Updates of
SKEKB MR Vacuum System
Y. Suetsugu, KEK
on behalf of KEKB Vacuum Group
Contents
Two major topics: Countermeasures against;
 Electron cloud effect in LER
 Pressure bursts accompanying beam loss in LER
 Other preparations for Phase-2 commissioning Installation of new beam collimators
 Replacement of beam pipes at LER injection region
 Etc.
 Summary

2018/3/14

22nd KEKB Review

1Electron cloud effect
Permanent magnet for bellows

Brief review:

The first ECE was observed at a beam current of~600 mA (1/1576/3.06RF), which was caused by
EC in the aluminum bellows chambers.
 This ECE was cured by permanent magnets
around the bellows chambers.
 However, ECE was again observed at a beam
current of ~900 mA (1/1576/3.06RF).
 As a source of EC, the TiN-coated beam pipe with
antechambers at drift spaces was suspected.
sy blowup

900 mA

Measured electron density is near to the threshold of the instability, ~3x1011 e- m-3
Permanent magnets around the beam pipes suppressed the non-linear pressure rise.

Any countermeasures were required before Phase-2.
 The dmax of TiN coating estimated from simulations was ~1.4,
which is relatively higher than expected.
Further analysis was necessary on this point.
2018/3/14

22nd KEKB Review

2Electron cloud effect
Countermeasures
We checked the behaviors of pressure against the beam current
all over the ring. (2016/6/27, 1/1576/3.06RF)

EC → Electron multipactoring → Non-linear behavior of pressure

Non-linear pressure rises were observed atAl and Cu beam pipes with/without
antechambers + TiN coating at Arc sections,
IR (Interaction Region), Tsukuba straight section,
Chicane sections, SRM (beam size monitor).

Al beam pipes reused from KEKB (without
TiN coating) at Fuji straight section andinjection section.
 Not observed in wiggler section .


Example of pressure
rise at an arc

Pressure

900 mA

Beam current

We decided to apply magnetic fields in the beam direction byusing solenoids or permanent magnets to these beam pipes,
which was planned from the design stage.
2018/3/14

22nd KEKB Review

3Electron cloud effect
Countermeasures

For drift spaces of Al or Cu pipes with antechambers + TiN coating
 Type-1 unit: Permanent magnets with iron yokes; 8 ferrite
magnets (f30) + iron plate (L160 mm), aligned with 40 mm pitch.
 This type was applied for the places more than 250 mm far fromthe ends of electromagnets (Q, SX, ST) to avoid the interference.
Application of Type-1 unit
Magnetic field of Type-1 unit
Typical strength ~ 60 G

BeforeAfter
Permanent
magnets

Type-1 unit
2018/3/14

Iron yoke(plate)22nd KEKB Review

4Electron cloud effect
Countermeasures

For drift spaces of Al or Cu pipes with antechambers + TiN coating
 Type-2 unit: Permanent magnets in Al cylinders; 21 ferrite
magnets (f30) in each Al cylinder (L180 mm).
 This type was applied for the places less than 250 mm from theends of electromagnets (Q, SX, ST).
Type-1 and Type-2 units near Q magnet
Type-1

Type-2Type-2 unit
2018/3/14

Magnetic field of Type-2 unit
Typical strength ~ 100 G

Aluminum
cylinderAluminum support

22nd KEKB Review

5Electron cloud effect
Countermeasures
Expected electron density in the Type-1 unit was calculated(modeled by Fukuma-san) for the beam current of 3.6 A with
1/2500/2RF by CLOUDLAND.
+
 dmax = 1.0~1.4, photoelectrons = 0.07~0.01x0.1x0.16 p.e./m/e /turn.
 The central electron density is on the order of 1010 m-3 for thedesign current.
Magnetic free
1/2500/2RF, 3.6 A
dmax=1.2



4.5x1011 m-3(r<8 mm)

Type-1 unit
1/2500/2RF, 3.6 A

4x1010 m-3(r<8 mm)dmax=1.2

Note: the central electron density in Al bellows chamber with thepermanent magnet is also estimated to be approximately
3.4x1010 m-3 for 3.6 A (dmax = 2.0, photoelectrons = 0.01x0.1x0.16 p.e./m/e+/turn).
2018/3/14

