Status of RF Accelerating Cavities
for SuperKEKB Positron Damping Ring
Tetsuo ABE (KEK)for SuperKEKB-RF / ARES Cavity Group(T. Abe, S. Enomoto, T. Kageyama, H. Sakai, Y. Takeuchi, and K. Yoshino)

The 22nd KEKB Accelerator Review Committee Meeting
2018-03-14

Contents
1. Overview and Fundamentals of the RF Accelerating Structure
2. Installation into the SuperKEKB Positron Damping Ring (DR)
3. Status of Cavity No.0 (prototype)
4. Summary

The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)11. Overview and Fundamentals ofthe RF Accelerating Structure
The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)2Overview of the RF Accelerating Structure

Operational frequency: 508.9 MHz (CW) (same as for the MRs)

Blue region: vacuum during operation
Gray region: HOM absorbers (SiC ceramics)

HOM-damped structure based on the successful ARES cavity system
Max. three cavities to be installed in a space originally designed for one
cavitye+ e+Max. total Vc = 2.4 MV to be supplied to the DR

“Multi Single Cell” structureElectromagnetic field in each cavity has high independence by the contribution theHOM absorbers between the cavities.
Number of cavities is variable; Cavities are replaceable.

Middle Cavity
Downstream
CavityUpstream Cavity
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Assembled in the DR tunnel like LEGO blocks→ One big mechanical structure with solid connections of the components

Vacuum pumps directly attached to each cavity
Design Parameters
Operational frequency

508.9 MHzRsh/Q0150 ΩQ0∼30000

Cavity Voltage (DR spec)

0.7 MV / cavity

Cavity Voltage (Cavity spec)

0.8 MV / cavity

Wall-loss power @ 0.7 - 0.8 MV / cavity~110 - 140 kW / cavity

Tetsuo ABE (KEK)

MR: Main Ring
HOM: Higher-Order Mode
SiC: Silicon Carbide3

Due to predicted CSR effects,
the required acceleration voltage was increased fivefold!
(N.Iida)RF section

CSR(CSR : Coherent Synchrotron Radiation)
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Injector Linac
To supply an accelerating voltage higher than 1.4 MV tothe DR in the limited space of a sole RF section, which wasoriginally designed for one cavity, this RF accelerating
structure has unique space-saving features ( next page).
Tetsuo ABE (KEK)4

The HOM absorbers in the grooved
beampipes absorb not only TE modes
but also higher-order TM modes.

Space-Saving Features
3.8 m

The HOM absorbers are allcompact tile-shaped SiC ceramics(48 x 48 x 20 mm3).

The neighboring cavities share agrooved beampipe in-between.

No additional HOM absorbers (such asSiC ducts) needed
E.g. SiC duct used at the MR / RF section

The cavity is connected directly to grooved
beampipes without bellows.
(280 mm long)

The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK) Fixed with 16 M12 bolts
 Vacuum-tight lip welding at theouter periphery (“weld ring gasket”)
5HOM impedances of the RF section
are low enough below CBI thresholds.

(CBI: Coupled Bunch Instability)

From wakepotentials calculated, using GdfidL, for the whole RF sectionCBI thresholds were calculated for the DR design (8 nC / bunch).

Longitudinal

Transverse

Growth Time > 20 ms> 5 ms (radiation damping time)The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)

Growth Time > 30 ms> 10 ms (radiation damping time)6

DR Cavities
No.2

No.1

0. Cavity No.0 (prototype) developed in JFY2011
 Surface protection of the endplates: acid cleaning followed by chromating

1. Cavity No.1 fabricated in JFY2012
 Surface protection of the endplates: Electropolishing (EP)

2. Cavity No.2 fabricated in JFY2013
 Surface protection of the endplates: Electropolishing (EP)No difference between No.1 and No.2 in the: Electric design
 Mechanical structure Fabrication method
 Low-power performance High-power performance
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2. Installation to the DR Tunnel(November, 2016)

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Tetsuo ABE (KEK)8Components of the RF Accelerating Structure for the DR
Upstream Cavity

Upstream
Middle Beampipe

Upstream
End Beampipe

Downstream Cavity

Downstream
Middle Beampipe

Downstream
End Beampipe

Dummy Pipee+e+ 

544

556

544

556

544

556

544

Seven components (= two cavities + five beampipes) to be aligned 
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Tetsuo ABE (KEK)9

