Present status of
the SuperKEKB Vacuum System
Y. Suetsugu, KEK
on behalf of KEKB Vacuum Group
Contents
Brief review of the vacuum system Requirements, features, construction, ….
 Present status Vacuum scrubbing
 Status of new components
 Major problems
Non-linear pressure rise (E-cloud effect)• Pressure bursts accompanying beam loss
• Etc.
•


2016/6/13

Summary and plan

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1Outline of upgrade
LER: Approximately 93% of beam pipes in length are renewed.
 HER: Approximately 82 % are reused.


LER

HER

New: 93%

New: 18%(2610m/2800m)(500m/2800m)

Sub systems, such as cooling water system, compressed air system,
were basically reutilized, with necessary upgrades.
 Control system was also reused, but the antique components are
updated.


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2Requirements to vacuum systemUltra‐high vacuum to keep small beam emittance, to reduce
background noise of detector, to avoid ion instability (HER)High tolerance against high current beams for stable operationBeam pipe with antechamber, step‐less (MO‐type) connection
flange, new collimators with low impedance

Suppression of electron cloud effect (ECE) to avoid beam size
blow up (LER)

New RF shields for bellows chambers and gate valves

Low beam impedance to keep small beam emittance and shortbunch length, and to avoid beam instabilities

Target pressure : ~ 10‐7 Pa with beams

Beam pipe with antechamber, solenoid field, coating of TiN film, etc.

Efficient use of KEKB resources as much as possible
82% of HER beam pipes are reutilized.
 Cooling water system , control system, etc. are also basically reused.


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Beam Pipe

New beam pipes with antechambers
Realize low beam impedance by putting SR mask and pump ports
in antechambers
 Effective to reduce photo electron effectTypical size: 90 x w220
 LER arcs: Aluminum alloy (A6063)
•

HER and Wigglers: OFC (C1011)•

Formed by extrusion method
Formed by cold‐drawing method~1230 pipes in total

Beam pipes for wigglers

Beam pipes forbending magnets

Beam pipes forquad magnets

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4Bellows, Gate Valve and FlangeNew bellows chambers and gate valves with comb‐type RFshield inside

High thermal strength Applicable to cross sections with antechambers
 Bellows chambers: ~1240, GVs: ~40
MO‐type flanges
RF‐shield
 Little step inside and low
beam impedance Adaptable to various typesof cross sections
 ~ 5000 flanges


Gate valve forantechamber

Aluminum alloy bellows chamber

Various types of MO flange

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5Pump system

Main: Three layers of NEG strips ST707 (arc)Installed into an antechamber: provide effective
distributed pumping system
 Activation by micro‐heaters (sheath heaters)
inserted between strips
 Screens (RF shield) between pump and beam
 Average pumping speed of 0.14 m3s‐1m‐1 for CO ~ 1130 pieces

3 layer NEG strips

Aux: Ion pumps


Noble pump with a pumping speed of ~ 0.4 m3s‐1
Photon line density at arc

Effective pumping
speed and pressure
=110‐6 mole. pho.‐1)
Eff. pumping speed

0.11 m3s‐1m‐1

Pressure
2.310‐7 Pain average
Bending magnets
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6Countermeasures against ECEElectron cloud instability can be a serious problem for LER (e+)
Blow up of beam size Deterioration of the luminosity

For the SuperKEKB, the threshold of electron density to excite
the head‐tail instability: ~2x1011 e‐ m‐3. (Ohmi‐san et al.)
 Various mitigation techniques were adopted in the SuperKEKB.

Antechambers
TiN coating

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Solenoid fieldClearing
electrode (w)21th KEKB Review

Groovedsurface (B)7Countermeasures against ECECountermeasures in SuperKEKB positron ring (Final configuration*)
Sections

L [m]

L [ %]Countermeasure

Material

Total

3016

100

Drift space (arc)1629 m54

TiN coating + Solenoid

Al (arc)

Steering mag.

316 m

10

TiN coating + SolenoidAl

Bending mag.

519 m

17

TiN coating + Grooved surface

Al

Wiggler mag.

154 m

5

Clearing ElectrodeCu

Q & SX mag.

254 m

9

TiN coating

Al (arc)

RF section

124 m

4

(TiN coating +) SolenoidCu

IR section

20 m

0.7

(TiN coating +) Solenoid

Cu or Al

*Solenoid coil has not yet applied.

