1. KEKB/SuperKEKB positron source a review and the status
KEK M. Fukuda
AWLC2017

2. 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
AWLC2017

3. Positron source for the KEKB and the SuperKEKB
IPAC10, THPD004, N. Iida
Phase space
Matching device
DC solenoid
Target
Positrons
Primary e- beam
Accelerator
AWLC2017
OHO07, Kamitani

4. Target material Requirements
T. Suwada et al., Phys.Rev.STAB
Target material
Requirements
High Z (Cross section of Bremsstrahlung ∝ Z2/A)
High melting point
Tantalum(73Ta),
Tungsten(74W),
Tungsten- rhenium alloy (W-Re)
KEKB, SuperKEKB
Target 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

5. Target destruction limit
Material: W75Re25 alloy
Target thickness
5.4X0
CLIC note 465
SLAC-TN
(CN-128)
Positron target material SLAC
Incident beam energy： GeV
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: GeV, σx: 0.91mm, σy: 0.35mm,
8x1010 e-/pulse, Area 1.0mm2
 1.95x1012[GeV/mm2]
AWLC2017

6. Matching Device
The generated positrons have the small beam size and the large divergence.
The matching device converts them to parallel beam.
QWT(Quarter wave transformer)
AMD(Adiabatic matching device)
QWT
Phase space
Matching device
DC solenoid
Target
Positrons
Primary e- beam
AWLC2017
Accelerator

7. QWT(Quarter wave transformer)
The QWT transforms 90deg in the phase space.
It captures the positrons satisfying this condition.
Magnetic field: Bi
QWT length: Li
Momentum: pz
(Pz = 10MeV/c in the KEKB)
Energy acceptance
OHO07, Kamitani
Energy acceptance
Transverse acceptance
a: Diameter of an accelerator iris
AWLC2017
CERN-94-1 Positron source, R. Chehab

8. AMD(Adiabatic matching device)
Adiabatic invariance is constant
during the motion.
Kamitani pdf
Primary coil
Conductor e+ e-
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.
adiabatic condition
Transverse acceptance
(B0 = 7.0 [Tesla], μ = 60[1/m])
AWLC2017
CERN94-1, R. Chehab
a: Diameter of an accelerator iris
OHO07, Kamitani

9. Debunching
Because the positrons have the large energy spread, the debunching is caused.
Debunching caused by speed difference
β of positrons is small before acceleration.
Debunching caused by spiral orbit
The positron has the spiral orbit in the solenoid field.
The diameter depends on the energy.
AWLC2017
OHO07, T. Kamitani

10. Matching device SuperKEKB KEKB The Matching device is the QWT in KEKB.
It has been changed to the flux concentrator to increase the positron
intensity in the SuperKEKB.
SuperKEKB
KEKB
Quarter wave transformer
Flux Concentrator
Pulse coil: 10kA
4GeV
OHO07, Kamitani
2017/05/22
Posipol2016 “SuperKEKB positron source status” Kamitani
OHO07, Kamitani
AWLC2017

11. QWT Target accelerator Pulse coil Field strength: 2.3 T Bridge coil
Coil length: mm
Inside diameter: 11mm
Number of turns: 8 turn
Peak current: 10kA
Pulse width: 100us
Bridge coils compensate for the field gaps
between the pulsed coil and the accelerator.
Current shape is a half sinusoidal wave
AWLC2017
OHO07, T. Kamitani

12. Flux concentrator Target: W 14mm(4X0)
Beam size on the target: σx,y 0.7mm
(The size is expanded by the spoiler)
Field strength: 3.5T
Length: mm
Outside diameter: 108mm
Aperture diameter: 7mm(min), 52mm(max)
Number of turns： 12turn
Spiral gap： 0.2mm
Peak current： 12kA
Pulse width: 6us
AWLC2017
PASJ2016 MOP063 Y. Enomoto et.al

13. Flux concentrator Work-hardening Procedure
1. 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)
AWLC2017
POSIPOL2016, T. Kamitani
PASJ2016, MOP063,T. Kamitani et. al.

14. FC power supply
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: 17kV
Pulse width: 5us
Peak current: 12kA
Repetition rate: 50Hz
PASJ2013, SUP057, M. Akemoto et.al
AWLC2017

15. Positron capture section
DC solenoid (KEKB, SuperKEKB)
Field strength: T
Coil length(1 module): 450mm
Number of turns(1 module): 301 turn
Current: A
Large Aperture S-band structure: LAS
Iris diameter: 30mm
(SuperKEKB)
IPAC14, THPRI047, T. Matsumoto et.al.
OHO07, T. Kamitani
POSIPOL2016, T. Kamitani
AWLC2017

16. Commissioning Positron yield optimization In 2015 Oct – Dec
FC Peak current: 6kA
Incident beam charge: 6.3nC (e-)
Positron 1.9nC , Yield: 30%
Design: 50%(at Sector2-end, FC 12kA)
(Yield 10% (at Linac end, QWT 10kA))
AWLC2017
POSIPOL2016, T. Kamitani

17. 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)
AWLC2017

18. 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
AWLC2017

19. Calculation of the QWT magnetic field
KEK M. Fukuda
AWLC2017

20. Motivation
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.pdf
W. Liu
AWLC2017

21. Pixel value picked up from the picture
193
630
180 93
311
381
853
224 75
162
206
293
346
818
348
357
382
391
427
514
518
609
619
Calibration: X: 50cm/( )px = cm/px
Y: 30cm/( )px = cm/px
Origin: 193px, 619px
AWLC2017

22. Conversion from the pixel value to the length (cm)50
-1.5
-11.4
13.5
21.5
75.5
3.5
-3.5
1.5
17.5
71.5
-13.5
11.4
31.0 30
27.1
26.1
22.0
12.0
11.6
1.1
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.
AWLC2017

23. Comparison of the geometries
Liu’s picture
Reproduced picture
AWLC2017

24. Calculation by the POISSON
Mesh: 0.5mm
Return yokes of Solenoids: Iron (Default: Steel 1010)
Ampere turn: Focusing, backing: , AT
Matching: AT
These ampere turns were tuned so that these peaks were reproduced.
10345 Gauss
5093 Gauss
Matching
Backing
Focusing
Matching
Focusing
AWLC2017

25. Magnetic field on the Z-axis
Z = 0 is the position of the target.
10448 Gauss
Focusing
5114 Gauss
Matching
4386 Gauss
Backing
AWLC2017

26. Comparison of the strength of the magnetic field on the z-axis
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
10345 Gauss
10448 Gauss
5093 Gauss
Focusing
5114 Gauss
Matching
4747 Gauss
4386 Gauss
AWLC2017

27. 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%.
AWLC2017

28. AWLC2017

29. Flux concentrator The positron yield is decreased due to the offset.
The FC is placed on the beamline with the offset of 2mm.
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
IPAC13, MOPFI017, L. Zang
AWLC2017
IPAC14, MOPRI003, L. Zang

30. AWLC2017