SuperKEKB RF Accelerating Cavity System(Improvements during LS1, etc.)

Tetsuo ABE<tetsuo.abe@kek.jp>

KEK / ACCLThe 27th KEKB Accelerator Review Committee
2024-03-26

RF Accelerating Cavity System for the SuperKEKB Rings (MR & DR)
Present RF Cavity Layout
TSUKUBA

D0430+1 RF stations
D11
NIKKO

D05-A

OHO

D10FUJI

D08

D07

D05

DR

ARES 1:2 station(one klystron drives two ARES cavities)
ARES 1:1 station(one klystron drives one ARES cavity)

RF Cavities for SuperKEKB (2024-03-26)

Tetsuo ABE (KEK/ACCL)

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Super-Conducting Cavities (SCCs)

𝒆𝒆−𝒆𝒆−∅150mmSingle-cell single-mode 508.9 MHz cavity
 All the HOMs are extracted to the beam ducts.
HOM dampers with:
Ferrite (used since KEKB)• 4mm-thick Ferrite (IB-004) bonded on the innersurface of a copper duct by HIPping• Ferrite Length: 120mm (150mm) for 220 (300) mm
diameter damper as “SBP” (“LBP”)• Power-handling capability demonstrated: 11.7(14.8) kW for 220 (300) mm diameter damper
SiC (new)• Added for higher HER beam currents at SuperKEKB∅150mmFerrite damper
Copper Pipefor Cooling

SiC

Stainless Steel Flange

SiC

SiC damper∅220mm duct
TM011 , TM020 , etc.

∅300mm duct TM110 , TE111 , etc.

eHIPped Ferrite (thickness: 4mm)

RF Cavities for SuperKEKB (2024-03-26)

Tetsuo ABE (KEK/ACCL)

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Improvement for SCC(M. Nishiwaki) The HER beam current is limited by the cooling capability of the chillers for
the HOM dampers (Ferrite & SiC) installed near the SCC modules.
 During LS1, the chillers have been replaced by new ones with a larger coolingcapacity.
→Allowable HER beam current due to the RF system: 1.7  2.0 A
 For higher HER beam currents than 2.0 A, we need to further upgrade thecooling capability and to add more SiC dampers.
 During LS2?

RF Cavities for SuperKEKB (2024-03-26)

Tetsuo ABE (KEK/ACCL)

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Normal-Conducting Cavities
Two cavities used for DR beam operation

30 ARES cavities used for MR beam operation

Single-cell damped cavity
Frequency: 508.9 MHz
Rsh/Q0 = 150 Ω
Q0 = ~3x104
Vc = 0.8 MV/cav (spec.)Pwall ≈ 140kW

Three-cavity system stabilized with the π/2mode operation
Frequency: 508.9 MHz (π/2 mode)Stored-energy ratio: USS/UAC = 9
Rsh/Q0 = 15 Ω
Q0 = ~1.1x105
Vc = 0.5 MV/cav (spec.)Pwall = 150kW (60kW in AC, 90kW in SC)

Equivalent mechanical model

SCCCAA𝑒𝑒 +C-damper

SSCC

AA

RF Cavities for SuperKEKB (2024-03-26)

No.1(Downstream CAV)

No.2
(Upstream CAV)

Tetsuo ABE (KEK/ACCL) In the DR tunnel

5Improvements of the Normal-Conducting Cavity System during LS1
 30 ARES cavities for the MR C-damper system (Attachment A) Replacement of water-cooling pumps by more powerful ones: ~25  40 L/min for > 20kW power capability / dummy load
̶ 3.6A LER beam current ⇔ 20 kW / dummy load in the C-damper system Replacement of support disks in the coaxial lines

̶ From cross-linked polyethylene (radiation-hard, but low heat resistance)
̶ To Teflon (high heat resistance) Replacement of high-power input couplers
 For four cavities: D08-C,D,E#1,E#2
 Followed by successful high-power tests

