Sudden Beam Loss
The 26th KEKB Accelerator Review Committee
2022/12/14 H.Ikeda1

Contents
 Motivation
 Observation
 Candidate of the beam loss reason
 Future plan Summary2Motivation

The biggest goal of SuperKEKB is to increase luminosity, but one of the obstacles is suddenbeam loss.
The cause of the sudden large beam loss is unclear.
Causes collimator (and other component)damage, QCS quench, Large B.G. to Belle-II.

Cannot storage a large current since it causes beam abort.

↓Start the task force to investigate and resolve the cause of the sudden beam loss.
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Observations : LMWhen we looked at the loss monitor when
the abort occurred, which was thought tobe caused by beam loss, beam loss looks
started within one turn at the whole ring
collimator part and Belle-II detector.

10μs

2min.

4

Observation : Belle dose

50μs

5

Observations : Bunch Oscillation Recorder (BOR)
Bunch Current Monitor (BCM)18K10 ADC board
VME 2W form factor with BLT data transfer function

8 bits resolution (MAX108 ADC)+Spartan6 FPGA card



5120x4096 turns of ring memory, or 5120x1 (BCM)

4.6s from 18k10（20M) to VME CPU, 1.4s from VME to NFS diskstop acquisition with (quick) beam loss event (DCCT)

LER Horizontal Vertical Longitudinal

HER Horizontal Vertical Longitudinal

BCM HER LER (shared memory)Long BCM HER LER(5120x4096)

Observations : Bunch Current Monitor

In order to investigate where in the train the beam loss started at the moment ofbeam loss, we recorded the bunch current 4096 turns before the abort trigger usingfeedback processors .

The beam loss measured by the bunch current monitor (BCM) occurs suddenly at acertain turn.

10μsAbort

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Observations : Bunch Oscillation Recorder
The Bunch oscillation is measured 4096 turns before the abort and the orbit is calculated
from the data.
 The orbit changed small ～1mm@FB position
17/MAY QCS quench , D6V1

03/JUN QCS quench, D2V1

14/JUN QCS quench, D2V1

Ibunch[ｍA]
H [mm]
V [mm]

0.2mm

0.5mm

0.4mm

1.5mm

0.2mm
0.2mm

L [a.u.]8(H.Fukuma)Beam loss occurs inboth HER and LER,
but the damage tothe hardware isparticularly large
when loss occurs inLER.

We don't know if itwill happen evenwith a single beam
operation, lowcurrent beam
because we haven't
operated for a longtime.

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Observations : Bunch Current of Operation

It is likely to occur when a certain bunch current is exceeded.
 The first four accidents of LER beam loss in 2022 happened at 𝐼𝑏 ≳ 0.7 mA/bunch within a day

after increasing the beam current at each different Nbunch.
 The threshold became somehow lower after the D06V1 damage on May 17.
 It might be due to the D06V1 damage, different collimator configuration than usual to mitigate

the beam background, or something else.

10(K.Matsuoka)

Observations : Beam Loss TimingIn order to find out where in the ring thebeam loss first started, we installed a loss
monitor specialized for timing measurement
inside the ring.
 7 beam loss sensors (CSI+PMT, EMT) installednear collimators
 Data aquisition
 WhiteRabbit (WR) : Distributed TDC system

synchronized to GPS Picoscope : ±1ms of abort trigger

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Observations : Beam Loss Timing

Beam loss occurs in collimator & IR, and where it occurs first depends on collimator tuning .

12(Y.Liu)Observations


There are no signs before beam loss starting.
 No small beam loss (beam loss monitor, BCM)
 No oscillation (Bunch Oscillation Recorder (BOR) ) No beam size change (X-ray monitor (XRM))
 It is not clear if the orbit changed significantly.(Libera)

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Observations : VacuumPressure bursts have been observed here and there, and it rarely occurs in the sameplace except in the collimator section. It may be the result, not the reason.

14(Y.Suetsugu)

Observations : Acoustic waves
Acoustic waves were detected at the same time with collimator beam loss.
 We measure a few event before shutdown

15(T.Abe)Candidate of the beam loss reason
Damage of vacuum component : measured @ KEKB & PEP-II phase changes (beam energy losses) had been observed ms ～ hundreds of μs before aborts.