Electron density along the ringWith above
countermeasures,ne ~2x1010 e‐ m‐3
is expected.
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8ConstructionAchievement of construction

Fabrication of main components
finished in FY2014.
 Pre‐installation work, including
TiN coating (LER) and baking,also finished in FY 2014.


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9Pre‐installation work
Basically all beam pipes are baked at 150C for ~24 hours including
NEG activation before installing them into the tunnel.
 Inside of LER beam pipes are coated with thin TiN to mitigate ECE.


Delivery

Clean room

UnpackingAssembling

TiN coating

Storage
Inside check
Clean room

HER
Assembling(pumps)Pre‐baking +NEG activation

LER
Pre‐baking
TiN coating

Storage

BakingStorage(Filled with N2)
Installation
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10ConstructionAchievement of construction

Fabrication of components
finished in FY2014.
 Pre‐installation work, including
TiN coating (LER) and baking,also finished in FY 2014.
 Installation of components into
the tunnel completed in 2015.
 The upgrade of sub systems
also completed.


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11Installation into the tunnelInstallation of beam pipes (2013)
Beam pipe for wiggler

Beam pipe for quad

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Gate valve at arc

Beam pipe for bend

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12Installation into the tunnelInstallation of bellows chambers follows the beam pipes (2014)
Al bellows chamber

Cu bellows chamber

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Cu‐Al bellows chamber for GV

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13ConstructionAchievement of construction

Fabrication of main components
finished in FY2014.
 Pre‐installation work, including
TiN coating (LER) and baking,also finished in FY 2014.
 Installation of components into
the tunnel completed in 2015.
 The upgrade of sub systems
also completed.
 NEG activation in the tunnel
finished by the end of 2015.


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14Activation of NEG pumpsRegions between gate valves in the tunnel are evacuated in series.


We have approximately 40 gate valves in a ring.

After rough pumping and He leak check, ion pumps are bakedfor one day. NEG pumps are finally activated at the last phase of
the baking.
 No beam pipe baking in the tunnel
 After NEG activation, average pressures of less than 1x10‐7 Pa
were successfully obtained.


NEG act

NEG act

NEG act

1E-7 Pa

1E-7 Pa
1E-7 Pa

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ConstructionAchievement of construction

Fabrication of main components
finished in FY2014.
 Pre‐installation work, including
TiN coating (LER) and baking,finished in FY 2014.
 Installation of components into
the tunnel completed in 2015.
 The update of sub systems also
completed.
 NEG activation in the tunnel
finished by the end of 2015.
 Beam commissioning (Phase‐1)
started in February, 2016.


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16Present status ‐ 1
Overall, the vacuum system made a good start.
 Beam currents and average pressures (‐ 2016/6/10)
[LER]• Base pressure ~5x10‐8 Pa• Max. Beam current: 910 mA• Avg.
~ 2x10‐6 Pa• Lifetime ~ 75 min.
(with emittance knob)[HER]• Base pressure ~3x10‐8 Pa• Max. Beam current: 830 mA• Avg.
~ 2x10‐7 Pa(whole ring)~ 1x10‐7 Pa(arc sections)• Lifetime ~ 600 min.
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Present status ‐ 2Status of new vacuum components
Confirming the stability of the new vacuum components was a major
subject for the Phase‐I beam commissioning.

No extra heating or abnormal pressure rise in these components has
been observed so far.
 The temperature rises in the bellows chambers, gate valves and flanges
are less than 3‐4 C at 890 mA, for example.
Gate valve

Bellows chamber

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Present status ‐ 3
Vacuum scrubbing: LER (1)The dP/dI value for arc section, where all of the beam pipes were newly
fabricated, was ~2x10−6 Pa A−1 at the beam dose of 500 Ah. The  was~1x10‐5 molecules photon‐1 [for pumping speed = 0.4 m3s‐1m‐1].
 The decrease rate of dP/dI slowed down from the beam dose of 100 Ah. 
Nonlinear behavior of pressures against the beam current (discussed later).

