Linac control monitors-- First direct measurement of electron and positronbunch characteristics at the positron source of theSuperKEKB B-Factory -Tsuyoshi Suwada (tsuyoshi.suwada@kek.jp)
Accelerator Laboratory, KEK

25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

SuperKEKB MAC@KEK

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Introductionü

In 2020, the positron (e+) source of the SuperKEKB (SKEKB) BFactory was upgraded to increase the positron yield.
•

üCountermeasure against electric discharge for the AMD, installations ofsteering magnets, and beam diagnostics.

There are three difficulties to install any beam diagnostics,•

in radiation hard environment, almost no space to install them, and difficult
technique to simultaneously and separately detect both secondary e- and e+bunches with very short time interval.

A wideband beam monitors (WBMs) were successfully installed in
the e+ source.
ü In this report, the motivation for introducing WBMs, and theirdetection technique, and some results are presented.
ü25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

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The SKEKB e+ source
SLs 0.4T, Acc.G 14-20 MV/m@unit1-5
SLs 0.5T, Acc.G 10 MV/m @unit1-6

14mm-thick W
Ee+~120MeV
Ee-~3.5 GeV
Qe-~10 nC

3.5 T

1.5 T

1. Secondary e+e- bunches are immediately captured in the transverse phase space by applying strong pulsed
and DC magnetic fields. On the other hand, in the longitudinal direction they are captured by applying
electromagnetic fields in accelerating structures. The e- bunch is stopped by a beam stopper at a chicane andthe e+ bunch passes through it. The bunch charges are first measured by a standard BPM after the chicane.
2. One of the important issues for beam diagnostics is to simultaneously and separately detect parallel
travelling e+e- bunches with very short distances by two WBMs in the capture section.
• How can we detect simultaneously and separately these secondary e- and e+ bunches?
• Which detection technique is suitable, in time domain or frequency domain?
• What kinds of bunch characteristics for a single bunch are measurable?
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

SuperKEKB MAC@KEK

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e- and e+ capture process in the e+ capture section
through dynamical phase-slip processrear e+front e-front e+rear eReverse
process••

The line order for the e- and e+ bunches are generated with very short time intervals (Dt~ 0 - 300 ps)
depending on the capture phase.
To understand the dynamical capture process, it is required to precisely measure the time interval Dt
between these two bunches.
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

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Historical detection technique in time domain
at the SLC e+ sources
e+ capture section
e+ production target
Ee- ~ 33 GeV
Ne- ~ 5x10^10 e-/pulsee+e-button pickup

400ps

1. Referred by J. E. Clendenin, et al., “SLC positronsource startup”; Procs. of 1988 Linear Accelerator Conf,
Williamsburg, Virginia, USA.
2. The commissioning of the e+ source began in early
September of 1986.

We can barely recognize both e- and e+ bunches, for which the signals were obtainedby a button pickup. The bunch separation is not very good in time domain.
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

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Historical detection technique in frequency domaine+ production target at the APS e+ sourcesee+e-Ee-~200 MeV
Qe-~50 nC/pulsee-e+e+ capture section
without any e+e- separator

Conventional heterodyne technique with afundamental 2856 MHz for e- and e+ 30-ns
pulsed beams for a stripline bpm
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

1. Referred by R. Fuja and Y. Chung,“APS linac beam position monitorsand electronics”
2. The linac commissioning began inearly October of 1993.
It is difficult to separately detect bothe- and e+ bunch characteristics.

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Choice from the viewpoint on the pros andcons for detection techniques
To the best of my knowledge in 2018,
1. Based on the detection technique in time domain,ü Any high frequency signal losses for all rf components, transmission lines
and signal pickups should be fully corrected in sufficient wide bandwidth.
ü Is such a correction scheme possible in time domain?
ü How much bandwidth is required at minimum?
ü This technique gives a great advantage in comparison with that infrequency domain?
2. Based on the detection technique in frequency domain,
ü any fundamental frequency is not a characteristic frequency for a single
bunch,
ü and a heterodyne technique is not so advantageous for a single bunch incomparison with a pulsed beam,ü and it is difficult to separately detect both e- and e+ bunch characteristics.
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

SuperKEKB MAC@KEK

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Wideband beam monitor (WBM)

38mm

Protrusion length of 1mm for SMA feedthroughantenna from the inner surface
ü Simultaneous and separate detection for both e- and e+ bunches is possible in time domain
with any rf loss corrections for all rf components and transmission lines.
ü Fundamental bunch characteristics, bunch interval, charges, transverse positions, and bunch
lengths can be separately measured for both e- and e+ bunches in time domain.
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

