Experiences on High Current
Y.Suetsugu, KEKB
Contents• Issues in High Current• Problems due to High Current– Vacuum-related– RF-related– Beam Instability• Summary

2004.2.16 9th KEKB Accelerator Review

1Issues in High Current
History of KEKB

1.5 A
1.1 A

2004.2.16 9th KEKB Accelerator Review

2Issues in High Current
Issues in High Current
SR Power[kW] (total)

For E = 3.5 GeV, I = 2.6 A, ρ =14.1m

Photon Number

For E = 3.5 GeV, I = 2.6 A

Max.15 kW m-1[photons s-1] (total)

3×1018 photons s-1 m-1

Gas loadFor η of 1×10-6 molecules photon-1~1×10-8 Pa m3 s-1

Photoelectron emission (LER)
HOM Power[W]For k = 1 V pC-1, I= 2.6 A

P = 14 kW (@τb =2 ns)

Beam Intensity
Wall current (~500 A m-1 at peak)Multipactoring (especially in positron ring)
RF power [Beam Power]
Beam instability
2004.2.16 9th KEKB Accelerator Review

3Issues in High Current
Beam Currents at Present
DesignLER HER
2.6
1.1
~5000
0.52
0.22

Typical Run
LER HER
1.5
1.11284
1.17 0.86

Total Current [A]Bunch #
Bunch Current[mA]
Beam currents are same or near to design value
Bunch number is lower than design value (1/4)
High bunch current
Intense HOM power:
Major issues for hardware at present

2004.2.16 9th KEKB Accelerator Review4

Vacuum-related Problems
High HOM Power
Heating of bellows and gate valves

Bellows near movable masks and IR(big HOM sources)
Especially for bellows and GV with arace-track cross section or big sizeShows resonance behavior
Objects beam injection, Beam oscillation
Cooling fan or exchange
Major problems at present

Damaged fingers in bellows
Pressure rise due to NEG

Heating of NEG elements
Near Movable masks
Abnormal pressure rise
Affect backgroundHOM absorber
RF shield gasket

NEG Element

2004.2.16 9th KEKB Accelerator Review

5Vacuum-related Problems
High HOM Power
Discharges in early versions ofmovable masks

Early versions of MM

Trapped mode
No HOM absorber
Excess heating & discharge Vacuum leak
Improvement of structure (version up)

Excitation of synchrotron oscillation
10 mm

Trapped mode around head

10 mm
Crack due to discharge

Discharge spot

2004.2.16 9th KEKB Accelerator Review

6Vacuum-related Problems
High HOM Power
Discharge in a test-type DCCT for upgrade
Test : stain-less steel → copper
Discharges at Kover joint blazedon ceramics ring
Rapid pressure rise vs. current
Improve structure

Rapid pressure rise even forpresent type[from M.Arinaga]
5 mmCuCeramics

Test model of DCCT
Pressure rise at DCCT region

Pressure [Pa]Over 1100 mA
Similar to test-type
keep watching

Inside2004.2.16 9th KEKB Accelerator Review

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Vacuum-related Problems
High SR Power

Finger in bellows

Melting of RF shield of bellows

SR hit RF-shield finger in bellwos → Leak
Re-alignment of beam chamber
Add SR mask at upstream

SR20 mm
Helico-flex

Melting of Helico-flex

SR hit Helico-flex gasket → Leak
Re-alignment of beam chamber
Add SR mask at upstream

SR5 mmVacuum leak at SR Masks

SR-Mask block

Welding point of SR-mask block
Due to heat cycle and defect of welding
Exchange of chamber
2004.2.16 9th KEKB Accelerator Review

Leak8Vacuum-related Problems
High HOM and SR Power at IR
IR: Complicated configuration
Deformation of beam chamber

Complex at IR

Al chamber at downstream side of IR
was heated and squashed by SR
Change structure & material (Cu)Heating of bellows and chambers
Due to mainly HOM
Increase cooling capacity

Movement of beam chamber

Due to heating by HOM or SR
Make collision tuning difficult
Cause of slow recovery of luminosity after beam abort ?
Feedback movement to BPM signal

2004.2.16 9th KEKB Accelerator Review

9Vacuum-related Problems
High Current (Direct damage)
Grooves at mask head

Damaged by steered beam
Change material copper →Ti
Rapid beam abort system
Not completely solved20 mmGroove on mask head (Cu)

Vacuum leak at abort window

10 mm

Discharge at RF-shield

20 mm

Beams pass just near the wall
Leak at transition from Al to Ti (window)
Thermal stress?
Groove on mask head (Ti)Change structure
Discharges due to wall current
Change structure & material
RF-shield fingers
2004.2.16 9th KEKB Accelerator Review

10Vacuum-related Problems
High Current (Ohmic loss)
Several problems on transverse kicker [from M.Tobiyama]

Damage at flexible structure of feedthrough (Ver.1)
Sagging of kicker plate(Cu), break down of fixing bolt (Ver.2)
Sagging and annealing of kicker plate (Ver.3)
Due to large ohm loss
Change of material (Cu+SUS)
Improve flexibility of feedthroughHeating of feedthrough itself islow enough up to now

30 mm

Kicker plates of transverse kicker
2004.2.16 9th KEKB Accelerator Review

11Vacuum-related Problems
High Current (Ohmic loss)
Heating of SR monitor chamber [from J.Flanagan]
Heating at Stainless-steel chamber
Heating by ohmic loss
Leads to drift of mirror orientation
with beam current
Change to Cu chamber

SRM chamber (Cu)
200 mmMirror orientation drift with beam
1.5
Horizontal

0-1.5700

Mirror Tilt [mrad]

Mirror Tilt [mrad]

