Electron cloud studies for the HL-LHC: where do we stand and the next steps

Electron cloud studies for the HL-LHC: where do we stand and the next steps

G. Iadarola, G. Rumolo

11th HiLumi WP2 Task 2.4 meeting - April 30, 2014

Many thanks to:H. Bartosik, R. De Maria, O. Dominguez, L. Mether, R. Tomas

Outline

• Studies for the arc main magnets

o Scaling with bunch intensity

o 200 MHz option

o 8b+4e scheme

o “Doublet” scrubbing beam

• Studies for the inner triplets

o New triplets in IP1/5

o Present triplets in IP1/8

• Studies for the matching sections – preliminary considerations

Outline



o 200 MHz option

o 8b+4e scheme






Bunch intensity dependence for the arc main magnets

0 0.5 1 1.5 2 2.50

10

20

30

40

50

60

Intensity [x 1e11 ppb]H

ea

t lo

ad [

W/m

]

quad_2x2808b_heatload_vs_int_lin

0 0.5 1 1.5 2 2.50

5

10

15

20

25

Intensity [x 1e11 ppb]

He

at

load

[W

/m]

dip_2x2808b_heatload_vs_int_lin

0 1 2 310

-6

10-4

10-2

100

102


Scr

ubb

ing

dos

e (

50e

V)

[mA

/m]

quad_simulated_beam_scrubdose_vs_int_log_legend

sey = 1.00sey = 1.20sey = 1.40sey = 1.60sey = 1.80sey = 2.00

Dipole Quadrupole


0 0.5 1 1.5 2 2.50

10

20

30

40

50

60


ea

t lo

ad [

W/m

]


0 0.5 1 1.5 2 2.50

5

10

15

20

25


He

at

load

[W

/m]


0 1 2 310

-6

10-4

10-2

100

102


Scr

ubb

ing

dos

e (

50e

V)

[mA

/m]



Dipole Quadrupole

Underlying mechanism:• When the SEY decreases the energy window for

multipacting becomes narrower

0 1 2 310

-6

10-4

10-2

100

102


Scr

ubb

ing

dos

e (

50e

V)

[mA

/m]



Quadrupole – SEY=1.3

• For high bunch intensity the e- spectrum drifts to higher energies


0 0.5 1 1.5 2 2.50

10

20

30

40

50

60


ea

t lo

ad [

W/m

]


0 0.5 1 1.5 2 2.50

5

10

15

20

25


He

at

load

[W

/m]


Dipole Quadrupole

0 1 2 310

-6

10-4

10-2

100

102


Scr

ubb

ing

dos

e (

50e

V)

[mA

/m]




0 0.5 1 1.5 2 2.50

10

20

30

40

50

60


ea

t lo

ad [

W/m

]


0 0.5 1 1.5 2 2.50

5

10

15

20

25


He

at

load

[W

/m]


Dipole Quadrupole

0 1 2 310

-6

10-4

10-2

100

102


Scr

ubb

ing

dos

e (

50e

V)

[mA

/m]



• In dipoles the multipacting region becomes wider for higher intensities


• Warning: dependencies rely on assumptions on Emax and uniform SEY on the wall

• Provided that we manage to access a low SEY regime, increased bunch intensity should be acceptable for heat load, effect on the beam still to be assessed

0 0.5 1 1.5 2 2.50

10

20

30

40

50

60


ea

t lo

ad [

W/m

]


0 0.5 1 1.5 2 2.50

5

10

15

20

25


He

at

load

[W

/m]


0 1 2 310

-6

10-4

10-2

100

102


Scr

ubb

ing

dos

e (

50e

V)

[mA

/m]



Dipole Quadrupole

200 MHz option: effect on e-cloud in the dipoles

• If option with 200 MHz main RF is adopted, longer bunches will have a positive impact also on electron cloud

• Impact of bunch length on e-cloud to be studied experimentally in Run 2 (e.g. to mitigate degradation at low energy)

Thanks to O. Dominguez

8b+4e fallback scenario

• Filling pattern with 25 ns spacing interleaving batches of 8 bunches with gaps of 4 slots, showed a significantly increased multipacting threshold compared the standard scheme

o backup scheme in case safe operation with 25 ns beam is hampered by e-cloud (still 50% more bunches wrt 50 ns)

