Electron Cloud at SuperKEKB

Hitoshi Fukuma, Kazuhito Ohmi, Yusuke Suetsugu, Makoto Tobiyama, KEK

58th ICFA Advanced Beam Dynamics Workshop on High Luminosity Circular e+e-Colliders, 24-27 October 2016 (eeFACT2016)

Cockcroft Institute at Daresbury Laboratory, UK

TUT3AH6

Measures considered at the design stage of SuperKEKB

•Electron density near the beam less than 1.0×1011 m-3 is expected.

•Simulation predicts the threshold electron density of instabilities of 2.2x1011 m-3.

Y. Suetsugu et al., , JVST A, 34 021605(2016)

Measures against the electron cloud in SuperKEKB

Aluminum beam pipe with grooves Clearing electrode

Inside view

Vacuum components related to the EC mitigation in SuperKEKB

Energy (GeV) 4Circumference(m) 3016Max. Beam current (mA) 870Max. Number of bunches 2363Single bunch current (mA) 1.44Min. bunch separation(ns) 4Bunch length (mm) 6RF frequency (MHz) 508.887Harmonic number (h) 5120Betatron tune(H/V) 45,54/43.56Synchrotron tune 0.018T. rad. damping time (ms) 43L. rad. damping time (ms) 22x-y coupling (%) 0.28Natural emittance (nm) 4.6

Phase-1 operation (Feb. –Jun. 2016)

•Without Belle-II detector and superconducting final focus quads

•Vacuum scrubbing (360-720 Ah)

•Optics tuning to achieve small emittance beam

•Background study with Beast detectors

Purpose

•No collision

EC related measurement at phase 1 Parameters in LER

Measures against the EC taken at the beginning of Phase 1 operation

•TiN coating on Aluminum surface

•Grooved surface in dipoles

•Clearing electrode in wiggler sections

•Ante-chamber

•No solenoid windings

Definition of fill pattern

XXX/YYY/ZZZ represents the number of train/bunches per train/bunch spacing in RF bucket.

For example, 4/150/4

Beam current [mA]

Pre

ssur

e [P

a]

•Vertical beam size blowup was observed at LER by an x-ray beam size monitor.

•At the same time, the pressures at whole LER ring showed a nonlinear behavior against the beam current above ~500 mA.

•The behavior was quite similar to that of electron currents measured at aluminum region without TiN coating.

•Aluminum bellows-chambers without TiN coating were suspected to be the source of the pressure rise.

The bellows-chamber has a length of 0.2 m and located every 3 m on average. They cover ~5 % of the ring in length.

EC related events

electron currentpressurebeam size

•Permanent magnets, which produce axial magnetic field, were installed at most bellows-chambers (~800 pieces).

The rate of pressure rise was relaxed in whole ring. Threshold current of the beam size blowup was increased.

SS

S

NN

N

S

S

N

N

Permanent magnets

•However, the nonlinear pressure rise and the beam size blowup were still seen at high beam current(1/1576/3.06).

Maximum axial field strength is about 60G on beam axis.

chamber surface

beam axis

pressure

beam size

Scaling of the blowup threshold in KEKB

•Threshold current is proportional to the linear current density (bunch current/bunch spacing in RF bucket).

Measurement of blowup threshold

Before permanent magnet installation

•Scaling of the threshold still holds in SuperKEKB.

•Threshold of the blowup increases by 1.7 after installation of permanent magnets at aluminum bellows-chambers.

After permanent magnet installation

SuperKEKB

Blowup in a long train after the installation of permanent magnets

•4 buckets spacing : blowup is not seen up to 1A.•3 buckets spacing : blowup is seen above 0.75A.

•Beam size seems slowly increased with the beam current.

4 spacing long train 3 spacing long train

θ

Ne

r

•The RFA has a retarding grid which repels electrons to select the electron's energy.

K. Kanazawa

[Al_Ante]

[Al_Ante+TiN]

Vb

Measurement of electron density RFA

•The retarding voltage determines the area near the beam where the monitored electrons are located.

•One RFA was installed at a bare aluminum chamber, while the other RFA at a TiN coated chamber.

•The electron density was measured by retarding field analyzers (RFA).

Simulated threshold electron density of single bunch instability

analytic formula

•The threshold density by an analytic estimate is constant if ωeσz/c>Q=6, while it increases with increasing bunch current in simulation.

