Giovanni Rumolo, G. Iadarola and O. Dominguez in LHC Beam Operation workshop - Evian 2011, 13 December 2011 For all LHC data shown (or referred to) in

Giovanni Rumolo,G. Iadarola and O. Dominguez

in LHC Beam Operation workshop - Evian 2011, 13 December 2011

For all LHC data shown (or referred to) in this presentation: V. Baglin, H. Bartosik, P. Baudrenghien, G. Bregliozzi, S. Claudet, J. Esteban-

Müller, G. Lanza, G. Papotti, F. Roncarolo, E. Shaposhnikova, L. Tavian

Electron cloud effects in the LHC

2

Outline

Focus of this talk Analysis of the 2011 observations and measurements

– How we observe electron cloud in the LHC– Resume of the effect of the scrubbing run with 50ns beams (1 – 11

April 2011) – Experience and progress with 25ns beams

→ Historical: MD sessions from the 29 June 2011 to the 24 October 2011

→ Scrubbing of unbaked/uncoated field free regions→ Scrubbing of the arcs→ Evolution of some beam observables

– Concluding remarks

3

Electron cloud observation in the LHC

→ The electron flux to the chamber wall Fe is revealed through

1) Pressure rise

2) Heat load

Beam chamber

4

Electron cloud observation in the LHC

→ The presence of electrons with density re around the beam causes

1) Beam coherent instabilities, single or coupled-bunch type, for the last bunches of a bunch train

2) Incoherent emittance growth, degrading lifetime, slow losses

Beam

Obviously, both Fe and re depend on the beam structure and on the surface properties, e.g. R0 and dmax

From the evolution of the observables during scrubbing, we can infer the decrease of dmax !

5

2011 scrubbing run in one slide!

⇒ A 10-days scrubbing run took place at the beginning of April 2011, during which 50ns spaced beams with up to 1020 bunches per beam were injected into the LHC and kept at 450 GeV/c.

⇒ It resulted into a very efficient machine cleaning– The dynamic vacuum decreased by one order of magnitude– The heat load on the beam screen in the arcs

→ was significant at the beginning of the scrubbing run→ disappeared at the end of the scrubbing run, even with higher number of

bunches injected– The average stable phase over the beam decreased by one order of magnitude– Instabilities and emittance growth, clearly visible at the beginning of the scrubbing

run, disappeared at the end even with low chromaticity settings ⇒ After the scrubbing run and first test ramps, the machine became ready to

operate for physics with 50ns beams⇒ The number of bunches per beam was ramped up to its maximum (1380)

within two months

6

dmax in the arcs after the 50ns scrubbing

⇒ Heat load measured during the ramp in physics fill 1704 compared with the one predicted by numerical simulations

⇒ The measured heat load is compatible with values of dmax=2.1-2.2 for R0=0.6-0.7

⇒ The expected dmax threshold is about 2.2 @ 450 GeV and 2.1 @ 3.5 TeV

40-60 mW/m

Measurement in fill 1704

Simulation scan

Simulations by H. Maury-Cuna

7

dmax in the uncoated and/or unbaked sections: estimation technique

• The evaluation of dmax is done in the field-free regions in proximity of the pressure gauges

– Used Beam1 data from gauges (Cu): VGI.141.6L4.B and VGPB.2.5L3.B– A solution (R0 , dmax) is found comparing the pressure rises DPi measured at different injections

with the electron fluxes Fi from simulations

Baked but uncoated: SEY ~1.6-1.9. Length 0.3 m Pumping speed from NEG and maximum for CH4 ≈ 10 L/s

NEG

Measured pressures

Simulated electron fluxes

8

dmax in the uncoated and/or unbaked sections: results

• Pressure rise measurements with 50ns beam to estimate dmax in the field-free regions in proximity of the pressure gauges (R0 ≈0.2)