Into one big mechanical structure (3.8 m long)

e+

e+

3844 mmSeven components (two cavities and five beampipes) to be aligned Connected
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Tetsuo ABE (KEK)1010

Alignment Policy Accuracy

 0.3 mm in the transverse directions

Same as for the ARES cavities at the MRs (1.0 mm in the longitudinal direction)

 What to be aligned: beam-port flanges (not cavities)
 Beam axis defined by the quadrupole magnets next
to the RF section (upstream and downstream) Position measurement of the Beam-port flanges
prior to the installationThe 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)11

Position measurement of the beam-port flanges
using FaroArm Edge (portable CMM):
 Measurement range: 1.8 m (in diameter)
 Five axes
 Precision at a fixed point: 0.024 mm (spec)

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Tetsuo ABE (KEK)

FaroArm Edge
from KEKB monitor group 12

Offsets of the component centerspredicted from the measurement results using FaroArm Edge
beam axis

We selected the best permutationout of 2048 permutations of the components
to make the offsets as small as possible,
or to make the structure as straight as possible.

1 mrad unexpected bend

beam axis(Cavity No. 2)

The 22nd KEKB Review (2018-03-14)(Cavity No. 1)

For details on the algorithm to select the best permutation,please see:T. Abe, Y. Takeuchi, H. Sakai, T. Kageyama, K. Yoshino, M. Masuzawa, T. Kawamoto,“Installation of Normal-Conducting RF Accelerating Cavities into the SuperKEKB
Positron Damping Ring”, in Proceedings of the 14th Annual Meeting of Particle
Accelerator Society of Japan, 2017 (Paper ID: TUOL06).

Tetsuo ABE (KEK)13

We also measured distances between the ∅150-duct centerand the reference planes of the beam-port flanges.
~245 mm~150 mm

To be used in the position
measurement after the installation

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We defined and marked
the beam axis (horizontal).

Upstream Q-mag

Downstream Q-mag

Boundary

The 22nd
KEKB Review (2018-03-14)
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Tetsuo ABE (KEK)

Boundary15

We defined and marked the beam axis (level).

Downstream Q-mag

Upstream Q-mag

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Connecting the grooved beampipes to the cavities

Viton O-Ring+=
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Alignment of the Beam-Port Flanges of the Cavities

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Then, we connected the dummy pipe.
Dummy pipe

Fixed with double
nuts to avoid too
much stress
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Position Measurement of the Beam-Port Flanges
after the Installation

Using FARO Laser Tracker ION

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to check the installation accuracy

Tetsuo ABE (KEK)

From KEKB Magnet Group20

Redefined the beam axis based on the reference baseplates of the Q-magnets(Downstream)(Upstream)

5660 mm
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Position Measurement
using the laser tracker


M6 tapThe 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)

Target Holder

1.5-inchi Reflector Target

22

Two-step measurement

Q-mag(upstream)
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Meas. 1

Meas. 2

Upstream CavityDownstream Cavity
Tetsuo ABE (KEK)

Q-mag(downstream)
23

Measurement using the laser tracker by Masuzawa-san and Kawamoto-san(From KEKB Magnet Group)

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Combined with the measurement results using FaroArm Edge on the distances
between ∅150-duct center and the reference planes of the beam-port flanges,
Using FaroArm Edge

~245 mm~150 mm
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We obtain deviations of the beampipe center from the beam axis.
(Inner)beam axis(Outer)■ Meas. 1 (using the laser tracker)▲ Meas. 2 (using the laser tracker)★ Measurements during installation(using a laser marker)- - - Theoretical prediction based on
the beam-port flange meas.

(Top)beam axis(Bottom)

The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)We achieved an alignment accuracyof 0.3 mm as a whole!
26

After all the installation works

Taken by Nobukazu Toge (KEK)The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)27 For two weeks from May 19, 2017 Only daytime, weekdays

Total Vc (blue) [MV]Klystron Output Power (red) [kW]High-Power RF Conditioning after the Installation
Note: High-power RF
conditioning of eachcavity was completed ata test stand beforehand.

Break due to other works in the DR tunnel

Smoothly reached Total Vc = 1.6 MV (350 kW klystron output)
over the DR spec: total Vc = 1.4 MV
The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)28

However,After maintaining total Vc = 1.6 MV (350 kW klystron output) during the conditioning,small vacuum leaks (10-9 to 10-8 Pa m3/sec)
occurred at the 7 rectangular flanges of the HOM waveguides .