→ not observed in SBL abnormal temperature risings at bellows chambers had been observed and the catastrophic

damages in the RF-finger had been confirmed.
Dust:Early stage @ SuperKEKB
 Beam aborts accompanied by local pressure bursts. →not observed the burst that causes itin SBL
 Beam loss lasted a few ms before the beam abort. → time scale is different Cleanor hit the vacuum chamber to remove as much dust as possible and fixed the problem.

 When the energy of the particles deviates greatly, the loss occurs in the horizontal collimator,

so it may be possible to find out something by examining the conditions that the loss occurs in16
the vertical collimator in more detail.

17(Y.Suetsugu)(Y.Suetsugu)

Candidate of the beam loss reason
Vertical abort kicker misfire
 We are using the same thyratron for horizontal kicker.

FB kicker trouble or lack of power : measured @ BEPC II
 Sinch the growth time of coupled bunch instability might be O(～several 10 turns),

our sbl was not caused by FB system problem.
Equilibrium of tuners, piezo's parameter, LLRF, noise from transmitter, 50Hz filter of RF
system could cause sudden beam loss. : measured @ BEPC II, DAFNE

RF system are monitored at each abort, and were not seen abnormal signal.

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Candidate of the beam loss reason
Electron cloud effect in collimator In this case, SBL should be measured at LER only, but

HER beam also lost.

 Looking at the pressure, D06H3 shows strange behavior. An electrical discharge or anelectron cloud (or rather, multipacting) may be occurring.
 But no pressure burst was observed at the timing of SBL

 Simulation of EC growth is on going.
 The electron density depends on pressure, secondary electron emission rate, gap spacing,bunch spacing, the incident electron energy that gives the maximum SEY, the bunch
current, etc.
 The electron density is on the order of 1E13-1E14 m^-3 at the highest with general(ordinary) parameters.
 Not sure how this relates to SBL.

 If there is a possibility that electron cloud in the collimators can cause
the sudden
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beam loss (under investigating), we may put solenoid coils around them. (T.Ishibashi)

Candidate of the beam loss reason
Fireball Breakdown triggering mechanism of the RF cavity (after conditioning).
 Necessary environmental conditions for fireball breakdown A metal with a high and low sublimation point coexist in the same place

Sublimation point in ultrahigh vacuum20

21(T. Abe)

Future plans
Increase the number of monitors.
Add new loss monitor for timing analysis and abort trigger

Add simplified BOR for another place on ring
 We tried at previous run, butposition resolution was not good.

 Expected much higher resolution on X, Y detection.
 Put them to two positions with the phase advance roughly 90 deg in order to check the orbit.
Setup acoustic sensors to most of LER collimator.

Check the temperature of vacuum component

Assuming the fireball hypothesis is correct,


Coat the surface of the LER collimator with copper when viewed from the vacuum side, all low
sublimation metals.

In the future, we may consider applying a high sublimation point carbon or tungsten coating on thecopper surface around the collimator head.

Simulations for each hypothesis are also underway.

22SummaryOne of the obstacles for luminosity increasing is sudden beam loss and the cause of thebeam loss is still unclear.
 We have been investigating with Loss Monitor, etc., and have been able to find the point

where the loss started, but no phenomena that clarify the cause have been found.
 Started the international task force to investigate and resolve the cause of the sudden beamloss.
So far, the fireball hypothesis is the dominant hypothesis to explain SBL.

In order to collect the information, we are working to improve monitors, and also
working to suppress the fireball hypothesis if it is correct.

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backup24

Injection related abort : BOR & BCM

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26

(Y. Suetsugu)

Simulation using PyECLOUD (2D)
(preliminary) The bunch current that gives themaximum ηey_aperture.

changes with The larger the aperture, the higher

the bunch current.

27(Y.Suetsugu)Calculation of electron cloud in collimator by modified version of CLOUDLAND(preliminary)Electrons gather near walls.

At t=4ns, the electron density at the center of the chamber is about 3.5 times
larger than the 2mm case.

(H. Fukuma)
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