– Measurements done at the beginning and at the end of the scrubbing run– Measurements done during the 50ns operation of LHC (19 May)– As expected, we are asymptotically approaching the dmax threshold for 50ns beams

29 June 2011, date of the first injections of 25ns beams in LHC

Calculated threshold for 50ns beam

On the 29 June, a new story begins, with the 25ns beams in LHC …

25ns experience in 2011

0 5 10 15 20 25 30 35 400

5

10

15

20x 10

13

Inte

nsity

Time [h]

0 5 10 15 20 25 30 35 40

0

10

20

30

40

Time [h]

Heat lo

ad

[W

/hcell]

S12S23S34S45S56S67S78S81

DATE SHORT DESCRIPTION

29 June Injections of 9 x 24b trains per beam with different spacings between them

28 August First attempt to inject a 48b train: fast instability dumps the beam within less than 1000 turns after injection

07 October High chromaticity (Q’x,y ≈15): Injection tests with trains of 72-144-216-288 bunches from the SPS + ramp & 5h store with 60b (12+24+24) per beam

14 October High chromaticity: injection of up to 1020 bunches per beam in 72b trains (decreasing spacings between trains: 4-3-2-1 ms)

24-25 October Injection of up to 2100 bunches in Beam 1 and 1020 in Beam 2 (1ms train spacing)

Scrubbing

29/06 14/10 24/10

0 5 10 15 20 25 30 35 400

5

10

15

20x 10

13

Inte

nsity

Time [h]

0 5 10 15 20 25 30 35 40

0

10

20

30

40

Time [h]

Heat lo

ad

[W

/hcell]

S12S23S34S45S56S67S78S81

29/06 14/10 24/10

• Pressure rise measurements during most of the 25ns fills were found hard to be used for the dmax estimation because of beam losses leading to rapidly changing regimes

• After considerable 25ns scrubbing, i.e. at the end of the 24/10 MD session, 8 x 72b batches with different spacings could be injected for Beam 1 into the LHC and remain stable to allow the pressure values to level

10

dmax in the uncoated and/or unbaked sections: results (II)

• Scrubbing with 25ns beam (~40h) has lowered dmax to 1.35 !• Again, we are not far from the threshold for 25ns beams, but further scrubbing is needed

11

Start of 25ns beams in LHC



dmax in the uncoated and/or unbaked sections: results (II)

12

dmax in the arcs: estimation technique

0 5 10 15 20 25 30 35 400

5

10

15

20x 10

13

Inte

nsity

Time [h]

0 5 10 15 20 25 30 35 40

0

10

20

30

40

Time [h]

Heat lo

ad

[W

/hcell]

S12S23S34S45S56S67S78S81

14/10 24/1029/06

Measured heat loads [W/hcell] averaged over sectors from cell by cell data

Five snapshots in the 25ns MDs to reproduce the measured heat load by simulations!

13


0 5 10 15 20 25 30 35 400

5

10

15

20x 10

13

Inte

nsity

Time [h]

0 5 10 15 20 25 30 35 40

0

10

20

30

40

Time [h]

Heat lo

ad

[W

/hcell]

S12S23S34S45S56S67S78S81

0 5 10 15 20 25 30 35 400

5

10

15

20x 10

13

Inte

nsity

Time [h]

0 5 10 15 20 25 30 35 40

0

10

20

30

40

Time [h]

Heat lo

ad

[W

/hcell]

S12S23S34S45S56S67S78S81

0 5 10 15 20 25 30 35 400

5

10

15

20x 10

13

Inte

nsity

Time [h]

0 5 10 15 20 25 30 35 40

0

10

20

30

40

Time [h]

Heat lo

ad

[W

/hcell]

S12S23S34S45S56S67S78S81

24/10

0 10 20 30 40 50 60 70 800

5

10

x 1010

Bunch position [us]

Bu

nch

inte

nsity [p

pb

]0 10 20 30 40 50 60 70 80

0

1

2

Bunch position [us]B

un

ch

len

gth

[n

s]

