Issues in SuperKEKB
Phase 2 Operation

Y. Funakoshi for the SuperKEKB commissioning teamAccelerator Laboratory, KEK
2018.10.15@B2GM

Contents• Missions of Phase 2 and achievements• QCS quench issue• High current issues• Detector beam backgroundMissions of Phase 2• Peak luminosity 1 x 1034 cm-2 s-1 (Validation of “nanobeam scheme”)• Squeezing by*• Ohnishi’s talk

• Specific luminosity (beam-beam parameter)• Ohnishi’s talk

• Increasing beam currents• This talk

• Beam background issues• Iida’s talk in BEAST session this afternoon, This talk

• QCS quench issue• This talk

• Tuning and Study of Injector Linac• Furukawa’s talk

QCS quench issues

Frequency of QCS quenches

Y. Ohnishi

2018/7/23 Monday Meeting

History of SuperKEKB Phase 2

by*= 80mm

by*= 8mm

6mm

by*= 4mm

by*= 3mm

5.55 x 1033/cm2/s (by*3mm, LER: 800mA, HER: 780mA, 1576 bunches/beam July 5th)
2.29 x 1033/cm2/s (by*3mm, LER: 270mA, HER: 225mA, 394 bunches/beam July 3rd)

List of QCS quenches (from QCS group)
38coils quenches, 26 events
Date

Time
2018/4/1
2018/4/2
2018/4/9
2018/4/9
2018/4/9
2018/4/9
2018/4/10
2018/4/10
2018/4/11
2018/4/11
2018/4/11
2018/4/11
2018/4/11
2018/4/20

2018/4/21
2018/5/6
2018/5/13
2018/5/17
2018/5/17
2018/5/24

7

Quenched Magnet Beam Line
20:55
QC1LP
LER
19:29
QC1LP
LER
17:31
QC1LE-a1
HER
20:06
QC1LE-a1
HER
20:53
QC1LE-a1
HER
21:40
QC1LE-a1
HER
17:44
QC1LE-a1
HER
21:56
QC1RE-b1
HER
14:21
QC1RE-b1
HER
15:25
Cancel-Mag-b3
HER
18:45
QC1RE-b1
HER
20:23
QC1RE-b1
HER
21:15
QC1RE-b1
HER
14:33
QC1RP
LER
14:33
QC1LP
LER  
14:33
QC1RP-b1
LER
0:21:49
QC1LP
LER
0:21:51
QC1RP
LER  
0:22:13
QC1RP-b1
LER
11:28
QC1LE-b1
HER
2:45
QC1RP-b1
LER
2:09
QC1RP-b1
LER
4:06
QC1RP-b1
LER
17:17
QCSL-Can-b3
HER

CausesInjection Kicer K1, K2 balance
Injection Kicer K1, K2 balance (EVR module)
Trial of by*=2.4mm
Trial of by*=2.4mm
Trial of by*=2.4mm
Trial of by*=2.4mm
Trial of by*=2.4mm (BT V steering tuning)Trial of by*=8mm
Trial of by*=8mm
Trial of by*=8mm
Trial of by*=8mm tune changer
Trial of by*=8mm local bump in downstream of IP
Trial of by*=8mm local bump in downstream of IP
RF Phase scan Mis-operation (big Phase jump)

Injection/strorage
Injection
Injection
Injection
Injection
Injection
Injection
Injection
Injection
Injection
Injection
Storage? (10mA)
Storage (5mA)
Storage (10mA)
Storage (48mA)

unknown (after end of RF phase scan)

Storage (18mA)

Waist knob test (locally large orbit or beta-beat)Beam injection with ECK=-2
by*=6mm K2-3 malfunction?
by*=6mm K2-3 malfunction?
Trial of by*=4mm, v-collimators not enough

Storage (35mA)
Injection
Injection
Injection
Injection

Narrower collimator setting to prevent QCS quench
May 28th Belle abort using diamond sensor was introduced.

