Norm[l-Con^u]ting RF C[viti_s
for Sup_rKEKB / MR & DR
Tetsuo ABE

MR: Main Ring
DR: Damping Ring

For SuperKEKB‐RF / ARES‐Cavity Group(T. Abe, T. Kageyama, H. Sakai, Y. Takeuchi, and K. Yoshino)

The 21th KEKB Accelerator Review Committee Meeting
2016‐06‐13

Contents
 RF cavities for the Main Rings (MRs) (“ARES”)
 Introduction
 Operational status in Phase 1
 Two serious troubles

 RF Cavities for the Damping Ring (DR) (“DR Cavity”)
 Introduction
 Results of the performance tests

2

ARES for the MRs
3

NC RF Cavity System for SuperKEKB (ARES)(Accelerator Resonantly coupled with Energy Storage)‐‐ Three cavity system stabilized with the accelerating /2 mode ‐‐High‐Purity Copper Resonators
Gap length: 256 mm
Three‐cavity system (unique)Rsh/Q0=15 Ohms
Q0=~1.1x105
Vc=0.5 MV/cav
Pc=150kW (60kW in AC, 90kW in SC)
Loss factor: 0.48 V/pC (bunch length: 5 mm)
Esp/Eacc = 3.8
(Cavity trip rate) < 1 /cav/3months
Us/Ua=9 (explained in the next page)

High energy (Us) stored as a low‐loss mode (TE013)

4

Three Main Characteristics of the ARES Cavity System
1. Large Storage Cavity (SC)Optimum detuning f   Optimum detuning suppressed by Us/Ua(=9)
 SC acts as an electromagnetic flywheel.
 f = fa / (1 + Us/Ua) = fa /10 < frev

 

I sin s Ra
fa
2Vc Q0Pb tan s4 U̶ fa = ‐200kHz (KEKB design), ‐280kHz (SuperKEKB design)
̶ frev = 99kHz

 Therefore, coupled bunch instabilities (CBIs) driven by
the accelerating mode are suppressed (Appendix A).

 /2‐mode operation

 Stable against the detuning and deformation
 The stored‐energy ratio: Us/Ua changeable Us/Ua = ka2/ ks2

 Parasitic 0 and  modes

 Damped selectively out of CC by an antenna‐typedamper (“Coupling‐Cavity Damper”)
 Have a almost symmetric impedance w.r.t. fRF

3. Hybrid HOM‐Damped Structure Explained in the next page

SC : Storage Cavity
CC : Coupling Cavity
AC : Accelerating Cavity

5

The ARES has‐‐ Two‐types of HOM‐Damped Structures. ‐‐To suppress CBIs driven by HOMs (Appendix A)
Type_1: HOM WG (HWG)

Type_2: Grooved Beam Pipe (GBP)

HOM absorber Bullet‐shaped SiC ceramics
 Directly water‐cooled

HOM absorber

Note:
No HWG in the horizontal
direction due to the (H)CC

TE 10

beam

TE 10

SiC ceramics tiles
Indirectly water‐cooled

H

H

For damping
Monopole HOMs
Vertically‐polarized dipole modes

For damping
Horizontally‐polarized dipole modes
T. Kageyama, “Grooved Beam Pipe for Damping Dipole
Modes in RF Cavities," KEK‐PREPRINT‐ 91‐133, 1991.

6

Type_2: Grooved Beam Pipe (GBP)

TM110Electric‐field Vectors

TE mode in the
Grooved Beam Pipe

7

Upgrade of the ARES for SuperKEKB(Selection)

8

NKlys:NCav=1:2

NKlys:NCav=1:1

KEKB

SuperKEKB Phase 1(Shown by T. Kageyama at KEKB Review 2011)

High‐Power Input Coupler / Performance Upgrade
1. Fine grooving inside the outerconductor of the coaxial line tosuppress multipactoring discharge.
(Appendix B)