0 10 20 30 40 50 60 70 800

1

2

x 109

Time [us]

e- p

er

un

it le

ng

th [m

-1]

0 10 20 30 40 50 60 70 800

5

10

x 1010

Bunch position [us]

Bu

nch

inte

nsity [p

pb

]

0 10 20 30 40 50 60 70 800

1

2

Bunch position [us]

Bu

nch

len

gth

[n

s]

0 10 20 30 40 50 60 70 800

1

2

x 109

Time [us]

e- p

er

un

it le

ng

th [m

-1]

fastBCT + bblength (B1)


14


Measured heat load

Simulated heat loads



Simulator PyECLOUD

0 10 20 30 40 50 60 70 800

5

10

x 1010

Bunch position [us]

Bu

nch

inte

nsity [p

pb

]

0 10 20 30 40 50 60 70 800

1

2

Bunch position [us]

Bu

nch

len

gth

[n

s]

0 10 20 30 40 50 60 70 800

1

2

x 109

Time [us]

e- p

er

un

it le

ng

th [m

-1]

0 10 20 30 40 50 60 70 800

5

10

x 1010

Bunch position [us]

Bu

nch

inte

nsity [p

pb

]

0 10 20 30 40 50 60 70 800

1

2

Bunch position [us]

Bu

nch

len

gth

[n

s]

0 10 20 30 40 50 60 70 800

1

2

x 109

Time [us]e

- per

un

it le

ng

th [m

-1]

15

dmax in the arcs: results

0 5 10 15 20 25 30 35 400

5

10

15

20x 10

13

Inte

nsity

Time [h]

0 5 10 15 20 25 30 35 40

1.4

1.6

1.8

2

2.2

max

Time [h]

dmax has decreased from the initial 2.1 to 1.55 in the arcs !

29/06 14/10 24/10

Calculated threshold for 25ns beam (450 GeV)

Calculated threshold for 25ns beam (3.5 TeV)

R0 = 0.7

16


Simulations dmax fixed to 1.55 for the last fill on the 25 October

0 500 1000 1500 2000 2500 3000 35000

0.5

1

1.5

2

Bu

nch

en

erg

y lo

ss [m

J/T

urn

]

25ns bucket number

Simulated

Measured

Measurements the energy loss per bunch is obtained from the stable phase shift

Beam 1

ZOOM


1550 1600 16500

0.5

1

1.5

Bu

nch

en

erg

y lo

ss [m

J/T

urn

]

25ns bucket number

Simulated

Measured

1860 1880 1900 1920 1940 1960 19800

0.5

1

1.5

Bu

nch

en

erg

y lo

ss [m

J/T

urn

]

25ns bucket number

Simulated

Measured

2420 2430 2440 2450 2460 2470 2480 2490 25000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Bu

nch

en

erg

y lo

ss [m

J/T

urn

]

25ns bucket number

Simulated

Measured

3060 3080 3100 3120 3140 31600

0.5

1

1.5

2

2.5

Bu

nch

en

erg

y lo

ss [m

J/T

urn

]

25ns bucket number

Simulated

Measured

⇒ Excellent agreement between bunch by bunch synchronous phase shift and simulated energy loss at the saturation of the e-cloud

⇒ Build up phase of the electron cloud still not reproduced by simulations with the same accuracy and level of detail

– Simulation underestimates the primary electron generation?– In reality, stronger memory effect between batches

→ Larger R0 ? unlikely, because 0.7 is already a high value→ Uncaptured beam between batches? (SPS experience)

– Dynamic range of the measurements?– Energy loss from impedance, dominant for the first bunches in each batch and for

the last batches in the full train?– Further check with the bunch by bunch position data from BQM

⇒ Model confirms cross-calibration between stable phase shift measurements to measured heat load data

Beam observables: Transverse emittances

14 October batches injected with 4 ms spacing, Q’x,y=15

24 October batches injected with 1 ms spacing, Q’x=3, Q’y=15

• Both beams still unstable in the two planes, or anyway affected by emittance growth

• Some visible benefits from scrubbing:– The effect of the electron cloud manifests itself later along the trains, in

spite of the closer spacing!– First 1 – 2 trains seem to be hardly affected now– In general, improvement in vertical

• Lowering horizontal chromaticity did not seem to degrade the beam horizontally, but rather it slightly improved it: effect of scrubbing?

