Sudden Beam Loss
The 27th KEKB Accelerator Review Committee
2024/3/25 H. Ikeda

Contents• What is Sudden Beam Loss
• Possible Reasons for SBL : fireball hypothesis• Work during LS1• SBL event after LS1• Summary“Sudden Beam Loss”
Beam signal measured by

Beam loss that occurs suddenly within 1
Beam Oscillation Recorder(BOR) & Bunch Current Monitor(BCM)
turn (10µs) without precursory phenomena.
＝ Sudden Beam Loss （SBL）• The cause of SBL is unknown.
• A significant percentage of the beam islost before the abort trigger is applied.
→ Harmful effects of SBL;
• Damage to collimators and other accelerator
components,• Quench of the final focusing
superconducting magnets (QCS),• Large backgrounds to the Belle-II detector,• Inability to store high current due to beamabort.

3

Observation• The beam suddenly disappears just before the abort.
• Beam loss occurs in both HER and LER, but the damage to the hardware is particularly large when loss
occurs in LER.
• We don't know if it will happen even with a single beam or low current beam because we haven't
operated for a long time.
• The starting point of beam loss depends on the tuning of the collimator and is not limited to a specificlocation.
• Just before the beam loss begins, the orbit appears to move, but its value is small～O(0.1 mm).
• The orbit is changing < O(1mm) after the beam loss.
• No oscillations that could be precursors to beam loss are observed.
• Pressure bursts have been observed all over the place and rarely occur in the same place except at
the collimator section.
• Regarding the pressure of D06H3 and H1 collimators there are rapid or nonlinear increase of pressure
depending on the beam current.
4

Possible Reasons for SBL
 Damage of vacuum component (RF Finger) @KEKB & PEP-II Beam phase changes (beam energy losses) observed ms to several hundred μs before aborts. → The me scale diﬀers from that of SBL.
 Abnormal temperature rise at bellows chambers had been observed and the catastrophic damages in the RF-finger had been
conﬁrmed. → We could not ﬁnd that damage.
 Dust : Early stage @ SuperKEKB Vacuum chambers were cleaned or tapped to remove as much dust as possible.
 Electron Cloud SBL should be measured only in LER.→ SBL is also measured in the HER beam.
 Curious behavior of the pressure in D06H3 collimator may suggest the formation of a discharge or electron cloud.
 Simulations show that the electron density distribution changes with time and a maximum electron density is on the order of
1E13/m3 to 1E14/m3 → How this relates to SBL?
 Fireball : Measured @ RF cavity
 The vacuum chamber is made of copper with low sublimation point and collimator head is made of tungsten or tantalum with highsublimation point.
→ The situa on has the poten al for a ﬁreball to be formed.
 This fireball hypothesis could explain SBL (~μs) due to the fast plasma evolution (~100 ns at the fastest).
5

The fireball hypothesis for SBL

For the details, see the following paper:

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

Step A

Sudden significant kick of beam bunches?

Beam-induced field

Huge current(> ~100A peak) needed
for significantly large
kicks of beam bunches(~ms or longer)

Step BT. Yamaguchi’s study results indicate such huge currents
emitted in fireball breakdown.
See his presentation at MeVArc2024：
https://indico.cern.ch/event/1298949/contributions/57838
82/High field applied
𝐶𝑢

𝑒
𝐶𝑢

𝑒

(~μs or shorter)(copper)(copper)

Stored beam↑

Step CBeam collimator head
(Ta or W or C)↓

Debris?

Aperture: ±1mm at min. at SuperKEKB

Fireball Hypothesis for SBL

Tetsuo ABE (KEK)6

CST PIC Simulation / Particle Sources

𝑌 = −4.5 mm
𝑍 = −17 mmbunches 4 GeV 7 nC/bunch 6 mm length in z with no transverse size
 4ns spacing

Simulating plasma generation(𝒆𝒎𝒊)

𝑰𝐩𝐞𝐚𝐤 𝐂𝐮

σ = 1 μs

(𝒆𝒎𝒊)

𝑻𝐢𝐧𝐢
(𝒆𝒎𝒊)
−𝑰𝐩𝐞𝐚𝐤 
Fireball Hypothesis for SBL

Tetsuo ABE (KEK)𝒆Time [μs]7+70 A[Examples of the simulation results]
Transverse kick angles for (𝒆𝒎𝒊)
𝒑𝒆𝒂𝒌
(𝒆𝒎𝒊)