€

ρe,th =2γν sωeσ z /c3KQr0βL

€

Q = min 6, ω eσ z /c( )

€

K =ω eσ z /c

Simulation (PEHTS)

Electron density and blowup threshold

◊Before installation of permanent magnetsBare aluminum ante-chamber

threshold electron density by the simulation blowup threshold

Plotted "threshold electron density by simulation" is 20 x simulated threshold.

Circumference / total bellows length = 3000m/160m = 20

•Electron density at a blowup threshold is consistent with the simulation (3, 4, 6 bucket spacing).

•For 6 bucket spacing, threshold increased. Probably due to higher bunch current as suggested by the simulation.

◊After installation of permanent magnets

TiN coated aluminum ante-chamber

threshold electron density by simulation

•Electron density at a blowup threshold is consistent with the simulation (2 and 3 bucket spacing).

Plotted "threshold electron density by simulation" are simulated thresholds assuming that the main EC is located at TiN coated chambers.

blowup threshold

Maximum secondary emission yield (SEY) δmax of TiN coated surface

◊Laboratory measurements

(electron energy 250eV)

•Estimated electron dose (Eelectron>500 eV) in phase 1 is 5 10-4 C mm-2 (pointed by arrows).

•At this dose, δmax in lab. measurement is 0.9 to 1.2.

Maximum secondary emission yield (SEY)

(KEKB vacuum group)

◊EC buildup simulation (by CLOUDLAND)

t(sec)

elec

tron

dens

ity n

ear b

eam

(m-3

)

δmax=1.0

δmax=1.2

δmax=1.4

δmax=1.5

EC buildup

δmax=1.3

SEY(δmax)

elec

tron

dens

ity n

ear b

eam

(m-3

)

+ : peak densityx : average density per train

measurement

•Simulation suggests δmax of 1.4.

e+/bunch Nb : 3.13x1010 (0.5mA/bunch)δmax@300eVrepeller Vr : -500V

Fill pattern: 1/150/3

•Possible reason of high maximum SEY in actual machine

◊Insufficiently conditioned place such as far downstream of bends and/or inside bends

◊High SEY at non-coated part

◊Better conditions of materials at laboratory

Pressure, baking of sample

•Further investigation is required in phase 2 operation.

€

Δνy =ρereβykγ

C = 0.005

€

k =

€

2 for Δνx = Δνy

€

1 for Δνx = 0

€

ρe = 8 ×1011m−3

€

ρe = 4 ×1011m−3

€

3.5 ×1011

•Tune shift is consistent with the measured electron density if the EC is flat.

Tune shift along a train

4/150/3

•Tune shift was measured by the bunch by bunch feedback system.

after installation of permanent magnets

•Bunches oscillate by coupled bunch motion mediated by the EC.

•Sideband spectrum of bunch oscillation reflects motion of the EC.

Electrons in a drift : short range wake ~10ns.Electrons in a solenoid : slow rotation around chamber surface (magnetron motion).

horizontal

without solenoid ("drift mode") with solenoid ("solenoid mode")

horizontal

Vertical spectrum is similar to horizontal.

KEKB

400 f01000 f0

Coupled bunch instability

2 bucket spacing, 300mA 4 bucket spacing, 350mA

2 bucket spacing, 300mA

Vertical sideband spectrum in SuperKEKB

before installation of permanent magnets

after installation of permanent magnets

•Vertical sideband spectrum changed after installation of the permanent magnets.

solenoid mode

drift mode

4 bucket spacing, 350mA

Remaining EC•The blowup and the nonlinear pressure rise are still observed in the fill pattern 1/1576/3.06 (for vacuum scrabbing) after installation of the permanent magnets.

•The installation of the permanent magnets in a long straight section improved pressure in that region.

4/150/2 Vertical 300 mA 4/150/2 Vertical 400 mA

•The drift mode in sideband spectrum still appeares at high current after installation of the permanent magnets for 2 bucket spacing where electron density is larger than other fill patterns.

•The above facts suggest the EC still remains in drift regions.

Plan toward phase 2

•Beam size blowup

At the end of phase 1, blowup started at Ib/sb=~0.2mA/rf bucket (Ib: bunch current, sb: bunch spacing in RF bucket).

