Vacuum Upgrade for Super KEKB
Y.Suetsugu, KEK
1. Brief Review of Vacuum Design (Arc)
2. Recent R&Ds
a. Beam Duct with Ante-chamber
b. Surfaces with low SEY (Beam Test)
c. Bellows Chamberd. Connection Flangee. Movable Mask (Collimator)
3. Summary
2005.02.22

KEKB Review 2005 @KEK

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Basic Design of Vacuum System _1
Main issues of vacuum system come from high currents(LER:9.4 A, HER:4.1 A) and a short bunch (σz = 3 mm):Intense SR power
Max.28 kW/m even for a beam duct with ante-chamber
Max.40 W/mm2, at 51 mrad incidenceHigh gas load
Ave.5x10-8 Pa m3/s/m, forη = 1x10-6 molecules/photon
Pumping speed of ave.0.1 m3/s/m to get ave.5x10-7 Pa
Lots of photoelectrons [e+]Photons: ave.~1x1019 photons/m/s
Intense HOM power
1 V/nC  ~200 W (Nb = 5120, fc = 100 kHz)High wall current
Peak:∼250 A (σz = 3 mm)
2005.02.22

KEKB Review 2005 @KEK

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Basic Design of Vacuum System _2
Proposed basic designs for arc are:Beam duct:

Q-mag

Copper beam duct with an ante-chamber
Distributed pumping by NEG strips
Inner surface with low SEY or/andsolenoid [e+]SR

Bellows and gate valves:
With comb-type RF shield

Connection flange:

MO-type flange or conventional
RF bridge + vacuum seal

Movable mask (collimator)

New design with low impedance(no concrete design)

Beam

NEG Strip·····

2005.02.22

KEKB Review 2005 @KEK

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Basic Design of Vacuum System _3
Recent R&Ds are focused on:
Copper beam duct with ante-chamber
Beam test of two trial models, Measurement of photoelectrons,
Application of NEG strips

Surface with low SEY
NEG coating and TiN coating
Test with beam

Bellows chamber with comb-type RF shield
Beam test of comb-type RF shield
Application to gate valve

Connection flange with smooth inner surface
Applying MO-type flange to beam duct with antechamber

Movable mask
Simulation study of a new structure with low impedance

2005.02.22

KEKB Review 2005 @KEK

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Beam Duct with Ante-chamber _1
Two test models of copper beam ducts with ante-chamber
were manufactured.
Oxygen Free Copper (OFC), Length = 5.2 m.

Two manufacturing methods were tried:
Pressing: Pressing from a Cu plate (assembly of four pieces)
Cold drawing: Drawn from a pipe (one piece)[Pressing]2005.02.22

φ = 94 mm
ha = 112 mm
t = 6 mmKEKB Review 2005 @KEK

[Drawing]

Rough surface

5Beam Duct with Ante-chamber _2
No essential problem was found for both manufacturingmethods.
Then, these two ducts were installed in LER last year, justdownstream side of a bending magnet.
Electron currents in the beam channel, temperatures andpressures were measured during a usual beam operation.

B e amB
Electron Monitor(DC, Collector:+100 V, Repeller:-30V)
2005.02.22

KEKB Review 2005 @KEK

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Beam Duct with Ante-chamber _3
Electrons in the beam channel
Photoelectrons decreased by factors at high current (Ib ≥1 000 mA).
The reduction was by orders at low current (Ib ≤ 100mA).
Multipactoring seems to become important at higher current.

Combination with solenoid field, and an inner surface with alow SEY will be required at higher current.
[Linear Scale]

Repeller Voltage = -30 V[Log Scale]

3.77 buckets spacing
3.77 buckets spacing

2005.02.22

Limit of measurement
KEKB Review 2005 @KEK

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Beam Duct with Ante-chamber _4
Beam duct with ante-chamber works almost as expected andthe manufacturing is found to be possible.
Next Step
Manufacturing of a beam duct for wiggler section isundergoing. (SR hit both sides)
Pressing, but one piece (4.5 m). Four NEG pockets.