Belle 2 beam abort based on
diamond sensors

8

List of QCS quenches (from QCS group)
38coils quenches, 26 events
Date

Time
2018/6/25
2018/7/3
2018/7/9

2018/7/15

2018/7/16

Quenched Magnet Beam Line
QC1RP
11:20
QC1RP-b1
LER
QC1LP
5:14
QC1RP-b1
LER
QC1LE
11:20
QC1LE-b1
HER
QCSL Cancel
QC1RP
LER
QC1LE
 
QC1LE-b1
QCSL Cancel
QC1LE-b1
17:53
HER
QCS Cancel

CausesInjection/storage

!&!$)% &&%! &!%%+ )% ' ('''$%&Storage (728mA))%!%$(Continuous bad injection?

Injection!&!$)% &&%! &!%%+ )% ' ('''$%&Storage(766mA))%!%$( #' "" $%&'&!! &'  %&&* ('''$%&)%!%$( 
Storage (LER: 793mA)#'  '!%%  #' ('''$%&& !&!$)%!%$(Storage (HER: 670mA)• 5 quenches happened after June 25th.
– 4 of them were induced stored beam accompanied with vacuum burst.
– In 2 cases, beam hit vertical collimators and gave some damages.
• The reason why beams hit collimators has not been understood.
– No beam orbit change, no beam oscillation.
– We suspect the dust trapping effect.
9

Locations of QCS quenches

Damage of collimator (LER D02V1)
BottomTop

Beam hit bottom of collimator.
Sputtered material (W) stuck to top?

S. TeruiVacuum burst when collimator wasdamagedBeam Current

Vacuum paressure

Damage of collimator (HER D01V1)

Summary of QCS quench in Phase 2• During Phase 2, QCS quenches happened 26 times. Once QCS
quench happens, it takes about 1.5~2 hours for recovery.
• Initial quenches in Phase 2 were mainly induced by injecting
beams.
– The quenches were almost prevented by setting movable collimators
properly and introducing the Belle 2 abort using diamond sensors.
– We felt that we had overcome the quenches, since we had noquenches for about a month after the quench on May 24th.

• However, on June 25th, the quench happened again by astored LER beam and 4 quenches followed in July.
– The reasons for the QCS quenches have not been understood well. I
suspect the dust events may have something to do with the quenches.

To do list for QCS quench• Install more collimators before Phase 3– 1 vertical collimator (LER)– 3 horizontal collimators (LER), 1 horizontal collimator (HER)• Understanding of mechanism of QCS quench– Ohuchi-san’s simple calculation: If ~8000 electrons (7GeV) lose their
entire energy at a small part of a coil, QCS quench can happen.
– Simulation on the more precise locations of particle loss near QCS.
• Collimator chip scattering, dust trapping…

– Simulation on the effect of continuous particle loss due to someprocesses (ex. Radiative Bhabha process).
– More experiences in early stage of Phase 3– A task force on the QCS quench issues has been established.

• W shields near QCS? (2019?)– Simulation works are in progress.

• Modification of QCS magnet system?

IR
e-

e+

HER
D01 H3, H4, 5
D01 V1HER

LER
D02 H1, H2, H3, H4
D02 V1TSUKUBALERLER
D03 H1

HER
D12 H1, H2, H3, H4
D12 V1, V2, V3, V4

HER
D09 H1, H2, H3, H4
D09 V1, V2, V3, V4

Phase-I

Phase-II

Phase-III: Vertical Collimator (KEKB type): Horizontal Collimator (SuperKEKB type): Vertical Collimator (SuperKEKB type): Horizontal Collimator (SuperKEKB type): Vertical Collimator (SuperKEKB type): Horizontal Collimator (SuperKEKB type): Vertical Collimator (SuperKEKB type)

LER
D06 H1, H3
D06 V2

FUJI
e-

e+
ARES

ARES

ARES
WIGGLER

ARES

SC: Horizontal Collimator (KEKB type)

WIGGLER

OHO

NIKKO

WIGGLERSC

SuperKEKB Main Ring

Additional tungsten(W) shield?

17Additional tungsten(W) shield?

18

LER beam envelop
bx* = 100mm, by* = 4mm

b2 coilse+horizontal : 80 sx
W shieldcollectors

vertical: 105 sy(5% coupling)

Quenches of downstream of IP: induced by horizontal oscillation?
Quenches of upstream of IP: induced by vertical oscillation?

