Beam background/MDI report
H. Nakayama (KEK),
S. Vahsen (Hawaii)Contents:• Latest background/MDI status• Improvements to background simulation• Improved data/MC agreement• Further background mitigation• MDI-related issues during 2020b

KEKB review, July 2020

Hiroyuki Nakayama (KEK)1

Background Status in 2020a,b Run

Iijima San’s talk

• Lpeak=2.402 x 1034/cm2/s on June 21st, 2020
• Achieved with LER 720mA, HER 610mA• beta*y = 1.0mm, 978 bunches• TOP is the detector currently most vulnerable to beam Limit, all bkg
backgrounds
Limit, all bkg• close to or at the rate limit in recent runs
but luminosity• Finite PMT lifetime + new SuperKEKB run plan dictates:
PMT rate from all bkg components except luminosity
needs be <1.2MHZ• Current composition, measured May 9• LER beam-gas is the dominant component,
approx. 50% of total• Reduced substantially since 2019c

ALD-type
MCP-PMT
Conventional
MCP-PMT

Kojima, Tsuzuki• Further reduction of TOP single-beam BG required forhigher beam currents in 2020c
KEKB review, July 2020

Hiroyuki Nakayama (KEK)2

MDI Status in 2020a,b Run

SuperKEKB

Belle IIMDI• New LER V collimator (D6V1)• Used D6V1 as “primary” (=narrowest) collimator to protect D2V1 near IP• Effectively suppress LER Beam-gas Coulomb (see next slide)• Plan to replace D6V1 head with Carbon in this summer

MDI taskforce unites
Belle II, SuperKEKB and LINAC

LINAC

Daily count of pulse magnet failures• Beam aborts due to bad injections: less frequent• Less machine downtime → more integrated luminosity• LINAC pulsed magnet misfire problem is solved on Apr. 9th• LINAC/BT orbit monitoring significantly improved

LINAC software
replaced on Apr. 9th• Injection BG duration• Shorter veto window → less dead time
• Many studies in 2020a,b, with newly developed monitoring tools
Date(MM/DD)• 2-bunch injections• Essential to achieve higher beam currents

PXD damage/QCS quench by beam loss on May 27th

• HER 2-bunch injection is successful and now set as default (LER needs more work)• Others•••••

Faster Diamond abort cycle 2.5 us in 2020 a+b (10 us before) ensuring safer operation
Two QCS quench events (May 27th, June 20th)PXD SR by HER injections: not critical right now, but need careful watching
CDC HV trip is much less frequent in 2020a,b
Collimator activations, collimator impedance

KEKB review, July 2020

Hiroyuki Nakayama (KEK)3

Good news: Background reduction, 2019c to 2020b
SVD• Previously dominant LER beam-gas significantly
reduced, by factors of approx.
• SVD: 2.3• PXD: 5• CDC: 3• TOP: 2.4*
*dynamic pressure component• Combined result of D6V1 collimator, moving othercollimators, vacuum scrubbing

CDC

PXD

Tanigawa

Stefkova

CDC
Nakagiri• Similar to our prediction (factor 2.5) by adding D6V1

• New: We now separate beam-gas into dynamic
and base• Both in simulation analysis• Main reduction seen in dynamic component.
Base component not always reduced.
• Important to understand evolution for future
BG predictions.
KEKB review, July 2020

Hiroyuki Nakayama (KEK)

KojimaTOP

4

Recent improvements to simulation• Andrii Natochii implemented an improved
framework for beam-particle tracking in SuperKEKB

Simplified aperturein the old simulation

NatochiiMore realistic aperture in
the new simulation• New features: apply collimation after particle tracking,pressure-weighted beam-gas simulation, custom beam pipe
aperture shapes, etc..
ellipse• Largest impact: implementation of correct
SuperKEKB collimator shape + tip scattering• Particles previously stopped by the collimators can now
reach the IP• Up to factor 1000(!) increase in simulated Belle IIdetector rates, resolving a longstanding HER
data/MC discrepancy• Surprisingly, largest effect from collimator shapechange transverse to beam axis
• This may imply we could benefit from wider collimator
heads for HER D1V1, in plane transverse to beam → should
be studied (kick factor, etc.)

Vertical collimator
Not wideenough?