‐2016/6/10‐2016/6/10

Pressure

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Ix

19

Present status ‐ 3
Vacuum scrubbing: LER (2)Compared to the case of the KEKB, which used circular copper beam pipes
without any coating, dP/dI value at the initial stage was lower by several
factors. Recently, however, dP/dI value is almost the same as the case of
KEKB at the same beam dose.

[KEKB]‐2016/6/10‐2005/7/1

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Present status ‐ 3
Vacuum scrubbing: HER (1)

The dP/dI value at the arc section, where most of the beam pipes wasreused from KEKB, was ~5x10−8 Pa A−1 at the beam dose of 500 Ah. The 
was ~1x10‐7 molecules photon‐1 [for pumping speed = 0.3 m3s‐1m‐1]. The 
is much smaller than that of LER.

‐2016/6/10‐2016/6/10

Pressure

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Ix

21

Present status ‐ 3
Vacuum scrubbing: HER (2)The dP/dI value is comparable to that in the case of KEKB at the final stage.
This means that the surface of the reused beam pipes “remembers” theconditions in the KEKB, even though the beam pipes were exposed to the
air for vacuum work.
 Now  is almost the same value with the final value of the KEKB HER.
[KEKB]‐2016/6/10‐2005/7/1

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Present status ‐ 4Residual gases
Residual gases during the beam operation have been monitored with aquadrupole mass analyzer (QMA) at an arc section.
 The QMA is located just above a sputter ion pump.
The main gases are hydrogen(m/e = 2), carbon monoxide(m/e = 28), methane (m/e =16), water (m/e = 18), andcarbon dioxide (m/e = 44).
••

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The high partial pressure ofmethane should be due to thepumping system using NEG as amain pump.
Because the beam pipes were
not baked in the tunnel, water
vapor still remains in the beampipe.
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H2
CO,N2
CH4
H2O
CO2
Ar
O2

23

Present status ‐ 5Effect of antechamber and TiN coating (1)In relation to the ECE, we set up two electron current monitors used in theexperiment in KEKB for measuring the electron numbers around thecirculating beam in the new aluminum beam pipe
• The measured electron currents almost reflect the electron density
around the beam orbit.
 The electron currents in an aluminum beam pipes with and without TiN
coating can be measured at the same time.
• Note that all of other new beam pipes were coated with TiN.


[Al_Ante+TiN][Al_Ante]

Retarding gridCurtesy of Kanazawa
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Present status ‐ 5Effect of antechamber and TiN coating (2)The electron currents were measured during the beam operation, andcompared with the data of KEKB.

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Present status ‐ 5Effect of antechamber and TiN coating (2)The electron currents were measured during the beam operation, andcompared with the data of KEKB.

Low current region (Ie < 400 mA)
Electron currents of the beam
pipe with antechambers aremuch lower than those without
antechambers.



Reduction of photoelectrons byantechamber structure wasreconfirmed.

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Present status ‐ 5Effect of antechamber and TiN coating (3)The electron currents were measured during the beam operation, andcompared with the data of KEKB.

High current region (Ie > 500 mA) (1)Electron current at aluminum with
TiN coating is much lower thanthat without it, and is comparablewith copper antechamber taking
into account the difference ofphoton flux.

TiN coating on aluminum is
working well.
 The estimation of SEY is inprogress. Furthermore, change in
SEY of the TiN‐coated surface
after long time storage is now
investigated in the laboratory.


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Present status ‐ 6Clearing electrode (preliminary results)
Clearing electrodes were prepared in the LER wiggler magnets to absorb
electrons, as a countermeasure against ECE.
 A DC voltage of 300 V were applied during the beam operation.
• The voltage was stably kept at a beam current of 910 mA.
 Electron current saturates at approximately 100 V.
 Electron currents is still independent of the bunch fill pattern ‐>photoelectrons?


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Present status ‐ 7Beam collimator (1)Two horizontal‐type collimators were manufactured for test.
 The collimators were installed at an arc section in the ring, and theperformance was tested with beams.
 No excess heating at bellows behind the jaw.
 Transverse impedance is almost coincident with the simulation result.


Schematic of a horizontal‐type collimator

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Collimator installed in the tunnel

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Present status ‐ 7
Beam collimator(2) A result in the BEASTII study (by Nakayama‐san)

As we change D06H3OUT width from 24mm to 17mm,
LYSO/CsI BG shows step‐like decrease at every time collimator get narrower.