The duct will be installed next summer, and tested
together with bellows chambers (described later).
[Cross Section][Pressed Plate (Cu)]

NEG Pockets
2005.02.22

KEKB Review 2005 @KEK

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Beam Duct with Ante-chamber _5
Manufacturing of beam duct with ante-chamber is possible,but, practically, leaves still room for improvement.
Main problems in the present design
Complex structure
Many welding lines
No pump in Q or Sx magnets[Present design]

2005.02.22

KEKB Review 2005 @KEK

High Cost
Low reliability
Low average pumping speed[Effective pumping speed (HER)]

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Beam Duct with Ante-chamber _6
Recent idea for beam duct design
Basically the same as the present
design for outer side, but pump is in
the antechamber at inner side.
A screen in the ante-chamber (∼70
mm from beam) hides the NEG
assembly (multi-strips, for ex.).
The same structure for B and
Straight duct, and omit NEG pockets. NEG

Q-mag. &Sx-mag.
SR
Screenassembly

FeaturesB-mag. &Straight

Pumps in Q and SX magnet
 Uniform effective pumping speed
Simpler structure and less welding
lines  Low cost
Easy install of NEG strip from end
flange  Multi-strips is possible
Impedance of screen is low:~104 V/C/m ~a few mW/m

Further considerations are
required.
Bending, NEG assembly…

2005.02.22

KEKB Review 2005 @KEK

Beam

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Surface with Low SEY _1
Surface with a low secondary electron yield

Necessary for e+ ring to suppress ECI. (Antechamber is not
sufficient)
Promising candidates are:

NEG (Ti,Zr,V) coating:Studied in CERN and applied to LHC
TiN coating: Applied to PEPII LER , , , [Test chamber in the tunnel]
Groove: Proposed by BINP, SLAC, , ,2.56 m

Two test chambers with NEG
coating and TiN coating wereprepared last year.
They were installed in LER andelectrons in the duct and pressures
were measured during a normal
operation.
The first experiment using e+ highcurrent machine
Photons: 6x1014 photons/mA/m[Electron Monitor]
2005.02.22

KEKB Review 2005 @KEK

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Surface with Low SEY _2
NEG (Ti,Zr,V) coating:[Set up of NEG coating]

Coated at BINP
Magnetron discharge
P ~ 0.3 Pa (Kr), B ~105 G
I = 150 mA, V = 400 V
Temperature is kept at 100°C
during coating
Thickness1 µm
Ti:Zr:V = 30:26:44 (atoms)
Activated in situ. at 220 °C for 2
hours

TiN coating:

Coated at BNL
Magnetron discharge
P ~ 1.3 Pa (Ar+N2)
I ~ 1700 mA, V ~ 380 V
Temperature is kept at 220°C
during coating
Thickness 0.3 ~1 µm
Ti:N =49 : 51
No baking in situ.

2005.02.22

KEKB Review 2005 @KEK

[Inner surface of NEG
coated chamber]

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Surface with Low SEY _3(Ie)

Measured electron current (Ie) vs. beam current (Ib)

3.77 bucket spacing
1293 bunches
Repeller -30V

Cu
NEG

TiN

“Bump”(Ib)
2005.02.22

Ie were measured after electron
bombardment of ~1x1020 e-/cm2(stable).
Ie for NEG coating is almost
same as that of Cu, except forhigh current.
Ie for TiN coating is clearly lower
than those of Cu and NEG.
They cannot be compared
simply, because the effect of SR(photoelectron yield) has to beconsidered.
A simulation was tried to explain
the behavior of curves and to
estimate δmax (Max. SEY) for
three surfaces, especiallypaying attentions to “bumps” inthe curves.

