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

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

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

Target material Requirements T. Suwada et al., Phys.Rev.STAB 10 073501 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

Target destruction limit Material: W75Re25 alloy Target thickness 5.4X0 CLIC note 465 SLAC-TN-15-006 (CN-128) Positron target material test @ SLAC Incident beam energy：20.5-24.4GeV 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] AWLC2017

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

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

AMD(Adiabatic matching device) Adiabatic invariance is constant during the motion. Kamitani20061101.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

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

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: 2.3T @ 10kA 4GeV OHO07, Kamitani 2017/05/22 Posipol2016 “SuperKEKB positron source status” Kamitani OHO07, Kamitani AWLC2017

QWT Target accelerator Pulse coil Field strength: 2.3 T Bridge coil Coil length: 42.5mm 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

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: 100mm 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

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.

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

Positron capture section DC solenoid (KEKB, SuperKEKB) Field strength: 0.4 T Coil length(1 module): 450mm Number of turns(1 module): 301 turn Current: 650A Large Aperture S-band structure: LAS Iris diameter: 30mm (SuperKEKB) IPAC14, THPRI047, T. Matsumoto et.al. OHO07, T. Kamitani POSIPOL2016, T. Kamitani AWLC2017

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

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

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

Calculation of the QWT magnetic field KEK M. Fukuda AWLC2017

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

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/(630-193)px = 0.1144 cm/px Y: 30cm/(619-357)px = 0.1145 cm/px Origin: 193px, 619px AWLC2017

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

Comparison of the geometries Liu’s picture Reproduced picture AWLC2017

Calculation by the POISSON Mesh: 0.5mm Return 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. 10345 Gauss 5093 Gauss Matching Backing Focusing Matching Focusing AWLC2017

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

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

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

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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

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