Fireball Hypothesis and Acoustic Observation
for Elucidation of the Mechanismof Sudden Beam Losses
at the SuperKEKB Main Ring
Tetsuo ABE<tetsuo.abe@kek.jp>

KEK / ACCL
B2GM
2024-01-29

SuperKEKB Accelerator~ Asymmetric-energy e+e- double-ring collider ~ Upgraded from KEKB B-factory (KEKB)
 Stored-beam energies

Belle IIHigh Energy Ring (HER) : 7.0 GeV (e-)


 Low Energy Ring (LER) : 4.0 GeV (e+)

 𝐸𝐸cms ≈ 𝑀𝑀Υ(4S) Stored-beam currents (design)Circumference: ~3 km

Main Ring (MR) HER : 2.6 A
 LER : 3.6 A

 Positron damping ring newly constructed Final target luminosity: 6.0×1035 cm−2⋅s−1

 Higher beam currents than those at KEKB Squeezing 𝛽𝛽𝑦𝑦∗ with the nano-beam collision
scheme Goal: 50-fold more integrated luminosity thanrecorded in KEKB To increase the luminosity in the future, we need toincrease the stored beam currents in addition tosqueezing 𝜷𝜷∗𝒚𝒚Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL) e+ sourcee+ damping ring 
e- gun 

Injector Linac
2

What’s Sudden Beam Loss (SBL)?

Belle II

Example of LER/SBL

Horizontal
Position [mm](b) Vertical bunch-by-by position

Current
Loss [mA]

Bunch
Current [mA](a) Horizontal bunch-by-by position

VerticalPosition [mm]Measured here

Last turn~10 μs(c) Bunch-by-by charge

ビームアボート(d) Loss in each bunch charge compared to that in the previous turnTime

One turn~10 μsCharacteristics
A)
B)
C)
D)
E)
F)

Beam bunches are suddenly kicked in the transverse direction, leading to significant beam losses.
Fast phenomena of the order of ~10μs
No change in the size or energy of the beam bunches
Observed not only in LER (𝑒𝑒 + ) but also in HER (𝑒𝑒 − ) (The bunch-charge loss patterns different between the two)Not observed before SuperKEKB / Phase 2
Largely depends on the bunch current (not total current)

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

3

Example of serious hardware damages due to SBL
LER / vertical beam collimator “D02V1” just upstream of the IR
TOP side
Phys. Rev. Accel. Beams 23, 053501 (2020)

Cf. An undamaged head (W)Damaged heads (Ta)A lot of rubble of the Ta heads strewn(Photos courtesy of Shinji TERUI)

Bottom side

 The impedance ↑ More difficult to suppress beam backgrounds at Belle II
Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

4

Investigation of the cause of SBL
 Machine performance failure?

 All of the relevant components are carefully monitored, and no suspicious one found Vacuum arc at RF contacts in vacuum components?
 In this case

 Any beam-phase change (= energy loss) should be observed in ~ms time scale.

 SBL occurred in ~10μs time scale, and no beam-phase change observed Dust-beam interaction?
 In this case, Vacuum pressure bursts and ~ms-time-scale beam loss should be observed.

 SBL occurred in ~10μs time scale mostly with no pressure burst

 Electron cloud?

 In this case, SBL should occur only in LER (e+), but SBL also occurred in HER (e-).
 Relevant simulation studies are on-going, and no clear relationship with SBL found sofar “Fireball”?
Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

5

FireBall-Triggered Vacuum BreakDown(FB BD)in RF Cavities

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

6

FB BD discovered in High-Power Tests of 509 MHz (CW) RF Cavities
Vacuum region in the cavity@ KEK / MR-D1-AT↓ Excited electromagnetic field↓

Max. RF power input to the cavity: ~200 kW (CW)

Accel. mode: 509 MHz Continuous Wave (CW) TM010



Made of Oxygen Free Copper (Class1)

Q0 = ~30000



Rsh/Q0 = 150 ΩSpec. Vc = 0.8 MV ( Eacc = 3.1 MV/m)

Wall-loss power: ~150 kW @Vc = 0.80 MV

For more details,
https://www2.kek.jp/accl/legacy/eng/topics/topics131007.html

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

7

Surface field at Vc = 0.9 MV (Input RF power: ~200 kW)(𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔)

𝑬𝑬𝒎𝒎𝒎𝒎𝒎𝒎 = 13 MV/m

This max. surface E-field is much lower than in: Field evaporation (> ~GV/m)
 X-band HG accel. structures (~200 MV/m)
 DC HV experiments (~100 MV/m)

 But breakdowns occur in UHF NC CW cavities.