Possible cause is a thermal cycle
accompanied by high temperature
at the rectangular flanges(~50 degC at max. for total Vc = 1.6 MV)
The vacuum leaks stopped byincreasing the clamping torque except
for one waveguide flange*.

HOM waveguide load(*The gasket was replaced, and then no leak)The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)29

Measures against thermal cycles(taken in Oct., 2017)

1. We added more cooling pipes on the E-bends of the HOM waveguides to suppress theHigh thermal-conductivity glue (Devcon)
thermal cycle at the rectangular flanges.
E-bendCooling pipe brazed on
the E-bend during thefabrication of the cavityCavity

2. We inserted rubber bushes at the end of the support bars to make small degree of flexibility.

(EPDM)

No vacuum leak since we took these measures
The 22nd KEKB Review (2018-03-14)(last leak test
performed
TetsuoABE (KEK) on Feb.22, 2018)30

3. Status of Cavity No.0 (prototype)The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)31

Cavity No.0 was used for Lip Welding Test in 2012.
Cavity No.0
Grooved beampipe (prototype)

We have demonstrated:(1) It is possible to perform vacuum-tight lip welding in the DR tunnel after the installation of the cavities, and(2) Lip welding  Leak test: OK  Disassembly -> Re-welding  Leak test: OK  Disassembly
The 22nd KEKB Review (2018-03-14)

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Cavity No.0 was re-tested on its high-power performance.

March, 2017

Conditioning Histories up to Vc = 0.90 MV/cavity

The light blue lines indicate the reference vacuum pressure specified by the computer controlled automatic aging.
If the vacuum pressure is higher than the reference, the input RF power (Pinp) is slightly stepped down until thevacuum pressure becomes lower than the reference, and then Pinp is slightly stepped up as long as the vacuum
pressure is lower than the reference. Prefl and Vc indicate the reflected RF power and cavity voltage, respectively.

The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)(1) Reached Vc = 0.90 MV (radiation limit) smoothly.
(2) Maintained Vc = 0.90 MV for six hours.
 Comparable performance with Cavity No.1 and No.2.
33

We decided to promote Cavity No.0 to be a spare cavity
for DR operation.
 Enough high-power performance Four HOM waveguide loads to be made soon Bellows for vacuum-tight lip welding to be replaced

The 22nd KEKB Review (2018-03-14)

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Status of the Bellows of Cavity No.0
To be replaced by new one!

Downstream side

Upstream side
Blank flange

Used in the lip welding test performed in 2012
Grooved beampipe (prototype)Cavity No.0

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Replacement Test of Bellows

Using the grooved beampipe prototype(performed in 2017)

Bellows for vacuum-tight welding is weldedon a SUS frame.
Flange
The end mill revolved around the beam axis (14 rounds),
scraping the SUS frame (0.5 mm / round).

End mill(0.5 mm / round)

The 22nd KEKB Review (2018-03-14)

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By End Milling
After the 14th round (last)

After the 4th round

Before…  …Bellows for vacuum-tight welding
FlangeThe bellows was removed
after the 1st round.

The SUS frame wascompletely removed.
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Tetsuo ABE (KEK)37 New bellows (0.2mm-thick bellows welded on a SUS frame)

Lip welding test

 No leak detected
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Summary
 Cavity No.1 and No.2

 Were successfully installed to the DR tunnel / RF section
 ~0.3 mm accuracy in the transverse directions achieved Position measurement of the beam-port flanges prior to the installation was successfuland useful.
 High-power performance re-checked: OK Now accelerating positrons without any serious problems

 Cavity No.0

 To be promoted to be a spare cavity for DR operation
 High-power performance re-checked: OK Its bellows for vacuum-tight lip welding to be replaced by new ones this coming spring Will be a spare cavity during Phase II

The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)39

Thank you for your attention!
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Tetsuo ABE (KEK)40

Backup Slides

The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)412  3 Cavity Configuration
Total Vc = 1.4 MV for nominal DR operation @ Phase II(0.7 MV/cavity)

Ultimate!