Old aperture
Simulated beam halo
New aperture

KEKB review, July 2020

Hiroyuki Nakayama (KEK)5

Improvement in data/MC agreement• Due to the improved collimator simulation,order 1000 increase in predicted HER
Touschek rates• Appears to largely resolve the long-standing
HER simulation problem• SVD, CDC shown here, but also holds forTOP, PXD• Measured luminosity bkg agrees withsimulation at the ~10% level in TOP, PXD.
Also agrees between continuous injection
and decay data (SVD see problem and morework need)• For the first time, data and MC agree within
one order of magnitude for all five leading
background components

Nakagiri

CDC data/MC ratio
BG sources

Old simulation

New simulation*

HER beam-gas (base)x30-130

x6-22

HER beam-gas (dynamic)

x20-50x4-12

HER Touschek

x30-80x0.6-1.2
Tanigawa

SVD data/MC ratio
BG sources

Old simulation

New simulation*

HER beam-gas (base)

x11

x3.4

HER beam-gas (dynamic)

x15

x6.3

HER Touschek

x130

x0.24

* New simulation includes realistic collimator shape and tip-scattering

KEKB review, July 2020

Hiroyuki Nakayama (KEK)6Implications for design luminosity• Once we correct design-luminosity rates by measured data/MC, the new rates predictions are slightly
lower than before (PXD)• Despite previous corrections factors of order 1000, our Phase 3 rate predictions seem to have beencorrect to factor ~3• Goal is to get to ~25% accuracy for single beam background, ~5% for luminosity backgrounds. This will
require more hard work, but now seems achievable.
• need gas injection study for better understanding of beam-gas background (Zeff=7 or 2.2 or ?)Old prediction

Stefkova

Two
PhotonPXD

Touschek LER

New prediction

Touschek
HER

Coulomb
LER

KEKB review, July 2020

Hiroyuki Nakayama (KEK)7

TOP BG extrapolation toward higher current
KojimaWithout further mitigation, TOP PMT
rate will hit the limit at LER~750mA(design current : 3.6A)

KEKB review, July 2020

Hiroyuki Nakayama (KEK)8

Mitigation idea example: Bellows shielding• To reach design luminosity, we need further
background mitigation.
• One of ongoing project is to add a shieldaround bellows pipe btw VXD and QCS,
where we see “hot spot” in data (also seenin simulation).
• Remaining space is quite limited there, somechanical design is challenging.
• Katsuro Nakamura prepared a CAD design ofthe shield and run beam loss simulation. He
estimates LER coulomb bkg can be reducedby 53% (CDC), 28% (TOP)• Aiming for installation in 2022

KEKB review, July 2020

Hiroyuki Nakayama (KEK)9Further background improvements
Carbon collimator head for D6V1

• Collimator work in mid-September 2020
• Replace D6V1 with carbon head, install LER D3V1 collimator• Simulation study including tip-scattering is ongoing• Additional shield around QCS bellows (2022)• Shown in the previous slide• Further BG reduction for TOP/CDC• QCS modification (2026)• Less overlap of solenoid and quads → suppress beam-beam blowup• Wider beam pipe aperture→ less BG=============QCS remodeling• LINAC work aiming for higher beam currents• Stable LER 2-bunch injections• Increase injection charge while keeping good emittance• VXD diamond & beam abort system upgrade•

Radiation Monitor & Beam Abort (RMBA) task force

KEKB review, July 2020

Hiroyuki Nakayama (KEK)10

MDI-related issues found in 2020b run

KEKB review, July 2020

Hiroyuki Nakayama (KEK)11

T. Koga

Issues: Injection BG duration

ECLTRG (#cluster>6)• Injection BG continues after injections• We apply trigger veto after each injection• Longer veto window-> more DAQ deadtime• Typical duration: LER: 6~12ms, HER:1~6ms••••

Single beam: BG duration∝bunch current
Colliding beams: BG duration longer than single-beam
Crab waist strength: no clear dependence
beta*y squeeze: BG duration longer with small beta*y• Dedicated machine study shows:• Not only the injected bunch, but also later bunches are lost
• However, “blank-shot ” injections don’t give any BG duration→ coupling btw an injected bunch and later bunches?
“blank-shot” injection: kickers are fired but no charge is injected
KEKB review, July 2020

Hiroyuki Nakayama (KEK)12

H. Ikeda

Issues: Beam aborts• “Beam abort” taskforce, led by Hitomi Ikeda, to investigate the possible cause of beam aborts andimprove integrated luminosity by reducing machine downtime• Beam aborts in-sync with injection has
significantly decreased thanks to LINAC
group’s effort• HER beam loss aborts not-in-sync with
injections are dominant now.
••

No clear correlation with injection timing→ Not caused by “delayed” injection losses
Small vacuum bursts? Or any unknown causes?