↑
Damping WG↑
Upstream end plate↑
Downstream end plate

Fireball Hypothesis for SBL (2024-01-29)

Hmax = 34 kA/m
Tetsuo ABE (KEK/ACCL)

Damping WG↓

Damping WG

8

Multi-directional and wide-field observation with Three Cameras

509 MHz CW cavity

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

9

Observation1: “Adherent”x fireballs
During high-power operation with Vc = 0.95 MV (Eacc = 3.7 MV/m)
Bright spots observed on the endplate are hightemperature (> 1,000℃) micro-particles (< 0.1 mm)

1316 ± 38℃

One of the bright spots

suddenly exploded
at the moment of breakdown.

Then, disappeared in thefollowing high-power
operation.

1028 ± 23 ℃

1349 ± 37℃
1155 ± 45℃

1207 ± 13℃
1440 ± 20℃
1057 ± 22℃

Observation of the emission spectra using a spectrum camera After various corrections, the temperatures were estimated.

Fireball Hypothesis for SBL (2024-01-29)

Endplate
E.g. Graphite sphere <Guess>with a 0.1mm diameter

E-field

40℃
H-field

A fireball adhering to a copper
surface exploded, then RF-cavity
breakdown

1,350℃

100℃
509 MHz cavity
Tetsuo ABE (KEK/ACCL)

10

Observation2: “Flying”xfireballs

Three high-speed cameras used for multi-directional observation
during HP operation of 509MHz (CW) NC cavity at Vc = 0.88 MV

Upstreamend plateSide view

Downstream
end plate

↓
1 ms
↓

↓
1 ms
↓

This video  https://youtu.be/VZe8dVLqWH0

Because the high speed cameras are not sensitive toinfrared, the flying bright spot should be a fireball with
> 1,000℃ if the size is small (≲ 0.1mm).

Fireball Hypothesis for SBL (2024-01-29)

↓
1 ms
↓

Tetsuo ABE (KEK/ACCL)

BD occurred at the moment of
the FB landing.

11Other flying fireballs

https://youtu.be/MqFYk4EVr7w

 Vc = 0.9 MV (CW)
(Downstream end plate)↑ Vc = 0.9 MV (CW)

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

12

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

13

Phase Diagram of Copper
Pressure(Data from https://www.iap.tuwien.ac.at/www/surface/vapor_pressure )MeltingFreezing

in the RF cavity

Vaporization
Condensation

LiquidTriple point

Solid
Gas
800℃

Solid
~0.05 Pa

2x10-5 Pa

Gas
Sublimation

Deposition

1085 degC
Fireball Hypothesis for SBL (2024-01-29)

Temperature Temperatures of copper surfaces exposed toultrahigh vacuum cannot be higher than 1,000℃.
 Adherent fireballs can emit significant light for >days.

 The fireball material is not copper!

Tetsuo ABE (KEK/ACCL)

14

Candidates of the Fireball Materials
Should have a Sublimation point > 1,000℃ in ultrahigh vacuum.

Element

Sublimation point [℃] @ 2 x 10-5 Pa

RemarksW

2258.6

Ta2123.4C (Graphite)

1769.9Mo

1705.7

Heater materials of vacuum furnaces for RFcavity fabrication

Zr

1565.7

Material of NEG pump strips (e.g. St707)

Ti

1162.6

Material of the KEKB collimator heads

Au

894.7

Cu

795.3

Al

765.0

Be

764.2

Ag

635.3

In

541.0

Materials of the SuperKEKB collimator heads

Material of normal-conducting RF cavities
No chance of leading to fireball breakdown in
RF cavities made of Cu

Data from https://www.iap.tuwien.ac.at/www/surface/vapor_pressure

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

15

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

16

Breakdown observable A: Fast drop of the accelerating field
Decay time: Normal RF-switch OFF  Decay time: 8 µs
 Fireball breakdown  Decay time: < ~500 ns

Yellow: large reflection Green: Accelerating field
Pickup antenna

 Fast drop
Green: Accelerating field

Absorption power: ~5 MW!
QL=13000@509MHz ⇒ Fill time: 8 µs
Fireball Hypothesis for SBL (2024-01-29)

If this power is absorbed in < 0.1mm2
 > ~GW/cm2
Tetsuo ABE (KEK/ACCL)

17

Breakdown Observable B: Current flash(During the high-power test of the RF cavity for the DR)

Field emitted e Impact on the metal surface
 X-ray radiation

X-ray detector(plastic scintillator + PMT)

Ch.2 : X-ray (UP) 
Ch.3 : X-ray (DN) 

200 nsYellow: Reflection wave

Ch.1 : Cav. Refl. 