𝑰𝒑𝒆𝒂𝒌 = 𝟕𝟎 𝑨𝐂𝐮
σ = 1 μs𝒆

-70 A

Time [μs]

(𝒆𝒎𝒊)

𝑻𝒊𝒏𝒊

(𝒆𝒎𝒊)

𝑻𝒊𝒏𝒊

= 𝟏, 𝟎𝟎𝟎℃

LER
Bunch Number

attracted by= 𝟏𝟎, 𝟎𝟎𝟎℃

(𝒆𝒎𝒊)

𝑻𝒊𝒏𝒊

emited from the fireball landing point
Bunch Number= 𝟑𝟎𝟎, 𝟎𝟎𝟎℃

∆𝑃 (Pink indicates one-signa

Bunch Number According to the Funakoshi-san’s study, several tens of μrad kick angles are needed for SBL.
 Further study is on-going for making larger kick angles based on the fireball hypothesis.
Fireball Hypothesis for SBL

Tetsuo ABE (KEK)8

Work during LS1
For preventing SBL• Replacing damaged collimator head.
• Copper coating of collimator heads (D6H3, D6V1, D5V1, D2V1). （Cover material with a high sublimation point,
which could be the seed of a fireball, with material with a low sublimation point.)• Installation of permanent magnets in all SuperKEKB-type horizontal collimators. （In order to reduce the electron
cloud effect…）• LER D02H4, D02H3, D02H2, D02H1, D03H1, D06H3, D06H4• HER D01H3, D01H4, D01H5
For investigation the cause of SBL• Add BOR to investigate beam orbit change in locations that may be the cause of SBL.
• Measure the orbit at two different locations with phase differences. ：Existing BOR• Add a simplified version to measure in phase with the collimator, although with less accuracy. ：New BOR• Add loss monitor for timing measurement• Install acoustic sensors to observe the sound when the Fireball occurs.(D2V1:minimum physical aperture,
D5V1:new collimator, QCS, D6V2)
9

BOR/BCM

D1

TL

筑波 TR

D2

12C
D01H5 (TL18)
D02H4 (TR14)
D02H2 (TR120)
D01H4 (TL34)
D02H3 (TR67)
D02H1 (TR142)
D01H3 (TL47)
D02V1 (TR80)
D03V1 (TR300)
D01V1 (TL60)
D12V4 (NR354)
D03H1 (OL352)
D12H4 (NR307)
D12V3 (NR285)
D12H3 (NR274)
D12H2 (NR236)
D12V2 (NR209)
D12H1 (NR199)
D12V1 (NR134)

D11

3C

D4 D4RF

NR

OL
赤：LER
青：HER

日光

NL

D05V1

Beam Dump
BOR/BCM

D10

9C

D09H4 (NL203)
D09V3 (NL232)
D09H3 (NL236)
D09H2 (NL279)
D09H1 (NL307)

D06V2(OR271)
D06V1 (FL362)

D09V2 (FR354)

D06H3 (FL276)

D09V1 (FR209)

D06H1 (FL200)
6C

D8FR 富士 FL

D7

Use BPM that is upstream of thecollimator in phase with the collimator
head.
D5RF
OR
• Same vertical betatron phase asA
D2V1 : MQN1OP D5 BM2 66.7m
B• Same vertical betatron phase as
D5 C
D6V1 : MQD5P26 D7 BM3 144.4

大穂

D
E
F

New BOR/BCM

D09V4 (NL156)• Use original FB
monitor (upstream
and downstream).

H
G
F•

LM

D12V1
PMT

D12V3
PMT

D02V1
PMT
D01V1
PMT

D05V1
PMT

D09V3
PMT
D06V2
EMT

D09H2
PMT
D09V1
EMT

D07 injEMT
D08 inj
EMT

D06V1
PMT

D06H3
EMT
D06H4
EMT

(K. Uno et al.)11

Acoustic-Emission Sensors for D02V1 collimator
Acoustic loss monitor

On the undersidein the same manner

I. Okada,
K. Uno, and
T. AbeCan hear AE around the head.

(mechanically isolated)

Can hear particle showers

In the same manner for D05V1 collimator (NCL)