HER aborts by bad injections havedisappeared since Apr. 9th
KEKB review, July 2020

Hiroyuki Nakayama (KEK)13

Improvements: LINAC/BT monitoring
LINAC/BT orbit trend

Miyahara,Mori,
Seimiya, Kaji,etc..

BT orbit web

2020/06/15 20:48
Diamond abort
Pulse Magnet failure(PC_6,181)

These new monitors allows us to easiy detect injector problems when bad
injections happens. → Effectively worked to reduce injection-related beam aborts
KEKB review, July 2020

14
Hiroyuki Nakayama (KEK)Nakayama

Weekly beam abort counts
HER aborts in-sync with injections due to
LINAC pulse magnet misfires (till April 9th)

LER aborts in-sync with injection due to
LINAC Klystron down or other unknown reasons

4/24~ HER CW start

5/11~14 off resonance
5/14~31 HER angle
5/21~ LER high εx
6/11 D6V2 open
6/10~16 off resonance

What is the causeof those HER
aborts not in-sync
with injections?

KEKB review, July 2020

Due to higher beam currents? or
D6V2 open on 6/11?

Hiroyuki Nakayama (KEK)

LINAC pulse magnet misfires
appeared again (June 11-)15

Issues: PXD SR during HER injection

Carsten• SR hit pattern on PXD forward –X modules• Becomes stronger when HER beta*_x was squeezed• Only visible during HER injections
•
•

•

•

SR NOT observed with “blank-shot” HER injections

HER horizontal tune adjustment shows no significant improvement within
acceptable tune range
HER D01H collimator adjustment didn’t improve SR

SR increased with the smaller HER IR orbit horitontal angle (May 1531), so we went back to the original angle• SR rate PV is now available.
• BCG/PXD experts monitors it.

KEKB review, July 2020

PXD SR is not critical right now, but we need to keep our eyes on it.
Hiroyuki Nakayama (KEK)16

Issues: QCS quench on May 27th, 2020
What happened?
• LER was aborted first. Diamond abort was not issued.
• Diamond system received the abort acknowledge signal and started
the data dump.
• Diamond was blind during this data dump, while still HER is
circulating the ring.
• ~0.7 sec later, iBump fast FB strongly kicked HER beam and caused
HER beam loss. Diamond abort was not issued.
• It resulted in QCS quench and damage on PXD/diamond readout.

PXD after QCS quench in May 27th

Solutions• Diamond system is modified.
• Dump the data only when both beams are aborted.
• iBump fast FB is also modified• Add the limiter on the FB power supply controller

KEKB review, July 2020

Hiroyuki Nakayama (KEK)

Another QCS quench occurred on June 20th. Diamondabort was issued. Caused by small LER vacuum burst?
17

Issues: LER D6V2 “mystery”• When we opened D6V2, injection BG duration (and injection BG on diamonds) improved.
• Now we use ~400um wider D6V2 settings.
Why?
• Tip-scattering of injection charge? → seems unlikely to reach IR from D6 or affect BG duration.
• We start to see collimator impedance issue ?
KEKB review, July 2020

Hiroyuki Nakayama (KEK)18

Summary• Beam backgrounds in 2020ab is significantly improved than 2019c• New LER V collimator, tighter collimators (thanks to stable injection), and vacuum scrubbing progress• Enables us to go higher beam currents and achieve peak luminosity record• Stable injections lead to less beam aborts/CDC HV trips → less downtime, more integrated luminosity• For the first time, data and MC agree within one order of magnitude••••

Simulation now handles true SuperKEKB collimator shapes and tip-scattering
The longstanding HER simulation problem has been resolved
Luminosity background in PXD, TOP agrees well with simulation (~10% level)~25% accuracy for single beam background, ~5% for luminosity is our goal

• To reach design luminosity with acceptable background, further background mitigation is needed.
• More LER collimators, bellows shield, QCS modification, etc..

• For more integrated luminosity and stable operation in 2020c and future runs, we need to
•
•
•
•
•

Achieve stable LER 2-bunch injection and larger injection charges with good emittance
Minimize injection BG duration to reduce DAQ deadtime
Understand the cause of beam aborts (not related to injections)Control SR background on PXD
etc…

KEKB review, July 2020

Hiroyuki Nakayama (KEK)19backup

KEKB review, July 2020

Hiroyuki Nakayama (KEK)20

KEKB review report 2019KEKB review, July 2020

CDC trips are much less frequent in 2020 thanks to stableinjections and higher CDC trip threshold.