Ch.4 : Cav. Pickup RF cavity
for the e+ DR

Fireball Hypothesis for SBL (2024-01-29)

Green: Accelerating field

Tetsuo ABE (KEK/ACCL)

Huge Current Flow!

18

Physical process of FB BD revealed by the observations
Step B
Step C

Step AA microparticle is heated by the RF field into a fireball.

(< ~0.1 mm)

E, H




𝑒𝑒

𝐶𝐶𝐶𝐶+

−

𝑒𝑒 −
𝐶𝐶𝐶𝐶+

Plasma is generated with eating the RF field in the cavity.

Huge current flow leads to vacuum breakdown.

Very high power density of ~GW/cm2
Temperature measurements
Fireball Hypothesis for SBL (2024-01-29)

Huge X-ray detected(Plasma generation and its exponential growth are needed for vacuum breakdown.)
Tetsuo ABE (KEK/ACCL)

19

Essential Condition for FB BD
Coexistence of different materials
with largely different sublimation/melting points in the same place𝑣𝑣 ≈ 10 m/s

Step B

Step B-1

𝑒𝑒

𝐶𝐶𝐶𝐶+

−

𝑒𝑒 −
𝐶𝐶𝐶𝐶+

If (fireball temperature) > (sublimation/melting point of the cavity material)

Step B-2


Step B-3

E,H-field

Local field enhancement
Local heating(my speculation)
Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

20

Essential Condition for FB BD
Coexistence of different materials
with largely different sublimation/melting points in the same place In the case of the beam collimators at SuperKEKB

 Heads made of W, Ta with a high sublimation point Chambers made of Cu with a low sublimation point

A lot of FB candidates from the damaged
collimator heads

Phenomena similar to FB BD can occur
around collimators.
Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

SuperKEKB beam collimator
21

A fireball could cause a sudden beam instability in an accelerator
with very small apertures.

T. Abe, “Fireball Hypothesis for the Trigger of Sudden Beam Losses at
SuperKEKB”, TUP01 in Proceedings of the 20th Annual Meeting of Particle
Accelerator Society of Japan (PASJ2023), 2023.

Sudden significant kick of beam bunches?

Beam-induced field

Step A(~ms or longer)

Step BHigh field applied𝑒𝑒Stored beam

Step C

𝐶𝐶𝐶𝐶+

−

𝑒𝑒 −
𝐶𝐶𝐶𝐶+

(~μs or shorter)

Beam collimator head
(Ta or W or C)↓

debrisAperture: ±1mm at min. at SuperKEKB

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

22

Cu coating applied to all the collimator heads during LS1
 Only the single material (Cu) seen by the beam
 Confinement of FB candidates on the head surfaces
Stored beam

Like the hybrid collimator

Like the robust carbon collimator

Extracted from Shinji TERUI et al,, NIMA 1059 (2024) 168971

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

23

Dust (microparticle) charging and lofting in Accelerators
https://indico.cern.ch/event/1272104/

The final report available at the CERN Document Server:
https://cds.cern.ch/record/2884112

There is a possibility that microparticles (debris) from the damaged heads,
which are far from collimators, could be charged, lofted, and moved along
the beam direction.
 Fireball?
 SBL?