22nd KEKB Review

6Electron cloud effect
Countermeasures

For Al pipes reused from KEKB (Fuji cross section)
 Retrieve solenoids at KEKB era + Type-1 and 2 permanent magnets
Solenoid + Type-2 unit

Type-2

SolenoidType-1 unit + Type-2 unit

Type-1

Type-2For new Al pipes with TiN coating for LER injection region
 Type-1 and 2 permanent magnets
Type-2 unit

Type-1 unit

Type-1
Type-2
2018/3/14

22nd KEKB Review

7Electron cloud effect
Countermeasures
For places where ECE has not been observed, but will be important Cu pipe at RF section without TiN coating: Retrieval of solenoids
at KEKB era or Type-2 permanent magnets.
 High by region at Tsukuba LC section: Adding more Type-2 units.
Type-2 units for RF section (Cu pipe)

Type-2Type-2 units for high by region

Type-2Type-2

Now, approximately 86 % of the drift space of the ring (~2000 m)
was covered by the magnetic fields with a strength higher than 20G.


Remained parts are around BPM, wiggler sections, ends of bending
magnets. Permanent magnets will be prepared until Phase-3.

2018/3/14

22nd KEKB Review

8Electron cloud effect
Estimated high value of dmaxPossible reasons of high dmax ~1.4:

Aging is still insufficient.
 No baking system, which usually shows high dmax, etc.


Other than these reasons, another possibility was recently
pointed out: More photoelectrons than expected in the beam
channel.
 The story is as follows:

The simulation starts from the premise that ne = 3x1011 m-3(threshold of ECE at ~900 mA).
 For ne on these orders, ne is not saturated. This means that thephotoelectrons are important as well as the secondary electrons(dmax). Actually, ne is almost proportional to the number ofphotoelectrons in the beam channel.
 If the number of photoelectron is different from that expected, theestimated value of dmax should change.


2018/3/14

22nd KEKB Review

9Electron cloud effect
Estimated high value of dmaxPossible reasons of high dmax ~1.4:

Aging is still insufficient.
 No baking system, which usually shows high dmax, etc.


Other than these reasons, another possibility was recently
pointed out: More photoelectrons than expected in the beam
channel.
 The story is as follows (cont’d):

So far, in the simulation using a circular pipe, the effectiveness ofantechamber (a = photoelectrons in beam channel/wholephotoelectrons) has been assumed to be 0.01, on the base of theexperiment in KEKB LER.
 However, recent simulation studies showed that a > 0.01 (0.03~0.1).
• Calculation of photon distribution using Synrad-3D under therealistic SKEKB layout and conditions (Fukuma-san)• Calculation of photoelectrons going out from the antechamberpart of actual beam pipe (Ohmi-san).
dmax might be smaller….

2018/3/14

22nd KEKB Review

10Electron cloud effectDependence of dmax on admax vs a. Calculation by Fukuma-san
a = 0.01
0.02
0.03

0.04



dmax

0.05
Present
simulation

There are various combinationsof dmax and a to give the samedensity, ne = 3x10 11 m-3.
 If a is 0.01, dmax is ~1.4.


ne = 3x1011 m-3

Quantum efficiency (he) = 0.1
Number of photons = 0.16 /m/e+/turnNumber of p.e. in beam channel = a x he x 0.16

Number of photoelectrons
in beam channel

2018/3/14

But, if a is 0.04, for example,
dmax is ~1.2.
Evaluation of actual numbersof photoelectrons (a) is very
important for estimating dmax!
We will try the following two
methods to estimate a in
the Phase-2 commissioning.

22nd KEKB Review

11Electron cloud effect
Estimation of a : Method 1
From the behavior of electron density against beam current.
 The behavior changes for the combinations of dmax and a.
Calculated ne for several combinations ofdmax vs a. Here ne = 3E11 m-3 at 0.18
mA/RF-bucket is fixed (1/300/3RF).