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Integrated WBM and steering magnets in a frame
Dipole magnets and
WBM fixed in a frame

Fixing structure of theframe in a solenoid coil

Frame inserted in a solenoid coile+@e-e+, e+@e+e25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

SuperKEKB MAC@KEK

9Signal detection system

All rf components should be corrected insuitable frequency region by using avector network analyzer.

pickups

ü Coax. cables, rf combiners, and rf loss inpickups

coaxialcablesrf combiners

Wideband oscilloscopeü Keysight Tech., BW13.5GHz, 40GSa/s.
25-27 Mar. 2024, Tsuyoshi Suwada/ KEK
Acc.Lab.

ü Any rf signal loss corrections are available
with normal and inverse FFT procedures
based on “de-embedding technique”.
ü The cut-off frequency was fixed to 9.1 GHz
in the inverse FFT procedure.
ü Any signal waveforms can be corrected infrequency domain and displayed pulse-bypulse in time domain.
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Results of the signal detection in time domain
Differential signal

Raw signal

1st @SP15-25
1st @SP16-25

wakefield
2nd @SP15-25

Raw signal (ch#1)
1) with rf-loss correction,2) at f_15=0 deg in nominal
e+ operation,3) 2 WBM signals combined,
4) 2 bunch acceleration withthe time interval of Dt2=96 ns

1st e-

1st e+

2nd @SP16-25

Integrated signal

1st e+

1st e-

25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

4ch-sum integrated signal
Integrated signal
for beam position calculation,
4ch-sum Integrated signal
for bunch length (pulse width),arrival timing (peak timing), andbunch charge (pulse area)
calculation.

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1st e+
1st e-

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Variations during the dynamical phase-slip process forboth the e+e- bunches depending on the capture phase
capture phases f15
main e+main e-350 pssatellite e-

25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

1st/SP-1525
main e+main e-satellite e+(1) The separate detection for e-e+ bunches in time
domain was successfully performed.
(2) The travelling order of e-e+ bunches is reversed
at certain capture phases through the dynamicalphase-slip process.
(3) Some satellite bunches can be identified whichare 350ps apart from the main bunch. The
generation of satellite bunches is caused by spillover from the longitudinal phase space.
(4) The results show that quite symmetricdynamical behaviors for both the bunchcharacteristics were observed.
(5) The ringing waves following from the bunch
signals are due to wakefields from upstream
accelerator structures.

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Time interval measurements as a function of the capture phase
1st bunch@SP15-25/SP16-25

2nd bunch@SP15-25/SP16-25

e+@e-e+dec. e+e+@e+eacc. e+

Tsuyoshi Suwada, “Direct Observation of Positron Capture Process at the Positron Source of
the SuperKEKB B-Factory”; Scientific Reports 12, 18554 (2022). Published online 2022
Nov. 03. https://doi.org/10.1038/s41598-022-22030-5.
25-27 Mar. 2024, Tsuyoshi Suwada/
KEK Acc.Lab.

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3D plots in the bunch characteristics as functions of time interval,
capture phase, and bunch intensity for e+e- bunches
Output [V]Output [V]e+e+Time [ns]Φ15 [deg]

Φ15 [deg]Time [ns]

e-

e-

e+

e+

(a) 1st bunch, SP15-25(b) 2nd bunch, SP15-25

1st bunch@SP15-25/SP16-25

2nd bunch@SP15-25/SP16-25

25-27 Mar. 2024, Tsuyoshi Suwada/
KEK Acc.Lab.

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e+e- bunch characteristics depending on the capture phasee- decelerating region

e- accelerating region e+ accelerating region e+ decelerating region

Time intervalTime interval

Bunch length (rms)

Bunch length (rms)

X

X

Y

Y
Charges

Charges

Capture phase

Capture phase

1st e-@e-e+,
1st e-@e+e- at SP15-25,
Step phase Df15=Df16=10o

1st e+@e-e+,
1st e+@e+e- at SP15-25

25-27 Mar. 2024, Tsuyoshi Suwada/
KEK Acc.Lab.

