Damping Ring
M.Tawada
On behalf of DR group

The 27th KEKB Accelerator Review Committee

1Contents
1. Introduction
2. Status & Issues
3. Work on DR during LS1
4. Summary

The 27th KEKB Accelerator Review Committee2

1. Damping Ring

The 27th KEKB Accelerator Review Committee3

1. Damping Ring Layout

Injection

C = 135m

Collimation system
ECSBCS

RF Cavity
Extraction
The 27th KEKB Accelerator Review Committee41. Pictures of DR
Normal Bend

LTR
DCCT

FB Kicker

Reverse Bend

Quadupole &Sextupole

Injection Kicker PSRTLInjection Septum

The 27th KEKB Accelerator Review Committee
RF Cavities51. Features of the DR• Large aperture enough to accept the Linac positron beam
• FODO with reverse-bend ensuring fast damping time of 12 ms, with lower bend field.
• Two pulses with 2 bunches each can be stored.
• Kickers with fast rise and fall-time of less than 100 ns and no ringing.
• Beam pipes with antechamber to accommodate photon masks, enabling very low transverse
impedance.
• TiN coating in the vacuum chambers to mitigate electron cloud effect.
• HOM-damped, high-voltage RF cavities to ensure a large energy acceptance and high bunchcharge.
• Collimation system installed at the switching yard in the Linac to cut the energy and emittance
tails.

The 27th KEKB Accelerator Review Committee

61. Parameters of DRParameters
Energy1.1

GeV

Number of bunch trains

2

Number of bunches /train

2

Circumference

135.498m

Max. stored current

70.8

mA

Energy loss per turn

0.0847MV

Damping time (τx/τy/τz)

11.5/11.7/5.8

ms

Injected beam emittance

1400

nm

Equilibrium emittance(εx/εy/εz)

29.2/1.5/3630

nm

nx/ny/nz

8.83/6.28/-0.018

Energy spread

5.5 x 10-4

Bunch length

6.6

Momentum compaction factor

0.01

Cavity voltage

1.0MV

RF frequency

509

MHzThe target equilibrium horizontal
emittance of 30 nm has been nearly
achieved.

mm

The 27th KEKB Accelerator Review Committee7

2. DR status & Issues
ØDR commissioning stated in Feb. 2018.
ØThe Operation of DR is ongoing smoothly without any major trouble after LS1.
ØIssues1.

2.
3.

Due to unexpected emittance growth after DR and small dynamic aperture of LER, lower
emittance of the DR is required. However, several attempts at lower emittance operation havefailed.
Beam orbit drifts and fluctuations in the DR, making optics corrections challenging.
Extracted beam orbit drifts are observed, stemming from the temperature dependence of
output current of extraction kickers.

The 27th KEKB Accelerator Review Committee

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H. Sugimoto(1) Lower emittance optics• Thanks to the FODO cell with reversed-bend, DR is capable of lower emittance operation by increasingarc cell tunes and lowering momentum compaction factor.
• Several attempts at lower emittance operation were conducted.
• These attempts failed due to unexpected large optics distortion, resulting in beam losses during beaminjection.

Low emittance optics

Nominaloperationpoint

The 27th KEKB Accelerator Review Committee9(1) Lower Emittance Operation Trials

H. Sugimoto• Several trials failed due to unexpected large optics distortion.
• The optics distortions could not be corrected by the quadrupole families.
• Adjustment of main bending magnets may improve the situation. (Not tried yet.)
Beta Beating

Residual Dispersions

The 27th KEKB Accelerator Review Committee10(1) Lower Emittance Operation Trials

H. Sugimoto• Uncertainty of the implemented excitation curves in the magnet system- Last year, It was reported that there was a mistake in the calculation of the excitation curve ofarc quadrupole magnets.
- The report is consistent with the correction factors identified by optics corrections.
-Using these correction factors, we attempted lower emittance operation again, but
unfortunately, it still didn’t succeed.
(1) Correction factors applied in the operation(Determined by optics corrections)(2) Correction factors anticipated due to themis-implementation of the excitation curve.
(3) = (2) - (1)The 27th KEKB Accelerator Review Committee11(1) Magnetic field measurement data comparison• Re-measurements of the spare magnets using different measurement system indicate
significant correction factors, a few % for bending magnet and 5 -10 % for quadrupole magnets.
• For quadrupole magnets, there is also a significant variation in implemented excitation curves.
• We intend to re-measure the remaining spare magnets using an alternative system and replace
excitation curves.
DR main dipole

DR-Arc QuadRTL-Quad @30A

■ data in use●re-measured data

The 27th KEKB Accelerator Review Committee

12Magnetic field measurement devices

Implemented data were measured by using thisharmonic coil.

Rotating stretched-wire coil as an alternative
measurement device

The 27th KEKB Accelerator Review Committee13(2) Orbit drift problem

QD11T. Ueda

• Beam orbit fluctuations and drifts were observed in a decay mode in the DR. The fluctuations depend on
the temperature of vacuum chamber, which is cooled by the water chiller system.
• Narrowing the temperature setting range of the cooling water chiller unit decreased the orbit fluctuations.
However, slow orbit drifts are still observed depending on the beam currents.
Beam position(QD11:horizontal)
Beam current
Chamber temperature

0.4℃
1.5℃

Before improvements

The 27th KEKB Accelerator Review Committee

After improvements

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T. Ueda(2) Orbit drift problem

Cooling water
channelBPM
Cooling water

Temperature
sensor
Water Chiller unit for vacuum chamber（Installed in power supply building）

BPM blocks, which are fixed to the Q-magnets.