This is the clear evidence of BG suppression by the collimator!

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Present status ‐ 8Control system
The control system were updated.
 For example, a CAMAC datalogger system were replaced witha CompactRIO (cRIO) system.
Control System Studio (CSS) is used
as GUI of the control panel on X
terminals. NEG activation, forexample, can be remotely controlledfrom control panels.
 Interlock logic with a PLC and beam abort
logic with a ladder sequence program areworking well so far.


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31Problems ‐ 1Non‐linear pressure rise against the beam current in LER ‐1


The behavior is quite similar to that of electron currents measured
at aluminum region without TiN coating.
Electron current in Al is much large that that in TiN, over 20 times!

Pressure [Pa]The pressures at whole LER ring showed a nonlinear behavior against thebeam current over ~500 mA.

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Pressures of 9 gauges inarc sections

Beam current [mA]32

Problems ‐ 1
Non‐linear pressure rise against the beam current in LER ‐ 2
 The behavior is quite similar to that of electron currents measured
at aluminum parts without TiN coating.
 Actually, we have aluminum
bellows chambers without TiN
coating along the ring. The
bellows chamber has a lengthof 0.2 m and located every 3 m
Pressures of 9 gauges inarc sections
on average. ~5 % in the ring.
Pressure [Pa]

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Beam current [mA]33

Problems ‐ 1
Non‐linear pressure rise against the beam current in LER ‐ 3
 The behavior is quite similar to that of electron currents measured
at aluminum parts without TiN coating.
 Actually, we have aluminum
bellows chambers without TiN
coating along the ring. The
bellows chamber has a lengthof 0.2 m and located every 3 m
Pressures of 9 gauges inarc sections
on average. ~5 % in the ring.
Pressure [Pa]Electron stimulated desorption(ESD) in the aluminum bellows
chambers by electrons (electron
cloud)?
 Actually, beam size blow up was
also observed at the same time.


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Beam current [mA]34

Problems ‐ 1
Non‐linear pressure rise against the beam current in LER ‐ 4
 As a test, we applied a magnetic field of axial direction bysolenoids or permanent magnets at nine aluminum bellows
chambers (~30 m section). The strength is 40 ~ 100 G near the
inner wall at the center of bellows.
 As a result, the rate of pressure rise at this section relaxed!
Pressure [Pa]Permanent magnets
Pressures of 9 gauges
in arc sections

Solenoid coils
Beam current [mA]
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35Problems ‐ 1
Non‐linear pressure rise against the beam current in LER ‐ 5
 We put permanent magnets on most of aluminum bellows
chambers (~800) all around the ring (arcs) on 2 ‐ 4, June.
 As a result, the rate of pressure rise relaxed in whole ring!


Pressure [Pa]Permanent magnets

Average pressuresin two arcs

Beam current [mA]
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36Problems ‐ 1
Non‐linear pressure rise against the beam current in LER ‐ 6
 We put permanent magnets on most of aluminum bellows
chambers (~800) all around the ring (arcs) on 2 ‐ 4, June.
 As a result, the rate of pressure rise relaxed in whole ring!
However, the non‐linear
pressure rise is still seen athigh beam current > 700 mA.

Electron cloud still remains!
 We will put permanent
magnets at a part of drift
region for test soon.
Pressure [Pa]

Average pressuresin two arcs

The ECE problem will be
discussed later by Ohmi‐san
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Beam current [mA]
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Problems ‐ 2
Pressure bursts accompanying beam loss in LER ‐1
 Beam aborts accompanied by local pressure bursts have beenfrequently observed in the LER.
The beam loss monitors trigger the beam aborts
 Beam loss started at several hundreds microseconds before thebeam abort.
Beam current

400 s /div.
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Problems ‐ 2
Pressure bursts accompanying beam loss in LER ‐ 2
 FeaturesThe locations of the pressure bursts have spread to more than 20 points
along the ring.
 More frequent in the Tsukuba straight section.
‐2016/6/10