Preliminary data:
Measured ne in Phase-1 [2016/6/20](4/150/3RF).

Fixed pointne~3x1011 at
900 mAThe behaviors are similar to the cases of (dmax, a) = (1.2, 0.04)~ (1.0,0.07). But, more data are required.
2018/3/14

22nd KEKB Review

12Electron cloud effect
Estimation of a : Method 2
From the difference of measured electron densities between theusual case (ne) and the case where the effect of photoelectrons fromantechamber is little (ne0)Usual casene0

nephotoelectronsXphotoelectrons

Magnets
We will be able to estimate a from the ratioof ne0 and ne, assuming that the density is
proportional to the number of photoelectrons.
 For example, we will be able to measure ne0
when weak magnetic fields are applied to only
the antechamber.
 We will continue the evaluation ofElectron monitor at bottom
properties of our TiN coating.


2018/3/14

22nd KEKB Review

13

Pressure bursts accompanying beam loss
Brief review
Beam aborts accompanied by local pressure bursts have beenfrequently observed in the LER.
The locations of the pressure bursts have spread to more than 20
points along the ring. More frequent in the Tsukuba straight section.

Typical pressure burst

2018/3/14

Locations of pressure bursts and the history

22nd KEKB Review

14

Pressure bursts accompanying beam loss
Brief review
Beam aborts accompanied by local pressure bursts have beenfrequently observed in the LER.
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.


Estimated detailed locations of pressure bursts
Groove in the beam pipe
Bending magnets

2018/3/14

22nd KEKB Review

15

Pressure bursts accompanying beam loss
Brief review
Beam aborts accompanied by local pressure bursts have beenfrequently observed in the LER.
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.
 The beam-loss monitors at collimators triggered the beam aborts.
 Beam loss lasted a few ms before the beam abort.
Typical abort log for the case pressure burst was observed. (Ikeda-san)LM (PIN) signal
LM (PIN) signal
RF phase
1 ms
Beam current

2018/3/14

Abort

22nd KEKB Review

16Pressure bursts accompanying beam loss
Brief review
Beam aborts accompanied by local pressure bursts have beenfrequently observed in the LER.
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.
 The beam-loss monitors at collimators triggered the beam aborts.
 Beam loss lasted a few ms before beam abort.


A possible cause: Collision of “dusts” with circulating beams.
Manufacturer of beam pipes in Tsukuba section is different from others.
 Groove structure is likely to catch the dusts.
 Aluminum grooves were formed at the first stage of beam pipe fabrication.
 A “knocker” was set at a beam pipe in a bending magnet, where the burst
had been observed frequently, and we could reproduce the phenomena byknocking the beam pipe.


How to deal with it?
2018/3/14

22nd KEKB Review

17

Pressure bursts accompanying beam loss
Estimation of dust size

Hints:

Gravity

Beam loss lasts for long time, ~ ms.
 Short beam lifetime, < 10 ms.
[Cross section]

Large dusts, over 100 mm~1 mm?.



Dusts in reserved beam pipes were checked.


Dust: sphereradius = r

Beam
Crossing time ~3 ms forr = 500 mm, sy = 1 mm.

Dusts larger than 100 mm were actually found, although smaller
dusts were dominant.
Example of a large dust (Al2O3) found in the reserved beam pipe.

2018/3/14

22nd KEKB Review

18

Pressure bursts accompanying beam loss
Estimation of dust size

Hints:

Beam loss lasts for long time, ~ ms.
 Short beam lifetime, < 10 ms.

