Stabilization of Beam Instabilities
by Intra-Bunch Feedback Systemat J-PARC MR*
Y.H. Chin, T. Obina, M. Okada, M. Tobiyama,T. Toyama (KEK), K. Nakamura (Kyoto University),
Y. Shobuda (JAEA)
The 11th Annual Meeting of PASJ
Aomori, Japan, on 9-11 August, 2014
*This work was supported by MEXT KAKENHI Grant Number 25105002, Grant-in-Aid for Scientific Research of Innovative
Areas titled “Unification and Development of the Neutrino Science Frontier”

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Intra-Bunch FB, Chin1

Transverse Instabilities andIntra-Bunch Feedback System

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Intra-Bunch FB, Chin2

Instabilities and Feedback Systemat J-PARC MR
The present bunch-by-bunch feedback system (BxBFB) at MR effectively suppresses observed
transverse dipole oscillations, together with help
from the chromaticities, allowing to attain the 230
kW beam power.
FB:On

FB:OffH:V:
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FB:Off20ms

H:
V:
Intra-Bunch FB, Chin

FB:On
3

Bunch by Bunch vs. Intra-Bunch FBThe BxB FB can damp only the dipole oscillations of
the center of mass motions of the whole bunches.
Even when it is on, internal bunch oscillations have been stillobserved, which are causing additional particle losses.

To suppress intra-bunch oscillations, a more wideband
and elaborate feedback system has been developed.
The new intra-bunch feedback system divides an RF bucket
into 64 segments (~10ns long).
 It acts on each segment (bin) as if it is a small bunch (bunchlet) in a narrowband mode, even if it is empty.
Bunch by bunch
Intra-bunch
BPM signals
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Kick signals
Intra-Bunch FB, Chin4

Schematic of Intra-Bunch FB System
Stripline kicker

Stripline BPMX+50ΩBeam50Ω

X-Y+
OscilloscopeHybrid100KHz-200MHzDAC-ADC+ADC-

DC offset
RF clock
Revolution clock
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Attenuator

x64

iGp12

DAC+x2Clock

Power Amp.
100KHz-100MHz

Injection timing

Revolution
Intra-Bunch FB, Chin

5

Exponentially Tapered ElectrodesA new stripline BPM for the intra-bunch feedback
system for J-PARC MR has exponentially tapered
electrodes for an improved frequency response,
compared to rectangular ones (Linnecar, CERN-SPS).
306mm

More details will be
discussed laterin this talk!
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Intra-Bunch FB, Chin

6Main Parameters@Routine Operation
Circumference
EnergyRepetition Period
Beam PowerRF Frequency
Number of Bunches
Synchrotron Tune
Betatron Tune (hor./ver.)Intensity (/pulse)
Bunch Length
Chromaticity (hor./ver.)3-30GeV
230kW (30GeV)
1.67-1.72MHz
80.002-0.0001
1.3x1014
50-200 ns-4 / -1

Horizontal Feedback
Vertical Feedback
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Intra-Bunch FB, Chin

@Beam Test in May
1568m
3GeV
2.48s
0.5 kW (3GeV)
1.67 MHz
10.001722.41/20.75
2.7x1012
150-200 ns+0.5/+1.2
BxB FB/Intra-bunch FB
on/offBxB FB Always on
7

Horizontal Beam Tests on May 7
Single bunch, Nb=2.7×1012

BxB off; Intra-Bunch off

BxB on; Intra-Bunch off

BxB off; Intra-Bunch on

Betatron sideband
At 76kHz

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Horizontal Oscillations inside a BunchBxB FB off and Intra-Bunch FB off
 The large horizontal oscillations are excitedaround the 262th turn due to the mismatching
field of the injection kicker magnets.

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Intra-Bunch FB, Chin

9

On/Off of BxB and Intra-Bunch FBs

 BxB on IntraBunch off

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 BxB off IntraBunch on

Impedance,
Intra-Bunch
FB, ChinChin10

Time Evolution of Oscillation
Envelops at Every 100 Turns
BxB off
 IB off

BxB on IB off

BxB off
 IB on

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11

Preliminary Results for Vertical
Instabilities at Onset of Acceleration
Nb=0.12x1013
 Vertical BxB off; IB off

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12

Vertical Intra-Bunch FB Turned On
Nb=0.9x1013
 Vertical BxB off; IB on

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13

More Tuning of IB FB andHelp from Chromaticity Needed

Still large particle losses observed during theacceleration in the case of (Nb=0.9x1013/IB on):

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14

Optimization of Stripline
Electrode Shape for a Flatter
Frequency Response

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15

Exponential Electrode:Prototype and Measurement at SPS(Linnecar, CERN-SPS-ARF-SPS/78/17)
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Frequency Response ofa Rectangular Electrode (Length=l)

Zstrip=R1=R2=50Ω
λ=2lFourier Transform

Zstrip=R1=R2=50Ω
λ=4l

Ib/2

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If the Electrode Becomes Very Narrow toward
Downstream, the Leaving Bunch Will Not See it.
No pair of green image currents will be generated.
Zstrip=R1=R2=50Ω

λ=2l9/10/2014

Fourier Transform

Intra-Bunch FB, Chin

λ=4lfrequency

18

TheoryTransfer function:

Example

Linnecar’s exponential electrodeTransfer function

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19

Improved Exponential ElectrodeTo make the end value of k(z) zero, let us subtractthe end value k(l) from k(z):Transfer function

Red: Improved exponential
Blue: Original exponential

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Electrode,
Intra-Bunch
Chin FB,& Shobuda
Chin

20

Blackman-Harris Window FunctionBlackman-Harris window function for k(z)zero derivativeat the both ends
Red: Blackman-Harris
Blue: Exponential

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Three Conditions for a Flat Response
Zero value at the end Smooth tapering toward zero derivative at the end Negative derivative at the beginningPolynomial Electrode- σ=1- σ=2- σ=2.63- σ=1- σ=2- σ=2.63

σ=39/10/2014Intra-Bunch FB, Chin

22

Optimum σLet us optimize σ which minimize the following
function:

Red: Polynomial with σ=2.63
Blue: Original exponential

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23

Simulations with CST Studio

Simulations with nearly full satisfaction of the
impedance matching conditions.
Correct width, height, thickness, resistors, etc.
RectangularExponential
Polynomial

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Summary

The first beam test successfully demonstrates thatthe new intra-bunch FB system is quite effective tosuppress intra-bunch oscillations.
The intra-bunch FB system is now used in routine
operation at J-PARC MR.
The beam loss at the injection is reduced from 350W to170W.

The polynomial electrode was proposed for a very flattransfer function of a stripline BPM.
Measurements for proof of its validity are underpreparation.

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Intra-Bunch FB, Chin25