KEKB/SuperKEKBpositron source

a review and the status

KEK M. Fukuda

1AWLC2017

Contents

• Positron source for the KEKB and the SuperKEKB• Positron target

• Matching device (QWT, FC)

• Commissioning of FC

• Calculation of the magnetic field of the QWT for ILC

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Positron source for the KEKB and the SuperKEKB

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IPAC10, THPD004, N. Iida

OHO07, KamitaniAWLC2017

Primary e- beam

DC solenoid

Phase spaceMatching device

Target

Accelerator

Positrons

Target material

4

RequirementsHigh Z (Cross section of Bremsstrahlung ∝ Z2/A)High melting pointTantalum(73Ta),

Tungsten(74W), Tungsten- rhenium alloy (W-Re)

KEKB, SuperKEKBTarget material: W 14mm (4c0)Primary e- beam energy: 4.0 GeV(KEKB), 3.3GeV(SuperKEKB)

Joining of tungsten crystal to a copper body by a hot isostatic pressing (HIP)

AWLC2017

T. Suwada et al., Phys.Rev.STAB 10 073501

Target destruction limit

5

Positron production for CLIC(CLIC note 465)

Threshold: 0.93x1010 [GeV/mm3] 76J/g(Peak energy-deposition density per volume)

After the target destruction was occurred in the SLC,This threshold : 76 35J/g (slac-r-571)

Threshold：2.0x1012 [GeV/mm2](Incident beam energy density per area)

B1: 24.4GeV, σx: 0.91mm, σy: 0.35mm, 8x1010 e-/pulse, Area 1.0mm2

1.95x1012[GeV/mm2]

CLIC note 465

Material: W75Re25 alloyTarget thickness5.4X0

Positron target material test @ SLACIncident beam energy：20.5-24.4GeV

SLAC-TN-15-006(CN-128)

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Matching Device

6

QWT

QWT(Quarter wave transformer) AMD(Adiabatic matching device)

AWLC2017

Primary e- beam

DC solenoidPhase spaceMatching device

Target

Accelerator

Positrons

The generated positrons have the small beam size and the large divergence.The matching device converts them to parallel beam.

QWT(Quarter wave transformer)

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(Pz = 10MeV/c in the KEKB)

OHO07, Kamitani

Magnetic field: BiQWT length: Li Momentum: pz

The QWT transforms 90deg in the phase space.It captures the positrons satisfying this condition.

Energy acceptance

AWLC2017a: Diameter of an accelerator iris

Transverse acceptance

Energy acceptance

CERN-94-1 Positron source, R. Chehab

AMD(Adiabatic matching device)

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OHO07, Kamitania: Diameter of an accelerator iris

(B0 = 7.0 [Tesla], μ = 60[1/m])

AMD field is produced by a flux-concentrator.The eddy current is induced in the tapered conductor by a changing magnetic field which is made by the primary coil.The magnetic field is concentrated due to the tapered shape of the FC head.

Transverse acceptanceadiabatic condition

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Adiabatic invariance is constantduring the motion.

Kamitani20061101.pdf

Primary coil

Conductor

e+e-

CERN94-1, R. Chehab

Debunching

9

Because the positrons have the large energy spread, the debunching is caused.

Debunching caused by speed differenceβ of positrons is small before acceleration.

Debunching caused by spiral orbitThe positron has the spiral orbit in the solenoid field.The diameter depends on the energy.

OHO07, T. KamitaniAWLC2017

Matching device

10

OHO07, Kamitani

2017/05/22 Posipol2016 “SuperKEKB positron source status” KamitaniOHO07, Kamitani

4GeV

Pulse coil: 2.3T @ 10kAFlux ConcentratorQuarter wave transformer

KEKB SuperKEKB

AWLC2017

The Matching device is the QWT in KEKB.It has been changed to the flux concentrator to increase the positron intensity in the SuperKEKB.

QWT

11

Pulse coil

Bridge coil

Current shape is a half sinusoidal waveOHO07, T. Kamitani

Target

accelerator

Bridge coils compensate for the field gapsbetween the pulsed coil and the accelerator.

Field strength: 2.3 TCoil length: 42.5mmInside diameter: 11mmNumber of turns: 8 turnPeak current: 10kAPulse width: 100us

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Flux concentrator

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Field strength: 3.5TLength: 100mmOutside diameter: 108mmAperture diameter: 7mm(min), 52mm(max)Number of turns： 12turnSpiral gap： 0.2mmPeak current： 12kAPulse width: 6us

Target: W 14mm(4X0)Beam size on the target: σx,y 0.7mm(The size is expanded by the spoiler)

AWLC2017PASJ2016 MOP063 Y. Enomoto et.al

Flux concentrator

13POSIPOL2016, T. Kamitani

Work-hardening Procedure1. Press FC-head till the gaps are contacted.2. Insert spacers into the slit.3. Remove the spacers.4. Measure the gap size.5. Repeat them from (1)

PASJ2016, MOP063,T. Kamitani et. al.AWLC2017

FC power supply

14

The 12kA modulator is developed based on the modulator for the S-band klystron.To reduce the cost, the modulator is consists of the common device(Switching power supply, thyratron and so on).