Capture phase

1st e-@e-e+,
1st e-@e+e- at SP16-25

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Capture phase

1st e+@e-e+,
1st e+@e+e- at SP16-25

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Plots for both the e+e- bunch positions
at SP15-25 and SP16-25

1st e-

1st e+

2mm

e+@e-e+, e+@e+e-

Offset target

2mm

2mm

Offset target

e-@e-e+, e-@e+e-

2mm・ It can be found that there are no characteristic differences in the transverse positions dependingon the capture phase under the nominal operation condition and those giving the maximum charges.
・ It is interesting that the cluster for each bunch rotates in the transverse plane owing to their
cyclotron motion in the longitudinal direction between SP15-25 and SP16-25.

25-27 Mar. 2024, Tsuyoshi Suwada/
KEK Acc.Lab.

SuperKEKB MAC@KEK

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Correlation plots for the time interval and bunch charges of bothe+e- bunches at SP15-25 and SP16-25
e-@e-e+e+@e-e+e-@e+e-

1st e-

e+@e+e-

1st e+

Accelerating e- (blue), e+ (green), decelerating e- (brown), e+ (red)・The time interval correlation is based on a 45o inclined line.
・This result means that the dynamical phase slip process was completed at the first WBM (SP15-25).
・The maximum charges are generated at the maximum time intervals even for the different bunches,
Dt~-268 ps (Dt~-268 ps)@ acc. e- (acc. e+), Dt~-264 ps (Dt~253 ps)@ dec. e- (dec. e+), respectively.
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

SuperKEKB MAC@KEK

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Correlation plots for the bunch length and bunch charges of bothe+e- bunches at SP15-25 and SP16-25
e+@e+eacc. e+e-@e-e+acc. e-e-@e+edec. e-

1st e-

e+@e-e+dec. e+

1st e+

・Although the dynamical phase-slip process was fixed at the front WBM (SP15-25), thedynamical exchange of particles in the longitudinal direction inside the bunch may be caused,called bunch lengthening (or shortening).
・ The correlation seems to be based on a 3rd-order polynomial function in the acceleratingphase region, and on the other hand, that shows a bunch shortening in the decelerating phaseregion.
・The maximum bunch charges are generated at the maximum bunch length.
25-27 Mar. 2024, Tsuyoshi Suwada/
KEK Acc.Lab.

SuperKEKB MAC@KEK

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Variations in e+e- bunch charges as a function of the capture
phase at SP15-25 and SP16-25

acc. e-@e-e+acc. e+@e+eope. pointacc. e-@e-e+acc. e+@e+e-

ope. pointdec. e+@ e-e+dec. e-@e+e-

dec. e-@e+e-

dec. e+@ e-e+

・ The present operation point is not on an optimized point.
・ The e+e- bunch charges in the accelerating phase region are greater than those in the
decelerating phase region in the capture section.
・ The e+ yields at the peak points in the acc. and dec. phase regions are 122% and 84%
greater than that at the nominal operation point, respectively.
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

SuperKEKB MAC@KEK

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Conclusions
•

•

•

•

•

Direct simultaneous measurements of the secondary e+e- bunchcharacteristics were successfully performed with WBMs at the e+source of the SuperKEKB B-factory.
Longitudinal and transverse bunch characteristics were obtained foreach bunch as a function of the capture phase to investigate theirdynamical capture process under two-bunch acceleration scheme.
The results show that quite symmetric behaviors in the e−e+ bunchcharacteristics were observed.
Such wideband detection techniques could be applied to conventional
and advanced e+ sources in future accelerator projects.
Based on this technique, the e+ intensity could be systematically
optimized in multidimensional parameter spaces towards highintensity e+ sources.
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

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Backup files

25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

SuperKEKB MAC@KEK

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Variations in the e+ bunch charges as a function of the capture
phase at SP15-25, SP16-25, and SP16-5・ It is clear that the present operation point is noton an optimized point.
・ The e+ yield at the maximum point obtained bya BPM after the capture section is only ~5%greater than that at the nominal operation point.
・ The capture phase at the max. e+ yield in thecapture section is shifted from that after thecapture section.
acc. e+@e+edec. e+@e-e+
・ The results show that only the e+ yield
SP1525acc. e+@e+e- SP1525
dec. e+@e-e+ measurement at the location after the capture
SP1625
SP1625section is not sufficient in the optimization
procedure. More detailed parameter tunings may
be required.
e+@SP165