 Cavity replacement in the D05-A station

 Due to un unusual cavity breakdown problem
 To be explained later in detail Two cavities for the DR Vacuum-sealing method between the cavity and duct changed
 O-ring replacement (Viton (rubber)  metal)
 To obtain much better vacuum in the DR / RF section
 Details follows…

RF Cavities for SuperKEKB (2024-03-26)

Tetsuo ABE (KEK/ACCL)

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(Extracted from the M. Tawada’s presentation in this ARC)

RF Cavities for SuperKEKB (2024-03-26)

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The vacuum-pressure level in the DR /RF section was high.
 Due to the Viton O-rings (rubber) used in the connection between the RF cavities
and beam ducts We used Turbo-Molecular Pumps (TMPs) in addition to sputter Ion Pumps (IPs) during beam
operation.
The dots indicate vacuum pressures in the sections other than the RF one (courtesy of K. Shibata) In the neighboring section to the RF (west)~1e-5 Pa in the DR / RF section

Turbo-Molecular Pump (TMP)(300L/s each)

sputter-Ion Pump (IP)
(400L/s each) TMPs needed in addition to IPs
RF Cavities for SuperKEKB (2024-03-26)

Tetsuo ABE (KEK/ACCL)

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Connection between the RF cavity and beam duct
Cavity side

Connected

Beam-duct side

Contact fingers (Au-coated BeCu)
spot-welded innermost in the flange
Viton O-ring(rubber) The vacuum is designed to be sealed with welding of the outermost bellows (like weld-ring gaskets)
 However, the welding is cost-ineffective (~100,000USD needed) and almost irreversible process.
 Before LS1, Viton O-rings (rubber) were used, which were not so radiation tolerant.
 SuperKEKB
We decided
to use metal O-rings toTetsuo
makehardware all-metal!
ABE(KEK/ACCL)
RF Cavities for(2024-03-26)
TetsuoABEthe(KEK/ACCL)
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Vacuum leak test at test bench

Custom-made HELICOFLEX gasket

 Flange mockups used
 Heat cycles applied with ribbon heaters
 No vacuum leak demonstrated
Test bench built by H. Sakai

The same flange as used in the DR / RF section
RF Cavities for SuperKEKB (2024-03-26)

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Vacuum leak test using the prototype cavity (No.0)
Vacseal applied Heat cycles with ∆T = 20℃ applied using
ribbon heaters
 No vacuum leak demonstrated
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O-ring replacementin the DR / RF section 2022-10-12

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High-power test of the DR / RF cavities up to tot. Vc = 1.5 MV> (Operational tot. Vc = 1.0 MV)

 No vacuum leak during or after the high-power test
RF Cavities for SuperKEKB (2024-03-26)

Tetsuo ABE (KEK/ACCL)

Vacuum in Upstream CAV [CCG V]Vacuum in Downstream CAV [CCG V]
Cooling-water temp. at CAV entrance [×10℃]

Klystron output RF power [kW]Vacuum pumping with
 Two IPs (400L/s each) Two TMPs (300L/s each) 6e-5 Pa 7e-6 Pa 8e-7 Pa 3e-7 Pa

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DR / RF Vacuum improvement by the O-ring replacement during LS1
2022-01-25 (just before LS1) 5e-6 Pa
 RF / GVs closed
 tot. Vc = 1.0 MV
 Pumping with IPs + TMPs

2023-11-24 (just after LS1) 5e-7 Pa
 RF/GVs closed
 tot. Vc = 1.0 MV
 Pumping with IPs + TMPs

~1e-5 Pa in the RF section before LS1 using both IPs and TMPs

2024-02-05 (recent status)
 2e-7 Pa
 RF/GVs closed
 tot. Vc = 1.0 MV
 Pumping with IPs only

GV: vacuum Gate Valve
IP: sputter-Ion Pump (400L/s each)
TMP: Turbo-Molecular Pump (300L/s each)~1e-7 Pa in the recent RF section using IPs only