Five Acoustic-Emission (AE) Sensors around QCSR-P
Three on the QCSR-P flange
Y. Arimoto,
K. Uno, and
T. AbeTwo on the
upstream beampipe
Can hear AE from the beam pipe in QCSR-P

Calibration• Acoustic sensor operation check and timing calibration.(T. Abe)
• Close the target collimator (D02V1 or D05V1) as much as possible, and open the collimator upstream as
much as possible.
• Set the orbit to hit near the collimator head by the vertical steering upstream of the collimator.
• Single bunch injection into LER.
• Total beam loss at target collimator without making one revolution→Loss monitor abort→Acoustic dataacquisition• The sound waves generated around the collimator head can be observed and confirmed using a beam 。
• Confirm how to determine the timing of sound wave generation near the collimator head from the observation
data of the acoustic sensor attached to the atmospheric cooling water pipe.
• BOR timing tuning and calibration.（M. Tobiyama）

SBL event after LS1• On March 8th, we observed thefirst SBL-like abort event after LS1.
• 2024/3/8 03:58:25• Mode : Physics Run• I_LER : 259[mA]• I_bunch :259/783=0.33[mA/bunch]• Beta at IP : 80 mm / 1 mm

Upstream BOR : V

Downstream BOR : V

Upstream BOR : V

Downstream BOR : V

Upstream BOR : V

Downstream BOR : V

Upstream BOR : V

Downstream BOR : V

0.22mm

Upstream BOR : V

Downstream BOR : V

Two turns before the abort, the orbit change wasmeasured at the downstream BOR.
The orbit change bunches were not the beam lost
bunches.

Upstream BOR : V

Downstream BOR : V

Upstream BOR : V

Downstream BOR : V

Beam loss was first measured at D5V1, but because the amount of beam loss was small, it was not aborted at thattime, and in the next turn, the loss monitor downstream of D6V2, which had a large amount of loss, fired an abort.
At the same time, a vacuum burst was also observed at D5.

Collimator

Belle dose (diamond sensor) was not so high.

赤：LER
青：HER
Beam Dump
BOR/BCM
New BOR/BCM

Beam Loss Monitor (Optical fiber) @downstream of
D6V2 issued the abort trigger first.

SBL_2024-03-08_03:58
Ch.2

Ch.1

(Green: LER abort signal)

QCSR-P chamber

D02V1 movable jaws
PMT

80 μs

No acoustic wave
80μs

(Time of propagation)

𝑒

EMT

PMT

80 μs

D05V1 (NLC) movable jaws

Ch.
3

80 μs

D06V2 movable jaws

No acoustic wave observed within 80 μs from each loss detection by PMT/EMT means:A) No acoustic generation before the loss started at each place.
B) All the acoustic waves are understood to be made by beam particles.

Collimator Setting :• D6V1(TOP/BTM) : 4.2 mm/-4.2 mm• D6V2(TOP/BTM) : 1.7 mm/-1.8 mm• D5V1(TOP/BTM) : 14 mm/-14 mm• D2V1(TOP/BTM) : 0.95 mm/-1.4 mm

Nsigma (beta)• QC1:149.0• D6V1 (TOP/BTM) : 103.4/117.9• D6V2 (TOP/BTM) : 101.5/65.5• D5V1 (TOP/BTM) : 125.4/125.9• D2V1 (TOP/BTM) : 115.5/90.8• The large beam loss occurred with D6V2 because it had the narrowest aperture.
• The background was cut well, so the doses of

D2V1, QCS and Belle-II were small.

• In order to protect the head of D2V1, we made D6V1 the narrowest, then D6V2, and finally D2V1 after
this event.
→The beam loss at D6V1 became largest, and the number of abort trigger increased at D6V1 LM (PIN) .

SBL has happened
several times afterthat.
We are currently inthe process ofadjusting trigger
settings andbrushing up on
analysis methods.
Some events haveseen beam
oscillation, orbitchange or nochanges, so wewould like toorganized thecondition at eachabort data and try
to determine thecause.