HEADTAIL simulations by Kevin Li

Electron Cloud InstabilityWhat do we expect ?

1.35 1.4 1.45 1.5 1.55 1.610

9

1010

1011

1012

1013

max

e- c

en

tral d

en

sity

450GeV

3.5TeV

• Calculated coherent ECI threshold for central density in dipoles is around re=1012 m-3 for nominal intensity at 450 GeV (simulations were run assuming the whole LHC made of dipoles)

• It can be stabilized with chromaticities Q’x,y>15, but emittance growth due to electron cloud + chromaticity remains!

• Right plot shows that this could be achieved only for dmax ≤ 1.5

Beam observables: Losses and lifetimes

24 October batches injected with 1 ms spacingBeam 1

Steady improvement visible on 2nd and 3rd train

Fast losses

Degrading lifetime

Lifetime degrades and then recovers

21

Concluding remarks (25ns)

⇒ Further scrubbing is needed to suppress the electron cloud⇒ Tricky, as the efficiency of scrubbing decreases with scrubbing itself…

– The electron dose measured in lab to decrease the dmax on Cu by an extra 0.1 from 1.55 to 1.45 is about the same needed to decrease it from 2.1 to 1.55.

– The flux of scrubbing electrons decreases with lowering dmax

⇒ Instability threshold for 25ns beams very close to the build up threshold− Not much margin to be in the comfortable situation of scrubbing without significant

beam degradation

⇒ Significant extra gain could be boosted by− Multi-train injections from the SPS− Find a comfortably stable filling pattern at 450 GeV and ramp to 3.5 TeV to benefit from

photoelectrons and from the lower electron cloud build up threshold

dmax (estimated)

dmax (threshold @450 GeV)

dmax (threshold @3.5 TeV)

StSt (straight section) 1.35 1.25

Beam screen (arcs) 1.55 1.45 1.37

22

Concluding remarks (50ns)

⇒ As could be expected, before the 25ns beams in the LHC, the dmax values were just about the build up threshold for nominal 50ns beams

⇒ After the 25ns MDs, the LHC beam chambers have been cleaned to dmax values well below the build up threshold for nominal 50ns beams

⇒ Simulation work on dmax thresholds as a function of bunch intensity is ongoing, but first results show little dependence at least up to bunch populations of 1.8 x 1011 ppb

⇒ If we keep the present level of conditioning, ‘ecloud-less’ operation of LHC with 50ns beams up to high intensities should be guaranteed (bar specific situations in common beam chambers, which need to be checked)

dmax (estimated)

dmax (threshold @450 GeV)

dmax (threshold @3.5 TeV)

StSt (straight section) 1.35 1.63

Beam screen (arcs) 1.55 2.2 2.1

Very special thanks to G. Iadarola, H. Bartosik, O. Dominguez, J. Esteban-Müller, and F. Roncarolo for their careful off-line analysis of large amounts of MD data and the

huge simulation effort that improved the general understanding of electron cloud and scrubbing!