1.5

Vertical

1200

HER Beam Current [mA]

0-1.5
700

Horizontal

Vertical

1200

HER Beam Current [mA]

2004.2.16 9th KEKB Accelerator Review

12Vacuum-related Problems
High Gas Load
Design : ΔP/ΔI =~1×10-10 Pa mA-1 (η = 1×10-6 )Pumping speed ~ 0.07 m3 s-1 m-1 with NEG + Ion pump
LER

HER

Almost achieved, but

Scrubbing is slowing down in HER. Why?
NEG activation >20 in some regions. Capacity?
2004.2.16 9th KEKB Accelerator Review

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Vacuum-related Problems
Abnormal Pressure Rise

Pressure rise at Al region

Pressure rise due to multipactoring
Observed in whole LER at
initial stage
Disappeared by solenoid
Especially aluminum chamber
region now
Injection region? (both rings)
More solenoid

Pressure burst

Pressure burst

Observed at new current frontier
Sometimes depends on fill pattern
Sometimes causes beam oscillation
and abort
Wait aging
Exchange of suspicious components
2004.2.16 9th KEKB Accelerator Review

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Vacuum-related Problems
Dust Trapping (electron ring)
Abrupt decrease of beam life time

Sometimes the life time recover soon but sometimes not
Usually recovered by re-fill
Seen frequently after long shut down (lots of vacuum work)
Improve environment during work
Dust trapping phenomena
Wait aging

2004.2.16 9th KEKB Accelerator Review

15RF-related Problems
Issues in RF System
Necessary RF Power ∝ ~SR and HOM power
RF power per cavity is large
LER
ARESHERARESSCC

Total Current [A]

1.5

1.1

# of Cavity

20

128

RF Voltage/Cavity [MV]

0.4

0.35

1. 4

Beam Power/Cavity [kW]130

170300

HOM due to cavity should be well damped
Instability due to acceleration mode should be welldamped
2004.2.16 9th KEKB Accelerator Review16

RF-related Problems
ARES Cavity : LER, HER[from K.Akai]

Discharges at input coupler

Sometimes lead to vacuum leaks
Arc sensors in vacuum andatmosphere
Rapid beam abort system
Regular maintenance
ARES cavity

Burn out of RF damper at
coupling cavity

Due to leak of cooling wateror bad RF contact at dummy load
Improve temperature monitoringsystem
Improve RF contact

No severe troubles these 2 years2004.2.16 9th KEKB Accelerator Review

17RF-related Problems
Super Conducting Cavity : HER[from K.Akai, T.Furuya]

SCCNo severe troubles so far
Heating of HOM absorber
Due to higher bunch current
than design value
Design: 5 kW
now:10 kW
No problem up to now
keep watchingCoupler
Design: 250 kW/cav.
now:300 kW/cav.
No problem up to now

2004.2.16 9th KEKB Accelerator Review

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RF-related Problems
Instabilities Caused by Cavities[from K.Akai]-1 mode coupled bunch instability

Due to low operating RF voltage
Expected at ~ 2A, but observed at ~1A
Prepare –1 mode damper

0 mode oscillation and instability
Due to noise?
Prepare 0 mode damper
Tuning of RF feed back system
Increase RF voltage

Coupled bunch instability due to HOM of cavities
Not observed yet

2004.2.16 9th KEKB Accelerator Review

19Beam Instability
Electron Cloud Effect in LER

Solenoid[from H.Fukuma]

Blow up of vertical beam size

Explained by head-tail instability
caused by electron cloudMitigated by solenoid winding,but not perfect yet
Continue solenoid winding further

Effect of solenoid on luminosity

525

2001 July : off

1 train, 1153 bunches,
4 rf bucket spacing
20

2001 July : on
CsI Specific/bunch

Vertical beam size@IP (micron)

432001 Dec. : on
212002 Feb. : on

C yokes

15Solenoid on

105Solenoid off
00200

400

600

800

1000

LER beam current (mA)

1200

1400

1600

0200

300

400

500

600

700

800

LER current (mA)

2004.2.16 9th KEKB Accelerator Review

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Beam Instability
Coupled Bunch Instability (CBI) in LER[from H.Fukuma]

CBI has been observed in LER

Mode spectrum and growth rate depend on solenoid
Horizontal/Vertical growth rate is 0.5/0.3 ms-1 at 600 mA withsolenoid
CBI is suppressed by a bunch by bunch feedback at present

16x 104Horizontal

44x10

Vertical

1400

600 mA

Solenoids on
Solenoids off

Solenoids off
1050

8700

435000200

400600600 mA

Solenoids on

1202500

800

1000

1200

1400

000200

400

600

800

ModeMode

Horizontal

Vertical

1000

1200

1400

2004.2.16 9th KEKB Accelerator Review

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Beam Instability
Coupled Bunch Instability in HER[from H.Fukuma]

CBI has been observed in HER
Simulation suggests that the horizontal oscillation is caused by CO+ ion
Amplitude grows along train and saturate
Peak is about 10th revolution harmonics in mode spectrum
Growth rate is about 10 ms-1 at 700 mA
Suppressed by a bunch by bunch feedback

Oscillation amplitude along a train(four trains in HER)

Time evolution of oscillation
2004.2.16 9th KEKB Accelerator Review

22Summary
We have experienced various problems in high currents
operation at KEKB and learned lots of things.
Some problems could be avoided by more delicate
consideration in design of components and more careful
installation of them.
But others were revealed for the first time under highcurrent and are indications of problems that will appear in
higher current operation.
The important thing is to study and understand the problems
accurately, which makes them valuable lessons for us
toward future high intensity machines.
Struggling against high currents will continue for higher
luminosity.
2004.2.16 9th KEKB Accelerator Review

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