2. The coupling loop is extended toincrease the input coupling factor(up to  = 6).
The optimum input coupling factor βopt is givenby βopt = 1 + Pbeam/Pc .
(T. Kageyama, et al., Proc. of PASJ, TUPS126, 2011)

Input Coupler
Power Handling Capability (Spec.)
PinputPc
Pbeam
SuperKEKB : 750 kW = 150 kW + 600 kW
KEKB: 350 kW = 150 kW + 200 kW
Pc = 150 kW generating Vc = 0.5 MV per ARES cavity.
T. Kageyama

3D Transparent View of ARES Cavity System

Upgraded Input Coupler
Increasing the Input Coupling Factor
Input couplers with increased input coupling (max = 56) needed for the stations with
the Nklys:Ncav=1:1 configuration to accelerate beams with the design current.

Used at the KEKB/MRs

With increased input coupling for the SuperKEKB

The high‐power performance was demonstrated both at the test
stand and in the KEKB operation.
Pinput = 740 kW (= Pc: 130kW + Pbeam: 610 kW)
11

ARES Cavity System / Upgraded Input Coupler / Production & Processing
ARES
Storage Cavity

Input Coupler
used asOutput Coupler

Input Coupler
Under Test

High Power Test Stand
To 1MW Water Load

RF Power from
1MW CW Klystron(TOSHIBA E3786)

Snapshot of Status Monitor for RF Processing(Photo and diagram by T. Kageyama) For the first two production couplers, stable power feeding was demonstrated
 Up to Pinput = 800 kW (= 750 kW + 50 kW) at the test stand.
 So far, 13 couplers have been processed up to 750‐800 kW.
 The same shown in the last report As of June, 2016, 5 couplers are waiting for high power test.
 Pinput = 850 kW (= 750 kW + 100 kW) to be demonstrated (after Phase 1)(Max. RF power stably supplied by the high power source)12 All the Input couplers for NKlys:NCav=1:1 stations in OHO D4&D5 have been replaced
by upgraded couplers.
 One coupler has been replaced by an upgraded one for one of the FUJI D8 stations.
 For other NKlys:NCav=1:1 stations, to be replaced before starting Phase 2.

Almost no vacuum break before T=0

13

Two ARES Cavities Newly Installed in OHO D4‐E,F

Pressure‐sensitive paper 
to check the RF contact
between CC and SC

14

Crack found on the welding lip of the D4‐F Cavity

 Vacuum sealing impossible
with lip welding

Sealed with a O‐ring (non‐metal)
The vacuum pressure in this cavity is one order ofmagnitude higher than others.

To be fixed after Phase 1

15

Monitoring ALL the Cavities with TV Cameras(Top View)

TV Camera (NTSC)

 TV camera attached in the
opposite side of the input couplerbeam During the KEKB operation, only
some cavities were monitored.
 For SuperKEKB, all the cavities areto be monitored and recorded.

Input Coupler

16

17

Example of the Recorded Videos
 D5‐B cavity
 Vacuum interlock activated with
RF input power of 60 kW during
the RF conditioning.
(Top View)

beam

TV Camera
Input Coupler

Useful for isolating problems caused by input couplers(Note: Input couplers are removable.)18

Operational Status of the ARES in Phase1

19

Operation Summary in Phase 1
Successful operation except for
Two serious troubles

20

Vacuum CCG [V]

RF Input Power [kW]Vacuum Trouble of the D8‐D Cavity

Vacuum‐pressure behavior for the ARES D8‐D cavity,
plotted together with the RF input power.
(T. Kageyama)21

Vacuum Trouble of the D8‐D Cavity
 The ARES cavity D8‐D at the FUJI RF section for LER could
not be operated due to abnormal vacuum pressure rise,sometimes observed other than during the RF operation.
 The gas source may be some subsurface defects, possibly
between the copper plating layer and the carbon steel
structure of the storage cavity.
 Now detuned  out of operation
 Plans toward Phase 2:<Notes>Successful operation at KEKBNo vacuum break since KEKB Plan A: hopefully locate and remove the defects in situ,
followed by high power testing for confirmation.
 Plan B: replace this ARES cavity by a spare one.
(Original version by T. Kageyama)
22

Other Serious Trouble: Chillers for RF Windows
Input Coupler

RF

Cooling Object to be cooled: RF Ceramic Windows
Channel Coolant: water + antirust(5%), pH: 6 to 7 % Coolant pressure: 0.2 to 0.3 MPa ( < 0.4 MPa)
 Put in the sub‐tunnel (not seen from the beam) No trouble related to the chillers used at KEKB

We have replaced all the 15 chillers
used at KEKB for a long time.
(Further details on the chillers aredescribed in Appendix C.)

