Vacuum System for KEKB Upgrade- mainly for arc section Y. Suetsugu
on behalf of KEKB Vacuum GroupDesign strategy– Change to the nano-beam scheme– Design of main componentsElectron cloud issues in the positron ring
– R&D of countermeasures– Mitigation strategy

2010/2/16

Summary and Remained issues
KEKB Review (KEK)1Design strategy_1
Change to the nano-beam scheme from the highcurrent scheme;– Energy (LER: 3.5 → 4 GeV, HER: 8 → 7 GeV)
– Emittance (εx 24 → 3.3 nm [LER])– Beam current (LER: 9.4 → 3.6 A, HER :4.1 → 2.6 A)– Bending radius (LER 15.3 → 71 m)– Bunch length (3 → LER: 6 mm, HER: 5 mm)Impact on the vacuum system_1– SR power for LER is reduced compared to the case in
the previous high-current scheme.
 Lower current, longer bending radiusÆAluminum beam pipe is now available for LER.
 ~3 W/mm2 (ref: HER ~15 W/mm2, if ha = 90 mm)
 Cost reduction (?)
 Gives more choices for the fabrication methods, andfor the countermeasures against electron cloud issues.
2010/2/16

KEKB Review (KEK)2

Design strategy_2
Impact on the vacuum system_2:– Photon density is reduced to one-half.
 Lower beam current– HOM power is also reduced.
 Longer bunch length, Lower beam current– Requirement to avoid beam instabilities is similar.
 Fast Ion and Electron Cloud are still key issues.
– Beam current may be increased in the future.
 Leave room for higher beam currents







The requirements to the vacuum system was actuallyrelaxed, but still severer than that of KEKB.
The design strategy fundamentally follows that considered
so far for high-current scheme.
Aluminum-alloy beam duct gives more choices in the design.
2010/2/16

KEKB Review (KEK)3

Design strategy_3Beam ducts are newly manufactured.
– Magnets are re-aligned.
– Beam impedances should be decreased further （pumpports, bellows, photon masks, etc.）.
– LER: More powerful countermeasures against theelectron cloud are required.



Main vacuum components are also newly designed.
– Bellows chambers, gate valves, movable mask(collimators), stoppers, etc. are designed to fit the beamducts.
– Higher strength against higher beam currents are
required.

2010/2/16

KEKB Review (KEK)4

Design strategy_4Main pumps (NEG) are replaced with new ones.
– Reset lifetime



Auxiliary pumps (ion pumps), rough pumping
system, vacuum gauges (CCG), L-angle valves,
thermo sensors, etc. are reused, with some new
backups.
– Ion pumps were found to be fine after 10 years operation.

Electric lines, cooling water pipes, control systems
are basically reused as it is, after maintenance.
– Cooling water pipes should be rearranged.

Capacity of water cooling system should beenhanced.
2010/2/16

KEKB Review (KEK)5

Design of main components_1
Beam ducts with ante-chambers– A countermeasure against the electron cloud.
– Low beam impedancePump ports and SR masks locate in an antechamber.

– Reduction of SR power is not a main purpose now.
– Fit to the existing (reused) magnets.
– LER: Aluminum alloy is now available. Copper is required
for wiggler sections.
– HER: Copper is required.


Pump (NEG) is installed into one of the antechamber (inside of the ring)Concept
NEG strip– Distributed pump system foreffective pumping. S ~ 80 l/s/m.
– Inserted from end flanges.

BeamSR
2010/2/16

KEKB Review (KEK)6

Design of main components_2


R&Ds of beam ducts_1– Copper beam ducts with antechambers have been
installed into the LER, and tested with beams.
– Cold drawing method for copper pipe was established.
Bent duct

Straight duct
Wiggler section

Arc section

BPM

2010/2/16

KEKB Review (KEK)7

Design of main components_3
R&Ds of beam ducts_2– Extrusion of aluminum-alloy beam duct is under going for
LER.
Aluminum-alloy duct
110

Aluminum-alloy duct

Example of cross section of– The design of HER beam
HER beam duct
duct has not yet fixed.
 Fit to existing magnets.
 If the half-aperture is ~90 mm
the SR power is the samelevel as the present HER.
ÆNegotiation with Mag. group
2010/2/16