Many thanks to V. Baglin, P. Baudrenghien, G. Bregliozzi, S. Claudet, G. Lanza, G. Papotti, E. Shaposhnikova, L. Tavian for all the beautiful data they kindly provided us with and the

numerous discussionsThanks to G. Arduini, B. Goddard, V. Kain, K. Li, H. Maury-Cuna, E. Métral, S. Redaelli, B. Salvant, F. Zimmermann, and all those who promoted and/or actively

participated in the MDs

Thank you for your attention

24

Some references⇒ Past observations, measurements and studies

– 2010 experience reviewed in the note “50 and 75 ns operation in the LHC: Vacuum and Cryogenics observations”, G. Arduini et al., CERN-ATS-Note-2011-046 MD

– 2010 experience + 2011 scrubbing run reviewed in several LBOC/LMC talks and in IPAC 2011 paper and talk “Electron Cloud observation in the LHC”, G. Rumolo et al., CERN-ATS-2011-105

– “Observations of Electron Cloud Effects with the LHC Vacuum System”, V. Baglin et al. TUPS01 in IPAC 2011

– “Electron Cloud Parameterization Studies in the LHC”, O. Domínguez et al., CERN-ATS-2011-142

– “Simulation of Electron-cloud Build-Up for the Cold Arcs of the LHC and Comparison with Measured Data”, H. Maury Cuna et al., CERN-ATS-2011-135

– “Review of beam instabilities in the presence of electron clouds in the LHC”, K. Li and G. Rumolo, CERN-ATS-2011-095

– “Injection into LHC of bunches at 25 ns spacing” G. Arduini, B. Goddard, et al., CERN-ATS-Note-2011-050 MD

– “Benchmarking Electron-Cloud Build-Up and Heat-Load Simulations against Large-Hadron-Collider Observations” H. Maury-Cuna et al., CERN-ATS-2011-218

25

dmax in the uncoated and/or unbaked sections: method of estimation


– Gauges explored (StSt): VGI.141.6L4.B and VGPB.2.5L3.B, Beam 1 data– Beams are injected with a filling pattern with different spacings, to try to extrapolate memory

effects

Baked but uncoated: SEY ~1.6-1.9. Length 0.3 m Pumping speed from NEG and maximum for CH4 ≈ 10

L/s

NEG

3 ms

2 ms

1 ms

16 ms

26


– The pressure rise DP is read at the gauge when injecting a new batch into LHC and is assumed to be proportional to the electron flux Fe

– Numerical simulations can be used for finding the pairs (R0, dmax) compatible with the measured pressure rise ratio

– By using more measured points, the point/region where the curves intersect will define the pair (R0, dmax), or narrow range, solution of of our problem

Simulated surface

Plane of measured ratio (161)

Intersection of the two in the (R0, dmax) plane

Plausible solution R0 ≈0.2, dmax =1.35 – 1.4

dmax in the uncoated and/or unbaked sections: method of estimation

0 500 1000 15000

0.2

0.4

0.6

0.8

1

1.2

1.4

Bu

nch

en

erg

y lo

ss [m

J/T

urn

]

25ns bucket number

Simulated

Measured

27


Simulations dmax fixed to 1.55 for the last fill on the 25 October

Measurements the energy loss per bunch is obtained from the stable phase shift

Beam 2 ZOOM


360 380 400 420 440 4600

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Bu

nch

en

erg

y lo

ss [m

J/T

urn

]

25ns bucket number

Simulated

Measured

580 600 620 640 6600

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Bu

nch

en

erg

y lo

ss [m

J/T

urn

]

25ns bucket number

Simulated

Measured

1000 1020 1040 1060 1080 11000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Bu

nch

en

erg

y lo

ss [m

J/T

urn

]

25ns bucket number

Simulated

Measured

1440 1460 1480 1500 15200

0.2

0.4

0.6

0.8

1

1.2

Bu

nch

en

erg

y lo

ss [m

J/T

urn

]

25ns bucket number

Simulated

Measured

Improvement visible on 2nd and 3rd train

Beam observables: Losses and lifetimes

24 October batches injected with 1 ms spacingBeam 2

Fast losses

Degrading lifetime

Documents

Giovanni Rumolo, G. Iadarola and O. Dominguez in LHC Beam Operation workshop - Evian 2011, 13 December 2011 For all LHC data shown (or referred to) in