Charging voltage: 17kVPulse width: 5usPeak current: 12kARepetition rate: 50Hz

PASJ2013, SUP057, M. Akemoto et.alAWLC2017

Positron capture section

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IPAC14, THPRI047, T. Matsumoto et.al.

Large Aperture S-band structure: LASIris diameter: 30mm(SuperKEKB)

POSIPOL2016, T. Kamitani

DC solenoid (KEKB, SuperKEKB)Field strength: 0.4 TCoil length(1 module): 450mmNumber of turns(1 module): 301 turnCurrent: 650A

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OHO07, T. Kamitani

Commissioning

16POSIPOL2016, T. Kamitani

Positron yield optimization In 2015 Oct – Dec FC Peak current: 6kAIncident beam charge: 6.3nC (e-)Positron 1.9nC , Yield: 30%

Design: 50%(at Sector2-end, FC 12kA)(Yield 10% (at Linac end, QWT 10kA))

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Summary of the positron source for the KEKB and the SuperKEKB

• Target• Material: W 14mm (4χ0)• Incident e-beam: 4GeV(KEKB), 3.3GeV(SuperKEKB),

σe- : 0.7mm

• Flux concentrator (SuperKEKB)• In 2015, the commissioning of the FC was carried out. Y(e+) = 30% at 6kA.• The peak current is limited by the breakdown in FC head.• Work-hardening is important to avoid the discharge.

• Capture section• DC Solenoid: 0.4T (KEKB, SuperKEKB)• Large Aperture S-band structure: LAS, Iris diameter: 30mm (SuperKEKB)

• Positron yield• KEKB: QWT(10kA): Y(e+) = 10% (Q(e+): 0.6nC, Q(e-): 6nC),• SuperKEKB: FC(6kA): Y(e+) = 30% (Q(e+): 1.9nC, Q(e-): 6.3nC), (in 2015)

FC(12kA): Y(e+) = 50% (Q(e+): 5nC, Q(e-): 10nC), (Design)

17AWLC2017

Acknowledgement

• I would like to thank Prof. T. Kamitani for giving useful information of the positron source in KEKB and SuperKEKB.

• Next :Calculation of the QWT magnetic field by POISSON

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Calculation of the QWT magnetic field

KEK M. Fukuda

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Motivation

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The aim is to reproduce the magnetic field of the QWT for ILC.It was calculated by Wanming Liu

Procedure ・Read the geometry from the below picture.・Calculate the magnetic field in the case of the geometry by POISSON.

Preliminary conceptual designing works regarding positron capture magnets.pdfW. Liu AWLC2017

93

75 162

180

193

224

206 293

311

346

381

818

853

630

609

518

427

391

357

619

348

382

514

Calibration: X: 50cm/(630-193)px = 0.1144 cm/pxY: 30cm/(619-357)px = 0.1145 cm/px

Origin: 193px, 619px

Pixel value picked up from the picture

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-11.4

-13.5-3.5

-1.5

0

3.5

1.5 11.4

13.5

17.5

21.5

71.5

75.5

50

1.1

11.6

22.0

26.1

30

0

31.0

27.1

12.0

Conversion from the pixel value to the length (cm)

The pixel value is converted to the real length by using the calibration.The origin is the middle between the backing coil and the focusing coil.This geometry is inputted to the POISSON. 22AWLC2017

Comparison of the geometries

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Reproduced picture

Liu’s picture

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Calculation by the POISSON

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Mesh: 0.5mmReturn yokes of Solenoids: Iron (Default: Steel 1010)Ampere turn: Focusing, backing: 121121, -121121 AT

Matching: 210800 AT

These ampere turns were tuned so that these peaks were reproduced.

Backing FocusingMatching

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10345 Gauss

5093 Gauss

FocusingMatching

Magnetic field on the Z-axis

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Z = 0 is the position of the target.

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Backing

Focusing

Matching

10448 Gauss

5114 Gauss

4386 Gauss

Comparison of the strength of the magnetic field on the z-axis

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The field strength at the focusing and the matching coils is identical within 1%.The strength of the middle part between these coils is different about 8%.

Liu’s result Reproduced result

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4386 Gauss4747 Gauss

10345 Gauss

5093 Gauss

10448 Gauss

5114 GaussFocusing

Matching

Summary

• I was able to almost reproduce the QWT magnetic field calculated by Liu-san.

• The field strength and the Ampere tune:• Focusing coil: 1.05T (121121AT)

• Matching coil: 0.51T (210800AT)

• The field strength at the middle part between the focusing and the matching coil is slightly different about 8%.

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28AWLC2017

Flux concentrator

29

The positron yield is decreased due to the offset.Yield: 0.87(ideal) 0.53

After the optimization of the slit position and the e- beam position on the target, the Yield is improved.Yield: 0.53 0.79

IPAC14, MOPRI003, L. Zang

IPAC13, MOPFI017, L. Zang

The FC is placed on the beamline with the offset of 2mm.

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