Small dust < 10 mm

Note: there is another possible story:

Dust is small, < 10 mm, such as NEG powder.
 The collision induces the synchrotron
oscillation, and it grows to the large betatronoscillation. That leads to the beam loss.
 Reason: the loss signal is observed behindthe synchrotron oscillation, ~ ms.
 We have to know the dinamic aperture.
 Further investigation is required, and somestudies are planed in Phase-2.
[Cross section]

Beam
Typical abort log

Here, we tried to explain the phenomena assuming that the dustis large.
2018/3/14

22nd KEKB Review

19

Pressure bursts accompanying beam loss
A simple modelr[Cross section]syDrop from above surface
Dust

simplificationr

Sliced sphere

Gravity
No charge up

Beam

sx

sx >> r

2𝜋𝜎𝑦

2𝜋𝜎𝑥

Calculate temperature of each slice every 10 bunches while thedust is interacting with beam. Height of one slice is ~ 55 nm.
 Assume that the slice evaporates when the temperature exceeds
the melting point or the vapor pressure exceeds 1x10-2 Pa.
Bunch #1

Bunch #11

10 bunches

2018/3/14

Bunch #21

22nd KEKB Review

Bunch #31

Bunch #41

Evaporate when the temperature
exceeds melting point.
20

Pressure bursts accompanying beam loss
Calculation using the model

Increase in the temperature is calculated from the energy
absorption at the overlapped region. (thermally insulated) Decrease in the temperature is calculated from the black bodyradiation. (much smaller than the increase rate) The beam lifetime is determined from the Bremsstrahlung, theRutherford scattering and the Möllar scattering.
 The beam intensity and the beam loss rate are calculated every 10
bunches.
 These calculations followed the method used in the dust
trapping analysis by F. Zimmermann. But, not trapped here.
Bunch #1

Bunch #11

10 bunches

2018/3/14

Bunch #21

22nd KEKB Review

Bunch #31

Bunch #41

Evaporate when the temperature
exceeds melting point.
21

Pressure bursts accompanying beam loss
Typical result

For Al2O3, which is the most probabledust in Al beam pipe
Min. lifetime and integrated loss against
dust radius for several beam currents.

Time change of integrated loss
and beam intensity
Limit

Abort

Abort log

Limit
for
abort
(assumption)

2018/3/14

22nd KEKB Review

22

Pressure bursts accompanying beam loss
Typical result

For Al2O3, which is the most probabledust in Al beam pipe
Min. lifetime and integrated loss against
dust radius for several beam currents.

Time change of integrated loss
and beam intensity
Limit

Abort

Abort log

Limit
for
abort
(assumption)

We can reproduce the log qualitatively.
2018/3/14

22nd KEKB Review

23

Pressure bursts accompanying beam loss
Gathering of dusts from actual beam pipes in the ringThe beam pipes where the bursts were frequently observed.

Location of beam pipe

2018/3/14

22nd KEKB Review

24

Pressure bursts accompanying beam loss
Gathering of dusts from actual beam pipes in the ring

The beam pipes where the bursts were frequently observed.
 A special tool to cleanup the inside of beam pipes with
antechambers was developed.
 The dusts was gathered by a powerful vacuum cleaner.


After knocking the beam pipe, the beam pipe was slowly filled with
N2. Then the dusts at the bottom of beam channel was vacuumed.
Special tool to cleanup the bottom of beam channel

2018/3/14

22nd KEKB Review

25

Pressure bursts accompanying beam loss
Gathering of dusts from actual beam pipes in the ring

The beam pipes where the bursts were frequently observed.
 A special tool to cleanup the inside of beam pipes with
antechambers was developed.
 The dusts was gathered by a powerful vacuum cleaner.
After knocking the beam pipe, the beam pipe was slowly filled with
N2. Then the dusts at the bottom of beam channel was vacuumed.

Lots of large dusts were found from one of the two beam pipes!
We did not check the inside of other beam pipes in which thebursts were not observed.
 But, this is one strong evidence for the assumption of large dusts.


Dust obtained from the beam pipe in question

2018/3/14

22nd KEKB Review

26

Pressure bursts accompanying beam loss
Countermeasures
It is hardly possible to cleanup the all dusts.
 But if the sizes of dusts are large, it is quite unlikely to imagine
that the dusts are picked up from the bottom of beam channel,
which might be charged up by electrons(?).
 We knocked 24 beam pipes for bending magnets (with groove)around Tsukuba straight section, where the pressure bursts were
frequently observed, and dropped dusts from the top.
Knocked 150 times for each beam pipe.

