Positron source
Y. Enomoto

2018.3.14

The 22nd KEKB Accelerator Review

1Present status


Toward phase 2 → ready

Required 0.5 nC x 2

Toward phase 3 → under development
Required 4 nC x 2

2018.3.14

The 22nd KEKB Accelerator Review

2Outline





Setup
Toward phase 2
Improvements from phase 1Quick Exchange mechanism
Toward phase 3
Requirement and improvements toward phase 3Increase primary electron charge and energy @ target
Increase positron production efficiency
Decrease loss between the target and the DR
Evaluate practical requirementSummary2018.3.14

The 22nd KEKB Accelerator Review

3Positron source setup 1
Positron target and capture section

2018.3.14

The 22nd KEKB Accelerator Review

4Positron source setup 2
FC head + BC + target = FC assembly
BC (bridge coil)target

2018.3.14

LAS (large aperture S-band)FC(flux concentrator) head

The 22nd KEKB Accelerator Review

solenoid5

Positron source setup 3
beam

W targetΦ2 mm hole for electron
Temp sensor
2018.3.14

Cooling water pipe The 22nd KEKB Accelerator Review

6Positron source setup 4
Bridge coil
DC magnetI = 600～750 A

How the FC works?

e•••Flux Concentrator
pulsed magnet
I = 6000～12000A

targete+Electron hit small point of the target.
Positron comes from small spot.
Angle distribution is large.
Calculated by T. Kamitani

targete+•••Adiabatically expand position distribution.
Angle distribution decrease.
Matched to the aperture of the following LAS.
2018.3.14

Ratio B @ target / B @ entrance of the LAS is importantstronger field is preferred in the following solenoid section
much stronger field is required @ target

The 22nd KEKB Accelerator Review

7Outline



Setup
Toward phase 2
Improvements from phase 1Quick Exchange mechanism
Toward phase 3
Requirement and improvements toward phase 3






Increase primary electron charge and energy @ target
Increase positron production efficiency
Decrease loss between the target and the DR
Evaluate practical requirement

Summary

2018.3.14

The 22nd KEKB Accelerator Review

8Improvements from phase 1
Phase 1

Phase 2 (now)

FC assembly

#1

#2

IFC

5～6 kA

5～6 kA

12 kA①Primary electron @ target

6.3 nC

6.9 nC

10 nC②Positron @ entrance of LTR

1.9 nC

2.0 nC>4 nC③Positron @ LINAC end

0.75 nC

1.4 nC>4 nC

Positron @ BT end

0.3 nC

1.4 nC?

4 nC

Replacement

Not considered

1 week①

Phase 3 (design)②③

More than four times positron to the ring
thanks to the DR
Quick replacement mechanism
Preparation against damage to the production system.
2018.3.14

The 22nd KEKB Accelerator Review

9Charge along the beam line
Phase 10.34 nC

LINAC

BT

0.75 nC

Electron gun

2018.3.14

Positron target

Linac end

The 22nd KEKB Accelerator Review

10Charge along the beam line
Phase 10.75 nC

Electron gun

Positron target

Linac end

Loss in the 3 sector decreased thanks to the DR

Phase 2

1.4 nCElectron gun

2018.3.14

Positron targetDR

The 22nd KEKB Accelerator Review

Linac end11

Quick exchange mechanismBefore the installation of the quick exchange mechanism(phase 1), in case of emergency on the target / FC system,
Heavy exposure to radiation is inevitable.
Need long (>1 month) cooling time.
Fortunately this did not happenThe mechanism is installed
In March 2017. (8 months after the end of phase 1 operation)
Old assembly (used in phase 1) was replaced by new one
using this mechanism.
We demonstrated that replacement work can be done within 1week.

2018.3.14

The 22nd KEKB Accelerator Review

122018.3.14

The 22nd KEKB Accelerator Review

132018.3.14

The 22nd KEKB Accelerator Review

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The 22nd KEKB Accelerator Review

15

Evaluation of radiation
Measured by S. Matsumoto
Expected (25 pps operation)

Measured (1 pps operation)

March 2017

Cooling timeRadiationdosecomment

8 months

50 uSv/h< 10 uSv/day

Measured in March 2017

1 hour3000 uSv/h< 600 uSv/day

100 hours

300 uSv/h< 60 uSv/dayRadiation rate gets 1/10 at 100 hours after stopping operation
2018.3.14

The 22nd KEKB Accelerator Review16

Outline




Setup
Toward phase 2
Improvements from phase 1Quick Exchange mechanism
Toward phase 3
Requirement and improvements toward phase 3






Increase primary electron charge and energy @ target
Increase positron production efficiency
Decrease loss between the target and the DR
Evaluate practical requirement

Summary

2018.3.14

The 22nd KEKB Accelerator Review

17Requirement and Improvements toward phase 3

Provide 4 nC x 2 positron 25 Hz at injection point ofthe LER.
Big difference among phase 2 (0.5 nC), phase 3 (4 nC) andpresent (1.4 nC).