Summary• There had been a sudden beam loss for an unknown reason from previous run.
• There is an urgent issue to investigate the cause of this SBL, as it may cause damage
to hardware or disrupt operations due to beam abort.
• For now, the fire-ball hypothesis remains a candidate for the cause and simulations
are on-going..
• During LS1, the damaged collimator head was replaced and coated to take measuresagainst SBL, and BOR, LM, and AS were added for preparation to investigate thecause of SBL.
• We looked at the data on the first SBL-like event afterLS1.
• We are currently in the process of establishing an analysis method for the monitoring
system, so we would like to determine the cause (position) of SBL from therelationship between orbit fluctuations, discharge sounds, and beam loss positions.

backup

31

Observation : Beam Loss Monitor• Checked the loss
monitor at the time ofabort occurrence
which was probably
due to beam loss.
↓• There were many
instances of sudden
beam loss occurring
simultaneously in theentire ring collimator
section and in the
Belle-II detector.

32

Observation :Bunch Current monitor & Bunch Oscillation Recorder• The beamsuddenly
disappears justbefore the abort.
• Beam loss
occurs in both
HER and LER,
but the damage
to the hardware
is particularly
large when loss
occurs in LER.

Abort

10µs=1turn33

Observation :Loss Monitor Specialized for Timing Measurements• We checked the starting point of
SBL in the ring by using loss
monitors specialized for position
identification• The beam loss mainly started at the
D06V1 collimator (a narrow aperture
to suppress background to Belle-IIdetector).
• When the D6 collimator wasdamaged and the aperture waswidened, beam loss began in the D2collimator section near the IR.
→The starting point of beam loss
depends on the tuning of thecollimator and is not limited to a
specific location.
34(Y.Liu)Observation : Beam/Bunch Current Dependence• It is likely to occur
when a certain
bunch current isexceeded.
• We don't know if itwill happen evenwith a single beam
or low current
beam because wehaven't operated
for a long time.
35(K.Matsuoka)

Observation : Beam Orbit• Just before the beam loss
begins, the orbit appears to
move, but its value is small～O(0.1 mm).
• The orbit is changing <O(1mm) after the beam loss.
• No oscillations that could be
precursors to beam loss areobserved.
36

Observation : Beam Size
• We checked for beam sizefluctuations by installing an
ultra-high speed CMOS camerain the X-ray monitor to take 1µs
data at 100kHz when the abort
is triggered.
• There was no sign of asignificant change in beam size
before the SBL.
37

Status of LER collimator

Name

Type

Tip Material(): longitudinal length in mm

Tip Condition

Remarks

D06H3
D06H4
D03H1
D02H1
D02H2
D02H3
D02H4

SuperKEKBSuperKEKBSuperKEKBSuperKEKBSuperKEKBSuperKEKBSuperKEKB

Cu coated C (160)
Ta (10)W (10)W (10)W (10)W (10)W (10)

healthyhealthyhealthyhealthyhealthyhealthyhealthyspoiler against inj. kickers’ accidental firings
absorber against inj. kickers’ accidental firings

D06V1
D06V2
D05V1
D02V1

SuperKEKBSuperKEKBSuperKEKBSuperKEKB

Cu coated Ti (10)hybrid (3)Cu coated Ta (4)Cu coated Ta (10)healthyhealthyhealthyhealthy(T. Ishibashi)38

Status of HER collimator
Name

Type

Tip Material(): longitudinal length in mm

Tip Condition

Drive Mechanism

D09H1
D09H2
D09H3
D09H4
D12H1
D12H2
D12H3
D12H4
D01H3
D01H4
D01H5

KEKB
KEKB
KEKB
KEKB
KEKB
KEKB
KEKB
KEKB
SuperKEKBSuperKEKBSuperKEKB

Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)Ti (40)Cu coated Ti (40)Ti (40)Cu coated Ti (40)W (10)W (10)W (10)

damageddamageddamageddamagedhealthydamagedhealthyhealthyhealthyhealthyhealthy

-

D09V1

KEKB

Cu coated Ti (40)damagedupgraded

D09V2
D09V3
D09V4
D12V1
D12V2
D12V3
D12V4
D01V1

KEKB
KEKB
KEKB
KEKB
KEKB
KEKB
KEKB
SuperKEKB

Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ta (10)healthyhealthyhealthydamageddamagedhealthyhealthyhealthy

Remarks

Plan to replace with new jaw (Cu coated Ti (40))

upgraded
upgraded
upgraded-(T. Ishibashi)

392022ab / LER1299mA sudden beam-loss abort :Example (CLAWS and Diamond)~300mm~210mm

~90mm~300mm~210mmコリメータ側面に貼った
この音響センサー（CTR(RingIN)）は外れていた！

(Longitudinal
distances)

外れていたCTR(RingIN)の音響センサーは粒子シャワー発生源（コリメタ・ヘッド）に近く、短い時間で大量の放射線を浴びたと考えられて、そのような場合、20μsくらいの時間で信号にオフセットが生じる（極性は正負両方あり）。

: Acoustic sensor

1

2

5

4
3
X
Acoustic loss monitor?