Vacuum improvement in the DR / RF section
by two orders of magnitude
during DR beam operation
 Only IPs used for beam operation
RF Cavities for SuperKEKB (2024-03-26)

Tetsuo ABE (KEK/ACCL)

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However, periodic vacuum-pressure rises appeared

2e-6 Pa 

3e-7 Pa 

5 days

Klystron output RF power [kW]2e-5 Pa 

Vacuum in Upstream CAV [CCG V]
Vacuum in Downstream CAV [CCG V]

after the O-ring replacement

1e-7 Pa 
 No impact on the DR beam operation (well below the interlock threshold) Appears even with no beam
 Under investigation
 The dominant component of the gas in the pressure rises: m=2 (H2)
RF Cavities for SuperKEKB (2024-03-26)

Tetsuo ABE (KEK/ACCL)

15 Arc section

D04

30 stations

OHO experimental hall

D11OHO

D10

D05
FUJI

D08

D07

RF Cavities for SuperKEKB (2024-03-26)

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Unusual Breakdown Problem on the D05-A Cavity
RF switch manually turned off

Usual cavity breakdown

Unusual cavity breakdown
seen only in the D05-A cavity
Field pickup (AC) ↓Field pickup (AC) ↑10 μs

Input RF ↓Input RF ↑10 μs
Field pickup (AC) ↓Input RF 

Field pickup (SC) ↑10 μs

 C damper power

Field pickup (SC) ↑Field pickup (SC) ↑Rapid drop of the RF field,
which is explained
as “Fireball Breakdown” in AC

Tail in the RF-field decay
determined by the fill time (~10μs)
The original paper↓

Slow drop prior to
the rapid drop of the RF field(E.g. see the Attachment B)

T. Abe, et al., "Direct Observation of Breakdown Trigger Seeds in a Normal-Conducting RF
Accelerating Cavity", Physical Review Accelerators and Beams 21, 122002, 2018.

RF Cavities for SuperKEKB (2024-03-26)

Tetsuo ABE (KEK/ACCL)

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Identification of the vacuum arc point in the D05-A cavity
at the moment of the unusual breakdown

𝑒𝑒 +ACCC All the light emission at the same point Vacuum arc in the coupling cavity (CC)cameraInput coupler

SCCamera

Unusual breakdown
at 2021-10-30_16:21

During stable high-power operation with 340 kW input
SCRF Cavities for SuperKEKB (2024-03-26)

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History of the D05-A unusual breakdowns
0.4

0.4

Frequent!

Frequent!

840.2
819.7
0.42
0.42
0.42
0.42
751.20.4
0.38

0.41

0.42

0.4
692.3

0.35
0.3

612.5
599.80.31591.2

0.3

0.25

0.34

500
0.22

400329301309.8
304.2
250.2

One month

202.7

0.1

0.11
0.1 0.1

100.5

LERビーム電流[mA]
LER beam current[mA]

Tetsuo ABE (KEK/ACCL)

2022/4/26 0:00

2022/3/26 0:00

2022/2/26 0:00

0 00 0 02022/1/26 0:00

02021/12/26 0:00

2021/11/26 0:00

2021/9/26 0:00

2021/8/26 0:00

2021/7/26 0:00

2021/6/26 0:00

2021/5/26 0:00

空洞電圧[MV][MV]
Cavity voltage

 Decided to replace the D05-A cavity (CC+AC)
RF Cavities for SuperKEKB (2024-03-26)

100

0 00

0

2021/4/26 0:00

2021/2/26 0:00

2021/1/26 0:00

2020/12/26 0:00

2020/11/26 0:00

2020/10/26 0:00

2020/9/26 0:00

2020/8/26 0:00

2020/7/26 0:00

2020/6/26 0:00

2020/5/26 0:00

2020/4/26 0:00

2020/3/26 0:00

2020/2/26 0:00

2020/1/26 0:00

0

2019/12/26 0:00

2019/10/26 0:00

2019/9/26 0:00

2019/8/26 0:00

2019/7/26 0:00

2019/6/26 0:00

2019/5/26 0:00

2019/11/26 0:00

29

0200

100.5
53.8

2021/3/26 0:00

0.05

0.15300

0.15

2021/10/26 0:00

0.15

700
600

0.3

0.28

Frequent!