1. Increase primary electron charge and energy @ target
2. Increase positron production efficiency
3. Decrease loss between the target and the DR
4. Evaluate practical requirement

We do not have plan to meet everything.
Now we are in the stage to reconsider strategy and decide priority.
2018.3.14

The 22nd KEKB Accelerator Review

181. Increase primary electron charge and energy @ target
energy

2018.3.14

Unfortunately 1 unit was down due towater leak when we took the data
This will be repaired shortlytarget

2.5

Q@SP_16_5 (nC)There are 21 (+1 standby B7)
acceleration unit before thetarget
We will re-install 1 unit in2018

5 % up in energy
Some units are not in fullpower operation

A few % up in energy

Positron charge @ downstream of the target

21.5
10.501617

The 22nd KEKB Accelerator Review

18
19unit # before target

2021191. Increase primary electron charge and energy @ targetcharge
Decrease loss in the firstsection
We have been trying butnot easy…
Increase 2nd bunch charge
Optimizing timing at thebunching section
Increase charge at gun
High efficiency cathodehas developed recently by
the injector group2018.3.14

1st bunch
2nd bunch

The 22nd KEKB Accelerator Review

20

2. Increase positron production efficiency
Simulation by F. Miyahara

Positron Yield=SP_16_5_Q/SP_15_T_Q

designPresent expectation

Parameter

simulation

ExperimentBC600 A

600 A

SL_15_11

650 A600 A

SL_16_11

650 A

400 A

SL_16_22

650 A

400 A

SL_16_33

650 A

650 A

ACC_15_1/2

10 MV/m

8.5 MV/m

ACC_16_1/2/3/4

12 MV/m

12 MV/mIncreasing FC current, positron yield of 0.45
(12 kA) is expected from experimental data.
Difference from design is still not clear.

FC current (kA)

SP_15_T_Q (nC)

SP_16_5_Q (nC)

Positron Yield

06.93

1.26

0.18

66.93

2.50

0.36

12 (expectation)

6.93

3.120.45

2018.3.14

The 22nd KEKB Accelerator Reviewx1.25
212. Increase positron production efficiency1 kV / turn @ 12 kA operation 6 us pulse width
Slit gap = 0.2 mm

5 MV/m
FC current waveform measured by CTFC current (kA)

6 us

2018.3.14

The 22nd KEKB Accelerator Review22

2. Increase positron production efficiency
What we did during 2016～2017Long term test with full assembly test standTest under almost the same condition as that in the tunnel.
Confirmed 12 kA operation in this test stand.

Work hardening process
Recommendation from specialist in SLAC.

Unfortunately situation was not improvedWhat we plan to do in 2018
Test with longer pulse but reduced voltageA modified pulsed power supply is under preparation by the RF group∝

FC current

Pulse widthPower supply voltage

Now

6 kA

6 us

9 kV

planned

12 kA

12 us

9 kV

Electromagnetic field simulation and optimization of the head shapeConsider other solution than FCRecommendation from the previous domestic review
Only 1.25 times gain is expected even 12 kA operation.
2018.3.14

The 22nd KEKB Accelerator Review

23

3. Decrease loss between the target and the DR

Increase some pulsed magnets in this section in 2018.
No other concrete plan now.
We need ideas and resources.

2018.3.14

The 22nd KEKB Accelerator Review

244. Evaluate practical requirement
Present working assumption
3.6 A360 s

Life360 s

Current

3.6 A

Injection bunch charge

4 nC

Injection rate

25* Hz x 2 bunch

Injection efficiency

50 %Circulation frequency

105

4 nC

25 Hz

0.01 A/s

2 bunch50 %0.01 A/s

10 Hz*The other 25 Hz will be used for HER injection
Injection rate balance between the HER and the LEC will be determined by the ratio of the life in each ring

No one knows precise life time in the LER now.
We expect the value gradually gets clear during phase 2 operation.

2018.3.14

The 22nd KEKB Accelerator Review25

SummaryWe are ready for phase 2
1.4 nC positron (requirement 0.5 nC)

Quick exchange mechanism
We are in the stage to reconsider strategy and decide priority
toward phase 3Increase primary electron charge and energy @ target
Increase positron production efficiency
Decrease loss between the target and the DR
Evaluate practical requirement

2018.3.14

The 22nd KEKB Accelerator Review

262018.3.14

The 22nd KEKB Accelerator Review

27Fighting against dischargeFC head #4
～2015/9 test w/o BC @ test stand
12 kA→OK
2015/9 Installed into the beamline
Discharge happened at 9.5 kA after a few days
operation.

2015/9～2016/7
Reduced current (6 kA) operation during phase 1.

2017/3
Replaced by FC head #5 (with assembly #2).

Visit specialist in SLAC.
Work-hardening process is important.
External magnetic field by BC is different.

2018.3.14

The 22nd KEKB Accelerator Review

28Fighting against dischargeFC head #5

2016/6Work-hardening process～2017/3 test w BC @ test stand
12 kA→OK
2017/3 Installed into the beamline
Discharge after a few days operation
2017/3～2017/5

Not in operation
2017/6～2017/9
Disassembled from the beamlineObserved
2017/9
Re-installed in the beamline
2017/10
6 kA operation

Reconstruct test stand
Try longer pulse operation
2018.3.14

The 22nd KEKB Accelerator Review

29Fighting against discharge
FC head #6

2017/10Work-hardening process～2018/3 reconstruct test stand
2018/4～
Try longer pulse operation∝
FC current

Pulse widthPower supply voltage

Now

6 kA

6 us

9 kV

planned

12 kA

12 us

9 kV

New power supply will beInstalled 2018/3

Previous test stand space was evacuated for the re-installationof 1-3 klystronEnergy of primary electron will be increased by 5 %→positron charge will expected to be increased by 5%2018.3.14

The 22nd KEKB Accelerator Review

30Capture section after target
FC+targetsolenoid

AC_15_1

AC_15_2

AC_16_1

LAS
AC_16_2

AC_16_3

AC_16_4

Normalsolenoid

Q

AC_17_1

2018.3.14

AC_17_2

AC_17_3

AC_17_4

AC_18_1

The 22nd KEKB Accelerator Review

AC_18_2

AC_18_3

AC_18_4

31

Positron yield vs Acc voltage

2018.3.14

now-20%1-538 kV

35 kV (-3 kV)

1-642 kV

39 kV (-3 kV)

1-740 kV

35 kV (-5 kV)

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2018.3.14

The 22nd KEKB Accelerator Review33