RF Ceramic
Window

RF Ceramic
Window

Cooling
ChannelCoupling‐
Cavity Damper(Solid Model by K. Yoshino)During Phase 1 Operation
Example of the filters with small amount of precipitates We observed precipitates growing in the coolant.
 Especially in the FUJI D8 stations
 Triggered LER beam aborts at D8‐B (3 times) The flow rate of the coolant decreased with precipitates.

 Principal component : para tertiary butyl benzoic acid
 Widely used as antirust Temporary treatment
 Cleaning the filers
 Replacement of the coolant by new one

Example of the filters withhuge amount of precipitates

 Measures toward Phase 2
 Understanding of the chemical process of the precipitate creation (on‐going) Including basic test (beaker test) Sealing the reservoir tanks against the (activated) air in the tunnel
 From the basic test (beaker test) with H2O+antirust(5%)+Cu+Mo̶ Precipitates created in the beaker open to the air
̶ No precipitates created in the beaker sealed against the air

 Monitoring of pH of the coolant during operation24

pH and Water Solubility
pH of the coolant in each chiller(measured on June 2, 2016)
7.5

Water solubility of the antirust‐ 12600 mg/L at 20degC (pH7)‐ 47.1 mg/L at 20degC (pH4.3)

6.5
6
5.5

D8‐E

D8‐C,D

D8‐B

D8‐A

D7‐E

D7‐C,D

D7‐B

D7‐A

D5‐E,F

D5‐C,D

D5‐A,B

D4‐G,H

D4‐E,F

D4‐C

5
D4‐A

pH

7

(K. Yoshino)

25

RF Cavities for the DR(“DR Cavities”)
26

NC RF Accelerating Structure for the DR
The blue, gray, green, and magenta regions indicate a vacuum,
HOM absorbers, coaxial lines of input couplers, and plungers of
movable tuners, respectively.
The colored arrows indicate the direction of the positron beam.

 RF operating frequency: 508.9 MHz
 Same as that of the MRs
 Based on the HOM‐damped structure of the successful ARES cavity system Three cavities at max. to be installed in a limited space originally designed for
one cavity (3.8 m in the beam direction, excluding the taper sections) Total Vc = 2.4 MV at max. with 0.8 MV/cavity

 Apart from the CC and SC of the ARES, this DR cavity has the following spacesaving features that are not included in the ARES: The HOM absorbers are all compact tile‐shaped SiC ceramics;
 The neighboring cavities share a GBP in‐between; and The cavity is connected directly to GBPs with lip welding for vacuum sealingat the outer periphery (“weld ring gasket”).

 “Multi Single Cell” structure Coupling of the Accel. mode and HOMs among the cavities significantly
suppressed by the HOM absorbers on the GBPs One big mechanical structure with solid connections of the components

 Loss factor of this structure: 2.3 V/pC (bunch length: 6.0mm)
 Vacuum pumps directly attached to each cavity

1225
1225

 In the DR tunnel, we will assemble the cavities separating them with
GBPs similar to LEGO blocks.
NC : Normal‐Conducting
HOM: Higher Order Mode
MR: Main Ring
DR : Damping Ring
CC: Coupling Cavity
SC: Storage Cavity
GBP: Grooved Beam Pipe

