Production of bunch doublets for scrubbing of the LHC J. Esteban Muller (simulations), E. Shaposhnikova 3 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 increaseBuckets are filled

Different options for bunch splitting 1)Bunch splitting in the LHC => 2.5 ns doublets 2)Bunch splitting in the SPS => 5.0 ns doublets. The latter can be done at a)injection b)flat bottom c)flat top d)intermediate flat portion (~200 GeV)

Main issues for all options Acceleration in the SPS – high intensity required for efficient scrubbing (>1.5x10 11 /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: -1 st injection: 3 MV  4 MV (~7% losses), 2 nd 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 1.5x10 11 /bunch

Splitting in SPS: flat portion Advantages Emittance blow-up required for beam stability > 200 GeV More bucket area available in the 2 nd 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