Presentation on theme: "K.A. Brown (M. Sivertz) Collider Accelerator Department, BNL"— Presentation transcript:
1K.A. Brown (M. Sivertz) Collider Accelerator Department, BNL Development of micro-bunching beams and application to rare K decay experimentsK.A. Brown (M. Sivertz) Collider Accelerator Department, BNL(M. Tomizawa)JPARC Project, KEK
2Outline Physics Motivations Parameters from KOPIO Micro-bunching at BNL AGSMicro-bunching at J-PARCRe-bucketing at RHIC (bunch compression)Re-bucketing at J-PARCSummary
3Separating signal from background Microbunching is crucial to themeasurement of the kaon momentum which allows for the kinematic suppression of backgrounds by transforming to the kaon rest frame.Make cuts on the pion energy and the difference in photon energies in the kaon rest frame.
4Physics Motivation: Microbunch Separation Microbunch separation determined bythe length of time required to clear out kaonsfrom the previous microbunch.Difference in time-of-flight between high momentum and low momentum kaons is ~30 nsec=> 40nsec (25MHz)Signal efficiency drops when neighboring microbunch too close
5Physics Motivation: Microbunch Width Fully reconstructs the neutral Kaon inKL p0 n n measuring the Kaon momentum by time-of-flight.Start when proton beam hits the targetEnd at thedecay time and decay pointreconstructed from thetwo photons.Timing uncertainty due to microbunch width should not dominate the measurement of the kaon momentum; requires RMS width < 300ps(of course the optimal width depends on the detector geometry)
6Physics Motivation: Interbunch Extinction Effects of Interbunch KaonsKL p0 p0 events, shifted in timeKinematic cuts are usedto reduce backgrounddue to KL p0 p0When KL does not comefrom the microbunch,incorrect kinematic fitdoes not allow for goodrejection. Panels showeffect of KL productionat varying interbunch times.P*(p)E*(g1) – E*(g2)Signal and p0 p0
7Physics Motivation: Intensity KOPIO had planned to study the very rare decayKL p0 n n which has a BR = 3x10-11.The goal was to collect ~100 events with a S/B > 2/1.This requires more than 1.5 x1014 decays and for cleanliness we wanted ~0.5 decay/spill in the decay region.Optimization of duty factor and running time indicates 100Tp/spill. Total integrated # protons to achieve the experiment goals was ~ 7 – 9 x Final value depended on inefficiencies.
8KOPIO Beam requirements Spill length with 100TP of ~3 seconds.Number of KL decays per microbunch: 3.57Yields ~0.5 KL decay in 10 < Z < 14 metersBoth are a flat optimumVariation of intensity between microbunches only impacts total run time (duty factor)Microbunch rms < 300psec (goal 200psec)Number of protons outside microbunches< 10-3 inside microbunches ( +/- 2 nsec)
9Time Structure of Beam AGS Cycle Spill Structure 0 sec 2.3 sec 5.3 sec Injection andaccelerationAGS CycleSpill Structure0 sec2.3 sec5.3 secExtractionStart of Cycle40 ns betweenmicrobunches200 ps RMSMicrobunch widthEnd of Spill and CyclembunchspacingstructureStart of SpillProton intensityTime
10BNL AGS: Micro-bunched slow extraction Empty buckets generate energy modulation of debunched beamHigher cavity voltage and/or smaller DP/P shorter bunchesNeed ~200 ps bunches every 40 nsFrequency200 ps40 nsTimeExtraction resonanceDebunched beam
11Microbunching the AGS Beam Extractedparticles25 MHz fundamental+ 100 MHz harmonicSimulation of the extraction process for MHz RF cavities.Impose a high frequency longitudinal oscillation on the beam.Slowly bring the beam into resonance (82/3) with RF.Beam is forced through the narrow phase region between the RF buckets.Adding the 100MHz harmonic cavity sharpens up the phase region in resonance.ExtractionRegion
12Test Beam Results: Microbunch Width 93 MHz cavity at 22 kVgave s = 217 ps.Microbunch time, in nsSimulation93 MHz cavity at 22 kVgave s = 240 ps.Microbunch time, in nsData
13Test Beam Results: Interbunch Extinction 4.5 MHz cavity at 130 kVgave e = 8 (+/- 6) x 10-6Microbunch time, in nsDataInterbuncheventsMicrobunch time, in nsSimulation4.5 MHz cavity at 130 kVgave e = 1.7 (+/- 0.9) x 10-3.Interbunchevents
1550GeV Synchrotron (Main Ring) •Imaginary Transition g • High Gradient Magnetic Alloy loaded RF cavity• Small Loss Slow Extraction Scheme• Both Side Fast Extraction for Neutrino and Abort line • hands on maintenance scheme for small radiation exposure•Injection Energy 3GeV•Output Energy 30GeV (slow)40GeV (fast)50GeV (Phase II) •Circumference m•Beam Power 0.75MW (Phase II)Particles 3.3x1014 ppp•Repetition 0.3Hz•Harmonic 9 •Bunch Number 8•Nominal Tune (22.4, 20.8)RFabortC2E3neutrinoD3M3M2E1cBT CollimatorsD2InjectionD1Slow extractionM1E2Ring CollimatorsInjection dumpC1
16J-PARC Slow Extraction •Dispersionhorizontal chromaticity Qx’=~0separatrix is independent of momentum•Bump orbit is moved during extraction (dynamic bump)small angular ESSfixed bumpdynamic bump
17Microbunch beams at J-PARC Microbunch technique developed for AGS Will NOT work for J-PARC, without some modifications.Large chromaticity extractionAlternatives? Bunched beam slow extraction.Bunch Compression using Re-bucketing (RHIC)Bunch Compression using chicanes (ERL technique)External Superconducting RF cavity (LEP, KEKB, CESR) followed by series of bend magnets: basic idea is to give bunch a time dependent momentum distribution. Different path lengths for different momenta will compress bunch.