High beam current issues issues

Coupled bunch instability in LER• The LER beam current was limited by the longitudinal
coupled bunch instability.
– It turned out that the source of the instability was not RF
cavities.
– It seemed that the one of the collimator was related to theinstability.
– The nature of the instability should be investigated in moredetails in Phase 3.
– In LER, we have a feedback system to suppress theinstability. But we didn’t have a time to tune the feedback
system.

• A task force which deal with the high beam current
issues has been established.

1372 bunch

1576 bunch
850 mA

When LER beam current exceeded 830mA, alongitudinal coupled bunch instability
started to be observed.
With 4trains the instability was notobserved with the same bunch current. But
with a higher (total) beam current, theinstability is induced again.
With changing D2H4 collimator setting, the23
instability
became stronger.

740 mA

780 mA

Spectrum when longitudinal instability occurred

Mode number = ~ 2300
This is not due to RF cavity.

Longitudinal Mode (2 bucket
spacing)Detector BG issues

Detector beam background issues•••

Iida-san gives a talk in BEAST session in the afternoon.
A task force the detector beam background was established during Phase 2 operation.
Injection BG–•

BG by storage beam–•–

It is unlikely that the “scraping” background is caused by the beam tail.
It may be caused by the off-momentum particles overfocused by QCS magnets. This seems to besupported by Nakayama’s (Touschek) simulation.
I asked Zhou-san (Beam-Beam) and Dima-san (Intra-beam) to do the beam tail simulations. The tails
may give some effects to SR BG.

Other issues
–
–
–

•

by* dependence seemed strong.

Scraping BG––•

The task force members did intensive injection tuning during Phase 2. The BG was lowered
effectively, although a stably good condition did not last for a long time.

BG storm or spikeHigh BG for outer layers of CDC, when we sqeezed by* from 4 to 3mm
SR BG?

Phase 3––New collimators will be installed.
Establishment of top-up (continuous) injection is a key issue at the beginning of Phase 3.

Injection BG••

BG was reduced rather effectively by collimator tuning.
BG was also reduced by the injector and injection tuning.
– Beam energy• An energy FB was introduced during Phase 2.

– Energy spread– LINAC and BT orbit– Optics correction in rings••

BG did not decreased drastically by using RF gun, although some BCG
member think that it had some effects.
Items to be introduced in Phase 3– LINAC beam orbit FB– Beam monitor for energy spread– Monitor for beam energy (RF phase monitor)••

A dedicated study on injection BG was done at the end of Phase 2.
Beast sensors like diamond sensors and CLAWS were essentially importantfor injection BG tuning.

BG by storage beam
• Tuning items when squeezing by*– Collimators– Optics corrections– Injection tuning• BG depends on tunes– We need more systematic study.

• Diamond senser abort– Effective to prevent QCS quench

Beam tail?
At the point (+/- 225mm from IP)
bx = 0.453m (bx* = 0.200m at IP)
ex = 1.73 nm (LER), 4.67 nm (HER)
sx = 28.0 µm (LER), 46.0 µm (HER)
Aperture = 4.9mm, -> 175 sx (LER), 107 sx (HER)
The aperture is too far from the beam!
Collimator Aperture (LER)
D2H3 OUT: 38.7 sx
D2H3 IN : 39.8 sx
D2H4 OUT: 59.3 sx
D2H4 IN : 55.7 sx
D6H3 OUT: 39.0 sx
D6H3 IN : 58.5 sx
It is unlikely that 100 sigma tails cause the background.