27

Main Characteristics of the DR Cavity
1. 3 cavities (max) in a limited space (3.8 m) originallydesigned for one cavity
 Required Total Vc = 0.26 MV (until 2010)  1.4 MV (from 2011)
 Three cavities with 0.7 MV/cav ( tot. 2.1 MV) in operation
 Up‐down separation of the HOM waveguides
 The neighboring cavities share a Grooved Beam Pipe in‐between.
 The cavity is connected directly to Grooved Beam Pipes with lip
welding for vacuum sealing at the outer periphery (weld ring gasket)
 Compact HOM absorbers (SiC tiles)

2. Complete HOM‐damped structure Based on the ARES cavity system
 CBIs suppressed (Appendix D)
28

Components of the RF Structure except for the cavity
HOM WG LoadOne Set of six SiC ceramic tiles

Solid connection between the cavity and GBP

GBP

29

RF Cavity for the DR (DR Cavity)
HOM Waveguide Load with SiC Tiles Inside
Grooved Beam Pipes
with SiC Tiles Inside

Turbo‐Molecular Pump

Design Parameters

Sputter Ion Pump

30

History of the DR Cavity Development
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, and Fabrication method(3. Cavity No.3 to be fabricated in JFY2017?)

31Vc = 0.95 MV/cav demonstrated at the Test Stand(> 0.7 MV/cav required for DR operation)

Cavity No.2

Cavity No.1

During the RF conditioning
During the stability test
after the RF conditioning

Vc

Wall‐Loss
Power
[kW]

Total
HoldingTime[hours]Numberof Cavity
Trips

0.80

144

30.5

1

0.85

164

18

0

0.90

186

14.5

3

0.95

210

8

1

[MV/cav] 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/cav successfully.

32

Configuration in the DR Tunnel
At the beginning of the DR commissioning

Total Vc = 1.4 MV

Dummy duct

Ultimate

Cavity No.3

Total Vc = 2.1 MV
If we need this,depending on results of the DR
commissioning and the budget.p

33

What We Learnt from the R&D of the DR Cavities(Note: No EP applied to the barrel part.)

1. Electro‐polishing (EP) for the end plates gives significant
improvement in: Q0
 Field emission during RF conditioning

2. Same performance between DR Cavity No.1 and No.2 in: Q0
 Field emission
 RF conditioning speed Vacuum performance High‐gradient performance:

Breakdown Rate at Vc=0.90 MV/cav (after RF conditioning)
Cavity No.1: 5.0+4.8‐2.7 /24hrs
Cavity No.2: 3.3+1.3‐1.0 /24hrs

3. Significant Findings directly related to cavity breakdown 34

1. Improvements with the EP

Q0(meas) / Q0(sim)Prototype92.9%IACS
Cavity No.1
97.1%IACS
Cavity No.2
97.3%IACS= Indirect observation of the dark current:
Field emission
Acceleration
Impact on the inner surface
Emission of X‐ray35

3. Significant Findings directly related to cavity breakdownby the direct in‐situ observation of the inside of DR Cavity No.2
using 3 TV cameras for Multi-directional and wide-field observation
during the high‐gradient test

All of the cavity‐breakdown events recorded in storage media automatically(5 seconds before, until 1 second after the moment of the breakdown)
36Significant Finding (1)60% of the cavity breakdown events are accompanied with a spot‐type explosion
without any other significant phenomena.

Intensity in thegreen‐square region

Example of the spot‐type explosion 
at the moment of the breakdown(Vc = 0.56 MV/cav)

Frame Number37Significant Finding (2)20% of the cavity breakdown events are accompanied with an explosion anddisappearance of a stable bright spot without any other significant phenomena.