We expect the reduction of the bursts in Phase-2 commissioning.
The study will be continued.

2018/3/14

22nd KEKB Review

27Other preparations for Phase-2
Installation of new 6 beam collimatorsTwo collimators, installed in the SuperKEKB positron ring for testin Phase-1 commissioning, worked well as expected.
Designed on the base of SLAC-type Low-impedance collimators.

Taking these promising results, six new collimators were
manufactured and installed in the positron ring (LER) and theelectron ring (HER) for Phase-2 commissioning.
A new vertical type collimator installedin the ring

Collimator system in Phase-IIHER
KEKB type

LER
D06 H3,H4

2018/3/14

22nd KEKB Review

28

Other preparations for Phase-2
Replacement of beam pipes for LER injection region

In order to accommodate the injection of low-emittance positron
beam, the septum magnet was renewed.
 LER abort system including kicker magnets is also upgraded totolerate the higher beam current with a smaller beam size.
 The beam pipes were replaced to new ones accordingly, including
the beam pipe inside the septum chamber.
septum and beam pipes for injection region

2018/3/14

22nd KEKB Review

29Other preparations for Phase-2
Countermeasures against problems found in Phase-1

Air leak from a flange of a tapered beam pipe at Fuji straightsection.
Replace to a new beam pipe with a “beam mask” as well as SRmask.
 Heating of flanges in wiggler sections
Re-alignment of beam pipes, and install new bellows chamberswith SR masks at the entrance of antechambers.


Heating of beam pipes at the downstream of wiggler sections.
Cooling-water paths are optimized to increase the flow rate.
OthersInstallation of bellows chambers with SR baffles to reduce photon
scattering to the mirror of SR beam size monitor.
 Installation of new test beam pipe for EC studies.
 Inside check of some bellows chambers
 Replacement of several vacuum gauges and ion pumps.


2018/3/14

22nd KEKB Review

30Summary
The MR vacuum system is now ready for the Phase-2.
 Various countermeasures were taken for problems
found in Phase-1.
Electron cloud effectApproximately 86 % of the drift spaces of the ring were coveredby magnetic fields using permanent magnets or solenoids.
 The analysis of the properties of countermeasures are on going.
Pressure bursts
The beam pipes in questions were knocked to drop the dusts
from the top of beam channel.
 The investigation on this phenomena is continuing.


Other preparations
Installation of new beam collimators
 Replacement of beam pies at LER injection region
 Etc.


2018/3/14

22nd KEKB Review

31Thank you for your attention.

2018/3/14

22nd KEKB Review

32Other preparations for Phase-2
Installation of new 6 beam collimators

One of the new collimators was equipped with a thermo-sensor tomeasure the temperature at the tip of the head.
 A special beam pipe with four pickup electrodes was installed justnear a collimator for measuring the characteristics of HOM(Higher Order Mode) generated by the collimator.
 Installation of lead shielding is also on going.
Collimator head equipped withthermo-sensor

Beam pipe with HOM pickups

Thermo-sensor

2018/3/14

Pickup22nd KEKB Review

33Design of key components_8
Countermeasures against electron cloud (EC)effect [LER]– Serious issues for recent positron and proton storage rings.
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 ?

– By using these countermeasures, the average electron density
on the order of 1010 e-/m3 will be obtained.


Threshold of head-tail instability: ~1.61011 e-/m3

2011/2/8

KEKB Review 2011 @KEK

34

Expected electron density

ne after applying measures described so far (Red)ne of approx. 1/5 of the threshold one is expected.
Compared with results of CLOUDLAND (Blue)– dmax=1.2, Solenoid field=50G (ne=0), Antechamber;photoelectron yield =0.01 (1/10)

Conditionne [m-3]Circular Cu chamber[KEKB beam pipe]

5.2E12+Solenoid at Drift (1/50)

4.7E11+Antechamber (1/5)+TiN (3/5)

5.7e10+Electrode in Wiggler (1/100)

3.5e10+Groove in Bend (1/4)

2.0E10Single Bunch Instability Threshold (~1x1011)

KEKB~3x1011

Low density in the simulation  ne=0 by solenoid. Careful solenoid winding is important.
2010/10/8-12

ECLOUD'10 @Cornell Univ.

