Beam background during SuperKEKB 2019 runs

SuperKEKB workshop

Jan. 31st, 2019

Hiroyuki NAKAYAMA (KEK)

Introduction

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Beam background

• Beam-induced background at SuperKEKB accelerator can be dangerous for Belle II detector

• Beam BG determines survival time of Belle II sensor components and might lead to severe instantaneous damage

• It also increases sensor occupancy and irreducible analysis BG

SuperKEKB Beam BG sources

• Single-beam BG: Touschek, beam-gas Coulomb/Brems, synchrotron radiation, injection BG

• Luminosity BG: Radiative Bhabha, two-photon BG, etc..

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How to cope with beam BG?

Movable collimators• Arc collimators and horizontal

collimators near IP is essentials

• Very narrow (d~2mm) vertical

collimators

Shielding structures • Thick tungsten structures inside final focus

cryostat and vertex detector volume

• Stops showers from beam loss

“hot spot”, at ~1m upstream from IP

• Polyethylene shield to reduce neutronstungsten

QC2RP

QC2RE

QC1RP

QC1RE

tungsten layer(15~70mm t)

e-

e+

“hot spot”

SuperKEKB horizontal collimator

Final focus magnet (QCS) cryostat, R-side

Tungsten head

Tapered beam pipe

Ramp~12°

Coolingchannels

Bellows

SuperKEKB Collimators

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LER(10):- 7 horizontal, 3 vertical “SuperKEKB type”

collimators- horizontal: D06H1, D06H3, D03H1

D02H1, D02H2, D02H3, D02H4- vertical: D06V2, D02V1, D06V1

HER(20):- 3 horizontal, 1 vertical “SuperKEKB type”

collimators- horizontal: D01H3, D01H4, D1H5- vertical: D01V1

- 8 horizontal, 8 vertical “KEKB type” collimators- horizontal:

D12{H1,H2,H3,H4},D09{H1,H2,H3,H4}- vertical: D12{V1, V2, V3,

V4},D09{V1,V2,V3,V4}

30 movable collimators installed

LER D06V1 was added in 2019-2020 winter shutdown to further reduce Beam-gas Coulomb BG

As of 2020 spring,

HERelectron (7GeV)

LERpositron (4GeV)

KL and muon detector (KLM)Resistive Plate Counter (barrel)Scintillator + WLSF + MPPC (end-caps)

Particle Identification (TOP,ARICH)Time-of-Propagation counter (barrel)Prox. focusing Aerogel RICH (fwd-endcap)

Central Drift Chamber (CDC)He(50%):C2H6(50%), Small cells, long lever arm, fast electronics

EM Calorimeter (ECL)Belle1 CsI(Tl) crystals + new waveform sampling

Vertex Detectors (PXD,SVD)2 layers DEPFET + 4 layers DSSD(Layer2 DEPFET partially installed)

Beryllium beam pipe2cm diameter

Reminder: Belle II sub-detectors

Beam background in 2019

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Latest BG situation

• BG rates at the end of 2019 (Dec. 8th, 2019)• beta_y*=1mm, 783bunch, 420+290mA, L~1.0*1034 on

(HER max current limited by poor injection at that time)

• Bottleneck: CDC and TOP• CDC HV leak currents: ~50uA (limit: 100uA)

• TOP PMT average rate: ~1.1MHz (limit: 2MHz until 2022)

• Remaining headroom: x2 until 2022

• Compared to 2019 spring run, beam BG has improved by factor of 2~3• Ring-total loss rate increases as beta_y* squeezed(3mm→1mm)

• So as IR loss rate, if no countermeasure is taken

• Tighter collimators settings can be used thanks to more stable injections ☺• More stable injection with smaller BT emittance growth

• Vacuum scrubbing progress ☺• collimator baking during summer shutdown also helped a lot 8

* Large CDC HV leak current causes HV trips and DAQ deadtime

* short-life TOP PMT photocathode should survive until replacement in 2021

and later 1MHz

and later x1

BG source point: LER FWD bellows

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• By reconstructing two random CDC tracks in data, we see a “hot spot” at z=~60cm around LER bellows,

• We believe this BG is caused by incoming secondary shower generated at z=1.1m inside QC1RP, where we have significant beam loss due to LER beam-gas.

• Showers are mainly suppressed by thick tungsten shields inside QCS cryostat, but leaks from z~60cm, where no shielding material exists

• A new shield around the LER bellows are being designed, although the space is quite limited….