0.20800

0.37

559.3

900
0.41

0.38

479.9

470.1

Frequent!

2022/5/26 0:00

0.4

2019/4/26 0:00

D05-A cavity voltage [MV]0.45

LER beam current [mA]

 Occurred even with no beam
 Getting worse and worse0(AC and CC are brazed into one.)19

Visual inspection  Leakage of brazing filler found at the vacuum-arc pointon 2022-09-28
Mechanicalconnection

Cameras ↓

Input coupler Leakage of brazing filler (BAu-11V)
 The vacuum-arc point is around hereC-damper

Light emission at the unusual breakdown

ACCCSC𝑒𝑒 +(3D figure made by T. Kageyama)SC
Brazing filler leakage (BAu-11V)

RF Cavities for SuperKEKB (2024-03-26)

Mechanical connection
Tetsuo ABE (KEK/ACCL)

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Check of the RF contact in the connection between SC and CC using pressure-sensitive paper
2014-05-22 (just after the relocation: HERLER between KEKB and SuperKEKB)

CC
↓

SC
↓

 Perfect contact around the vacuum arc point
 Any contact failure was not a cause of theunusual cavity breakdown of the D05-Acavity.

Pressure-sensitive paper

RF Cavities for SuperKEKB (2024-03-26)

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Moving the spare cavity out of the FUJI stock area

2023-01-20

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Moving the D05-A cavity (AC+CC) out of the D05 straight section
AC+CC↓

RF Cavities for SuperKEKB (2024-03-26)

SC↓

2023-01-25

Tetsuo ABE (KEK/ACCL)

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Moving the spare cavity into the D05-A station in the D05 straight section
2023-01-26

Alignment
Tiny clearance!

Landing in the D05-A

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Successfully finishedNo vacuum leak
No vacuum arc
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Tetsuo ABE (KEK/ACCL)

Klystron output RF power [kW]Vacuum pressure [CCG V]

High-power test of the new D05-A cavity up to 0.5 MV/cav25

The vacuum sealing made with a metal O-ringMechanical connection surface in the CC side

 The vacuum is designed to be sealed at the connection
between SC and CC with outermost bellows welded.
 After the cavity replacement during LS1, no welding
bellow left due to:

No welding bellow leftMetal O-ring First welding during the KEKB era
 Second welding during the relocation (HER  LER) between KEKBand SuperKEKB

Bump for RF contact
Nose cone in CC From the test bench result, vacuum leak occurs if wemake a perfect contact at the bump for RF contact
between SC and CC.
 ARES cavity was not designed for using metal O-rings

 Decided to make a gap of ~0.7 mm at the bump for RFcontact
 No RF contact here RF contact made through the metal O-ring

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ARES three-cavity system
Accelerating π/2 mode in the ARES cavity system for ILER =0TE013 in SCf-1250-1000

𝑒𝑒 +Equivalent mechanical model

ACInput coupler

C-damper

SCXCC-750-500-250

250for ILER > 0500750

1000

This node shifted toward X>0 for higher beam currents
 Higher C-damper power Higher RF field applied to the connection at X=0
Surface electric field during operation with 0.5 MV/cav and ILER=0
 At the connection (X=-130mm) : ~0.3 MV/m
 In AC: 7.2 MV/m (max.)

C-damperSC C-damper antenna

0Electric Field [a.u.]

