Beam Background, MDIHiro Nakayama (KEK)The 26th KEKB Accelerator Review Committee
2022.12.13

2022a: Feb-Mar 2022
2022b: Apr-June 2022

Beam background in 2022a and 2022b• Beam background rates have been
well below the limit during 2022ab•

TOP PMT rate limit was raised from 3→5 MHz• Belle II background rates did not limit thebeam currents••

Limited by injection power
Also limited by increased number of sudden
beam loss events at higher currents• Collimators were severely damage by
sudden beam loss events•

It became difficult to control (injection) BG• Longer LER injection BG duration•

Larger DAQ deadtime fraction (~10%@23Hz inj.)

2

Belle II beam background in recent runs
- Belle II beam BG didn’t limit beam currents in 2021 and 2022
- Thanks to successful BG mitigation by collimators, vacuum scrubbing progress, etc..
- However, it will be a problem at higher luminosity without further BG mitigation- TOP counter is the most vulnerable sub-detector to beam backgrounds- Finite PMT photocathode lifetime, replacement work during long shutdown needed- Major contribution from LER beam-gas, LER Touschek, Luminosity BG, etc..
TOP background breakdown
during recent physics runs
PMT
rate
[MHz]

June 28, 2020
beta*y = 0.8mm

e- (7GeV,HER)
e+ (4GeV,LER)

K. Kojima3

Data/MC ratio of each BG component
A.Natochii

BG

-

BG

BG

Each BG component measured in dedicated machine studies in 2020/2021
Data/MC ratio stays within one order of magnitude from unity
Measured lumi-BG stays consistent with prediction (will dominate at full luminosity)
This confirms our good understandings on beam loss processes at SuperKEKB
Those ratios are used to rescale simulated beam background rates toward higher luminosity
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Future prediction of beam BG• Data/MC ratio from recent BG measurements
can be applied to improve BG predictions at
future optics• Latest prediction can be found in our
Snowmass WhitePaper (arXiv:2203.05731)• Up to L~3x1035, beam BG will remain highbut acceptable• For the target luminosity (L~6x1035),
machine condition is very uncertain to
make accurate prediction• major modification of IR?

A.Natochii
5New: Real-time Background Composition Monitoring
based on machine-learning approach
U. Gottingen (S. Schwenker)
used for training•

The neural network (BGNet) is fed witharchived 1Hz EPICS PVs••

The trained network can successfully
reproduce BG levels at “future” timing(which is not used in training)••

Feature attribution says
D6V1 collimator width
most contributed the BGchange•Larger prediction error for “far” future

BGNet can provide real-time backgroundcomposition monitoring•

D6V1 widthsome domain knowledge is used in the model(i.e. beam gas BG rate ∝ current x pressure)instead of BG studies which needs long machine time

“Feature attribution” method can tell us
which parameter most contributed to thechange of the BG rate•quite useful feedback for machine operators

Issue: duration of injection BG••

Very bad(~20ms)
LER duration !!
LER duration [ms]HER duration [ms]LER current
HER current

2022a

As beam currents increased (> 1A), duration of
injection BG became worseDue to severe collimator damage by sudden beamloss events, poor quality of 2-bunch injection, etc..

•

Belle II trigger veto window had to be widened toreject injection BG with longer duration• DAQ deadtime reached ~10% (!)•

Recorded physics data (outside injection veto)shows some performance degradation

Reduced Ks
efficiency
after injection

2022b• Improvement of injection quality is crucial for physics runs after LS1 !
• Lots of injection improvement works planned during LS1 (injection talk tomorrow)More major issues in 2022ab• Sudden beam loss (mainly in LER) → sensor/collimator damage, QCS quench• Storage beam suddenly becomes unstable and lost within 2~3 turns (20~30us).
• Large loss events caused QCS quenches x8 times (!) in 2022ab.
D6V1/D2V1 collimators were severely damaged.
• Abort timing analysis using fast loss monitors made a good progress.
• The cause of beam loss is still unknown, but several new hypotheses are on the table.

• Injection: frequent injection aborts on “bad injection” days → operation time loss
• Sometimes injection becomes unstable and caused frequent injection aborts•

Due to LINAC energy drift, poor 2-bunch injection, collimator damage, narrow dynamic aperture at b*y=0.8mm, etc….

• Mostly just above the diamond abort threshold. Slightly relaxing the threshold can suppress such
aborts, but the possible risks should be carefully accessed.