KEKB Review 2005 @KEK

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Surface with Low SEY _4
Assumptions in simulation

Energy spectrum of
secondary electrons

2-dimensional
Energy spectrum of emitted electrons(same for photoelectrons and secondary
electrons)
Energy spectrum of SEY(M.A.Furman at al.*)Direct photons:6x1014 photons/m/mA
Reflected photons:6x1014 photons/m/mA
Space charge assuming a uniform electron
density
Energy spectrum of SEY“Macro” electrons (≤104 electrons) aretraced from the emission and thenumber of electrons hitting the bottom ofduct (position of electron monitor) withan almost normal incidence angle arecounted.

Emitting point of*Proc. MBI97, Tsukuba, 1997
photoelectrons
Position of monitor
2005.02.22
KEKB Review 2005 @KEK

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Surface with Low SEY _5
Simulation: electron current (Ie) vs. beam current (Ib)
Main parameters are q (photoelectron yield) and d (δmax).
Large d (2.0)
3.77 bucket spacing
1293 bunches
Repeller -30V

Small d (0.0)
Low q (0.1)

High q (0.4)

Ie strongly depends on d for low q.
Ie is independent of d for high q, especially for low Ib.
Ie is almost constant for high d, which is limited by space charge.
Ie mainly determined by q for very low Ib (≤ 30 mA)
The “bumps” are clear for low q or low d, and the shape and theposition depend on q and d.
 Estimations of q and d are possible by curve fittings.
2005.02.22

KEKB Review 2005 @KEK

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Surface with Low SEY _6
Curve fitting
Curve fitting by scanning q (0.1≤q ≤ 0.4) and d (0.8 ≤ d ≤ 2.0).
Constraints : q [Cu]=0.3~0.4, q [Cu] : q [NEG] : q [TiN]=1:~0.8:~0.55(from exp. at PF)(from data at Ib = 30 mA)
q
3.77 bucket spacing
1293 bunches
Repeller -30V

Cu

0.28-0.34 1.3-1.55

NEG 0.23-0.3
TiN

δmax

0.95-1.2

0.14-0.16 0.85-1.05

High photons in e+ machine makes the effect of low SEY unclear.
TiN coating seems better from view points of low SEY and small q.
The pressure, however, was low for NEG coating.
Further investigations, such as tests at little photon region, at straightsection or beam duct with ante-chamber, will be continued.
2005.02.22

KEKB Review 2005 @KEK

16

Bellows Chamber _1
The comb-type RF shield has been developed in KEK.
[Comb-type RF shield]
Two circular bellows chamber
was installed in LER two years ago.
Good results were obtained.
Temperature decreased to <1/6
Temperature of comb ~ 50 °C at 1.6 A
No damage after 1.5 year operation[Circular Type Bellows][Temp. of corrugation]
Without cooling fan

2005.02.22

KEKB Review 2005 @KEK

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Bellows Chamber _2
Last summer, four more circular bellows chambers were
installed just near movable masks, where a bending(max.~30 mrad) is required. The intense heating was cured.
A Race-track (48 x 64) bellows chamber was also installed.
The most severe section in the ring.
No problem was found so far.
[Near Movable Mask]

2005.02.22

KEKB Review 2005 @KEK

[48x64 Race-track]

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Bellows Chamber _3
Next Step
Beam test of a bellows chamber for wiggler section.
Manufacturing is undergoing.
Install next summer?
[Conceptual Design]

OptionWe do not eliminate a possibility to use a finger-type RF shielding
with SiC inside the bellows.
But, the comb-type RF shielding seems more basic remedy forheating problem, if things go well.
2005.02.22

KEKB Review 2005 @KEK

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Gate Valve _1
Gate valve has the same problem to bellows chamber.
Heating problem has been observed already in KEKB.
Application of comb-type RF-shield to gate valve is studied.
A test model (circular type) is manufactured and installed in LER
this winter.

Fingers:
Ag plated SS

Teeth:
Cu[Collaboration with VAT Co.]2005.02.22

KEKB Review 2005 @KEK

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Gate Valve _2
Temperatures at several points on the body were measured
Temperature rise at up and down side of flange decreased to 60% ofconventional one.
Temperature rise of body was 2~3 °C, decreased to 30 %.