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H. Sugimoto(2) Orbit drift problem on DR Optics Control• Beam current-dependent (apparent?) dispersion function- BPM reading depends on beam current, especially when RF frequency is changed.
- Three consecutive dispersion measurements show systematic dispersion changes.
Red: Beam current
Blue: RF frequency#1#2

Dispersion #2- #1

Dispersion #3- #1#3

The 27th KEKB Accelerator Review Committee

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H. Sugimoto(2) Orbit drift problem on DR Optics Control• Two examples of BPM readings vs beam current with different RF frequencies.
- The beam current dependency depends on RF frequency.
- A hundred-micron difference in beam position at frequencies of +-50KHz results in a 10 mmdifference in the measured dispersion functions.
- What is the best way to extract the dispersion function for optics correction?

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S. Miyahara(3) Extracted Beam Orbit Drift Issue• A slow orbit drift of extracted beam was observed at the Linac BPM. These orbit drift changes when therepetition changed, which is much clearer for the 2nd bunch.
• It takes a long time to reach equilibrium for the high repetition rate (> 2 hour for 12.5Hz).
• The cause is identified as the temperature dependence of the output current of the extraction kicker.
LinacBPM
SP_DC_4

1 hourDR ext.
kicker

0.1 mmRTL
The 27th KEKB Accelerator Review Committee18

(3) DR extraction kickers
•
•
•
•
•
•

No. of magnets: 2Deflection angle: 4.7 mrad
Pulse shape: double half sine
Pulse width: < 300 ns
Rise /Fall time: < 100 ns
Peak currents: 1546 AThyratronC1in the tunnel
in the tunnel

The 27th KEKB Accelerator Review Committee19(3) R&D for reducing temperature dependence• The temperature dependence has decreased to about 1/3 by• Using mica capacitors in stead of ceramic capacitors,• Switching from natural oil cooling to forced oil cooling.

• Successfully operated it for over 100 hours at rated voltage using mica capacitors.
• However, the test didn’t meet the installation deadline, unfortunately.

The 27th KEKB Accelerator Review Committee20

(3) R&D for reducing temperature dependenceCeramic

Capacitor temp.
(℃ )C1-C2 time(s)C2-time (s)C1Volt ( kV)

C2 Volt (kV)

Output Cur(A)Start time

24.5

3.36E-07

1.877E-07

27.24

36.3

1044

End time

30

3.337E-07

1.851E-07

27.24536.545

1023

ΔT

5.5

-2.3E-09-2.6E-09

0.005

0.245

-21

ΔI(%)

-0.685%-1.385%0.018%

0.675%-2.011%

ΔI/ΔT(%)-0.124%-0.252%0.003%

0.123%-0.366%mica

Capacitor :temp.
(℃ )C1-C2time(s)C2-time (s)C1 Volt (kV)C2 Volt (kV)

Output Cur (A)Temp.
depedendence
724

Start time

11.5

3.196E-07

2.131E-07

27.24235.79

End time

26.1

3.202E-07

2.114E-07

27.265

35.74

736.5

ΔT

14.6

6.000E-10-1.700E-09

0.023

-0.05

12.5

ΔI(%)

0.19%

-0.80%

0.08%-0.14%

1.73%

ΔI/ΔT(%)

0.013%-0.055%0.006%-0.010%0.118%

Stability specification is < ±0.1%
The 27th KEKB Accelerator Review Committee21(3) R&D for reducing temperature dependenceMica capacitors

The 27th KEKB Accelerator Review Committee

Force oil cooling system

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3. Work on DR during LS1ØDuring LS1, two tasks were carried out.
ØChange the material of the vacuum seal from Viton to metal. It will bereported later.
ØRepair of minor oil leaks in the magnetic switch tanks of the extraction kickers.

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DR / RF Vacuum improvement by O-ring replacement during LS1T. Abefor the six connections
between the cavities
and beam ducts

Metal O-ring

Viton O-ring(~1e-5 Pa in the RF section before LS1 using both IPs and TMPs

Changing from Viton O-rings to metal seals improved
vacuum levels significantly.
More details will be reported by Abe-san later.

~1e-7 Pa in the recent RF section using IPs only

Vacuum improvement in the DR / RF section
by two orders of magnitude
during DR beam operation
The 27th KEKB Accelerator Review Committee

IP: sputter-Ion Pump (400L/s)
24
TMP: Turbo-Molecular Pump (300L/s)

Fixing oil leaks from welded bead
• Minor oil leaks were detected in the magnetic switch tanks of the extraction kicker.
• The exact leak location couldn’t be identified in the tunnel.
• Tanks were removed from the tunnel for repair during LS1 and subsequentlyreinstalled.

The 27th KEKB Accelerator Review Committee25

4. Summary• DR operation is ongoing without any major trouble.
• We will attempt lower emittance optics again by replacing excitation curves.
• Due to the temperature dependence of the beam orbit, optics corrections havebecome challenging.
• The vacuum level of the RF cavity has ben significantly improved.
• To enhance reliability of kickers, we are conducting further tests using a micacapacitor at higher voltages for a longer durations.

The 27th KEKB Accelerator Review Committee

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