1. K.A. Brown (M. Sivertz) Collider Accelerator Department, BNL
Development of micro-bunching beams and application to rare K decay experiments
K.A. Brown (M. Sivertz) Collider Accelerator Department, BNL
(M. Tomizawa)
JPARC Project, KEK

2. Outline Physics Motivations Parameters from KOPIO
Micro-bunching at BNL AGS
Micro-bunching at J-PARC
Re-bucketing at RHIC (bunch compression)
Re-bucketing at J-PARC
Summary

3. Separating signal from background
Microbunching is crucial to the
measurement 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.

4. Physics Motivation: Microbunch Separation
Microbunch separation determined by
the length of time required to clear out kaons
from 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

5. Physics Motivation: Microbunch Width
Fully reconstructs the neutral Kaon in
KL  p0 n n measuring the Kaon momentum by time-of-flight.
Start when proton beam hits the target
End at the
decay time and decay point
reconstructed from the
two 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)

6. Physics Motivation: Interbunch Extinction
Effects of Interbunch Kaons
KL  p0 p0 events, shifted in time
Kinematic cuts are used
to reduce background
due to KL  p0 p0
When KL does not come
from the microbunch,
incorrect kinematic fit
does not allow for good
rejection. Panels show
effect of KL production
at varying interbunch times.
P*(p)
E*(g1) – E*(g2)
Signal and p0 p0

7. Physics Motivation: Intensity
KOPIO had planned to study the very rare decay
KL  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.

8. KOPIO Beam requirements
Spill length with 100TP of ~3 seconds.
Number of KL decays per microbunch: 3.57
Yields ~0.5 KL decay in 10 < Z < 14 meters
Both are a flat optimum
Variation 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)

9. Time Structure of Beam AGS Cycle Spill Structure 0 sec 2.3 sec 5.3 sec
Injection and
acceleration
AGS Cycle
Spill Structure
0 sec
2.3 sec
5.3 sec
Extraction
Start of Cycle
40 ns between
microbunches
200 ps RMS
Microbunch width
End of Spill and Cycle
mbunch
spacing
structure
Start of Spill
Proton intensity
Time

10. BNL AGS: Micro-bunched slow extraction
Empty buckets generate energy modulation of debunched beam
Higher cavity voltage and/or smaller DP/P  shorter bunches
Need ~200 ps bunches every 40 ns
Frequency
200 ps
40 ns
Time
Extraction resonance
Debunched beam

11. Microbunching the AGS Beam
Extracted
particles
25 MHz fundamental
+ 100 MHz harmonic
Simulation 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.
Extraction
Region

12. Test Beam Results: Microbunch Width
93 MHz cavity at 22 kV
gave s = 217 ps.
Microbunch time, in ns
Simulation
93 MHz cavity at 22 kV
gave s = 240 ps.
Microbunch time, in ns
Data

13. Test Beam Results: Interbunch Extinction
4.5 MHz cavity at 130 kV
gave e = 8 (+/- 6) x 10-6
Microbunch time, in ns
Data
Interbunch
events
Microbunch time, in ns
Simulation
4.5 MHz cavity at 130 kV
gave e = 1.7 (+/- 0.9) x 10-3.
Interbunch
events

14. Microbunch Beams at J-PARC

15. 50GeV 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) RF
abort C2 E3
neutrino D3 M3 M2
E1c
BT Collimators D2
Injection D1
Slow extraction M1 E2
Ring Collimators
Injection dump C1

16. J-PARC Slow Extraction
•Dispersion
horizontal chromaticity Qx’=~0
separatrix is independent of momentum
•Bump orbit is moved during extraction (dynamic bump)
small angular ESS
fixed bump
dynamic bump

17. Microbunch beams at J-PARC
Microbunch technique developed for AGS Will NOT work for J-PARC, without some modifications.
Large chromaticity extraction
Alternatives? 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.

18. Re-bucketing at RHIC Basic Idea:
Lengthen the bunch by placing on the unstable fixed point
Rotate 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?

19. Re-bucketing in RHIC

20. Tomographic reconstruction of re-bucketing in RHIC

21. Re-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 Voltage
Rebunch ~25 MHz
Debunch/rebunch at high intensity = beam loading compensation in the 25 MHz system must be very good.
Re-bucket at ~200 MHz
End 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..

22. Re-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 splitting
BNL: h=6 to h=12 for high intensity = use bunch splitting
25 MHz bunched beam extraction at high intensity.
Debunched beams have lower peak current, avoid instabilities
BNL experience: coherent effects become significant.
Bunches are still too long.

23. External Chicane for Bunch Compression
Imposed Time dependent momentum distribution
Differences in Time of flight compresses bunch.
Superconducting RF Cavity
Series of Sector Bends

24. External 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.

25. kinematic suppression of background
Summary
For rare K-decay experiments, very short bunched beams provide:
kinematic suppression of background
momentum resolution via time of flight
Short bunched beams from J-PARC are feasible.
RF phase displacement technique, as developed at BNL, is still the best option, but requires some modifications
Re-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.

26. Supplemental Material

27. Overview of AGS Slow Extraction

28. J-PARC Slow Extraction
•3.3x1014 protons per pulse(15uA)
full beam power :

29. Microwave instability seen at KEK
Instabilities
Microwave instability
Longitudinal Space Charge
Below transition, longitudinal space charge opposes the effect of the RF voltage, perturbing longitudinal phase space (Good thing!)
Microwave instability seen at KEK

30. Instabilities As seen at CERN PS
e-p instability
As 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, 1999
R. Cappi, et al, proceedings of the 2001 Particle Accelerator Conference, Chigago
As seen at CERN PS

31. Instabilities 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.

32. AGS performance for g-2 operation
6 single bunch transfers from Booster
Peak intensity reached: 72  1012 ppp
Bunch area: 3 eVs at injection eVs at extraction
Intensity for g-2 ops:  1012 ppp
Strong space charge effects during accumulation in AGS
Dilution needed for beam stability
2 seconds
Peak current
Intensity
40 A
5 x 1013 protons

33. Longitudinal Phase Space Dilution at Injection
A key parameter is peak beam current.
Bunch Dilution using
93 MHz VHF cavity

34. High Intensity Slow Extraction
70 TP Slow extracted beam observations. Vertical Chromaticity is kept positive after transition.

35. Slow Extraction Dynamics
A particle with a magnetic rigidity Br receives (thin lens) kicks by a sextupole of length L,

36. Slow Extraction Dynamicsh
stable X
unstable

37. Slow Extraction Dynamics
Unstable region
Stable region
Distrib. Of particles
Extraction Methods:
Move particles into resonance by changing betatron tune of particle distribution (AGS).
Increase particle amplitudes until encounters the unstable region (RF knockout method).

38. K0PI0 Experiment

39. Spill and Ripple

40. Spill and Ripple

41. Spill and Ripple

42. Spill and Ripple