Unfortunately, from 2020b, we had to move the BCG
shifter booth to another building amid this COVID-19
situation. However, communication between machine
operators and BCG was basically smooth.
Belle II diamond abort cycle in 2020 runs is 2.5 us, fasterthan 2019(10us). It can detect a surge in dose and issue abeam abort faster.
Details in Iida-san’s report. We made a significant progressin 2020 runs.

KEKB review, July 2020

Hiroyuki Nakayama (KEK)21

SuperKEKB collimators

KEKB review, July 2020

Hiroyuki Nakayama (KEK)

Andrii Natochii

22

Matsuoka

KEKB review, July 2020

Hiroyuki Nakayama (KEK)23

Single-beam BG study
for measuring Touschek and Beam-gas component separately
𝐼2
𝑅𝑎𝑡𝑒 = 𝑇+ 𝐵𝑍𝑒2 𝐼𝑃
𝜎𝑦 𝑛𝑏

𝑅𝑎𝑡𝑒/𝑍𝑒2 𝐼𝑃 = 𝑇

T, B: Touschek/Beam-gas coefficientsy: vertical beam size, nb: number of bunches
P: pressure, I: beam current
Ze: effective atomic number of residual gas

𝐼𝜎𝑦 𝑛𝑏 𝑃𝑍𝑒2

+𝐵

Linear function

P. Lewis

Strategy:• Single-beam• Assume Touschek + Beam-gas and noother BG component• Vary number of bunches (or beam size),
which should affect Touschek component
only• Fit for T and B coefficients and comparethem against estimation by MC• Use measured data/MC ratio for scaling
BG simulation at future optics

Slope: T

Touschek

Intercept: B
Beam gas𝐼

𝜎n𝑦b𝑛𝑏 𝑃𝑍𝑒2
FCC-ee meeting, July 6th, 2020

Hiroyuki Nakayama (KEK)[mA/Pa/μm]
24

A snapshot from a single-beam BG study
Example: LER/HER single-beam study on May 9th, 2020

•
•
•

•

Number of bunches: Nb=783/1565/393.
As we increase number of bunches, Belle II BG rates at the same beam current becomes smaller (dueto decrease in Touschek BG)
Beam size scan is not used recently, since unexpected BG increase was observed at larger beam size.

Observed dependency are consistent with the “Touschek+ Beam-gas” model (no significant indicationof other BG sources)

FCC-ee meeting, July 6th, 2020

Hiroyuki Nakayama (KEK)25

A snapshot from a Lumi-BG study••••
FCC-ee meeting, July 6th, 2020

“Continuous injection” runs
• L=1.5→1.0→0.5e34, by vertically displacing two beams (“ibump V-offset”)• Beam sizes slightly changes as luminosity changes“Beam decay” runs (no injections)• Measurement not affected by injection BG
Measure lumi-BG component by subtracting single-beam BG components scaledwith current, beam size, etc..
Measured Lumi-BG agrees with simulation at the ~10% level in TOP, PXD !!
• Also agrees between “continuous injection” and “beam decay” data
Hiroyuki Nakayama (KEK)26

Recent improvements to BG measurement and analysis• To reduce systematics from different pressure
conditions during continuous injection,
perform more extensive beam studies••••

Single beams, cont. injection
Single beam, decaying
Colliding beams w/ continuous injection
Colling beam, decaying• We use three fill-patterns to vary Touschek
bkg, rather than emittance control knob
• We now account static and dynamic pressure
separately in fit• Tanigawa San (SVD) found that bunch-length
correction can improve background evenmodel further→fit model matches all data at
0.5% level

KEKB review, July 2020

Hiroyuki Nakayama (KEK)27

KEKB review, July 2020

Hiroyuki Nakayama (KEK)28

Data/MC agreement: full tables
SVD

CDC

KEKB review, July 2020

Hiroyuki Nakayama (KEK)29

Recent improvements to simulation• Andrii Natochii implemented an improved
framework for beam-particle tracking inSuperKEKB: SAD+G4+C++ → SAD++• This allows applying collimation after particle
tracking, custom beam pipe aperture shapes,
and more• This and related work has led to multiple
crucial simulation improvements• More CPU-efficient collimation system optimization• Re-weighting of beam-gas background withposition-dependent (CCG) pressure measurement→ improved LER beam-gas simulation• IR beam-pipe aperture correction• Realistic collimator mask profile → improved HER
simulation• Collimator tip-scattering → improved HER
simulation

KEKB review, July 2020

Hiroyuki Nakayama (KEK)30

Luminosity background• Measured lumin. bkg agrees with simulation at the few-% level in TOP, PXD• Agrees between continuous injection and decay data
• Highly encouraging!
• However, SVD, which has the most detailed bkg model, sees a problem. More workneeded.