On the undersidein the same mannerCollimator gap [mm]D05V1 half gap [mm]D05V1 injection halg gap [mm]D02V1 half gap [mm]3/8 03:58:25

25

D06V1 は両方とも4.1 mm
オフセット250μmを入れても4.35ｍｍ（125σ）

20

D05V1 は14 mm（129σ）

15
10
5
0

1

2

3

4

5

D06V1 half gap [mm]6Results of the CST PIC Simulation
Cu↓
Fireball landing↓

Full simulation of the dynamics of the LER

𝒆 bunch↑Collimator heads (Ta)with ±2 mm apertureand emitted
and interactions among them↑Cu

+ positive charge− negative charge

All the particles

All the particles(Short time range)(long time range)𝑡 = 0−50 ns

𝑡 = 0−10 μs

Cu only

Cu only
https://youtu.be/5pHMwFmjSyo

https://youtu.be/Ow0IX8bEB9g
Fireball Hypothesis for SBL

Tetsuo ABE (KEK)43Calculation formula of the transverse kickFinal state

Initial state()( : macro-particle,)()

σbunch = 6 mm

Charge−30[mm]

𝑝 : Y-momentum of particles in the 𝑖-th macro-particle
𝑞 ( ) : Charge of the 𝑖-th macro-particle
𝑄 : Bunch charge

−18

−6

Fireball Hypothesis for SBL

6

18

30

Tetsuo ABE (KEK)44(𝒆𝒎𝒊)
𝑰𝒑𝒆𝒂𝒌 = 𝟕𝟎 𝑨
Comparison between w/ and w/o
(𝒆𝒎𝒊)

𝑻𝒊𝒏𝒊

= 𝟑𝟎𝟎, 𝟎𝟎𝟎℃

No 𝐶𝑢 emitted(only 𝑒 emitted)

Bunch ID Number

Bunch ID Number

pushes up the space charge limit of emitted
Fireball Hypothesis for SBL

Tetsuo ABE (KEK)45

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

Numerical noise level in the PIC simulation:
Fireball Hypothesis for SBL

Tetsuo ABE (KEK)

(

)

)

0.1μrad46

Transverse kicks for HER（

） beams bunches
(𝒆𝒎𝒊)

(𝒆𝒎𝒊)

𝑰𝒑𝒆𝒂𝒌 = 𝟐𝟖𝟎 𝑨

𝑰𝒑𝒆𝒂𝒌 = 𝟕𝟎 𝑨
(𝒆𝒎𝒊)

𝑻𝒊𝒏𝒊

= 𝟑𝟎𝟎, 𝟎𝟎𝟎℃

(𝒆𝒎𝒊)

𝑻𝒊𝒏𝒊

= 𝟏𝟎𝟎, 𝟎𝟎𝟎℃

 Looks different from the results for LER(𝒆 ) Under detailed study
Fireball Hypothesis for SBL

Tetsuo ABE (KEK)47

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

𝜙

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

Tetsuo ABE (KEK)48

Essential Condition for FB BD
Coexistence of different materials
with largely different sublimation/melting points in the same place
Step B-1
Step B𝑣 ≈ 10 m/s

𝐶𝑢𝑒

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

Step B-2


Step B-3

E,H-field

Local field enhancement
Local heating(Just my speculation; detailed investigations needed)
Fireball Hypothesis for SBL

Tetsuo ABE (KEK)49

Physical process of the fireball breakdown
revealed by the observations
Step B
Step C

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

𝐶𝑢
𝑒

𝑒
𝐶𝑢

(< ~0.1 mm)

E, H




Proven by

Plasma is generated with eating the RF field in thecavity.

Proven by

Huge current flow leads to vacuum breakdown.

Proven by

Temperature measurements

Fast drop of the accelerating field, andVery high power density of ~GW/cm2

Huge X-ray detection(Plasma generation and its exponential growth are needed for vacuum breakdown.)
Fireball Hypothesis for SBL

Tetsuo ABE (KEK)50