Mini‐Supernova

Example of the bright‐spot explosion 
at the moment of the breakdown(Vc = 0.95 MV/cav)

Intensity in thegreen‐square region

 This bright spot disappeared

Frame Number

38

Pyrotechnical phenomena are minority!
Flash

Lightning

A few % of the cavity breakdown events
For more details T. Abe et al., “Breakdown study based on the direct in‐situ observation of inner surfaces of an RF accelerating cavity
during a high‐gradient test”, submitted to PRAB.
(Preprint: KEK‐Preprint_2015‐25 )39

Summary ARES cavities for the MRs
 Successful operation except for the two serious troubles: Vacuum trouble on the D8‐D cavity
 Precipitate growth of the antirust in the chillers

 DR Cavities

MR: Main Ring
DR: Damping Ring
NC: Normal‐Conducting
CW: Continuous Wave
UHF: Ultra High Frequency

 Good performance demonstrated
 Significant findings directly related to cavity breakdown on the
NC CW UHF cavities obtained through the high‐gradient test40

Photo on June 10, 2016, in the DR/RF section(Assembly testing)Thank You!
41

Appendices42

Appendix A (1/5)

Longitudinal CBI driven by the accelerating /2 mode

To be cured by the RF feedback systems(shown in the next page)43

Appendix A (2/5)(S. Yoshimoto)

S. Yoshimoto, et al., presented in the 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, 2003 (PaperID: 1P072)

44

Appendix A (3/5)

Longitudinal CBI driven by the parasitic 0 and  modes

To be cured by the longitudinalbunch‐by‐bunch feedback system
45

Appendix A (4/5)

Longitudinal HOM Impedance (Simulation Results) and
CBI Threshold at SuperKEKB/LER
Both impedances fromwakepotentials in time‐domain With the full structure (including SC) Only AC + half CC (without SC)
τ=13ms for SuperKEKB/LER / 22 ARES
cavitieswith 3.6 A beam current< 21.6 ms (Longitudinal rad. damping time of LER)Fastest longitudinal CBI sourceTo be cured by the longitudinal bunch‐by‐bunch
feedback system

46

Appendix A (5/5)

Horizontal HOM Impedance (Simulation Result)and CBI Threshold at SuperKEKB/LER(by T. Kageyama)

τ=300ms for SuperKEKB/LER / 22 ARES cavities
with 3.6 A beam current>> Transverse radiation damping timeof LER: 43.2 ms
From wakepotentials in time‐domain
From eigenmode analysis

Frequency

0.6GHz

0

1GHz

47

Appendix B

Multipactoring suppression by fine grooving of conductor surfaces of
coaxial-line input couplers

Ø77mm

Outer conductor of
the coaxial line
T. Abe, et al., Phys. Rev. ST Accel. Beams 13, 102001, 201048

We have replaced all the 15 chillers
used at KEKB for a long time.

Appendix C (1/2)

 First, we used the same type of chiller with the almost
the same specifications (ORION RKE‐1500A‐VW) at thetest stand (D1‐A) on the ground.
 Performance and stability demonstrated
 No trouble so far for 8‐year operation

 We purchased succession machines (ORION RKE‐
1500B1‐V) for MR and DR operation, and replaced theold 15 chillers (ORION RKS‐1500‐V‐C) by these (ORION
RKE‐1500B1‐V).
 Small amount of precipitates observed in the reservoir tanks
in two of the chillers in FUJI D8 on 2015‐10‐0749Appendix C (2/2)

Comparison between the Chillers
Capacity of
Top cover on
the reservoir the reservoir
tank
tank
ORION RKS‐1500‐V‐C(used in the KEKB
tunnel)45 L

Feed‐water
inlet

Overflow‐wateroutletYes
Closed(in addition to in operation
the top coveron thechassis)Closedin operation

30 L
ORION RKE‐1500A‐
VW(used at the test stand(D1‐A) on the ground)None(only the top
cover on the
chassis)Closedin operation

Open
alwaysORION RKE‐1500B1‐V(used in the
SuperKEKB tunnel)

None(only the top
cover on the
chassis)Closedin operation

Open
always

15 L

50

Appendix D

CBIs driven by the HOM
ImpedancesFrom wakepotentials calculated, using GdfidL, for the whole RF sectionCBI thresholds for the DR design

Longitudinal

Transverse

Growth Time ＞ 20ms
＞ 5ms (rad. damping time)

Growth Time ＞ 30ms
＞ 10ms (rad. damping time)

51

EOF
52