KEKB review, July 2020

Hiroyuki Nakayama (KEK)31

Neutrons• In KLM, data/MC agreement is stillquite poor.
• My suspicion is that this is due tocavern neutrons, which are not yet
simulated• Such neutrons may be delayed
w.r.t. accelerator parameters when
they reach KLM, hence also defy
our usual analysis procedure• We are now starting a neutroncampaign to accurately measureand simulate this component
KEKB review, July 2020

Hiroyuki Nakayama (KEK)32

Origin of cavern neutrons? TPCs and He-3
concrete
Poly-shield

Schueler
Black rectangles: positions of TPC directional neutron detectors

NoshieldFWD

June 2019 B2GM

BWD

TPC analysis by Schueler is measuring neutron (recoil) energy spectra and directional distribution (identifying hot spots).
Miller has moved spare He-3 tube far into tunnel for more information. Cavern neutron BKG component is not included in
KEKB review, July 2020
Hiroyuki Nakayama (KEK)33
the simulation.
New simulation effort started (Natochii,Liventsev). Liventsev has already updated geometry.

Issues: activation of collimators• LER survey (June 2020)
D06H3：400 uSv/h
D06V1：400 uSv/h
D06V2：260 uSv/h
D02V1: 130 uSv/h
D02H3: 950 uSv/h• D6V1: “primary” (=narrowest)
LER vertical collimator• D2V1: Low activation is thanks to
D6V1• HER survey (Apr. 2020)
D12V1 200µSv/h
D09V4 : 80µSv/h
D12H1 15µSv/h
D09H4 : 60µSv/h
D12V2 35µSv/h
D09V3 : 40µSv/h
D12H2 20µSv/h
D09H3 : 9µSv/h
D12H3 65µSv/h
D09V1 : 380µSv/h
D12V3 350µSv/h
D09V2 : 15µSv/h
D12H4 45µSv/h
D09H1 : 25µSv/h
D12V4
2µSv/h
D09H2 : 75µSv/h
KEKB review, July 2020

Tanaka, Terui• HER D09V1(and D12V1,3) show large activation, but
the loss monitors at those collimators show smallvalues• Several collimators are opened, especially ones withhigher activation, by carefully looking at injection BG

Hiroyuki Nakayama (KEK)34

CDC HV trips – much less frequent in 2020a,b

KEKB review, July 2020

Hiroyuki Nakayama (KEK)•

Only few CDC HV trips in 2020ab(using higher trip thresholds)•

Inner layers(∈SL0) were tripped•

Mostly caused by HER injections•

Trip frequency seems to be decreasingover time, although
the beam currents gets higher•

Still acceptable trip rates at higher
beam currents?

35

We prepared a scheme to obtain the “shotID” of bad
injection causing beam abort, using the abort
timestamp. It makes the injector diagnosis much easier.

BPAC
reviewer’scomments
in 2020 Feb

We implement faster Diamond abort cycle 2.5 us in 2020
a+b (10 us before) ensuring safer operation. We also plan tosend beam aborts using the plastic scintillators installed on
the surface of QCS cryostat (see CLAWS++ report in MDI
session). Preliminary study indicates the possibility to send
aborts earlier than diamonds.
LER crab waist strength is now LER 80% and HER 40%.
It seems crab waist well suppresses beam-beam blowup,
and we reach higher bunch current.
We added D6V1 in LER and use it as a “primary”(=narrowest) vertical collimator, to protect D2V1 near IP,
which was damaged in the past.
To ease a load on D6V1, we will replace with Carbontype head during the 2020 summer shutdown.

The luminosity BG study on May 9th was conducted
at a high luminosity (L~1.5e34) and matches
simulation very well. We will repeat the studies athigher luminosities.
KEKB review, July 2020

Hiroyuki Nakayama (KEK)36