• Injection: difficult to keeping a good condition for a long time → limit max currents• HER injection efficiency sometimes became poor and limited the max beam current.
• HER 2-bunch injection cannot be used in 2022ab (poor quality of 2nd bunch).

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Beam aborts in 2022ab
•
(75)

(139)

(90)

(51)

(37)

(18)

(26)

(12)

•

2022a

CCG: pressure burst
EQ: earthquake

Frequent
EQ aborts

LER/HER beam loss aborts got more frequent as beam
currents became higher.
LER large beam loss aborts caused QCS quenches(8 times in 2022ab).
- If QCS quench happens, it takes ~3 hours to recover QCS andmore time to reproduce a good machine condition•

Large beam loss caused damage in LER verticalcollimators (D6V1,D2V1) and Belle II sensors•

HER/LER injection aborts become frequent on several“bad injection” days

“bad injection” days2022b

- Due to LINAC energy, damaged collimator, beta*y=0.8mm, etc…

•

Not dangerous, but it takes 20~30minutes per abort torefill beams → loss of integrated luminosity•

Large earthquake on Mar 16th. Frequent earthquake
aborts due to the aftershocks continued for several
weeks.
LER Injection kicker accidental fire aborts happened inearly 2022a. D6H3 collimator was damaged (again).

b*y=0.8mm trial (May19-26)•: QCS quench by LER loss

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Major LER beam loss events in 2022ab (1/3)
2022-03-11 10:08

2022-03-23 22:20
1turn

1turn

2022-04-08 11:55
1turnHorizontal orbit

Vertical orbit

Vertical oscillation

Bunch current

Bunch current loss
- QCS quench (#1 in 2022ab)- HUGE IR loss (544mRad)- Severe D2V1 damage(pressure burst >10-5 Pa)>50% of stored beam
lost in one turn

- IR loss was small (12mRad)- This is not QCS quench, but..
- Severe D6V1 damage(pressure burst >10-4 Pa !)- QCS quench (#2 in 2022ab)- QCS quench occurred although
IR loss was rather small (91mrad).
WHY??
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Major LER beam loss events in 2022ab (2/3)
2022-05-17 14:38
1turn2022-06-01 22:05

2022-06-03 14:48
1turn

1turn

Vertical oscillation!!!
>50% of stored beam
lost in one turn

- QCS quench (#3 in 2022ab)- HUGE IR loss (471mRad)- Severe D6V1 damage(pressure burst >10-5 Pa)- After this abort, D6V1 cannot
be closed and wider than D2V1

>80% of stored beam
lost before abort- QCS quench (#4 in 2022ab)- HUGE IR loss (739mRad)- No pressure burst- QCS quench (#5 in 2022ab)- HUGE IR loss (>>1000mRad)(diamond saturated !)- Severe D2V1 damage(pressure burst > 10-5 Pa)
11Major LER beam loss events in 2022ab (3/3)
2022-06-09 00:37

2022-06-09 04:26

2022-06-14 14:34
1turn

1turn

1turn

!!
>50% of stored beam
lost before abort- QCS quench (#6 in 2022ab)- HUGE IR loss (644mRad)- QCS quench (#7 in 2022ab)- HUGE IR loss (>>1000mRad)(diamonds saturated !)- Severe D6V1 damage(pressure burst ~10-5 Pa)- PXD sensor damage- QCS quench (#8 in 2022ab)- HUGE IR loss (1056mRad)- Severe D2V1 damage(pressure burst > 10-5 Pa)

What is the cause of those “catastrophic” beam loss ??

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Severe collimator damage

A direct hit from thecrazy beam melted
the collimator headand splattered it all
over, even onto theviewport window.

Sudden Beam Loss (SBL)

A dedicated talk by Ikeda-san
tomorrow morning• Sudden Beam Loss (SBL)••••Stored beam is suddenly lost within a few turns
The cause is not fully understood yet
Sometimes lead to QCS quench, or severe damage on collimator head and VXD sensors
Major limitation for the machine operation, especially at higher beam currents• Ongoing efforts to understand the cause of SBLs
• Fast loss monitors installed around LER collimators• “More eyes” to find the hint for SBL• SBL documentation now available (link)• SBL subgroup formed under the SuperKEKB International Task Force (ITF)• 1st meeting on Aug 31st, 2nd meeting on Oct. 14th
• Fruitful discussions inviting experts outside B2/SKB collaboration

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Fast loss monitors and timing analysis• 7 beam loss sensors (CSI+PMT, EMT) installed near
LER collimators (shown in thering map on the left)•

more sensors will be added during LS1, at injection points, NLC, HER, etc..

