Vacuum System Upgrade
Y.Suetsugu, KEKB Vacuum GroupContents
1. Basic Design

1.1 Beam Chambers: Ante-chamber Scheme
1.2 Pumps: Pump, Pressure, Vacuum Scrubbing
1.3 Connections: Flange+Bellows
1.4 Special Components: Movable Masks, HOM Dampers
2. R&D Status: Ante-chamber, Bellows, Masks

3. Summary
2004.02.16

2004 9th KEKB Accelerator Review

11. Issues for Vacuum System
Parameters Considered Here
Energy [GeV]
Beam Current [A]Bunch Length [mm]Bunch Number
Bending Radius [m]

LER
3.5
9.4
35018
16.31

HER
8.0
4.1
35018
104.46

Key points in designing beam chambers and components
How to deal with intense SR?
How to reduce beam impedance?
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21.1 Beam Chamber_1
Beam chamberAnte-chamber scheme[Ante-chamber]

NEG Pocket

NEG Strip

BeamCooling ChannelSR[Beam Channel]
2004.02.16[SR Channel]

NEG Strip

2004 9th KEKB Accelerator Review

31.1 Beam Chamber_2
Merits of Ante-Chamber(1) Low power density of SR
For LER,[9.4A]

134 W mm-2 (ha = 47 mm) [Max.475 °C]
39 W mm-2 (ha = 112 mm) [Max.156 °C](2) Low Impedance
1 pumping port of KEKB (φ 94): k = 6×107 V C-1
k = 9×105 V C-1 per 1 m (~1/100)
SR mask of KEKB (5mm height): k = ~5×109 V C-1
k = 1×105 V C-1 per 1 m (~1/10000)

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41.1 Beam Chamber_3
Merits of Ante-Chamber (positron ring)(3) Less photoelectrons in beam channel
Not perfect~ 1/10(simulation)(4) Low photoelectron yield by using a rough surface
on side wall (< 1/10)
Rough surface at far side wall has little effect on beam.
Reflection of SR is also reduced by a rough surface.

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51.1 Beam Chamber_4
Cross Sections - LER

φ 90 mm(now φ 94 mm)

2004.02.16

Half height= 7 mm

B, normal

Q, Sx

Ion Pump

NEG Feed-through2004 9th KEKB Accelerator Review

61.1 Beam Chamber_5
Cross Sections - HER

98×50 mm(now 104×50 mm)

2004.02.16

Half height= 7 mm

B, normal

Q, Sx

Ion Pump

NEG Feed-through2004 9th KEKB Accelerator Review

71.1 Beam Chamber_6
Large sizes of beam chamber is better,Decrease SR heat load
Increase conductance, capacity of pump

[SX]

But the sizes are limited by magnets
Q, SX:
Half aperture, Diameter
Height of SR channel
B:
Height of NEG pocket (for HER)

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2004 9th KEKB Accelerator Review[B]81.1 Beam Chamber_7
Thermal analysis
40°C
39 W mm-2
156°C
270 MPa

Considering HOM power or ohmic
loss, extra cooling channel will be
required
2004.02.16

2004 9th KEKB Accelerator Review[Example]91.2 Pumping_1
Goal Pressure : 5×10-7 Pa in average
Beam life time ~ 10 hours (Luminosity life time ~2 hour)
Necessary pumping speed = 0.1 m3 s-1 m-1(for η = 1×10-6 mol. Photon-1)

Pumps : Distributed Pumping Scheme
NEG strip (+ lumped ion pump)[Feed-trough for NEG activation][ST 707 NEG Strip](30 mm in width)
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101.2 Pumping_2
Estimation of Pumping Speed
NEG: 2.0 (1.25) m3 s-1 m-2 (after some saturation)
Lumped pumps between adjacent Q and Sx (0.1 m3 s-1 )LER

2004.02.16

HER

2004 9th KEKB Accelerator Review

111.2 Pumping_3For more pumping speed
Double NEG strip as for chambers at Wiggler section
Put more lumped pumps near Q and Sx
Enlarge bore size of magnets and put pumps in magnets (?)

Assumption of η = 5×10-7 molecules photon-1 may bereasonable nowadays
η = 1×10-6 may be conservative[Example at wiggler section]

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2004 9th KEKB Accelerator Review

121.2 Pumping_4
Vacuum Scrubbing (simulation)
LERAssumption
η : Based on KEKB
I : Increases gradually(18.8 mA stepwhen τ > 5 hours)(τ : Brems., CO)
NEG : Activate whenS < 0.02 m-3 s-1m-1
Activation >20
Need more capacity of NEG

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2004 9th KEKB Accelerator Review

131.3 Connection_1
Use flange + bellows to connect chambers
Make handling of beam chambers easy by using bellows
and flanges
Several configurations are under consideration

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141.3 Connection_2
FlangeUse special RF-bridge andHelico-flex
Or develop a special gasket
that doubles as vacuum seal
and RF shield

Common Problem:
How to absorb tilt between adjacent chambers
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151.3 Connection_3
Bellows
With new or improved RF shield toavoid HOM heating (see R&D)
Position feed back system for BPM isunnecessaryGate valvesSame problem with bellows
New or improved RF shield is required
Ante-chamber cross section ?
Tapers at both sides