Stored beam
Collimator

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

24

[Step A] Beam-induced field can generate fireballs?
Simulation using a first-principle simulator (CST Wakefield Solver)<Results>𝑑𝑑

𝜙𝜙
For a bunch length of 6 mm, total current of 900 mA, and 1272 bunches
per ring, which is a typical set of operational parameters for
sudden beam losses at SuperKEKB/LER.
Fireball Hypothesis for SBL (2024-01-29)

The results show that fireballs can be generated
realistically if a submillimeter or smaller micro-particle getsto a few millimeters from beam bunches for a second with alow emissivity of the fireball material.
Tetsuo ABE (KEK/ACCL)

25

[Step C] Large transverse kick?
Simulation using a first-principle simulator CST / Particle-In-Cell (PIC) Solver𝒆𝒆+Step C

Fireball Hypothesis for SBL (2024-01-29)↑
Collimator headwith ±2 mm aperture

SuperKEKB Collimator Model

Tetsuo ABE (KEK/ACCL)

26

CST PIC Simulation / Particle Sources

𝑌𝑌 = −4.5 mm
𝑍𝑍 = −17 mm

𝒆𝒆+ bunches

Simulating plasma generation
 4 GeV(𝒆𝒆𝒆𝒆𝒆𝒆) 7 nC/bunch
𝑰𝑰𝐩𝐩𝐩𝐩𝐩𝐩𝐩𝐩 𝐂𝐂𝐂𝐂+
 6 mm length in z with no transverse size
= 35, 70, 140 A
 4ns spacingなど⋯⋯
Fireball Hypothesis for SBL (2024-01-29)

σ = 1 μs
(𝒆𝒆𝒆𝒆𝒆𝒆)−𝑰𝑰𝐩𝐩𝐩𝐩𝐩𝐩𝐩𝐩 

Tetsuo ABE (KEK/ACCL)

Initial velocities of emitted
particles are determined from(𝒆𝒆𝒆𝒆𝒆𝒆)𝒆𝒆−

the initial temperature (𝑻𝑻𝐢𝐢𝐢𝐢𝐢𝐢of the plasma according to
Maxwellian distribution)

Time [μs]27Results of the CST PIC SimulationCu↓

𝒆𝒆+ bunch

Fireball landing↓↑
Collimator heads (Ta)with ±2 mm aperture↑Cu

All the particles

Full simulation of the dynamics of the LER 𝒆𝒆++ positive charge− negative chargeand emitted 𝒆𝒆− , 𝑪𝑪𝑪𝑪+
and interactions among them
All the particles(Short time range)(long time range)

𝑡𝑡 = 0−50 ns

𝑡𝑡 = 0−10 μs

https://youtu.be/Ow0IX8bEB9g

Fireball Hypothesis for SBL (2024-01-29)

Cu+ only

Cu+ only
Tetsuo ABE (KEK/ACCL)

https://youtu.be/5pHMwFmjSyo

28

Calculation formula of the transverse kickFinal state

Initial state(𝑖𝑖)

𝑝𝑝𝑦𝑦 = 0(𝑖𝑖: macro-particle, 1, 2, ⋯ , 𝟐𝟐𝟐𝟐𝟐𝟐)(𝑖𝑖)

Chargeσbunch = 6 mm

−30
[mm]

−18

−6

Fireball Hypothesis for SBL (2024-01-29)

6

18

30

Tetsuo ABE (KEK/ACCL)

𝑝𝑝𝑦𝑦 : Y-momentum of particles in the 𝑖𝑖-th macro-particle
𝑞𝑞 (𝑖𝑖) : Charge of the 𝑖𝑖-th macro-particle
𝑄𝑄𝑏𝑏 : Bunch charge

29

Transverse kick angles for(𝒆𝒆𝒆𝒆𝒆𝒆)𝑰𝑰𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 = 𝟕𝟕𝟕𝟕 𝑨𝑨(𝒆𝒆𝒆𝒆𝒆𝒆)𝑰𝑰𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 = 𝟕𝟕𝟕𝟕 𝑨𝑨(𝒆𝒆𝒆𝒆𝒆𝒆)𝑻𝑻𝒊𝒊𝒊𝒊𝒊𝒊= 𝟏𝟏, 𝟎𝟎𝟎𝟎𝟎𝟎℃(𝒆𝒆𝒆𝒆𝒆𝒆)𝑻𝑻𝒊𝒊𝒊𝒊𝒊𝒊= 𝟏𝟏𝟏𝟏, 𝟎𝟎𝟎𝟎𝟎𝟎℃

+70 A

𝐂𝐂𝐂𝐂+

σ = 1 μs

-70 A

𝒆𝒆−

Time [μs](𝒆𝒆𝒆𝒆𝒆𝒆)𝑻𝑻𝒊𝒊𝒊𝒊𝒊𝒊= 𝟑𝟑𝟑𝟑𝟑𝟑, 𝟎𝟎𝟎𝟎𝟎𝟎℃

∆𝑃𝑃𝑦𝑦 (Pink indicates one-signa)