25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

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Layout of the e+ source of the SKEKB injector linac

new(e-)

new(e+) new (e-) new (e+)

e+/e- separator BPM

・Total length of the capture section ~15 m
・Primary Ee- = 3.5 GeV, ~10 nC (single-bunch)・Ee+ ~ 120 MeV after the capture section
・ e+ target 14mm-thick W
・Solenoidal fields of FC/BC 3.5/1.5 T
・Solenoidal fields of DC SLs 0.4 T(unit1-5) /0.5 T(unit1-6)
・LAS Acc. gains 14-20 MV/m (unit1-5) / 10 MV/m (unit1-6)
・Wideband monitors (new for e+) and conventional BPMs (new for e-)
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

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SuperKEKB MAC@KEKSuper
KEKB
Accelerator
The
SuperKEKB
B-Factory:
an electronpositron collider
with asymmetric energies
Complex
Upgraded components for SuperKEKB
Upgrade of SuperBelle New QCS(Final focus system)
Crossing angle
22 mrad (KEKB)
30 mrad (SuperKEKB)

7 GeV positron beam
3.6 A

e+ E = 4 GeV

Ante-chambers,
comb-type bellows, HOM absorbers

e- E = 7 GeV
4.0 GeV electron beam
2.6 A

3km
500m

Damping ring for e+
New e+ source

New RF Gun+ Cathode materials: CsTe, Lab6, Cu

Flux concentrator (Superconducting coil)+ L-band capture section

100m
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

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References
1.

2.

3.

4.

5.

6.

7.

Tsuyoshi Suwada, Muhammad Abdul Rehman, and Fusashi Miyahara, “First simultaneous detection of electron andpositron bunches at the positron capture section of the SuperKEKB factory”; Scientific Reports 11, 12751 (2021).
https://doi.org/10.1038/s41598-021-91707-0.
Tsuyoshi Suwada, “Modal Analysis of Electromagnetic Couplings between SMA-Feedthrough Electrode and Beam for
Wideband Beam Monitor”; Poster presented at the 10th International Beam Instrumentation Conference (IBIC2021),
Video meeting hosted by PAL, Korea, Sep. 13-16, 2021, pp. 392-395 (JACoW Publishing, 2021) [ISBN 978-3-95450230-1, doi:10.18429/JACoW-IBIC2021-WEPP12].
M. A. Rehman and T. Suwada, “Observation of Wakefireld Effects with Wideband Feedthrough-BPM at the Positron
Capture Section of the SuperKEKB Injector Linac”; Proceedings of the 10th International Beam Instrumentation
Conference (IBIC2021), Pohang, Rep. of Korea, Sep. 13-16, 2021, pp. 52-55 (JACoW Publishing, 2021) [ISBN 978-395450-230-1, doi:10.18429/JACoW-IBIC2021-MOPP10].
M.A. Rehman, T. Suwada, and M. Miyahara; “First synchronous measurement of single-bunched electron and positron
beams with a wideband feedthrough-BPM at the positron capture section of the SuperKEKB inejctor linac”;
Proceedings of the 2021 Particle Accelerator Conference (IPAC’21), hosted by the Brazilian Center for Research in
Energy and Materials (CNPEM), Campinas, Brazil, May 24-28, 2021, pp.557-560 (MOPAB163); ISBN: 978-3-95450214-1, ISSN: 2673-5490, doi:10.18429/JACoW-IPAC2021-MOPAB163.
T. Suwada, “Characteristic Analysis of Wideband Beam Monitor with High Frequency Pickups”; Rev. Sci. Instrum. 93,
093301 (2022) [DOI: 10.1063/5.0087321].
Tsuyoshi Suwada, “Direct Observation of Positron Capture Process at the Positron Source of the SuperKEKB BFactory”; Scientific Reports 12, 18554 (2022). Published online 2022 Nov. 03. https://doi.org/10.1038/s41598-02222030-5.
T. Suwada, “First direct measurement of electron and positron bunch characteristics at the positron source of theSuperKEKB B-Factory”; AIP Advances 13, 045021 (2023); [DOI: 10.1063/5.0145776].
25-27 Mar. 2024, Tsuyoshi Suwada/ KEK
Acc.Lab.

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Frequency characteristics for the bunch characteristics
fc ~10 GHz

Time interval Dt between e- ande+ bunches, fc ~10 GHz
systematic error
sDt = Dt (20 GHz) - Dt (10 GHz)
Bunch lengths le- and le+
fc ~10 GHz
systematic error
sl = l (25 GHz) - l (10 GHz)
Bunch charges Qe- and Qe+
fc ~20 GHz
systematic errorsQ = Q (25 GHz) - Q (20 GHz)

25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.
SuperKEKB MAC@KEK

Tsuyoshi Suwada, Muhammad Abdul Rehman, and Fusashi Miyahara, “First
simultaneous detection of electron and positron bunches at the positron capture
section of the SuperKEKB factory”; Scientific Reports 11, 12751 (2021).
26
Published online 2021 June 17. https://doi.org/10.1038/s41598-021-91707-0.

