittance blow-up:
– from 0.5 eVs to 1.76 eVs• Particle loss
– During splitting: ~5%– LHC injection: ~30%
• Voltage program:– During splitting: 2 MV 4MV– Extraction at 7 MV– LHC injection: 6 MV
• Total time needed ~ 0.1 s• LHC buckets filled with this intensity
distribution (3 satellites):3.5% - 43% - 7% - 43% -3.5%
• Triplet in case of lower voltage at extraction in the SPS and less losses
• Very small final intensity
Splitting in SPS: flat top
Advantages• Issues with doublets only on
flat top, at the last stage• Less problems with
injection into LHC• Sufficient RF voltage/bucket
in LHC
Disadvantages• Losses at high energy in SPS• Extraction of uncaptured
beam to LHC• Minimum voltage during
splitting limited by beam loading => large emittance blow-up
• Full SPS bucket after splitting => long bunches– Losses at injection into LHC– Satellite bunches in the LHC=> Less favorable scenario
Splitting in SPS: flat portion• Voltage program:
– During splitting (200 GeV):1 MV 2MV
– Extraction to LHC: 7 MV– LHC injection: 6 MV
• Emittance blow-up:– from 0.35 eVs to 0.82 eVs
• Particle losses– During splitting ~7 %– LHC injection < 2%
• No satellites in LHC (&SPS)• Total time needed ~ 0.1s +
even slower ramp
Power limitation during cycle
• Similar limitations for
0.4 eVs and 0.8 eVs bunches above 300 GeV
• Intensity limited to 1.5x1011/bunch
Splitting in SPS: flat portion
Advantages• Emittance blow-up required
for beam stability > 200 GeV• More bucket area available
in the 2nd part of the ramp• Uncaptured beam is not
injected into LHC
Disadvantages• Acceleration of the large
emittance => beam loading limitation to the total intensity
• More complicated magnetic cycle with additional flat portion
• Losses at relatively high energy in SPS
=> Better scenario than splitting at the flat top or LHC injection
Conclusion• Main limitations are expected to be from high intensity (beam loading)
and beam losses (full bucket after splitting)• Splitting at SPS injection seems to be the most feasible scenario: minimum
emittance blow-up. Can be used after efficient scrubbing of the SPS (1-2 weeks)?
• Intermediate flat portion seems to be the 2nd interesting option (if splitting at SPS injection is rejected due to e-cloud)
• Need more detailed simulations (no intensity effects included)• Need new hardware • Need to be tested in MDs• Acceleration of high intensity 25 ns beam in the SPS will be itself very
challenging task