Fireball Hypothesis for SBL (2024-01-29)

LER 𝒆𝒆+ attracted by 𝒆𝒆− emited from the FB landing point
Tetsuo ABE (KEK/ACCL)(Kick angle does not increase
with any further increase ininitial temperature.)30

Comparison between w/ and w/o 𝐶𝐶𝐶𝐶+(𝒆𝒆𝒆𝒆𝒆𝒆)𝑰𝑰𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 = 𝟕𝟕𝟕𝟕 𝑨𝑨(𝒆𝒆𝒆𝒆𝒆𝒆)𝑻𝑻𝒊𝒊𝒊𝒊𝒊𝒊= 𝟑𝟑𝟑𝟑𝟑𝟑, 𝟎𝟎𝟎𝟎𝟎𝟎℃

No 𝐶𝐶𝐶𝐶+ emitted(only 𝑒𝑒 − emitted)

Fireball Hypothesis for SBL (2024-01-29)

𝐂𝐂𝐂𝐂+ pushes up the space charge limit of emitted 𝒆𝒆−
Tetsuo ABE (KEK/ACCL)

31

Transverse kicks for HER（𝒆𝒆− ） beams bunches(𝒆𝒆𝒆𝒆𝒆𝒆)𝑰𝑰𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 = 𝟕𝟕𝟕𝟕 𝑨𝑨(𝒆𝒆𝒆𝒆𝒆𝒆)𝑻𝑻𝒊𝒊𝒊𝒊𝒊𝒊= 𝟑𝟑𝟑𝟑𝟑𝟑, 𝟎𝟎𝟎𝟎𝟎𝟎℃(𝒆𝒆𝒆𝒆𝒆𝒆)𝑰𝑰𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 = 𝟐𝟐𝟐𝟐𝟐𝟐 𝑨𝑨(𝒆𝒆𝒆𝒆𝒆𝒆)𝑻𝑻𝒊𝒊𝒊𝒊𝒊𝒊= 𝟏𝟏𝟏𝟏𝟏𝟏, 𝟎𝟎𝟎𝟎𝟎𝟎℃

� 𝐂𝐂𝐂𝐂+� 𝒆𝒆−

 Looks different from LER(𝒆𝒆+ ) Under detailed study
Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

32

Relevant experiment on-going!
High-power RF-cavity test stand (MR-D1-AT)
509MHz accelerating cavity prototype for the e+ DR↓

The RF-field level and time structure in the RF cavity are roughly the same as those at the collimators,
so that we can estimate the fundamental parameters of the FB hypothesis from the RF-cavity high-power test.
(𝒆𝒆𝒆𝒆𝒆𝒆)

We are now trying to measure ”total” breakdown current ( ≈ 𝑰𝑰𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 )Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

33

Measurement of total breakdown current

E-field (TM01)

Insulator (alumina)

Camera 2

Camera 1

Insulator (alumina)

509 MHz CW cavity(being conducted by Takaaki YAMAGUCHI (KEK))

Insulated

Suggests huge currents (peak)(> 100A!?)∅150 mmEmission point 1

256 mmCT1
Emission point 2
Mirror (Al)

Mirror (Al)① Assuming “(light emission point) = (𝑒𝑒 − emission point)”② In this case, Camera 2 can identify the emission point.
③ Percentage of 𝑒𝑒 − reaching the mirror to be estimated with PIC sim.

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

CT2Emission point 2
34

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

35

Why Acoustic Observation?
 How to detect vacuum arcs through:RF-field change  Impossible (no RF monitor port, low Q-value)
X-ray emission  Difficult to detect due to the stored beam
Light emission  Impossible due to strong synchrotron radiation
Acoustic emission: the only one to be detected Acoustic emission is generated by thermal shock when an arc current impacts a metal surface.
(Air)

Acoustic sensor

Ampl. [dB]

1.
2.
3.
4.

Detectablewith a high S/N ratio

A lot of noises
from cooling-water flows,
switching power supplies, etc.