“Hot spot” at z=60cm

Significant beam loss at z=1.1m are suppressed by tungsten shield

“Hot spot” at z=60cm

2019b

Dominant source: LER Beam-gas BG

• Touschek/Beam-gas BG can be measured separately by dedicated machine studies

• Data/MC ratio for each BG source can be obtained

• BG extrapolation for future machine parameters can be performed by scaling simulated future BG rates with the latest data/MC ratio

Details of machine studies in Sven’s talk• Recipe of machine study

• Measurements in 2019 runs

• Extrapolation for future optics

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CD

C c

urr

ent

[uA

]

0

2

4

6

8

10

12

0 0.5 1 1.5 2

Beam-gas

Touschek

LER single-beam study I=250mA, beta_y* = 1mm

Touschek scaling

TOP BG on Dec. 8th, 2019

Issues during 2019 operations

• LER injection BG causes CDC HV trips

• Long duration of LER injection BG

• LER injection spikes on vert. collimators and beam aborts

• Soft photon increase in PXD sensors

• Damage on collimator head

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Issues: LER injection BG on CDC leak current

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- Special BG studies (Dec. 3rd and 6th) with external analog meters- LER injections give large impact (HER is quiet)- LER 22Hz injections give larger impact than 12.5Hz injection- Even firing injection kicker only (no injection charge) gives

some impact !

Fast monitoring of CDC BG is important. For 2020 runs, we will prepare:- current meters with faster readout- 1Hz real-time PV calculated from CDC wire rates

- Large CDC leak current cause HV trips (DAQ deadtime), which limits maximum beam currents for physic runs

- CDC leak current values are updated very slowly(~0.1Hz), due to the bottleneck of CDC PS hardware readout. No direct measurement is currently available for injection BG component on CDC leak current

CDC HV trips in 2019 autumn and spring runs

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autumn spring

Blue short spikes at the bottom: CDC HV trip events

LER/HER current

CDC currents

- CDC HV trips during 2019c was much less frequent than 2019b- CDC leak currents are smaller, since we are running at smaller beam currents due to shorter

beam life and poor injections- Difficult to judge whether injection BG impact on CDC has improved or not - Need to see carefully after we increase beam currents in 2020a

- 2-bunch injection started only from Dec. 9, need more optimization

Issues: Duration of LER injection BG

14

T. Koga

LER: 6ms + 14ms (3.2μs per 10μs)

→ ~13% deadtime @ 12.5Hz injection(lose integrated luminosity)

“Full veto” “Gated veto”

- Not only injection BG amount seen by diamonds etc., injection BG durationshould also be suppressed

- Quick and easy-to-understand feedback to machine operator is important(oscilloscope view, epics PV for logging, FFT, etc…)

- 2-bunch injections?

Issues: LER injection loss on D2V1/D6V2

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• Why constant injection loss on D02V1/D06V2? • vertical oscillation due to X-Y coupling?

• Headache: (infrequent) large injection spikes cause loss monitor beam aborts

• If such vertical loss can be suppressed, we could further close these collimators and reduce beam-gas BG.

• tip-scattering from D2V1 is not yet seen

Issues: damage on LER D06V2 head

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• Damage on D06V2 found during winter shutdown• Caused by the beam-dust event at Dec. 11th? • Newly installed D06V1 can mitigate the load on D06V2/D02V1?• Or, we need to install low-Z collimator?

→ More details in Ishibashi-san’s talk

Issues: PXD SR increase in Dec. 2019

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• Just after we change HER beta*_x from 80mm to 60mm, PXD sensors on –X side start to see significant SR increase

• Similar SR increase in June had disappeared by HER tune adjustment, but it didn’t help this time.

• Correlated with injections. Why?• We rotated HER horizontal orbit

angle clockwise, assuming the SR hit downstream +X side first and then back-scattered to –X side (magenta arrows in left picture) and SR reduced by 1/2

• Then appears another back-scattering from further downstream (light blue arrows), from the storage beam

• PXD total dose increased by x2

w/o injectionswith injections

Prospect for 2020 runs

Improvement• Newly added D06V1 will reduce LER beam-gas BG by 2.5 (simulation)

• should be confirmed by the machine study at early 2020

• Vacuum scrubbing progress will also reduce beam-gas BG• Crab-waist might reduce beam tails and BG?

Issues• Injection related issues: most important for stable operation in 2020

• CDC HV trip by LER injections (DAQ deadtime)• Long LER injection BG duration (DAQ deadtime)• LER injection loss on vertical collimators (beam aborts)• Stable/clean 2-bunch injection

• Another collimator damage by beam-dust event?• PXD SR increase again, when HER moves to smaller beta_x*?