18Re-bucketing at RHIC Basic Idea: Lengthen the bunch by placing on the unstable fixed pointRotate elongated bunch to upright (high in dE, short in dt)Turn on higher harmonic RF with voltage matched to dE of the elongated bunch.What does it look like?
20Tomographic reconstruction of re-bucketing in RHIC
21Re-bucketing at J-PARC The basic method needs simulation studies to develop further:Debunch 1.7 MHz beam to DC (continuous distribution in time)This is the hard part! Beam loading goes as 1/RF VoltageRebunch ~25 MHzDebunch/rebunch at high intensity = beam loading compensation in the 25 MHz system must be very good.Re-bucket at ~200 MHzEnd product is shorter bunches (~5 nsec) with 25 MHz spacing.Finally, need to develop slow extraction of this bunched beam, that will further reduce bunch widths by another factor of 4 (or so).To get to 200 psec requires more thinking..
22Re-bucketing at J-PARC: Problems De-bunching: beam loading is inversely proportional to RF voltage. As RF volts are decreased, instabilities become greater.CERN: problem was too significant = use bunch splittingBNL: h=6 to h=12 for high intensity = use bunch splitting25 MHz bunched beam extraction at high intensity.Debunched beams have lower peak current, avoid instabilitiesBNL experience: coherent effects become significant.Bunches are still too long.
23External Chicane for Bunch Compression Imposed Time dependent momentum distributionDifferences in Time of flight compresses bunch.Superconducting RF CavitySeries of Sector Bends
24External Chicane for Bunch Compression To get even a 100 to 200 psec compression requires a very long system of magnets! Clever techniques can reduce the size, but only by relatively small factors.It can work very well as an “after-burner” system, to get another 50 to 100 psec in compression.
25kinematic suppression of background SummaryFor rare K-decay experiments, very short bunched beams provide:kinematic suppression of backgroundmomentum resolution via time of flightShort bunched beams from J-PARC are feasible.RF phase displacement technique, as developed at BNL, is still the best option, but requires some modificationsRe-bucketing, as done at RHIC, will require addition of two (and possibly a third) RF systems at J-PARC.Most difficult problem for J-PARC will be beam loading compensation for the RF systems. It must be very good, to keep intra-bunch extinction low.
28J-PARC Slow Extraction •3.3x1014 protons per pulse(15uA)full beam power :
29Microwave instability seen at KEK InstabilitiesMicrowave instabilityLongitudinal Space ChargeBelow transition, longitudinal space charge opposes the effect of the RF voltage, perturbing longitudinal phase space (Good thing!)Microwave instability seen at KEK
30Instabilities As seen at CERN PS e-p instabilityAs bunch lengths get very short and peak beam currents get high, the probability of higher mode interactions with electrons increases.V. Danilov et al, LANL, proceedings of the 1999 Particle Accelerator Conference, New York, 1999R. Cappi, et al, proceedings of the 2001 Particle Accelerator Conference, ChigagoAs seen at CERN PS
31Instabilities Transverse space charge Main effect is on the betatron tune. Two components, the incoherent tune shift ( effectively the tune spread) and the coherent tune, or the change in the frequencies of the beam centroid.Will change as beam is extracted and average current decreases. A tune shift during extraction and a change in the tune spread during extraction will affect the bunching and possibly the intra-bunch extinction (needs simulations).Resistive wall ? Well known not to be a problem when g<gtr.
32AGS performance for g-2 operation 6 single bunch transfers from BoosterPeak intensity reached: 72 1012 pppBunch area: 3 eVs at injection eVs at extractionIntensity for g-2 ops: 1012 pppStrong space charge effects during accumulation in AGSDilution needed for beam stability2 secondsPeak currentIntensity40 A5 x 1013 protons
33Longitudinal Phase Space Dilution at Injection A key parameter is peak beam current.Bunch Dilution using93 MHz VHF cavity
34High Intensity Slow Extraction 70 TP Slow extracted beam observations. Vertical Chromaticity is kept positive after transition.
35Slow Extraction Dynamics A particle with a magnetic rigidity Br receives (thin lens) kicks by a sextupole of length L,
37Slow Extraction Dynamics Unstable regionStable regionDistrib. Of particlesExtraction Methods:Move particles into resonance by changing betatron tune of particle distribution (AGS).Increase particle amplitudes until encounters the unstable region (RF knockout method).