(no noise)

Arc current(Vacuum)

20
Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

50

100

Frequency [kHz]36

Acoustic sensor used in this observation: AE124AT
 Resonator type
 Sensitive only around 120 kHz(Selection by Toshiyasu HIGO (KEK/e+e-Linac))Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

37

Six sensors attached toLER D06V1 collimator
at the end of 2022ab
Hot melt glue was used

Fireball Hypothesis for SBL (2024-01-29)

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2022ab / LER1299mA sudden beam-loss abort : Example (CLAWS and Diamond)~300mm~210mm~210mm~300mm~90mm(Longitudinal
distances)

Fireball Hypothesis for SBL (2024-01-29)

Velocity of acoustic longitudinal (transversal) wave
in copper:
4.65(2.26) mm/us
This data suggests that the particle shower produced at thecollimator head generated widespread acoustic wave in thedownstream of the head, which propagated upstream.
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39

During LS1, we attached acoustic sensors for D02V1 andIori OKADA (Nara Women's University) attached all the sensors
D05V1 (NLC) collimatorin collaboration with K. Uno and T. Abe.
Acoustic loss monitor

On the undersidein the same manner

Can hear AE around the head.

(mechanically isolated)

Hypothesis
for SBL (2024-01-29)
Can Fireballhear particle
showers

Tetsuo ABE (KEK/ACCL)

In the same manner for D05V1 collimator (NCL) 40

We can hear acoustic waves from the collimator head.
Acoustic sensor

Fireball Hypothesis for SBL (2024-01-29)

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41

Five Acoustic-Emission (AE) Sensors around QCSR-P

QCSR

Three on the QCSR-P flange
Y. Arimoto,
K. Uno, and
T. AbeTwo on the
upstream beam pipe

Can hear acoustic waves from the beam pipe in QCSR-P(A possible trigger of vacuum arcs in QCS was noted by Shinji TERUI.)
Fireball Hypothesis for SBL (2024-01-29)

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42

The oscilloscopes to be triggeredby the LER beam-abort signal
or “Manual signal”

Oscilloscopes and DAQ
 Created and maintained by Shunto OGASAWARA
 Updated every LER beam abort (auto.) or “Manual get”

In the D5 sub-tunnel

http://kekb-co-web.kek.jp/group/RF/AEsensor/index.php

In the D2 sub-tunnel

Oscilloscopes should be put near thecollimators because of the small
analog signals from the acoustic
sensors (~0.1 mV at min.)

In Tsukuba B4

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Summary The SuperKEKB collimators satisfy the occurrence conditions of FB-triggered SBL: Coexistence of different materials with largely different sublimation/melting points in the same place Small physical apertures (± 1 mm at min.) More chance of producing fireballs First principle simulation study of the FB hypothesis on-going Several μrad kicks at the collimator possible
 Funakoshi-san’s study based on the loss pattern at the SBLs suggests several tens of μrad kicks needed for SBL.
 Further study with other simulation parameters on-going We are conducting the high-power experiment to measure the fundamental parameters of thefireball hypothesis to prove it.
 We have established the acoustic observation system to detect a vacuum arc when SBL occurs.
 25 acoustic sensors attached to the two collimators with a sub-tunnel nearby D02V1 (smallest physical aperture) D05V1 (largest amount of scraped particles?) 5 to the QCSR-P flange and chamber
 Another mechanism?

 Just started the observation in the 2024ab beam operation

 We will make a new strategy for the next step based on observation results early in 2024ab.

Fireball Hypothesis for SBL (2024-01-29)

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44

Thank you for your attention
For more details on:

 The fireball hypothesis

 T. Abe, "Fireball Hypothesis for the Trigger of Sudden Beam Losses at SuperKEKB", in Proceedings ofthe 20th Annual Meeting of Particle Accelerator Society of Japan, PASJ2023-TUP01 (2023).

 Fireball breakdown of RF cavities
 KEK Accl. Lab. Topics (web article)

 “Minuscule Gremlins Cause Vacuum Breakdown in Radio-Frequency Accelerating Cavities”
̶ https://www2.kek.jp/accl/eng/topics/topics190122.html

 Presentations at Workshops

 T. Abe, "Direct Observation of Breakdown Phenomena in Normal-Conducting Accelerating Structures: 509-MHz Continuous-Wave Cavity and11.4-GHz Pulsed-Wave Cavity", presented at the 12th International Workshop on Breakdown Science and High Gradient Technology(HG2019), Chamonix, France, June 2019.
 T. Abe, "Updated Results of Breakdown Study for 509-MHz Continuous-Wave Accelerating Cavities based on Direct In-situ Observation",presented at the 7th International Workshop on Mechanisms of Vacuum Arcs (MeVArc 2018), Puerto Rico, May 20-24, 2018.