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→ see Andrii’s talk for details

Needs dedicated machine studies

Possible issues for long-term operation

• HER current can be kept as required? • RF gun charge decrease seen in 2019 autumn run

• Photocathode replaced during winter shutdown (Ir2Ce→Ir7Ce2)

• Charge-recovery work (“activation”) will need few days

• Hourly limit of integrated Linac charge (radiation safety)• Reached the limit in late 2019 and injection stopped until next hour

• Poor HER injection efficiency (40~50%) should be improved

• TMC instability due to LER vertical collimators?• 3 vertical collimators in LER

• beta_y*=1mm is OK, but we will see a problem at smaller beta_y* optics

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Mechanical R&D efforts to cope with background

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• Low-Z head can be used for LER vertical collimator at D6 section(far upstream IP).• Particles losing >2% energy are lost before IP• Aiming for install in 2020 fall/winter• Activity lead by SKB vacuum group

• Additional shield around QCS bellows• Although quite challenging to find space, serious consideration ongoing• Aiming for install together with VXD2021• Activity lead by VXD mech group

Katsuro Nakamura S. Terui

→ See Nakamura’s talk in the next session→ See Ishibashi’s talk in the later session

Summary

• Beam background endangers detector components and give a limit on maximum beam currents for luminosity runs

• Significant BG improvement achieved in 2019 autumn run• Tighter collimator settings thanks to stable injection

• To reach higher luminosity in 2020, effort to achieve further stable injection is critically important

• Still bumpy road toward the design optics• BG extrapolation in the next talk

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Backup

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Impact of D6V1

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SAD beam-loss simulation (Andrii Natochii)D06V2(1.6/1.8mm)

D02V1(1.4/1.4mm)

D06V2(1.6/1.8mm)

D02V1(1.4/1.4mm)

D06V1(2.6/2.6mm)

IR loss = 87MHz

IR loss = 32MHz

w/o D6V1

w/ D6V1

Apply CCG pressures

9MHz

Scattering positions of beam-gas Coulomb IR loss

Loss from upstream of D6V1 are well

suppressed

12MHz 17MHz

x103 [Hz]

2MHz

Assume 1nTorr uniform pressure

x103 [Hz]x1/2.5

TMC instability

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beta_y d Nsigma k beta*k

[m] [mm] [e+15 V/C/m] [e+15 V/C]D06V1 61.4 2.6 72 0.30 18.3

D06V2 19.2 1.6 80 0.62 11.9D02V1 14.6 1.4 80 0.75 11.0

LER, beta_y*=1.0mm

Kick factor by GdfidL(k=0.442 at d=2mm, k∝d^-1.5)

sum(beta*k) = 41.2Ib_thresh = 1.65 mA/bunch

beta_y d Nsigma k beta*k[m] [mm] [e+15 V/C/m] [e+15 V/C]

D06V1 61.4 2.5 70 0.32 19.4

D06V2 19.2 1.4 70 0.75 14.5

D02V1 40? 2.05? 71 0.43 17.0

LER, beta_y*=0.5mm

beta_y d Nsigma k beta*k

[m] [mm] [e+15 V/C/m] [e+15 V/C]

D06V1 61.4 1.85 52 0.50 30.5D06V2 19.2 1.05 52 1.16 22.3

D02V1 111.6 2.50 52 0.32 35.3

LER, beta_y*=0.27mm (design)

QC1 780 13.5 105

QC1 1560 13.5 75

QC1 2890 13.5 55

Assume ey=21pm (1% coupling)

Cf.0.5A/783 (2019c)= 0.64mA/bunch

2.5A/2500 (“2A”)= 1.00mA/bunch

3.6A/2500 (design)= 1.44mA/bunch

sum(beta*k) = 50.9Ib_thresh = 1.34 mA/bunch

sum(beta*k) = 88.1Ib_thresh = 0.77 mA/bunch

D06V1/D2V1 only

sum=65.81.04mA/bunch

At design optics, we might need to give up using D6V2

☺

☺

Luminosity record runs in the final week

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HER current can be kept as required?

Date Dec. 4, 23:00 Dec. 7, 08:00 Dec. 11, 01:00

Lumi 1.0e34 1.1e34 1.0e34

Nbunch 783 783 1565

LER current 440mA 500mA 600mA

HER current 310mA 360mA 420mA

HER life 22min 16min 28min

beam loss ratedue to beam life

-0.25mA/s -0.40mA/s -0.25mA/s

HER inj. Charge 0.4nC 0.4nC 0.4nC

HER inj. rep 12.5Hz 25Hz 25Hz

HER inj. eff (at I=0mA) 40~60% ~60% ~40%

Current increase rateby injection

0.20~0.30mA/s 0.60mA/s 0.40mA/s

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With larger Nbunch, we need higher beam currents to achieve same luminosity and hence encounter higher BG. Beam life is mainly determined by Touschek, so it gets longer thanks to smaller bunch current. The total beam loss rate will be more or less similarand required injection rate is also similar.