KEKB Review (KEK)8Design of main components_4
MO-type Flanges– Thermally strong, sure RF bridge, applicable to antechamber scheme, low beam impedance– In addition to SS flanges, copper alloy and aluminumalloy flanges has been developed.
Easy welding to pipes, reduction in heating by joule loss

– Several flanges have been installed into the ring andtested.
Cu-alloy flange (CrZrCu)

2010/2/16

Al-alloy flange (A2219, A2024)

KEKB Review (KEK)9Design of main components_5
Bellows and gate valves with comb-type RF-shield– Sure RF shielding, thermally strong– applicable to ante-chamber scheme– Finger-type for some cases, if flexibility is required.



Trial models have been installed into the ring and tested.

– Reduction in the temperature of bellows has been demonstrated.



Copper RF shield will be used even foraluminum-alloy beam ducts.
Bellows chamber

2010/2/16
2010/2/17

Gate valve

RF-shield (gate valve)

KEKB Review (KEK)10Design of main components_6
Movable mask: Stealth type has been tested.
Very sorry to say, but it met an severe obstacle.

– Even a diamond support could not withstand the intense
beam power, although the principle was demonstrated at
low beam currents --- suspend the R&D.



Modified present type (Ver.4) will be used in thecommissioning stage at least.

– Or advanced type of that used in the PEP-II ? (fit to theantechamber scheme?)– HOM absorbers will be reinforced.
PEPII type
Temperature of bellows near mask

Ver.4

Stealth type

Ver.6

2010/2/16

KEKB Review (KEK)11Electron cloud issues_1
Electron cloud instability can be a serious problem
for LER (positron ring).
The threshold of electron density to excite the singlebunch instability:
K. Ohmi , KEK Preprint 2005-100 (2006)
Here,

E [GeV] = 4.0= 7828γ= 0.0185
νs

σz [m] = 6.E-03
c [m/s] = 3.E+08
K
= 11
Q
=7
re [m] = 2.80E-15
β y [m] = 25
L [m] = 3016

Nb
= 6.25E+10
Qb[C] = 1.4E-08(1.4 mA/bunch)
Sb [m] = 1.2(4ns)
λ [C/m] = 5.2E+12 (Qb/2/σz)
σ y [m] = 2.E-05
σ x [m] = 2.E-04= 5.46E+11 K = ωe σz/cωe
Q = Min(Qnl, ωe σz/c)
ωe σz/c = 10.9
Qnl ~7ρth [e-/m3] =1.13E11 ÆTarget: 1E11
2010/2/16

KEKB Review (KEK)12Electron cloud issues_2Expected electron density without any cures– Estimated from experiments so far at KEKB.
– For a circular Cu pipe (φ 94mm), 4 ns spacing, 1
mA/bunch, No solenoid.
Sections

L [m]

L [ %]ne [e-/m3]

nex L [%]

Total

3016

100

Ave.5E12100

Drift space (arc)1629 m

54

8E12

78

Steering mag.

316 m

10

8E12

15

Bending mag.

519 m

17

1E12

3.1

Wiggler mag.

154 m

5

4E12

3.6

Q & SX mag.

254 m

9

4E10

0.063

RF section

124 m

4

1E11

0.072

IR section

20 m

0.7

5E11

0.063

Main partAny countermeasures are required to reduce ne down to 2%!
(5E12 Æ 1E11 [e-/m3])
2010/2/16

KEKB Review (KEK)13

Electron cloud issues_3
Established counter measures– Solenoid filed at drift section (~50 G): Effective to bothphotoelectrons and secondary electrons.
– Ante-chamber scheme：Effective to photoelectrons.
Adopted at PEPII LER– TiN coating (Reduction in SEY)：Effective to secondary
electrons. Adopted at PEPII LER
Effect of solenoid[by K. Kanazawa]<1/1000

Effect of antechamber

Effect of TiN coating

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

Cu

Circular chamber~1/5
TiN coating

8/100/2 (800)
4 ns spacing
R = 47 mm (Circular)

2010/2/16~1/100With
antechamber

KEKB Review (KEK)14

Electron cloud issues_4
Comparison among these countermeasures– Standard = bare copper ducts (circular)
Materials, methods

Relative EC density

Al

~20

Cu (Circular pipe)

1

Notes
Coatings are indispensable.