• Each sensor equipped with precise timing system•

Beam loss signal waveforms are recorded by the oscilloscopes & beam loss
timings are precisely measured by timestamps of White Rabbit modules• By comparing beam loss timings at different ring
positions, we can tell where in the ring the beam loss
initially started• A Joint project with Belle II and SuperKEKB members•

Also, by comparing beam loss timing w.r.t the KEKB revolution signal timing, we cantell which bunch caused the beam loss (calibration done by injection loss)

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Example of beam loss timing analysis

in previous turns

Y. Liu
D6V1
D6V2

D6H3
D2V1
BCM loss

was the earliest sensor to see beam loss
• BCM loss started from the next turn=> First beam loss occurred within BCM~D6V1, not BCM upstream

seen before the BCM loss=> First beam disturbance also occurred within BCM~D6?

Similar observations for most of LER beam loss events
BOR = Beam Orbit Recorder
BCM = Bunch Current Monitor

Find more plots at http://bpc-3-406-2.kek.jp/image_new/2022b_TVL/index.html

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Y. Liu

In most cases in April/May, earliest beam loss is detected by D6 loss monitors.
→ The initial beam disturbance occurs at the upstream of D6 section?
After D6V1 was severely damaged by May 17 abort, it had to be used very wide (BG *increased* when we closed D6V1).
In some of recent aborts which caused QCS quenches, D2V1 saw the earliest loss (D2V1 was not protected by D6V1)

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Timing analysis summary• Timing analysis result of 2022ab events:• The initial loss was found to be detected
mainly at either D06V1 or D06V2.
• D06V2 or D02V1 loss became more frequent
as damaged D06V1 collimator’s aperture getswider and D06V2 narrower.
• If the initial loss is detected at D02V1, thelarge beam loss tends to happen at IR
resulting in QCS quench.

More details in Liu-san’s talks at the MDI meetings
https://kds.kek.jp/event/42104/
https://kds.kek.jp/event/42881/

New SBL subgroup under ITF• A new SBL subgroup was formed under the SuperKEKB International Task Force (ITF)• Contact person: H. Ikeda (SuperKEKB)• Sub-contact: H. Nakayama (Belle II)• Invite experts outside our collaboration to the meeting for wider discussion• 1st meeting on Aug 31st (indico)• Fruitful discussions on proposed hypotheses for SBL:
- “X-aborts” in KEKB, due to RF finger failure ? → seems not (SBLs are much faster)- “Dust events”, as seen in Phase1/2 ?
- “Fireball hypothesis” (vacuum arching) ?
- Feedback system issue, as in BEPC II ?

→ seems not (SBL loss is mainly vertical , not horizontal)→ not sure (need more data with acoustic sensors)→ seems not (SBLs are much faster)• 2nd meeting held on Oct. 14th (indico)

MDI plenary (H.Nakayama)

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SBL documentation
Coordinator: H. Nakayama• SBL documentation available (1st version)• Prepared for the 1st SBL International taskforce (ITF) meeting• Shared with the external experts invited to the SBL ITF• Outline of the documentation:
1.
2.
3.
4.
5.

Introduction
Beam loss sensors
Beam loss timing analysis
Observation by other sensors (BCM, TbT BPM, etc..)
Hypothesis of the cause(collective effects, beam dust, fireball, etc..)• Plans• Include discussions at the recent SBL ITF meetings• Belle II note, publication
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Recent new efforts for beam loss location hunt
• Fiber loss monitors around D6V1/D6V2 (Ikeda)• It can issue beam aborts now (could be faster than IR sensors)

Fiber loss monitors• Additional PINs around D6V1/D6V2 (Ikeda)• Acoustic sensors around D6V1/D6H3 (Abe)• To monitor the sound of electric discharge (“fireball” hypothesis)• Simplified BOR/BCM around D6V1 (Fukuma)• Limited accuracy for 2-bucket-spacing bunches

BOR = Beam Orbit Recorder
BCM = Bunch Current Monitor• Others• CT (Current Transformer) as a beam current monitor• QCS bellows diamonds with larger dynamic range

Acoustic sensors• And more …?
- We really appreciate those new contributions, especiallyfrom SKB experts !
- We are now trying to incorporate those new inputs to ourbeam loss timing analysis
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Faster beam abort delivery
Faster beam abort can mitigate the damage from SBLs.