Comb-type RF shield is also available to gate valve, may besuitable compared to bellows chamber.
[Finger type]U[Comb type]UD

3.77 bucket spacing
1293 bunches

DUD

2005.02.22

KEKB Review 2005 @KEK

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Connection Flange _1
Gap between connection flanges becomes a big problem forhigh current and short bunch.
Conventional RF-fingers or metal O-rings are insufficient.
Heating has been observed in KEKB

MO-type Flange

Developed for C-band wave guide in Japan (circular and rectangular)
Features
Smooth inner surface (Less than 0.2 mm step)High thermal strength (1.5 mm copper gasket)
Reliable electrical contact (Vacuum seal = RF bridge)
Simple structure (Uni-sex structure)

Application to a beam duct with ante-chamber is studied.
Cu Gasket

Positioning
Pin

SUS Flange

2005.02.22

KEKB Review 2005 @KEK

[Concept of MO-type flange]

22Connection Flange _2
Experiment using a test flange[Inside view of MO-type flange]

Only flange (without duct)

Promising results were obtained.
Vacuum sealing was possible with areasonable fastening force.
Baking up to 250°C, twist up to 5 mradwere OK.
[He Leak Test][Test Flange]

SS316

OFC

2005.02.22

KEKB Review 2005 @KEK

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Connection Flange _3
MO-type flange for a wiggler duct was also tested.
Baking at 150°C was OK. Twist (5 mrad) was OK.
No problem was found.

Next steps
Application to test chambers and test with beam
Test together with beam duct next summer?
Copper-alloy flange  Easy to weld (in future)[MO-type flange for wiggler duct][He Leak Test]

SS316

OFC

2005.02.22

KEKB Review 2005 @KEK24

Movable Mask _1
Design Base: Ver.4 Type in KEKB
ProblemHigh loss factor(~1V/pC  ~200 kW)
Damage of headAny drastic improvement is required.
An idea  A head supported by ceramics= Reduce interaction of head with beam field
E

E

Easily imagined
problems:

Mask Head• Trapped mode
• Heating• Charge up
time

Ceramics(Fixed)Ceramics (er=4)
2005.02.22

Metal (Cu)

KEKB Review 2005 @KEK

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Movable Mask _2
Trapped Mode
Grooved duct to couple with TE11 mode (head is off center)
Use HOM damper, but still Q~2000

HeatingHeat conduction is mainly by radiation
Estimated input :200W 1000 °C (emissivity = 0.5)
Apply blackened surface (ex. CuO, as in the case of PEPII)

Charge up

Thin coating of low conductivity material on ceramics support,such as TiN[Trapped mode]

2005.02.22

KEKB Review 2005 @KEK

[Coupling to TE11 mode]

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Movable Mask _3
Conceptual drawing

Mask head = Carbon (graphite)
Coated by copper?
L=80 mm (~0.4 R.L.)
Support ceramics = BN (low er)
Loss factor ~0.1 V/pC (1/10, but still
high)[Loss factors]

Need further considerationand test.
Any ideas are welcome!
2005.02.22

KEKB Review 2005 @KEK

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Summary
R&Ds for the vacuum system are proceeding steadily.
Copper Beam Duct with ante-chamber:
Both pressing and drawing were well available for manufacturing.
Reduction of photoelectrons were confirmed, but other method tosuppress the multiplication of secondary electrons are necessary forhigh currents.
Improvement of structure is considered.

Surface with low SEY
NEG coating and TiN coating were studied in the positron ring.
Photoelectrons weaken the effect of low SEY.
TiN seems better but further investigations are required.

Connection flange
Application of MO-type flange is studied.
Bench test showed good results and the beam test is planned.

Bellows chamber
Comb-type RF-shield has been studied and no essential problem
was found so far.

Movable masks
Conceptual study has just begun.
2005.02.22

KEKB Review 2005 @KEK28