Solenoid [Drift space]~1/50~50 G, considering gaps(<1/1000 if uniform)

Ante-chamber scheme~1/5<1/100 for photoelectrons

Cu (Al)+TiN coating~3/5

Relatively high gas desorption

Any coatings (TiN, NEG, DLC, etc.) is effective independentof the underlying materials (Cu or Al). But the effect seems
small for its work (time and labor).
Note that aluminum beam duct requires any coatings to
reduce SEY.
2010/2/16

KEKB Review (KEK)15

Electron cloud issues_5
Application of these countermeasures
ne [e-/m3]

nexL [%]

Total (3016 m)

5E12

100

Drift space (1629)

8E12

78

Steering mag. (316)

8E12

15

Bending mag. (519)

1E12

3.1

Wiggler mag. (154)

4E12

3.6

Q & SX mag. (254)

4E10

0.06

ne [e-/m3]

nexL [%]

Total (3016 m)

9E10

100

Drift space (1629)

3E10

18

Steering mag. (316)

3E10

3.4

Bending mag. (519)

2E11

38

Wiggler mag. (154)

7E11

39

Q & SX mag. (254)

8E9

0.81

Sections

Sections

2010/2/16

Ante-chamber (Cu or Al+coating)+ Solenoid
×1/5
×1/50
Ante-chamber (Cu or Al+coating)×1/5

Average ne is stillborderline region
If with TiN-coating, ne ~6E10 e-/m3

Further countermeasures
Main in bending and wiggler
magnets are desirable.


KEKB Review (KEK)16

Electron cloud issues_6
Countermeasures in bending and wiggler magnets
Candidates:– Clearing electrode Attract electrons by electro-static field– Grooved surface
 Lower effective SEY geometrically

Experimental R&Ds have been performed using KEKB
LER
– Also as a part of US-Japan collaboration for ILC DR
B

B

by L. Wang et al.

Electrode
2010/2/16

KEKB Review (KEK)

Grooved surface
17Electron cloud issues_7
Clearing electrode has been said to be very effective to
reduce EC in magnetic field.
– Impedance and heating of electrode have been serious
problems for intense e+ beam.

Very thin electrode structure was developed.

– 0.2 mm Al2O3 and 0.1 mm tungsten (W) electrode formed
by a thermal spray method. ±1 kV is OK.
– Good heat transfer and low beam impedance– Flat connection between feed-through and electrodeAn insertion for test with a thin electrode

400 mmx
40 mm

Connection to feed throughTo feed-throughTungsten(t0.1)

Al2O3(t0.2)
Feed-throughStainless steel
2010/2/16

Y. Suetsugu, H. Fukuma, M. Pivi and L. Wang, NIM-PR-A, 598 (2008) 372
KEKB Review (KEK)18

Electron cloud issues_8A test chamber was installed in a wiggler magnet.
– Magnetic field: 0.78 T– Effective length: 346 mm– Aperture (height): 110 mm– Photons: 1x1014 photons/s/m/mA

Test chamber in a wiggler magnet

Wiggler magnet

Inside view

Electrode

Test chamber

Monitor2010/2/16

An electron monitor and an insertion
with an electrode are placed at thecenter of a pole, face to face.
Electron monitor has an RFA and 7
strips to measure spatial electron
distribution (~40 mm width in total).

KEKB Review (KEK)19

Electron cloud issues_9
Results: Effect of electrode potential (Velec)– Drastic decrease in the electron density by applying Velec
was observed. (For negative large Velec, electrons flows into
the monitor)– Similar effect was observed for 2 ~ 16 ns spacings.
(b)
1585 bunches(Bs ~ 6 ns)~1600 mA(~1mA/bunch)

Velec = 0 V

Velec = 0 V[Log scale][Log scale]

Center+500 V+500 V[Spatial distribution]
2010/2/16[Spatial distribution]
KEKB Review (KEK)20

Electron cloud issues_10
Results: Change with beam dose

– The electron density decreased to less than ~1/100 at
Velec > ~+300 V compared to the values at Velec = 0 V (W)and a TiN-coated flat surface.
(0.9~1.0 mA/bunch)~1x1013 m-3

~1x1012 m-3~1x1011 m-3
~1x1010 m-3


2010/2/16

KEKB Review (KEK)

Two-time experiments.
Electron currents for thethermal-sprayed tungsten(Velec = 0V) is similar tothe case of flat TiNcoated surface. ÅRoughsurface?
The second result was
lower than the first one.
Å Aging of surface?
No extra heating ofelectrode and feedthrough was observed.
21

Electron cloud issues_11
Application to a real beam pipe with antechambers.
– Final check of feed through and heating of electrode.