Note: abort delivery has been already
improved significantly in the past years(for example, by introducing two abortgaps). Further improvement by the items
shown below would not be so dramatic.

1. Add an abort sensor in
the upstream section

2. Abort sensor withfaster response

3. Shorter transmission
path of abort request

•

•

•

•

Loss monitor in D06 section
should provide faster beamaborts (thanks to better
location)
Confirmed by the D06 fiber loss
monitors and D06V1 fast loss
monitors installed in 2022b

Faster beam aborts by D6V1 loss monitor•

CLAWS response time is very
fast (~200ns)
Additional CLAWS system canbe installed at D05(NLC), or
D06 section, etc..

CLAWS at NLC•

Abort request sent directly tothe abort kicker, not via CCR
R&D started for field wirelesssystem or laser in the tunnel

Abort request directly sent to the kicker

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Other MDI-related topics• CLAWS abort system•••

CLAWS injection veto works as intended to suppress large number of (unnecessary) injection aborts.
Discussion started for another CLAWS abort system around NLC, for faster abort delivery.
Discussion also started for direct abort delivery from sensor to the abort kicker (not via SKB central control room)• Diamond abort system•••

Recent frequent injection aborts on “bad injection days” are issued by diamonds only. Dose are just above the 4mrad threshold
We had intensive discussions for relaxing the diamond threshold, with all risks considered (threshold not yet changed so far)
QCS diamond range was changed to provide dose/timing measurement for large loss without saturation (from June 15th)• Injection status•••

HER injection efficiency fluctuate a lot even within a day (temperature?) → makes injection tuning quite difficult
To reach the target current, HER 2-bunch injection (now 2nd bunch has poor quality) is must
To further increase LER injection charge, unnecessary injection aborts should be suppressed• NLC (Non-linear collimator)••

NLC should reduce vertical impedance significantly, while keeping BG at similar level.
We have decided to install NLC during LS1.

• How to cope with “crazy beam” and avoid collimator head damage is under discussion.

• Machine-Learning approach for beam background prediction••

Using variety of SuperKEKB 1Hz EPICS PVs as input to the neural net
Also applicable to injection tuning or luminosity tuning?
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Summary• Beam background in 2022ab: below the Belle II detector limit• TOP PMT rates dominated by LER single-beam BG and luminosity BG• Belle II did not limit the max beam currents for operation• Injection BG duration got worse at higher beam currents• Need wider injection veto window → large DAQ deadtime: ~10%!
• Also affected recorded data: some degradation seen in CDC performance• Major issue: Sudden Beam Loss events (SBLs)

A dedicated talk by Ikeda-san
tomorrow morning• QCS quenches x8, with severe collimator damage• Become frequent at higher (bunch) current? → limit the max beam currents for operation• Investigation on SBL issue made good progress• Timing analysis using fast beam loss monitors shows initial beam loss location• International taskforce launched: fruitful discussion inviting experts from other collaborations• Several hypotheses are on the table, but conclusion not reached yet (homework for runs after LS1)

• Another major issue: stability of injection performance• Difficult to keep good condition for a long time → limit the max beam currents for operation• Many improvement works planned during LS1 (inj. efficiency, emittance, inj. kicker, etc…)
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backupGave up HER
2-bunch injectionRequest from BPAC reviewers:> 2) Some more details on the injection background issues such as the radiation issueand dead time would be appreciated.
Dead time?
- Dead time fraction due to injection loss is determined by:[duration of injection background] x [ averaged injection repetition rate].
* The dead time fraction is getting larger recently, since we need maximum repetition rate (23Hz) to reach the highest currents.

- In 2020, we conducted series of machine studies to investigate the injection BG duration.
- This slide by Koga-san nicely summarizes the discussion on injection BG duration.
https://kds.kek.jp/event/41955/contributions/211658/attachments/155748/198302/koga_injectionveto_2022_5_26.pdf* When we fired injection kicker but did not inject any charge, no beam loss was observed.
* The injection background duration becomes longer with higher bunch current.
* The duration is longer with colliding beams than single-beam.
* The duration depends on various main ring optics parameters and injection parameters.
* The duration can also change by the collimator settings. If the collimator is damaged and had to be used wider than desired,it could increase the duration of injection background.