FUJI

TSUKUBANIKKO

D01 D02
D12

D03

D11

D04

NIKKO

OHO

D10

D05

D09

TSUKUBA

OHO

D06
D08

D07
FUJI

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FUJI

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Problems ‐ 2
Pressure bursts accompanying beam loss in LER ‐ 3
 FeaturesThe locations of the pressure bursts have spread to more than 20 points
along the ring. More frequent in the Tsukuba straight section.
 In most cases, the bursts were observed near aluminum beam pipes in
dipole magnets, with groove structure. (except for wiggler section)Position of pressure
burst estimated by the
distribution ofpressures observed inseveral vacuum gauges

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Quadrupoles Bending magnets

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40Problems ‐ 2
Pressure bursts accompanying beam loss in LER ‐ 4
 FeaturesThe bursts seems likely to occur when the maximum beam current isincreased.
 The locations spreads for high beam current, but still near bending magnets.
 Beam orbits (horizontal) and bunch fill patters had no effect.
‐2016/6/10

FUJI

TSUKUBANIKKO

D01 D02
D12

D03

D11

D04

NIKKO

OHO

D10

D05

D09

D06
D08

TSUKUBA

OHO

D07
FUJI

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FUJI

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Problems ‐ 2
Pressure bursts accompanying beam loss in LER ‐ 5
 One possibility was discharges by wall current at inner wall ofbeam pipes, but….


No extra heating around these points.
 Almost no small pressure bursts, which should be likely observed indischarging phenomena.
 Do discharges emit heavy materials leading to the beam loss toward
beam orbit direction?


Bunch

Discharging at small
gaps, dusts, whiskers
or poor electricalcontacts
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42Problems ‐ 2
Pressure bursts accompanying beam loss in LER ‐ 6
 One possibility was discharges by wall current at inner wall ofbeam pipes, but….


No extra heating around these points.
 Almost no small pressure bursts, which should be likely observed indischarging phenomena.
 Do discharges emit heavy materials leading to the beam loss toward
beam orbit direction?


Another possibility: Collision of “dusts” with circulating beams.
Groove structure is likely to catch the dusts.
 Aluminum grooves were formed by extrusion process, at the first stage ofbeam pipe fabrication.
 Beam pipes in wiggler sections have clearing electrodes at the top side.
UFO problem!
Or
Bunch
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43Problems ‐ 2
Pressure bursts accompanying beam loss in LER ‐ 7
 Knocking test

A “knocker” was set at a beam pipe in a bending magnet, where
the burst had been observed frequently.
 We succeeded in reproducing the phenomena three times byknocking the beam pipe!
Artificial beam aborts

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Problems ‐ 2
Pressure bursts accompanying beam loss in LER ‐ 8
 Knocking test

A “knocker” was set at a beam pipe in a bending magnet, where
the burst had been observed frequently.
 We succeeded in reproducing the phenomena three times byknocking the beam pipe!
Collision with dusts is the most possible cause of the pressure
bursts, and then the beam aborts.
 However, It is not yet clear whether the dusts are falling or
attracted by the positive beam.
 Countermeasure is now in consideration.


Wait? : Dusts should be limited. Fortunately, the frequency of bursts
seems to gradually decrease.
 Knocking all beam pipes during a long shut down.?


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Problems ‐ 3
Air leak at a flange of a tapered beam pipe ‐1Occurred at a flange of a tapered beam pipe where the cross section of
104 mm x 50 mm reduces to 60 mm x 40 mm.

NIKKO

Air leake‐ beamTSUKUBASR maskOHO

104x50race track

60x40
race track

FUJI

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Problems ‐ 3
Air leak at a flange of a tapered beam pipe ‐1

Occurred at a flange of a tapered beam pipe where the cross section 104
mm x 50 mm reduces to 60 mm x 40 mm.

A trace of “beam” was found on the inner wall across the flange.
The SR from the bending magnet hit the opposite side.
 High residual radioactivity had been reported.
 Instantaneous temperature rise was observed at the flange at the timing of
beam abort.
 Loss monitor (PIN diode) signals were observed at this point.
 Pressure bursts have been sometimes observed around here at the timingof beam aborts.


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Problems ‐ 3
Air leak at a flange of a tapered beam pipe‐2


Occurred at a flange of a tapered beam pipe where the cross section 104
mm x 50 mm reduces to 60 mm x 40 mm.

The cause is not completely understood yet.
 One possible process isThe beam loses the energy at other point ‐> veers from the normal
orbit (oscillation?) ‐> hit the pipe wall.