 Original paper 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.
Fireball Hypothesis for SBL (2024-01-29)

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Backup slides

Fireball Hypothesis for SBL (2024-01-29)

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Example of Fireballs observed in the ARES Cavities
C-damper power𝑒𝑒 − beam

A-cav. pickup
↓

S-cav

C-cav

C-damperVideo file: https://youtu.be/qtIB6kLf2j4
Fireball Hypothesis for SBL (2024-01-29)

A-cav

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Observation of the FB size
≲ ~0.1mm

Compact TV camera

1 mm
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Statistics on all the 205 breakdown events• 10% “Pyrotechnic” breakdowns• Observed only in the initial stage of RF conditioning“Pyrotechnic”
Nothing seen25%

• 25% accompanied by a bright-spot (BS) explosion
BS explosion• 40% accompanied by a spot-type explosion not
originating from a stable bright spot
No observed breakdown events were accompanied by two or more explosions.
 Such an explosion must be a breakdown trigger!
For more details:
Fireball Hypothesis for SBL (2024-01-29)

10%

25%

Spot-type
explosion

40%(BS: Bright Spot) T. Abe, "Visual Imaging of Radio-Frequency Cavity Breakdown“, KEK Accl. Lab. Topics 2016/10/5 (web article):
http://www2.kek.jp/accl/eng/topics/topics161005.html
 T. Abe, et al., "Breakdown Study Based on Direct In-Situ Observation of Inner Surfaces of an RF Accelerating Cavity during a HighGradient Test", Physical Review Accelerators and Beams 19, 102001 (2016).

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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.

∅0.01mm Cu∅0.10mm Cu∅1.00mm Cu∅0.01mm Μο
∅0.10mm Μο
∅1.00mm Μο
∅0.01mm C∅0.10mm C∅1.00mm C

Temperature of the micro-particle

A graphite particle in theconstant and uniform
magnetic field (509 MHz)

Fireball Hypothesis for SBL (2024-01-29)(509 MHz)In the constantmagnetic fieldNo field (radiation cooling)

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CST PIC simulation / particle sources

𝒆𝒆+ bunches

Fireball Hypothesis for SBL (2024-01-29)

Tetsuo ABE (KEK/ACCL)

Magnetic field [A/m]⋯⋯

The peak field strength at the FB landing point
is comparable with that in the RF cavity.
Electric field [V/m] 4 GeV 7 nC/bunch 6 mm length in z with no transverse size
 4ns spacing51

(𝑒𝑒𝑒𝑒𝑒𝑒)

In the special case of no particle emission from the FB landing point (i.e., 𝐼𝐼peak = 0)

Fireball Hypothesis for SBL (2024-01-29)

Numerical noise level in the PIC simulation: ≲ 0.1μrad
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An Example of Carbonic Microparticles on Copper Surface
Surface inspection by EPMA (Electron Probe Micro Analyzer)
Size of this microparticle : 1 μｍ after ultrasonic cleaning in isopropyl alcohol.
Copper Coupon∅ 18 mm

 Material : Oxygen Free Copper (Class1)
 The top surface has Ra=0.03μm.

Base
Inspection conducted by T. Higo (KEK)

Although carbon cannot be melt in copper,
It can adhere to copper surface.
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Tetsuo ABE (KEK/ACCL)53

Details on the RF cavity used in this experiment• Normal fabrication method• Precision machining of OFC (class1)• Chemical etching for the barrel• Electropolishing (EP) for the endplates< Before EP >< After EP >Ra = ~1.5 μm
Ry = ~8 μm

Ra < ~0.2 μm
Ry < ~1 μm• The inner surface is very smooth.

• Brazed in a vacuum furnace• Fabricated by an accelerator manufacturer• Good high-power performance• The breakdown rates are the same as thoseof other UHF CW cavities• The vacuum pressure level is normal duringhigh-power operation.

PAC’05, TPPT007

The results obtained in this study have generality.
Fireball-Triggered Vacuum Breakdown (HG2023)

Tetsuo ABE (KEK)54

End of This File
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