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2004 9th KEKB Accelerator Review

161.3 Connection_4
General Layout (two bellows between Q magnets)

LERHER
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171.4 Special Components_1
Movable Mask
Necessary to reduce background of detector
Basically the same structure with present type
Material of head : C or Be ?
Bellows Chamber[Ver.4 for KEKB]
Mask Chamber
Bellows Chamber

Beam
Mask Head
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2004 9th KEKB Accelerator Review

181.4 Special Components_2
Movable Mask
Problem : High Impedance
k = 1 V pC-1
@ σz = 3 mm

[Calculated loss factor]

Loss ~200 kW !!
@5000 bunches and 9.4 A

Reduce number ?
Powerful HOM absorber ??

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2004 9th KEKB Accelerator Review

191.4 Special Components_3
HOM absorbers for various components
Become more and more
important components
More powerful absorber isrequired[HOM absorber for KEKB
movable masks]

SiCSlotWing
Beam Chamber
SiC

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2004 9th KEKB Accelerator Review

202. R&D Status_1
Ante-Chamber_Type-1
Length:5.2 m
Manufactured in BINP
Material : Copper (OFC)
Forming : Stamp
Welding : EBW
Saw-tooth at side wall

EBW
EBW
Pressed from a OFC plate2004.02.16

Coolingchannel Saw-tooth

2004 9th KEKB Accelerator Review

212. R&D Status_2
Ante-Chamber_Type-1
Installed in LER last month
Electrons in beam channel and temperatures of chambers were
measured
Ante-chamber in tunnel

B e amB
Electron Monitor(DC, +100 V)
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2004 9th KEKB Accelerator Review

222. R&D Status_3
Ante-Chamber_Type-1
First result of electron measurement (without solenoid)[Linear scale]
Measured in DC mode (average)
Bias voltage = 100 V
Non-linear dependence on beam current
Multi-pactoring
Electron current for ante-chamber issmaller than that for normal chamber.
Reduction rate is about 1/4 at 1.4 A

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2. R&D Status_4
First result[Linear scale]

Limit of measurement
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Normal chamber:Photoelectrons are dominantAnte-Chamber:Multiplied second electrons are
dominant
Reduction rate is about 1/4 at 1.4 A, but
1/300 at 20 mA
Almost in agreement with expectation:
Ante-chamber:~1/10
Saw-tooth: ~1/20
Ante-chamber is insufficient tosuppress multiplication of secondary
electrons at high current.
Solenoid, Surface treatment ?
Need further investigation

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2. R&D Status_5
Ante-Chamber_Type-2

Under manufacturing : Almost same structure with Type-1
Material : Copper (OFC)[Drawn pipe]
Forming : Drawing
Welding : EBW

Experiments and studies are under going
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2. R&D Status_6
Bellows (RF Shield)
Comb-type RF Shield
High thermal strength, Low impedance
Back Fingers

Inside viewa = 10 mm, b = 15 mm, c = 2 mm
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Outside view

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2. R&D Status_7
Bellows with comb-type RF ShieldTwo trial models were installed in LER last summer at
downstream side of movable masks (most severe location)
Temperatures of corrugation and comb were measured andcompared with finger-type bellows at the same location[Trial model]

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2. R&D Status_8
Results of beam test
Temperature at corrugation reduced to ~1/6
Temperature of comb is about 50°C at 1.5 A, that of finger isestimated to be ~130°C at the same condition.
[Corrugation]

2004.02.16[Comb]

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2. R&D Status_9
Bellows with comb-type RF Shield
Working very well up to 1.5 A

Problems
Small longitudinal movement (+3/- 4 mm at present)
Control of chamber temperature is required
Rather complex structure

Future plan
Application to more small diameter chamber (~48 mm), to arace-track cross section (104×50 mm) and to gate valves
Simplify the structure and expand stroke
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2. R&D Status_10
Movable Mask
A big HOM source (200 kW / mask)
There is a limitation on the present way of thinking (make slopes gentle,for example)

An idea for mask with lower impedance
Slender mask head supported by ceramics rods
[Ver.4 Mask Chamber][New Mask Chamber]
HOM Absorber

Beam
Mask Head (Ti)2004.02.16

Support(Ceramics)
Mask Head (Ti for ex.)2004 9th KEKB Accelerator Review

30

2. R&D Status_11
Small loss factor
SiC HOM Absorber

GrooveMask Head : Metal[w:8mm, t:3mm, l:80mm]
Support :Ceramics[ε:6, 2×(2mm × 3mm)]

Same configuration to Ver.4
Need further investigation and tests
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3. Summary
Basic design of vacuum system is taking shape
The design is based on that of KEKB and introducing newideas at the same time
Antechamber scheme is suitable for beam duct
Pumps are NEG strips and ion pumps
Bellows and flanges are used to connect ducts
R&Ds for ante-chamber, RF-shield, flange and movablemasks are undergoing

Remained issues
How to suppress electron multipactoring
Solenoid (+ coating ?), How is inside of Q or Sx?

Design of straight sections, IR
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