• The effectiveness of this scheme will have to be confirmed during Run 2

Thanks to O. Dominguez

25 ns (72b. per PS batch)8b + 4e (48b. per PS batch)8b + 4e (56b. per PS batch)

Measured in 2012 (Fill 3429)

“Doublet” scrubbing beam

• Operation with 25 ns will rely on SEY reduction by beam induced scrubbing

• To operate with 25 ns beams (~2800b.) it is mandatory to achieve lower values in SEY compared to what was achieved in Run

o Dedicated scrubbing beams with hybrid bunch spacing (5+20 ns) are presently under study (e-cloud enhancement shown experimentally at SPS)

o If possible to be used for scrubbing already during Run 2 (SPS and LHC)

1.1 1.2 1.3 1.4 1.510

-6

10-4

10-2

100

102

SEY

Scr

ubb

ing

dos

e (5

0eV

) [m

A/m

]

0.50e11ppb0.60e11ppb0.70e11ppb0.80e11ppb0.90e11ppb1.00e11ppb1.10e11ppbStd. 25 ns

10 20 30 40 50 60 700

2

4

x 1011

Bea

m p

rof.

[p/m

]

10 20 30 40 50 60 701

1.5

2

2.5

3

3.5

Time [ns]

Ne /

Ne(0

)

PyECLOUD simulations

LHC dipole

Ele

ctro

n de

nsity

[a.

u.] Scrubbing beam (5+20) ns

Standard beam 25 ns

Outline



o 200 MHz option

o 8b+4e scheme






Q1 (A/B)

Q2 (A/B)

Q3 (A/B) D1

Q1

Q2 (A/B)

Q3 D1Present

HiLumi

IP1, 4 TeV, β* = 0.6 m

IP1, 7 TeV, β* = 0.15 m

Inner triplets for HL-LHC upgrade

More details on electron cloud studies for the inner triplets can be found in:

• For the present triplets:

http://indico.cern.ch/event/270441/

• For the HL LHC triplets (including IP2 and IP8):

https://indico.cern.ch/event/278323/


http://indico.cern.ch/event/270441/






A look to the EC buildup – HL-LHC triplets

Few snapshots of the electron distribution HL-LHC triplets develop thicker stripes along field lines farther from the center of the chamber

HL-LHC (2.20 x 1011 ppb)

Present (1.15 x 1011 ppb)

Distribution of heat load – HL-LHC triplets

Heat load distribution along HL-LHC triplets + D1 • Build up more or less efficient at different locations mainly due to the different

hybrid bunch spacings• The least efficient build up, i.e. lower heat load, at the locations of the long-range

encounters (vertical dashed lines)• Values in D1 are comparable or higher than values in the quads

Total heat load per element – HL-LHC triplets

Total heat load per element in HL-LHC triplets + D1 • Similar thresholds for quads and D1• Values in D1 higher than values in the quads for high SEY values

1 1.2 1.4 1.6 1.8 20

200

400

600

800

1000

1200

1400

1600

SEY

Hea

t lo

ad

[W]

25 ns - 2800 bunches

1 1.2 1.4 1.6 1.8 20

200

400

600

800

1000

1200

1400

1600

SEY

Hea

t lo

ad

[W]

25 ns - 2800 bunches

Effect of larger bunch population and chamber size. For the same SEY:- Similar energy of multipacting electrons- Larger number of impacting electrons

⇒ Total heat load about x3 largere-cloud suppression can be obtained using low SEY coatings and/or clearing electrodes

Total heat load on the triplet beam screen

Present triplets(1.15 x 1011 ppb)

HiLumi triplets(2.20 x 1011 ppb)

Cu beam scr.

SEY like 2012

Cu beam scr.