TM010 in AC

AC(The surface field at the connection between SC and CC) ∝ (square of the C-damper power)

RF Cavities for SuperKEKB (2024-03-26)

Tetsuo ABE (KEK/ACCL)27

D05-A high-power test with a large tuning offset
20 kW C-damper power corresponds to beam loading with LER 3.6 A

Tuner position [V]10kW maintained

C-damper power [W]

P_kout=~180kW↓-85° -84°

-83°

2023-04-07
P_kout=~300kW↓-82°

Tuning offset

0°

Wall-loss power (= Input minus reflection) [W] Successful simulation of the beam loading of a 3.6 A LER beam current with a 20kW C-damper power!
RF Cavities for SuperKEKB (2024-03-26)

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~0.7 mm gap

Klystron output power [kW] Input coupler

ACSC

C-damper𝑒𝑒 +RF Cavities for SuperKEKB (2024-03-26)

C-damper power [kW] CC

7.9e-7 Pa 1h

RF Power [kW]2.4e-6 Pa Vacuum Pressure [CCG V]

D05-A high-power test with 20 kW C-damper power maintained for 6.5 h Vacuum pressure [CCG V]

 No breakdown occurred No vacuum arc observed in any frame of the recorded video
 No breakdown
during the 2024a beam operation so far
Tetsuo ABE (KEK/ACCL)

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Allowable LER beam current due to the RF system(Shown in the previous ARC)

Uncertainty not verified↓

 Limited by the power absorption capability of theSiC dampers installed at the end of each RF
section for ARES We had been working to investigate the actual
capability limit of the SiC dampers, and todevelop how to increase the capability using anL-band (1.25GHz, CW, 50kW) high-power test
stand (D1-CT).
 However, the klystron power supply (for D1AT&CT) was broken half way through high-power
testing.
 This R&D bas been halted.
 At least, we can guarantee 2.0 A.
 ~2.5 A not sureD1-CT∅150mm duct for ARES cavities Taper duct

SiC damper
being used forbeam operation

RF Cavities for SuperKEKB (2024-03-26)

SiC damper
to be tested

L-band klystron This kind of R&D for high-power components
takes a long time (e.g. serval years).
 The broken klystron power supply is shared with
D1-AT (509MHz test stand) where an important
experiment was on-going to measure
fundamental parameters in the fireballhypothesis for SBL.
See  https://indico.cern.ch/event/1298949/contributions/5783882/

Tetsuo ABE (KEK/ACCL)

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Summary

 SCC

 The cooling capability for the HOM dampers (Ferrite & SiC) has been upgraded.
 Allowable HER beam current due to the RF system: 1.7  2.0 A

 ARES D05-A cavity had the unusual breakdown problem.

 Vacuum arc in CC (not in AC)
 CC+AC replaced by spare one Successfully recovered with no breakdown until now including the current 2024a beam operation

 RF cavity for DR

 The O-ring replacement (Viton  Metal) led to the significant vacuum improvement.
 The strange periodic vacuum pressure rises appeared after the O-ring replacement
 Under investigation
 No impact on the DR beam operation

 The klystron power supply for the high-power test stands (D1-AT, D1-CT ) was broken.
 Can not perform high-power tests toward higher beam currents

RF Cavities for SuperKEKB (2024-03-26)

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Thank you for your attention

RF Cavities for SuperKEKB (2024-03-26)

Tetsuo ABE (KEK/ACCL)

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Backup slides

RF Cavities for SuperKEKB (2024-03-26)

Tetsuo ABE (KEK/ACCL)

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[Attachment A] ARES C-damper system↓↓↓

Antenna coupler
with COAX 120D out.

77D in Air

COAX Taper
120D  77D120D77Din Air

COAX 77D
Water-cooled dummy loadThese support disks have been replaced:
Cross-linked polyethylene  Teflon↑C-damper(Antenna + Coaxial line + Dummy load)
RF Cavities for SuperKEKB (2024-03-26)

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[Attachment B] Example of fireball breakdowns observed in the
ARES Cavities
C-damper power

Field pickup (AC)↓𝑒𝑒 − beam

10 μs

SSAC

CCC-damperVideo file  https://youtu.be/qtIB6kLf2j4
RF Cavities for SuperKEKB (2024-03-26)

D04-F
Tetsuo ABE (KEK/ACCL)

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