- Taken all together, these observations suggest the following scenario:* Oscillation of the injected beam remains for a while by interaction between the injected beam, storage beam and collision* The oscillation propagates to vertical plane with x-y and chromatic coupling.
* Ohmi-san's simulation also shows such effect.

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Request from BPAC reviewers:> 2) Some more details on the injection background issues such as the radiation issue and deadtime would be appreciated.
Impact on data quality?
ECL data loss

CDC (dE/dx)

CDCTRG- Data quality seems affected by injection background.
- Further investigations are needed.
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Flexible Boron sheet around QCS cryostat?
(for neutron SEU mitigation)

More details can be found in Andrii’s talk at the Background meetinghttps://indico.belle2.org/event/8224/

New SBL subgroup under ITF• A new SBL subgroup was formed under the SuperKEKB International Task Force (ITF)• Contact person: H. Ikeda (SuperKEKB)• Sub-contact: H. Nakayama (Belle II)• Invite experts outside our collaboration to the meeting for wider discussion• 1st meeting on Aug 31st (indico)• Fruitful discussions on proposed hypotheses for SBL:
- “X-aborts” in KEKB, due to RF finger failure ? → seems not (SBLs are much faster)- “Dust events”, as seen in Phase1/2 ?
- “Fireball hypothesis” (vacuum arching) ?
- Feedback system issue, as in BEPC II ?

→ seems not (SBL loss is mainly vertical , not horizontal)→ not sure (need more data with acoustic sensors)→ seems not (SBLs are much faster)• 2nd meeting held on Oct. 14th (indico)33

Y. Funakoshi, at the abort meeting on May 31th, 2022
https://kds.kek.jp/event/42366/

Having more eyes isimportant. Not only
loss monitors, butalso beam monitors
such as BOR/BCM
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A new “Fireball” hypothesis (by T. Abe-san)Observed by camera in RF
cavity. How about collimators?

When a tiny particle is heated up by thebeam-induced field (particle should beclose enough to the beam) and thentouches some metal surface, it generates
plasma and could causes a vacuum arc,
which could interact with beam particles.

From the abort meeting on May 31th, 2022
https://kds.kek.jp/event/42366/(different from the dust-capturedin-beam hypothesis)
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Mike Sullivan

Ulrich Wienands?

36

Injection upgrade plans
From the minutes of August MDI meeting, input from Iida-san and Mori-sana) Items to be implemented during LS1* Electron two-bunch injection suffered from vertical orbit shift and emittance growth of the 2nd bunch.
--> Fast kicker in LINAC will be installed to solve the orbit shift.
* HER beam chamber aperture at the injection point was found to be insufficient.
--> The chamber will be modified and replaced.
* LER injection kicker accidental fire ruins D6H3 collimator.
--> To mitigate the damage, carbon-head collimator will be installed.
( For a longer term, using a common power supply for all kickers is considered)* LER kicker waveforms have larger residual than HER.
--> Ceramic ducts will be modified or metal film will be installed to reduce the residual.
* We need continuous monitoring of energy spread and vertical beam size in BT.
--> SRM are already installed in BT.
* Electron injection amplitude is found to be too large.
--> Most downstream HER septum magnet (SE1) will be replaced to reduce injection beam loss and improve injection efficiency .

37

Injection upgrade plans (contd.)
From the minutes of August MDI meeting, input from Iida-san and Mori-sanb) Items which might be implemented during LS1 or later* Electron emittance growth in BT is a serious issue.
The cause of vertical emittance blowup is still unknown.
Horizontal emittance growth should be understood by ongoing CSR simulation--> Beam chambers might be replaced to suppress CSR if confirmed by the simulation* Positron emittance growth in BT is also a serious issue, especially above 2.6nC.
CSR simulation is ongoing too. CSR in DR-sector3 is also suspected as a source.
The cause of vertical emittance blowup is still unknown.

c) Other items* Injection tuning is quite complicated and can be performed by a limited number of experts.
--> If we can establish an automatic tuning scheme, it might help non-experts.
* In LER, injection beam loss in the LER continues for ~10msec, while in HER it lasts only for ~1msec.
Why and where in the ring the LER injection loss occurs should be investigated.
--> Dedicated analysis using beam loss monitors might help understand this.

38