D6H4 OUT: 48.7 sx
D6H4 IN : 53.7 sx

Task forces• Several task forces have been established orare being planned.
– Detector beam background issues– Linac BT emittance preservation issues– QCS quench issues– High beam current issues– (Beam-beam issues)

Spare slides

R

8

•

)(
•

1

•
• R
1

8

L

1
e

)(

M

1

a

)-

1
2

1

1B2

3

P1E

8

s

L

1

2
M

1

h


9
•
•
0

m
4

6

U3

0

7

R

3

U3

•

1
1

3
•
•
• (

2
S3E
) 2
2
~20 m

S3
R1
S3
S3

7

3 1 9

5S

3 1

7
7

3 1 9
3 1H

5S
R

3 1

1

~15 m

QCS




L

A

•

aWd

•

R
4

B

2L

aWdL

•

H
•

22C
2( 5

• )

•

4

AH

4 2301
aWd
L
5

r

E
W

•
•

5
@
Lr

am
L
5
L
@ H

5

5
B

5

5
5

L
LA



L

5

aWdL



Comparison of machine parameters
bet ween design and Phase2
parameters

DesignPhase 2units

factorIbeam (LER/HER)

3.6/2.6

0.8/0.78(0.27/0.225)

A

4.5/3.3

xy (LER/HER)

0.0881/0.0807

0.03/0.02

by*

0.27/0.30

3/3 (2/2)# of bunches

2500

1576 (394)

Ibunch (LER/HER)

1.44/1.04

0.508/0.495(0.685/0.571)

mA

2.8/2.1
2.1/1.8

Luminosity

8 x 1035

5.55 x 1033

cm-2 s-1

145

2.9/4.0mm

11/10
1.6(6.3)

2018/5/11 strategy meeting[1] Luminosity of 1 x 1034 cm-2 s-1•

Simple scaling
– 5/9
•
•
•
•
•

Luminosity: 4.7 x 1032
Beam currents: 250mA, 220mA
by* = 8mm
Beam-beam parameter: ~0.014
Number of bunches: 600– Possible parameter set
•
•
•
•
•

Beam currents: 1A, 0.88A (x 4)by* = 3mm (x 8/3)
Beam-beam parameters: ~0.03 (x 2)
Luminosity = (4.7 x 1032 ) x 4 x 8/3 x 2 = 1.0 x 1034
Number of bunches: 1576 (for example)• We need– Squeezing by*– Increasing beam currents– Luminosity tuning to raise the beam-beam parameters

Machine Parameters of SuperKEKB
Phase 2 (July 5th 2018)
Horizontal EmittanceBeam current @Maximum Luminosity
Maximum Beam current in Phase2
Number of bunches
Averaged bunch spacing
Total RF voltage Vc
Synchrotron tune ns
Calculated bunch length sz @zero current
Betatron tune nx / ny
Beta function at IP bx* / by*
Measured vertical beam size (XRM) @IP sy*
Vertical beam-beam parameters xy
Beam lifetime
Luminosity (Belle 2 CsI)

LER
1.64
788
860

1576
1.80

HER
4.54
778
800

8.8
12.8
-0.0226-0.0258
4.64
5.33
44.562/46.614 45.545/43.612
200/3
100/3
1.48
0.610
0.050
0.010
40
65
5.55

nm
mA
mA
m
MV
mm
mm
µm
min.
1033 cm-2 s-1

Phase 3 (2019 March – June)•

Identify what limits the luminosity or machine operation.
–

What happens with squeezing by*?
•–

What limits beam-beam parameter?
•––

Longitudinal coupled bunch instability…
Effects of electron cloud…

Understanding Belle 2 beam background and how to suppress it?
•

With SVD, Pixel detector•••

Establishment of continuous injection
Collimator tuning
Injector and injection tuning

QCS quench•••

IP Chromatics coupling…?

What limits beam current?
••–

Lifetime decrease?, bad injection efficiency?, QCS quench?

Mechanism of QCS quench
LER vertical collimator tuning

Physics Run– Next week we will discuss with Belle 2 group a guide line of physics run (how muchluminosity the accelerator group assure to them) in the first year of Phase 3.

•

We need to set target parameters––

Beam currentex.1.5A (LER), 1.2A (HER)
Luminosity: ex. 2 x 1034 cm-2 s-1

Efforts to prevent QCS quench••

Countermeasure meetings were held several times.
Narrower collimator setting from the viewpoint of QCS quench protection(April 11th)– Our feeling is that HER QCS is well protected by collimators but we need morevertical collimators in LER. Vertical collimator setting was not enough, whenthe quench occurred on May 24th in HER.

