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

Outline

Physics MotivationsParameters from KOPIO

Micro-bunching at BNL AGSMicro-bunching at J-PARC

Re-bucketing at RHIC (bunch compression)Re-bucketing at J-PARC

Summary

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.

0* * *

1 2.E vs E E

0 0L K

0 0 0L K

Physics 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

Physics Motivation: Microbunch Width

Fully reconstructs the neutral Kaon in KL measuring the Kaon momentum by time-of-flight.

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)

Start when proton beam hits the target

End at the

decay time and decay point

reconstructed from the

two photons.

Physics Motivation: Interbunch ExtinctionEffects of Interbunch Kaons

Kinematic cuts are usedto reduce background due to KL 0 0

When KL does not comefrom the microbunch, incorrect kinematic fitdoes not allow for goodrejection. Panels showeffect of KL production at varying interbunch times.

P*()

E*(1) – E*(2)

KL events, shifted in time

Signal and 0 0

Physics Motivation: Intensity

KOPIO had planned to study the very rare decay

KL 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 1020. Final value depended on inefficiencies.

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)

Time Structure of Beam

TimePro

ton in

tensi

ty

Injection andacceleration

AGS CycleSpill Structure

0 sec 2.3 sec 5.3 secExtraction

Start of Cycle

40 ns between microbunches

200 ps RMS Microbunch width

End of Spill and Cycle

bunchspacing

bunchstructure

Start of Spill

BNL AGS: Micro-bunched slow extraction

TimeExtraction resonance

Fre

qu

en

cy

Empty buckets generate energy modulation of debunched beamHigher cavity voltage and/or smaller P/P shorter bunchesNeed ~200 ps bunches every 40 ns

Debunched beam

40 ns200 ps

Extracted particles

25 MHz fundamental+ 100 MHz harmonic

Microbunching the AGS Beam

Simulation of the extraction process for 25+100 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

Test Beam Results: Microbunch Width

93 MHz cavity at 22 kV gave = 217 ps.

Microbunch time, in ns

Simulation

Simulation

93 MHz cavity at 22 kV gave = 240 ps.

Microbunch time, in ns

Data

Data

Test Beam Results: Interbunch Extinction

4.5 MHz cavity at 130 kV gave = 8 (+/- 6) x 10-6

Microbunch time, in ns

Data

Data

Interbunch events

Microbunch time, in ns

Simulation

4.5 MHz cavity at 130 kV gave = 1.7 (+/- 0.9) x 10-3.

Simulation

Interbunch events

Microbunch Beams at J-PARC

50GeV Synchrotron (Main Ring)

•Imaginary Transition • 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 1567.5m•Beam Power 0.75MW (Phase II)• Particles 3.3x1014 ppp•Repetition 0.3Hz•Harmonic 9•Bunch Number 8•Nominal Tune (22.4, 20.8)

Slow

ext

ract

ion

Injection

Ring Collimators

BT Collimators

RFabort

neutrino

C2

C1

M1

M2M3

D1

D2

D3

E1c

E2

E3

Injection dump

J-PARC Slow Extraction

•Dispersion free @LSShorizontal chromaticity Qx’=~0 separatrix is independent of momentum•Bump orbit is moved during extraction (dynamic bump) small angular spread @ ESS

fixed bump dynamic bump

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.

Re-bucketing at RHIC

Basic Idea:

1. Lengthen the bunch by placing on the unstable fixed point

2. Rotate elongated bunch to upright (high in dE, short in dt)

3. Turn on higher harmonic RF with voltage matched to dE of the elongated bunch.

What does it look like?

Re-bucketing in RHIC

Tomographic reconstruction of re-bucketing in RHIC

Re-bucketing at J-PARC

The basic method needs simulation studies to develop further:

1. Debunch 1.7 MHz beam to DC (continuous distribution in time) This is the hard part! Beam loading goes as 1/RF Voltage

2. Rebunch ~25 MHz Debunch/rebunch at high intensity = beam loading

compensation in the 25 MHz system must be very good.

3.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..

Re-bucketing at J-PARC: Problems

1. 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

2. 25 MHz bunched beam extraction at high intensity. Debunched beams have lower peak current, avoid instabilities BNL experience: coherent effects become significant.

3. Bunches are still too long.

External Chicane for Bunch Compression

Superconducting RF Cavity

Series of Sector Bends

Imposed Time dependent momentum distribution

Differences in Time of flight compresses bunch.

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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.

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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.

Supplemental Material

Overview of AGS Slow Extraction

J-PARC Slow Extraction

•3.3x1014 protons per pulse(15uA)full beam power : 750kW @50GeV

Instabilities

Microwave instability

Longitudinal Space Charge

Below transition, longitudinal space charge opposes the effect of the RF voltage, perturbing longitudinal phase space (Good thing!)

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form a is and y,instabilit of thresholdat thebunch per particles of

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Microwave instability seen

at KEK

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

Instabilities

R. Cappi, et al, proceedings of the 2001 Particle Accelerator Conference, Chigago

As seen at CERN PS

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 tr.

Instabilities

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AGS performance for g-2 operation

6 single bunch transfers from Booster Peak intensity reached: 72 1012 ppp Bunch area: 3 eVs at injection

10 eVs at extraction Intensity for g-2 ops: 50-60 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

pro

tons

Longitudinal Phase Space Dilution at Injection

A key parameter is peak beam current.

Bunch Dilution using 93 MHz VHF cavity

High Intensity Slow Extraction

70 TP Slow extracted beam observations. Vertical Chromaticity is kept positive after transition.

Early in SEB run.

2 % ineff.

8 % ineff.

Later in SEB run.

2 % ineff.

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Extraction Methods:

1. Move particles into resonance by changing betatron tune of particle distribution (AGS).

2. Increase particle amplitudes until encounters the unstable region (RF knockout method).

Distrib. Of particles

K0PI0 Experiment

Spill and Ripple

Spill and Ripple

Spill and Ripple

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