Installation of WPM into a frame
Frame for dipole and WBM
WBM support（upstream）
WBM support（center）
STX/STY magnets

210mm
430mm

25-27 Mar. 2024, Tsuyoshi Suwada/
KEK Acc.Lab.

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Installation of solenoid coil into the e+ source(upper) e+
capture section

Cable extraction(left) Solenoid coil installed
the frame
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

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Dipole magnet and WBM fixed in a frame
Inserting a WBM and dipole magnet in
an aluminum frame

Extracting coax. cables with a bending
angle of 90o in the forward direction.

25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

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Integrated WBM and semi-rigid coaxial cables
filled with PEEK as an insulator
Integrated aluminum frame
fixed with WBM and dipole
magnet2m-long signal-extraction
coax. cable filled with aninsulator made of PEEK

25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

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Typical measurement results of there different coax.
cables connected in series/amplitude and phase data
S21 amplitude

S21 phase

T. Suwada, “Characteristic Analysis of Wideband Beam Monitor with High Frequency Pickups”;
Rev. Sci. Instrum. 93, 093301 (2022) [DOI: 10.1063/5.0087321].
25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

SuperKEKB MAC@KEK

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RF loss correction technique for pickups (I)ü rf induced technique by which S21 between
channels is measurable by a VNA
ü T. Suwada, “Characteristic Analysis of Wideband
Beam Monitor with High Frequency Pickups”;
Rev. Sci. Instrum. 93, 093301 (2022) [DOI:
10.1063/5.0087321].

25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

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RF loss correction technique for pickups (II)Equivalent circuit model of WBM taking into
account electromagnetic couplings between
feedthrough electrodes. The couplings fordifferent modes are shown with different
colors.

25-27 Mar. 2024, Tsuyoshi
Suwada/ KEK Acc.Lab.

Typical measurement results of amplitude
S21 between electrodes No. 1 and No. 2(blue) [also No. 1 and No. 3 (red)].

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Electromagnetic coupling analysis

Detection principle

Figure 3:(a) Schematic drawing of electromagnetic couplings
between SMA feedthrough and coaxial structure.
(b) Electromagnetic couplings between SMA feedthroughand a thin ring beam. Inner radius of the coaxial structure:a = 19 mm, ring-beam radius: b, radii of the SMA innerand outer conductor: bi = 0.9 mm, bo = 2.05 mm, and thecharacteristic impedance of the SMA is Z0 = 50 𝛀.
Figure 4: Equivalent circuit of electromagnetic couplings
between SMA feedthrough and coaxial structure. The
arrows indicate the couplings between TEM (blue), TE(red), and TM (green).
Tsuyoshi Suwada, “Modal Analysis of Electromagnetic Couplings between SMA-Feedthrough Electrode and Beam for Wideband
Beam Monitor”; Poster presented at the 10th International Beam Instrumentation Conference (IBIC2021), Video meeting hosted by
PAL, Korea, Sep. 13-16, 2021 (WEPP12).

25-27 Mar. 2024, Tsuyoshi Suwada/
KEK Acc.Lab.

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Numericalresults
Calculations of input impedances for
TEM+TE and TEM+TM modes

Figure 5:(a) Variations in the input
impedance (TEM+TE
modes) as a functionof frequency and thebeam size.

(b) Variations in that
(TEM+TM modes) as afunction of frequency andthe beam size.
The subscripts n
andmaximum m are fixed to 1
and 5, respectively.
The extruding length isfixed to 1 mm.

Figure 6:
Variations in the couplingstrengths between the
TEM modes as a functionof frequency and thebeam size.

Figure 7:
Variations in the couplingstrengths between the
TEM modes as functionsof frequency and thelength. The transverse
charge distribution of 𝛔b= 4 mm is fixed.

Tsuyoshi Suwada, “Modal Analysis of Electromagnetic Couplings between SMA-Feedthrough Electrode and Beam for Wideband
Beam Monitor”; Poster presented at the 10th International Beam Instrumentation Conference (IBIC2021), Video meeting hosted by
PAL, Korea, Sep. 13-16, 2021 (WEPP12).

25-27 Mar. 2024, Tsuyoshi Suwada/
KEK Acc.Lab.

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