Feed throughEBW#1 and #2

Installed chamber

Inside view

#1
#2

– No extra-heating, degradation of insulation are found after
one-month operation. (the experiment will be continued)
2010/2/16

KEKB Review (KEK)22

Electron cloud issues_12Grooved surfaces: geometrically reduce SEY.
– The properties were studied in a wiggler magnet using thesame experimental setup to that of the clearing electrode.
(Roundness)– B = 0.78 TRtParameters of grooves– Material: Cu, Al-alloy, SS– β : 20~30°, Rt:0.1~0.2 mm– d: 2.5~5 mm

B

dby L. Wang et al.

(Depth)GrooveCu

Al+TiNAl

MonitorSS

Y. Suetsugu, H. Fukuma, M. Pivi and L. Wang, NIM-PR-A, 604 (2009) 449
2010/2/16

KEKB Review (KEK)23

Electron cloud issues_13Results: EC growth– A significant reduction in the electron density wasobserved by using grooved surfaces.
– Similar effect was observed for 2 ~ 16 ns spacings.
Flat

β20°, d5 mm Groove

1585 bunches(Bs ~ 6 ns)[Linear scale]

4x10-7

4x10-7

Center[Spatial distribution]
2010/2/16[Spatial distribution]
KEKB Review (KEK)24

Electron cloud issues_14
Results: Change with beam dose

– The electron density decreased to 1/6~1/10 compared to thecase of a flat TiN-coated surface (β = 20). That is, less than
~1/10 compared to flat copper.
(0.9~1.0 mA/bunch)Electron densities forgrooves surfaces in~1x10 mthese parameters werelower than the case of aflat TiN-coated surface.
~1x10 m
Smaller electrons even
R =0.2mm
if no-coating: TiN
coating improves the~1x10 meffect, but the groove
R =0.1mm
structure seems much
~1x10 meffective to reduce SEY.

Less density for smaller
β and Rt.
13

-3

12

-3

t

11

-3

t

10

-3

2010/2/16

KEKB Review (KEK)25

Electron cloud issues_15Comparison between clearing electrode and groove– All data so far are plotted in one figure(0.9~1.0 mA/bunch)

B = 0.78 T
Measured with thesame monitor at thesame location.
Clearing electrode ismuch effective in
reducing electron
density compared toother methods.

TiN-coatedflat surface

Groovedsurface(β~20°)

1/6~1/10
2010/2/16

KEKB Review (KEK)

Clearing
electrode

~1/10
26

Electron cloud issues_16Updated comparison among mitigation techniques– Based on the experiments so far. Standard = Cu (circular pipe)Materials, methods

Relative effect Notes

Al

~20

Cu (Circular pipe)

1

Solenoid [Drift space]~1/50~50 G, considering gaps(<1/1000 if uniform)

Antechamber~1/5<1/100 for photoelectrons

Cu (Al) +TiN coating~3/5

Relatively high gas desorption

Groove (β~20°) [in B]~1/10

Top and bottom

Electrode [in B]~1/100

Most effective against EC.
Expensive?

Coatings are indispensable.