Countermeasure
Optimize the collimator positions.
• We closed the horizontal and vertical beam collimators after theaccident. The pressure bursts still remains, but the magnitude becomes
small. ‐> seems working!
 Add mask in the tapered beam pipe in the future.
Beam mask?

SR mask
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We have ceramics
chambers for beamaborts at thedownstream of thischamber.
48

Problems ‐ 4Other problems
Air leak at a gate valve• Leak was found when the valve open just before starting thecommissioning. ‐> Replace with new gate valve.
 Heating of beam pipes in wiggler sections• The temperature of beam pipes in wiggler sections are very
sensitive to misalignment of beam pipes or bump of beam orbit.
‐> Re‐alignment of beam pipes and adjustment of orbit
 Stop of water flow meters• Wheels in flow meters stop the rotation due to any debris inwater (copper oxide?).‐> Cleaning, replacement
 Fake abort signals• Noise? or malfunction of circuit? ‐> still investigation.
 Heating of HER bellows chambers• Damage? ‐> Cooling fan, temporally.
 Malfunction of temperature sensors, and so on.


We are dealing with these problems according to the situation.
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Summary and planConstruction of SuperKEKB vacuum system had finished in 2015.
and the commissioning started in February, 2016.

Overall, the vacuum system made a good start.
 Present statusNo abnormal temperature rises or vacuum pressure has been
observed for the new vacuum components so far.
 Vacuum scrubbing is progressing steadily.
• The reused beam pipes for HER remembered the condition in the
KEKB.
 Residual gases are typical one for the NEG pump system.
 Effects of the antechambers and TiN coating against ECE wereconfirmed.
 Beam collimators are working well up to now.
 Control system is also working well.


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50Summary and plan

Major problems
Nonlinear behavior of pressures were observed in the LER, which was
due to ESD in aluminum bellows chambers. ‐> cured by setting
permanent magnets.
• But the effect of ECE is still seen at higher beam current.
 Pressure burst accompanying beam aborts is a major concern recently.
Most probable cause is the collision of beam with dusts.
• The frequency seems to decrease recently.
 A fraction of steered beams seems to hit a narrow tapered‐beam pipe.
• Beam collimators seems helpful.
Lots of useful information have been obtained in Phase‐I.
 The stored beam currents will be gradually increased to ~1 A during
the Phase‐I commissioning.
Checking the soundness of the system.

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Summary and planPlans for Phase‐II
Installation of new IR beam pipes (Kanazawa‐san’s talk) Fabrication of five beam collimators and the installation.
 Change of beam pipes at the LER injection region.
 Replacement of damaged bellows chambers and other components.
 Rearrange of cooling water pass at wiggler section.
 Etc.
KEKB Vacuum group would like to thank all of KEKB
accelerator staff for their cooperation and kindconsideration for allowing the long enough time forvacuum scrubbing.

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Thank you for your attention.

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Main parameters related to vacuum system

Material of new beam pipe
Cross section of
new beam pipe
Main pumps
Total Power of SR
Critical Energy of SR
Max. SR power line density
Avg. photon flux line density
Linear pumping speed
Ave. pressure with beam
Ave. base pressure
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Unit
HER (electron)
OFC (arc),
OFC (wiggler)
Racetrack (50104)
50 + Antechambers
NEG
NEG (strip)(strip + cartridge)
1.1 (arc:2200 m)
5.2 (arc: 2200 m)
MW
6.3 (wiggler:300 m) 1.1 (wiggler:100 m)
1.9 (arc)
7.2 (arc)keV
9.2 (wiggler)
17 (wiggler)
2.6 (arc)
7.7 (arc)
kW m‐113 (wiggler)
9 (wiggler)~5.51018(arc)~6.81018(arc)photons s‐1 m‐1
19
19~4.710 (wiggler)~1.310 (wiggler)m3 s‐1 m‐1~0.1 (arc)~0.06 (arc)
Pa
~10‐7
Pa
~10‐8

LER (positron)
Al‐alloy (arc)
OFC (wiggler)90 +Antechambers

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History of pressure bursts for HER

FUJI

TSUKUBANIKKO

D01 D02
D12

D03

D11

D04

NIKKO

OHO

D10

D05

D09

TSUKUBA

OHO

D06
D08

D07
FUJI

FUJI

[h]
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