In that sense, we prefer smaller Nbunch for the same beam current. However, specific luminosity starts to get smaller at higher bunch current. In that region, optimal Nbunch should be carefully decided.

Even with 25Hz injection, HER=~500mA will be a limit with Nb=783

HER max current is limited with 12.5Hz injection We can reach higher HER

currents with Nb=1565, but BG level gets higher

2-bunch injection? Need more tuning on injection BG

Large loss even after manual beam aborts

2019 spring 2019 autumn

From 2019 autumn run, we start to see significant loss on collimators/diamonds even after *manual* beam aborts. Loss on collimators are so large to issue loss monitor aborts. This is probably due to the new fill pattern with two abort gaps.

This will be a critical problem at higher beam currents.

Number of loss monitor aborts after manual beam aborts with I>60mA

TOP BG in 2019 autumn and spring run

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N.Tsuzuki(Nagoya)

TOP BG rates in Dec. 2019 cf. TOP BG rates in May 2019

Tighter collimator settings and better vacuum pressure improve BG rates (x2), even with smaller beta_y* (more ring-total loss)

Luminosity BG seen by TOP

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N.Tsuzuki(Nagoya)

(BGtotal – BGsingle-beam) vs. Lumi plot shows positive non-zero slope(p1), which corresponds to Lumi-BG term (~0.18MHz at L=1e34)

Measured lumi-BG rates (upper, scaled to full luminosity) seems consistent with 17th BG MC rates (lower)

Lumi-BG is dominant in MC

data/MC~1 for TOP lumi-BG

Luminosity BG seen by SVD

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H. Tanigawa

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Improvement: Faster beam abort delivery

Abort delivery time

21~39us (2019b) → 17~30us (2019c)

Already faster by 4~9us• Two beam abort gaps (by 0~5us)

• Improved sync with abort gap timing (by 4us)

• QCS PS failure trigger is much faster now

More to come• 400kHz diamond sampling (by 0~7.5us)

• Firmware being tested at Trieste

• Apply tighter loss monitor threshold (by few us?)• Injection veto already demonstrated on test using LM at LER D06V2

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2019c

30us

All diamond aborts during 2019c emptied the rings within ~30us

Important to protect QCS/collimator/Belle II

LER D02 pressures (Nov. 4th vs. June 25th)

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Nov June

D02_L12(D02H1 collimator) → significantly improved (collimator baking during shutdown)

D02_L18(D02V1 collimator), D02_L25(nearest to IP) → increased (vacuum work during shutdown)

2. BG studies in 2019c run

a. Single-beam BG studies • beta_y*=2.0mm (Nov. 4th, 9th, 11th), 1.2mm(Nov.14th),

• beta_y*=1.0mm (Nov. 28th, Dec. 7th)

• Touschek/Beam-gas composition and comparison with BG MC (data/MC ratio for scaling future MC)

b. Lumi-BG study• L=0.8e34 (Dec. 8th)

• Vary luminosity by changing vertical beam displacement to lumi-BG component

• Need careful subtraction of single-beam BG components

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a. LER single-beam BG study on Dec. 7th, 2019

LER current

BG rates

789 bunch gdl trig.

1565 bunch gdl trig.

393 bunch gdl trig.

- Vary N_bunch:789→1565→393 - beam size scan is not used (avoid unexpected BG increase due to possible scraping)

LER single-beam BG study on Dec. 7th, 2019

~15% of Belle II LER storage BG is from Touschek(37% for SVD)

64% of LER life is from Touschek

Due to a hot spot at z=+20cm?

HER single-beam BG study on Dec. 7th, 2019

Magnet PS failure at 00:37BG increased by x2

789 bunch gdl trig.

1565 bunch gdl trig.

393 bunch gdl trig.

D08 pressure increase

HER single-beam BG study on Dec. 7th, 2019

5~15% of Belle II HER storage BG is from Touschek(22% for SVD)(26% for CDC)

77% of HER life is from Touschek

10~20% of Belle II storage BG is from HER(29% for PXD)

SR shown as B-component?

SR in HER?

b. Lumi-BG study on Dec. 8th, 2019

- Nb=783, LER/HER 420/300mA, 11.5/25Hz injection, 1x80mm/1x60mm optics - Luminosity is changed from L=0.8→0.5→ 0.25→0 using ibump vertical offset- Beam sizes are adjusted to ‘almost’ follow L=0.8 values using LER ECK and/or HER YaECK

- SVD/PXD occupancy seems to decrease as L gets smaller, at least by eye- Outer detectors also seems slightly decreased?- Need further careful investigation to subtract single-beam components