Clearing electrodes and grooved surfaces can be strong
countermeasures in dipole fields, more effective than any
coatings.
2010/2/16

KEKB Review (KEK)27

Electron cloud issues_17Application of further countermeasures
ne [e-/m3]

nexL [%]

Total (3016 m)

9E10

100

Drift space (1629)

3E10

18

Steering mag. (316)

3E10

3.4

Bending mag. (519)

2E11

38

Wiggler mag. (154)

7E11

39

Q & SX mag. (254)

8E9

0.81

ne [e-/m3]

nexL [%]

Total (3016 m)

3E10

100

Drift space (1629)

3E10

68

Steering mag. (316)

3E10

13

Bending mag. (519)

2E10

14

Wiggler mag. (154)

4E9

1.5

Q & SX mag. (254)

8E9

2.8

Sections

Sections

2010/2/16

×1/10+ Groove?
Ante-chamber (Cu or Al+coating)Ante-chamber (Cu) + Electrode×1/100

KEKB Review (KEK)

Further low average ne.
If with TiN-coating, ne ~2E10 e-/m3
This is the result with the
maximum possible curesat present.
28

Electron cloud issues_18Expected reduction in the electron density– Major electron cloud will be reduced by antechamber
scheme and solenoid field at arc section.
– Electrodes in wiggler and grooves in bending magnets will
decrease EC further and increase the safety margin.

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

5.2E12

Solenoid at Drift (1/50) +Cu
Antechamber (1/4)

9.4E105.7e10+Electrode in Wiggler(1/100) +Groove in B (1/10)

2.7E101.7e102010/2/16

Single Bunch Instability Threshold (~1x1011)

With TiN coating

KEKB Review (KEK)29

Electron cloud issues_19
Impact on the beam impedance– The evaluation has just started (Æback up slides).
– Compared to the resistive wall (φ90 mm, Cu)
Further– Clearing electrode, assuming;
One electrode, proportional to the length.
 Resistive wall of W is included (over estimate).


Loss factor

investigationis required.

1m

Total (~80 m / 3000 m)[in wiggler magnets]~x 2.1~x 1.03

Wake potential (height) ~x 6 (5+1)~x 1.11– Grooved surface, assuming;

Grooves at top and bottom, proportional to the length (over estimate).
 Increase in resistive wall of 50 % by grooved surface is included.


Loss factor
Wake potential (height)
2010/2/16

1m

Total (~520 m / 3000 m)[in Dipole magnets]~x 1.3~x 1.05~x 2 (1+1)~x 1.17KEKB Review (KEK)30

Summary & Remained issues_1
Super KEKB is challenging for the vacuum system.
The design will fundamentally follows thatconsidered in the high-current scheme.
– Basic R&D on components has almost finished.
– Optimization of design should be required considering the cost.
– Aluminum beam ducts can be used for LER now.


Radiation survey is undergoing using KEKB.

For countermeasures against electron cloud in a
dipole field, clearing electrodes and grooved
surface are found to be very effective.
– Ante-chamber and solenoid reduce main part of electrons.
– By using clearing electrodes and grooved surface in
dipole field, the average electron density further decrease.


Grooves were also effective in drift space (experiment of SLAC).

– The affect on the beam impedance should be considered
carefully.
2010/2/16

KEKB Review (KEK)31

Summary & Remained issues_2
Choice of material for LER beam ducts– Should be considered together with countermeasuresagainst EC, constructing scenario (schedule), cost, etc..
– Present plan: The Right Material in the Right Place
Drift space

Cu

or

Al+Coating (or Groove)

Q and Sx mag.

Cu

or

Al+Coating (or Groove)

B mag.

Al+Groove (with coating?) or Cu+Groove

Wiggler mag.

Cu+ElectrodeNotes:

Copper is a good material, i.e., low SEY (after scrubbing),
good radiation shielding, low gas desorption, experiences in
KEKB.
 (TiN) coatings have relatively high gas desorption. Wait
scrubbing? They are effective, but the effect seems small for
its labor. Coating can be done after the commissioning (?)
 Depends on the beam test in the next run.


2010/2/16

KEKB Review (KEK)32

Summary & Remained issues_3
R&D for aluminum beam duct with grooves isundergoing– Welding groove blocks at top and bottom.

– Extrusion: if possible, the manufacturing process becomesvery simple.

– Beam test at a straight section (drift space)in the next run.
Æ Grooves at whole surface
2010/2/16

KEKB Review (KEK)33

Summary & Remained issues_4
Design of HER beam ducts– Fix the cross section.
– Components will be the same with those in LER.

Design of special component– Movable mask, Septum, Abort window, HOM absorber,etc.