•

Belle 2 diamond sensor beam abort was introduced (May 28th).
– Our feeling is that this abort system helps to prevent QCS quenches.

••Continuous efforts to improve beam injection (to reduce Belle 2 BG)
Others– Move loss monitors to the place where the betatron phase is same as QC1s
and the beta function is large.
– A fiber loss monitor was installed in upstream of QCSL in LER.
– I ask Belle 2 group that the 40 scintillators on QCS are available for monitoring
beam loss at QC1s.
– More steps in setting local orbit bumps or luminosity tuning knobs• Synchronized magnet setting system will be introduced shortly.

– Careful operation in RF phase scanFurther countermeasures for QCS quench•

New collimators before Phase 3– LER: 1 new vertical collimator, 3 new horizontal collimators– HER: 1 new horizontal collimator•

Installation of heavy metal (W) shields was proposed by Ohuchi-san.
– We are estimating their effectiveness. More realistic beam loss scenario isneeded. If needed, we will perform some machine study in Phase 2.

••

More simulations are needed to simulate effects of ``chip scattering” ofcollimators.
Are there any alternatives of QC1 dipole corrector coils?
– It seems that luminosity performance is degraded, if we use other correctors
instead of QC1 dipoles.

•

Remodeling QC1 magnets?
– We should consider it as a part of a long-term upgrade plan of SuperKEKB.

•QCS quench due to continuous beam loss?
– We started estimation.

LER beam envelop
bx* = 100mm, by* = 4mm
QC2RP QC1RP

QC1LP
b2 coilse+horizontal : 80 sx
W shieldvertical: 105 sy(5% coupling)collectors

QC2LP

"(LER)
BG 
!$&&" ""
#%&
$&&

2mm2mm
 QCS Finally t wo crab cavities were installed in KEKB,
one for each ring in January 2007

HER (e-, 8 GeV)LER (e+, 3.5 GeV)…..after 13 years’ R&D from 1994

Troubles• 11:17:19 HER/LER Abort (766 mA/487 mA)•   
• BOR 

()HER current (Pintriggerabort)

LER current
Ion chambertriggerabort)~50 µsTroubles• 11:17:19 HER/LER Abort (766 mA/487 mA)• D01_V1'&) ")
(*1E-6 Pa)• D01_H4'&) ((") • $!#)% 

Troubles• 11:17:19
HER/LER Abort (766 mA/487 mA)• QCS/ 5OKM&:> (7860$241X• BT=*+24A9160⇒IVNRWM:)0•
>0• 6/25 =BPWL+QCSFDVJ=#!;91@000•'• 6/25>/11:20:30,•"2.3/Linac A3ESTUW="2491@HCQVGXX(-%)• 674/=HCQVG:><10?6/ >LERH. Nakayama
Phase2.1

105sy w/
5% coupling
by*=4mm

e+

e-

e+

e-

For phase3,
nsigma_x(LER) = 25, nsigma_x(HER) = 16,
nsigma_y(LER) = 45, nsigma_y(HER) = 25,

In case of LER vertical, aperture at QC1 is narrowest.
In other cases, edges of vacuum chambers are narrowest.

Ohuchi-san’s estimation
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

6
QCS+"!OSQ]U;>MGF,E7 GeV5 G O-%<I<@GC0&<I=7:G-%O)A@GHGFELI=7+"!G.'WZX_Ta
a0.35 ∩
CuaNbaTib0.5a0.25a0.25
4;10mmG.'F5 9*$<B5 GQV[R_`GeV9F.'F8KNMD<I=7
SC\PY_G#b0.962mm36SC\PY_GCuG23b4.31 X 10-3 g6NbTiG23b3.16 X 10-3 g
CuGb0.1 J/kg^K6NbTiGb0.87 J/kg^K @4K
JL6.'G3b3.18 X 10-6 J/K
5 1 GQV[R_`GeVb7 X 109 X 1.6 X 10-19 JouleJLO1;>MGF,E5 G H
2696 DELI=7
1/CH63GCSQ]U=MD(8MD?G H3 DEL8087 C=7