Starting with 2 cavities

The 22nd KEKB Review (2018-03-14)

Total Vc = 2.4 MV at max. @ Phase III?
(0.8 MV/cavity)(if necessary)

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The Endplates of DR Cavity No.1 and No.2 were Electropolished (EP).
Material: OFC (class1), 40µm etching, Skin depth(δ)@500MHz: 3µm

After EP

Before EP
Ra=1.5µm, Ry=8µm

Ra=0.2µm, Ry=1µm ( < δ = 3µm)

(Upstream)

Fixed End Plate (FEP)
w/o tuning bump
φ500mm(Downstream)

Tuning End Plate (TEP)w/ tuning bump

The 22nd KEKB Review (2018-03-14)

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Low-Power Measurements of Unloaded Q-factor (Q0)

Prototype
Cavity No.1
Cavity No.2

Q0(meas) / Q0(sim)92.9%IACS
97.1%IACS
97.3%IACS

4% improvement with EP(Note: No EP applied to the barrel)of the coaxial-type input coupler
The 22nd KEKB Review (2018-03-14)

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Setup of High-Gradient (HG) TestToshiba
CW KlystronE3732(1MW, 508.9 MHz)(No beam injected into the cavity during the HG test)
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Detection of Cavity Breakdownby the Decay Time in Pickup Signal
1. The interlock system was activated with a reflection level
over the threshold.
2. Check the decay time of the pickup signal of the acceleratingmode ~8 µs  Not cavity breakdown << 8 µs  Cavity breakdown

QL = ~13000 

T. Abe, T. Kageyama, H. Sakai, Y. Takeuchi, and K. Yoshino,“Breakdown Study based on Direct In Situ Observation ofInner Surfaces of an RF Accelerating Cavity during a HighGradient Test”, Phys. Rev. Accel. Beams 19, 102001 (2016).

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Histories of the RF Conditioning
 83 hours to reach Vc=0.90 MV

 95 hours to reach Vc=0.90MV
 107 hours to reach Vc=0.95MV

The light blue lines indicate the reference vacuum pressure
specified by the computer controlled automatic aging.
If the vacuum pressure is higher than the reference, Pin isslightly stepped down until the vacuum pressure becomeslower than the reference, and then Pin is slightly steppedup as long as the vacuum pressure is lower than thereference.

 Pin (Prefl) : input power to (reflected power from) the cavity
 Wall-loss power: Pwall = Pin – Prefl = ~0.99 x Pin
 Cavity No.2 reached 0.95MV/cavity successfully.
 Comparable conditioning speeds btwn Cavity No. 1 and 2

The 22nd KEKB Review (2018-03-14)

Tetsuo ABE (KEK)47

After the RF Conditioning (up to Vc=0.95MV) completed,Stability Test with Holding Vc = 0.90 MV> Vc = 0.70 MV(required for DR operation)

Example of the daily histories  Cavity No.1: 3 breakdowns for 14.5 hours in total = 5.0+4.8-2.7 /24hrs
 Cavity No.2: 11 breakdowns for 80 hours in total = 3.3+1.3-1.0 /24hrs

Same high-power performance between DR Cavities No. 1 and No. 2 within the statistics
The 22nd KEKB Review (2018-03-14)

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Radiation Dose Rate

= Indirect observation of the dark current:
Field emission
Acceleration
Impact on the inner surface
Emission of X-rayfor the DR Cavity No.2

No significant difference
between DR Cavity No. 1 and No.2
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Constant during the stability test
(20 cavity breakdowns in this period)
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Permittivity of the SiC ceramic tiles
Typical Measurements of SiC ceramics

Permittivity of SiC ceramics largely depends on• Source SiC powders• Sintering conditions of SiC ceramics

Relative permittivity of the SiC ceramics (CERASIC-B), as a function offrequency, used in designing this accelerating structure.
These permittivity values are typical measurements on the SiC ceramictiles used for the GBPs of the ARES.

The 22nd KEKB Review (2018-03-14)

We aggressively controlled the permittivity bychanging the amount of aluminum containedin the SiC powder so that the permittivity
should be close to that used in designing theaccelerating structure.
Y. Takeuchi, et al., “Control of RF Dielectric Properties of SiC Ceramics for HOM Absorbers", in Proceedingsof the 8th Annual Meeting of Particle Accelerator Society of Japan, Aug. 2011 (Paper ID: TUPS137)

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End of This File
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