35

Expected electron density
Expected electron density without solenoid (from KEKB)
赤： ne expected from experiments in KEKB
青： ne calculated using CLOUDLAND
Conditions：f 94 mm Cu pipe、dmax=1.2, Solenoid field=50G (ne=0),
Antechamber (photoelectron yield =0.01:1/10 of circular pipe)、
4ns spacing (2RF)、1 mA/bunch = 600 mA/600 bunch
Conditionne [m-3]Circular Cu chamber

5.2E12Single Bunch Instability Threshold (~1x1011)[KEKB beam pipe]+Solenoid at Drift (1/50)

4.7E11+Antechamber (1/5)+TiN (3/5)

5.7e10+Electrode in Wiggler (1/100)

3.5e10+Groove in Bend (1/4)

2.0E10No solenoid

6.2E11

KEKB~3x1011

Without solenoid, ne is on the order of 1011 m-3, near to themeasured value. SuperKEKB Internal Review @KEK
2017/9/8
36

Drift section_1
Beam pipe with antechambers– Effective to reduce photoelectrons
Adopted in PEP-II LER– Also effective to reduce photon scattering (with roughsurface)– Contribute to decrease impedance
Depth: 65 mm
Height:14 mm– Reduction rate: ~ 1/5 at high current region
Arc

Effect of antechamber

Conceptual drawing

1/1284/3.07 (1284)
6~8 ns spacing
R = 4７ mm (Circular)
Vr = 30 V
Ph = 81015 Ph/s/m/ms

Pump

Circular chamber

~1/5

Beam
SR

2010/10/8-12~1/100Roughsurface(Ra10~20)

ECLOUD'10 @Cornell Univ.

With
antechamber

37

Electron cloud effect
Estimation of a : Method 1
From the behavior of electron density against beam current.
 The behavior changes for the combinations of dmax and a.
Calculated ne for several combinations ofdmax vs a. (1/300/2RF).

Preliminary data:
Measured ne in Phase-1 [2016/6/20](4/150/2RF).

The behaviors are similar to the cases of (dmax, a) = (1.2, 0.04)~ (1.0,0.07). But, more data are required.
2018/3/14

22nd KEKB Review

38

Electron cloud effect
Estimation of a : Method 2

From the difference of measured electron densities between thecase where the effect of photoelectrons from antechamber is small(ne0) and the usual case (ne).
 For example, we will measure the electron density when weakmagnetic field is applied to only the antechamber.
Assuming that ne is proportional to thenumber of photoelectrons in the beam
channel, and also that the quantum
efficiency is constant,
𝑛𝑒0
𝑃𝐸𝑏
=
𝑛𝑒
𝑃𝐸𝑏 + 𝛽 × 𝑃𝐸𝑎
 PEb and PEa: number of photoelectrons in
the beam channel and in the antechamber.
b: probability that photoelectrons in theantechamber go out to the beam channel.
Apply weak magnets at antechamber near electron monitor
Magnets

Electron monitor at bottom
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39

Pressure bursts accompanying beam loss
Typical results


For Al2O3, which is the most probabledust in Al beam pipeTime change of integrated loss
and beam intensity
Limit

Integrated loss and min. lifetime against
dust radius for seal beam currents.

Limit
for
abort
(assumption)

We can reproduce the log qualitatively.
2018/3/14

22nd KEKB Review

40Electron cloud effectModel of permanent magnet Type-1 (Fukuma san)
1/8 model

Magnetic field~40 G

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41

Electron cloud effectDensity of electrons in the Type-1 unit
Central density of electrons inside of the Type-1 unit for dmax = 1.0~1.4
and a =0.007~0.001.
2RF bucket, 2500 bunch, 4 A

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42Electron cloud effect
Effect of magnetic field

1A ~ 8G~20 G

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43Electron cloud effect
Effect of Baking on dmax of TiN coating (in laboratory)

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44