2010/2/16

KEKB Review (KEK)34

Summary & Remained issues_5
Scheduling of manufacturing– Manufacture ~2000 beam ducts (2 ~ 6 m) and installthem in 3 years?!
Make 4 ducts per day! (200 days per year) Install 8 ducts per day!


– Discover and nurture new vendors.
– Establish detailed procedures of preparation (baking, any
coating, solenoid winding,,,).
– Make consistent schedule with magnet installation,,,.

2010/2/16

KEKB Review (KEK)

35

END
2010/2/16

KEKB Review (KEK)36

Back up2010/2/16

KEKB Review (KEK)37

Impedance of electrode_1


Dependence of loss factor and wakepotential on the thickness of Al2O3
insulator– Calculated by using GdfidL (1m model,
half chamber)– t0.2 mm ~ 2.0 mm
Loss factor

2010/2/16

Longitudinal Impedance

KEKB Review (KEK)38

Impedance of electrode_2


Effect of tilt on loss factor– Calculated by GdfidL (0.5 m model, halfchamber)– Comparison with resistive wall impedance– Up to 20 mrad
D

Loss factor

2010/2/16

Model

Tilt

R

Loss factor is smaller (~1/2)
compared to that of theresistive wall (for 1 m).
Loss factor is almost
constant against the tilt
angle.

KEKB Review (KEK)39

Impedance of electrode_3


Effect of tilt on loss factor– Calculated by GdfidL (0.5 m model, halfchamber)– Comparison with resistive wall impedance– Up to 20 mrad

Model

Tilt

Loss factor and kick factorÅLoss factor

Kick factorÆ


2010/2/16

KEKB Review (KEK)

Loss factor is almost
constant against the tilt
angle.
Kick factor increases withthe tilt angle.

40

Impedance of electrode_4


Effect of length on loss factor– Calculated by GdfidL ( half chamber)– Comparison with resistive wall impedance– Up to 3m.

Model

Loss factor

2010/2/16

KEKB Review (KEK)

Loss factor is almost
constant against the length.

41

Impedance of electrode_5

Wake potential– Calculated by GdfidL (0.5 m model, halfchamber)– Comparison with resistive wall

Model

Wake potential







2010/2/16

KEKB Review (KEK)

The wake potential is
inductive.
The peak value of the wakepotential is higher by afactor of 5~10, though theloss factor is small.
Should be carful in using
the electrode for longsection, in relation to single
bunch instability
42

Impedance of groove_1


Effect of tilt on loss factor– Calculated by GdfidL(0.5 m model, half chamber)– Comparison with resistive wall impedance– Up to 20 mrad
Loss factor

D

R

Kick factorb =30, d=5 mm

b =20, d=3 mm

2010/2/16

The grooves are located atboth up and bottom.
The loss factor is very small
for tilt angle = 0 ~ 20mrad.
(~10 % of resistive wall)
The loss factor is almost
constant against tilt (a little
bit increases).

KEKB Review (KEK)43

Impedance of groove_2

Effect of tilt on loss factor– Calculated by GdfidL(0.5 m model, half chamber)– Comparison with resistive wall impedance– Up to 20 mrad
R
Loss factor and kick factor

D

Kick factorÆ

b =30, d=5 mm

Loss factor is almost
constant against the tilt
angle.
Kick factor increases withthe tilt angle. Larger thanthat in the case of electrode.

ÅLoss factor

b =20, d=3 mm

2010/2/16

KEKB Review (KEK)44Impedance of groove_3

Effect of length on loss factor– Calculated by GdfidL (half chamber)– Comparison with resistive wall impedance– Up to 3 m.
Loss factorD

2010/2/16

KEKB Review (KEK)R

Loss factor is almost
constant against the length.

45

Impedance of groove_4

Wake potential– Calculated by GdfidL (0.5 m model, halfchamber)– Comparison with resistive wallR
Wake potential

D





2010/2/16

KEKB Review (KEK)

Wake potential is inductive,
and almost the same tothat for resistive wall.
Should be carful in using
the electrode for longsection, in relation to single
bunch instability.

46

Impedance of groove_5Longitudinal impedance– Calculated by using GdfidL (1m model,
half chamber)– σz = 20 mm, s = 20 m
R
ImpedanceD

2010/2/16

KEKB Review (KEK)

Little trapped mode47