€

Itot,th = Ib,thNb ≈ Ib,thhsb

= h Ibsb

⎛

⎝ ⎜

⎞

⎠ ⎟ th

= 5120 × 0.2mA =1024mA

•Phase 2 tentative parameters (with final focus quads, Belle II)

Target luminosity: 1 x 1034 cm-2 s-1

Beam current: 1 / 0.8 A (LER/HER)

(Y. Funakoshi, 2015 SuperKEKB Review)

Total current at threshold

The threshold is marginal for the phase 2 operation.

Target values of IP beta’s: βx*: x4, βy*: x8 of design

•Coupled bunch instability (CBI)

•Budget has been requested to install permanent magnets in most of drift sections (mainly arc sections) until phase 2 operation.

◊Fill pattern of 4 RF buckets spacing fits this condition since range of the wake is ~10ns.

◊The maximum growth rate of the CBI due to the EC is proportional to the bunch current if the wake affects only next bunch.

◊Growth time of the CBI at phase 1 was 0.8ms to 1ms at the bunch current of 1mA in the fill pattern of 4 trains, 150 bunches in a train and bunch separation of 4 RF buckets.

◊The damping time of bunch feedback system was about 0.5 ms near 1A.

◊If the bunch spacing is 4 RF buckets, the present feedback system would suppress the CBI at phase 2 where the maximum bunch current will be around 1mA.

S. S. Win et al., PRST-AB 8, 094401 (2005)

€

Ωm −ωβ( )L /c =Nprec2γωβ

W1 −Lsp( ) × exp 2πi m +νβ

M⎛

⎝ ⎜

⎞

⎠ ⎟ A. W. Chao's textbook

(Exercise 4.12)

Summary

•Many measures to reduce the EC, such as ante-chamber, TiN coating, grooved surface and clearing electrode have been applied in SuperKEKB.

•Permanent magnets at bellows chambers were very effective in reducing the EC.

•Measurement in phase 1 operation shows that no blowup is observed up to 1A in 4 bucket spacing without solenoid windings.

•Measurements of the electron density and the CBI sideband spectrum suggest the EC still remains in drift regions when the current increases further.

•Installation of permanent magnets in drift regions before phase 2 is proposed.

•EC effects in high beta sections will be studied in phase 2.

•In case of no solenoid windings, the threshold linear current density of the blowup was increased from 0.04mA/RF bucket in KEKB to 0.17mA/RF bucket in SuperKEKB owing to the measures mentioned above.

backup

4 spacing long train (1/1200/4)

◊Long train (2016/6/27)

•No blowup up to 1A was observed, which is consistent with the simulation.

before installation of permanent magnets

horizontal verticalGrowth time

axial field origin

400 f0

1000 f0

400 f0

after installation of permanent magnets

"drift mode"

"drift mode"

700 f0

•Growth time is larger than 0.8ms after installation of permanent magnets at 600mA (4/150/4).

4/150/2,3,4

EC at high beta sections

•Cloud densities were estimated in two cases(Y. Suetsugu).

Case 1; low density in high β section (green in Fig.)

Case 2; high density in high β section (cyan in Fig.)

Case2 + strengthened solenoid (factor 1/2), groove (factor 1/5) in high β section

Baseline mitigation methods

cloud density

•The EC in high beta function region might give strong impact on the blowup because of the large kick to the beam by the EC.

Case 1: low density in high β Case 2: high density in high β

ρe,th=6xdesign ρe,th=4xdesign

•"Design" means the electron density distribution in Fig.

•The EC in high beta region lowers the threshold electron density by ~70%.

•Threshold density was calculated by a simulation.

K. Ohmi, ARC2014

Electron cloud in QC1RP

•It was pointed out that if photon scattering is considered, the number of synchrotron light photon incident on the QC1RP chamber is 30 times larger compared with the case of no photon scattering (J. A. Crittenden, IPAC2015 TUPTY080, 081) .

"Cloud densities averaged over the transversebeam cross section at bunch arrival time to be a factor of 4 lower than the beam pipe averages."

βy=~3000m at QC1RP

Length of QC1RP 0.33m 1.3x1013 m-3electron density at beam

•Since vertical beta function is large (~3000m), EC in the quad may be a source of blowup.

The QC1RP chamber is TiN coated.