SEY like 2012

SPS like a-C coating

Full suppression(SEY≈1 or clearing

electrodes)

Scaling with bunch population - IP2 and IP8

1.1e11 ppb 2.1e11 ppb

Electron cloud in present inner triplets, scaling with bunch population for one cut:• Wider multipacting region for high bunch intensity

Scaling with bunch population - IP2 and IP8

Electron cloud in present inner triplets, scaling with bunch population for one cut:• Wider multipacting region for high bunch intensity • Doubling bunch population leads to about x3 larger heat load • e-cloud suppression strategies needed also for these magnets

Detailed study for precise heat load estimation still needs to be performed

0 0.5 1 1.5 2 2.50

20

40

60

80

100

120

140


Hea

t lo

ad

[W/m

]

B2R1_cut_8_2808b_heatload_vs_int_lin

Section far from long range encounter

0 1 2 310

-4

10-3

10-2

10-1

100

101

102


Scr

ubb

ing

dos

e (5

0eV

) [m

A/m

]

B2R1_cut_8_simulated_beam_scrubdose_vs_int_log_legend

sey = 1.00sey = 1.10sey = 1.20sey = 1.30sey = 1.40sey = 1.50sey = 1.60sey = 1.70sey = 1.80sey = 1.90

Outline



o 200 MHz option

o 8b+4e scheme






Matching section IP5 - Present LHC

ab

BS1a=22.5, b=17.6

BS2a=28.9, b=24

BSArca=22, b=17.15

BSD2a=31.3, b=26.4

Beam screens can be rotated

Q4BS2

Q5BS1

Q7BSArc

Q6BS1

D2 - BSD2

Beam off center

Matching section IP5 – HL-LHC

ab

BS1a=22.5, b=17.6

BS2a=28.9, b=24

BSHLa=37, b=32

BSArca=22, b=17.15

BSHLD2a=41, b=36

Q4BSHL

Q5BS2

Q7BSArc

Q6BS1

D2 - BSHLD2

Matching section IP2 – Present LHC

ab

Q4BS2

Q5BS1

Q7BSArc

Q6BS1

D2 - BSD2

Q4BS2

Q5BS2

Q7BSArc

Q6BS1

D2 - BSD2

BS1a=22.5, b=17.6

BS2a=28.9, b=24

BSArca=22, b=17.15

BSD2a=31.3, b=26.4

Beam screens can be rotated

Inj. beam

Matching sections

abBS1

a=22.5, b=17.6BS2

a=28.9, b=24BSArc

a=22, b=17.15BSD2

a=31.3, b=26.4

BSHLa=37, b=32

BSHLD2a=41, b=36

In total 6 beam screen shapes (all rect-ellipse):

Many, many, many configurations in terms of B field/gradient, beam size and beam position considering energy ramp, squeeze, separation, β*-leveling etc…

Need for parametric scans to asses which of these dependencies strongly impact the EC buildup

Matching sections

abBS1

a=22.5, b=17.6BS2

a=28.9, b=24BSArc

a=22, b=17.15BSD2

a=31.3, b=26.4

BSHLa=37, b=32

BSHLD2a=41, b=36


Many many many configurations in terms of B field/gradient, beam size and beam position considering energy ramp, squeeze, separation, β*-leveling etc…

Need for parametric scans to asses which of these dependencies strongly impact the EC buildup1 1.1 1.2 1.3 1.4

10-4

10-3

10-2

10-1

100

101

SEY

He

at l

oa

d [W

/m]

QF nemitt=2.50 um

0.45TeV1.34TeV2.22TeV3.11TeV4.00TeV

1.3 1.35 1.4 1.45 1.510

-4

10-3

10-2

10-1

100

SEY

He

at l

oa

d [W

/m]

MB2 nemitt=2.50 um

0.45TeV1.34TeV2.22TeV3.11TeV4.00TeV

Some preliminary indication: effect of beam size and B field variation during the energy ramp (nominal intensity)

Arc quadrupoleArc dipole

Thanks to L. Mether

Preliminary

Uniform train of 600b.

Matching sections

abBS1

a=22.5, b=17.6BS2

a=28.9, b=24BSArc

a=22, b=17.15BSD2

a=31.3, b=26.4

BSHLa=37, b=32

BSHLD2a=41, b=36


Many many many configurations in terms of B field/gradient, beam size and beam position considering energy ramp, squeeze, separation, β*-leveling etc…

Next steps:Effects of beam sizeEffect of B gradient

Effect of beam position

Arc quadrupole (practically also matching quad with BS1)

Q4 IP2 (where the effect is stronger)

Thanks for your attention!

Documents

Electron cloud studies for the HL-LHC: where do we stand and the next steps