electron cloud and scrubbing in the lhc

Electron cloud and scrubbing in the LHCG. Arduini, H.Bartosik, G. Iadarola, G. Rumolo

LHC Operation Meeting, Evian, 19/12/2012

Many thanks to:Many thanks to Cryogenics, Damper, EN/STI, Injection,

Operation, Vacuum teamsSeveral ABP, BI and RF colleagues contributing to measurements

Outline

• Introduction on electron cloud (EC) and scrubbing effects

• EC effects in the LHC: 50ns vs. 25ns

• Experience with 25ns beams:

• Observations in 2011

• The 2012 Scrubbing Run (450GeV)

• 25ns beams at 4TeV• Scenarios for post-LS1 operation

• Conclusions

Introduction

When the an accelerator is operated with close bunch spacing an Electron Cloud

(EC) can develop in the beam chamber due to the Secondary Emission from the

chamber’s wall.

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Seco

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SEYmax

Secondary Electron Yield (SEY) of the

chamber’s surface:

• ratio between emitted and impacting

electrons

• function of the energy of the primary

electron

Introduction

When the an accelerator is operated with close bunch spacing an Electron Cloud

(EC) can develop in the beam chamber due to the Secondary Emission from the

chamber’s wall.

• Strong impact on beam quality (EC

induced instabilities, particle losses,

emittance growth)

• Dynamic pressure rise

• Heat load (on cryogenic sections)

Dipole chamber @ 7TeV

Introduction: electron cloud effects and scrubbing

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ax

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Scrubbing of Cu measured with

e- at 500eV (CERN TE-VSC)

Scrubbing is a mitigation for the e-cloud effects:

• Keeping a significant e- flux on the chamber’s walls causes a decrease of the

SEY (and hence the e-cloud)

• The dependence of the SEY on the accumulated dose is logarithm - like

Outline







• Conclusions

Electron cloud effects in the LHC dipoles: 50ns vs. 25ns

• Main focus on the dipole magnets (~60% of the machine) they determine the performance in terms of beam quality

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• The “multipacting threshold” for 25ns beams is significantly lower than for 50ns

• In 2011, 4 days of scrubbing with 50ns beams + 2 days of tests with 25ns beams have lowered the SEY in the arcs well below 2.0 allowing an “EC free” operation also in 2012

Outline







• Conclusions

Electron cloud effects with 25ns: flashback into 2011

up to ±5mm

~ bunch 25 is the first unstable

First injection tests with a train of 48b. on 26/08/2011:

• Beam unstable right after injection (dump due to losses)

• Probably driven by e-cloud in the dipoles (mainly vertical motion, trailing bunches of the train)

• Beam stable with high chromaticity settings (Q’=15)

Thanks to W. Höfle and D. Valuch


up to ±5mm

~ bunch 25 is the first unstable

First injection tests with a train of 48b. on 26/08/2011:

• Beam unstable right after injection (dump due to losses)

• Probably driven by e-cloud in the dipoles (mainly vertical motion, trailing bunches of the train)

• Beam stable with high chromaticity settings (Q’=15)

Thanks to W. Höfle and D. Valuch

The instability could be successfully reproduced in HEADTAIL simulations (with electron distributions from PyECLOUD)

bunch 26 is the first unstable


First scrubbing tests with 25ns (14 & 24-25 Oct. 2011):• Injected up to 2100b. for B1 and 1020b. for B2

• SEY in the arcs could be decreased down to 1.52

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25ns threshold @450 GeV

Reconstructed comparing heat load meas. and PyECLOUD sims.


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Beam degradation still important:

• Poor bunch by bunch lifetimes• Large emittance growth

Thanks to F. Roncarolo




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An important benchmark for our models came from bunch by bunch energy loss estimation based on stable phase measurements.

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25ns bucket number

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Thanks to J. Esteban-Müller and E. Shaposhnikova



Outline







• Conclusions

Preliminary!

Outline







• Conclusions

The 2012 scrubbing run

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3.5 days of scrubbing with 25ns beams at 450GeV (6 - 9 Dec. 2012):

• Regularly filling the ring with up to 2748b. per beam (up to 2.7x1014 p)

• Overall very good efficiency: injection rate determined by MKI vacuum interlocks (in the beginning) and by time required by the cryogenic system to adapt to the increasing heat load (mainly in stand alones)


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Scrubbing effects in the arcs:

• Quite rapid conditioning observed in the first stages• The SEY evolution significantly slows down during the last scrubbing

fills (more than expected by estimates from lab. measurements and simulations)

Thanks to L. TavianSector 5-6

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Thanks to L. Tavian

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Measured

Scrubbing effects:

• Evident improvement on beam lifetime

Sunday 10 Dec

Friday 7 Dec


Scrubbing effects:



Scrubbing effects:


• Important losses do no appear before 1h after injection

Beam 1

Beam 2


Scrubbing effects:

• Evident improvement on beam lifetime• After the scrubbing, emittances are ok for trains of 72b. (3.6µs spacing)

but a significant blow-up is still visible when longer trains are injected

Fill with trains of 72b. Fill with trains of 288b.

Thanks to T. Rijoff and H. Maury Cuna


Outline







• Conclusions

Electron cloud observations at 4TeV

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84b.

156b. 372b.

804b.804b.

After Scrubbing Run machine studies with 25ns beams at 4TeV were possible. Main observations:

• The heat-load strongly increases during the ramp since the EC is enhanced by the photoelectrons due to synchrotron radiation This violent transient on the heat load in the arcs limits the number of bunches which can be accelerated

After Scrubbing Run machine studies with 25ns beams at 4TeV were possible. Main observations:

• Despite the larger number of electrons, at high energy the beam becomes less affected by EC the beam quality achievable at collisions is determined by the EC effects at 450GeV

Thanks to T. Rijoff and H. Maury Cuna

Begin. flat top After 8h. At flat top


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108b. 204b. 396b. 396b.

780b.

A pilot physics run with 25 ns beams took place in the last two days of the 2012 run:

• High brightness BCMS 25ns beam from the injectors (batches of 48 bunches 1.1e11ppb within transverse emittances of ~1.4µm)

• Stable beams with 396b. (squeezed up to 780b.)


Electron cloud effects with 25ns: 2012 experience

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From M. Lamont, LHC Morning Meeting, 17/12/2012




Electron cloud effects with 25ns: 2012 experience

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Outline







• Conclusions

Possible scrubbing scenarios for 2015

Comm. Vacuum conditioning

(50ns or short trains 25ns) (5-7 days)

Scrubbing with 25ns(2 days)

50ns physics

δarc≥2.2 δarc≈2.2 δarc≤2.

450GeV 6.5TeVRequirements for operation with 50ns beams:





50ns physics


450GeV 6.5TeV


Test ramps with 25ns(2 days)

δarc≤2. δarc≈1.45

450GeV 6.5TeV

Requirements for operation with 50ns beams:

Further requirements for operation with 25ns beams:

Same conditions as in 2012 pilot physics run but with enhanced heat load due to higher energy





50ns physics


450GeV 6.5TeV



25ns physics (intensity ramp up and further scrubbing)


450GeV 6.5TeV



Same conditions as in 2012 pilot physics run but with enhanced heat load due to higher energy





50ns physics


450GeV 6.5TeV



25ns physics (intensity ramp up and further scrubbing)


450GeV 6.5TeV



Remark:In case of long periods without beam de-conditioning is likely to take place:

• No action needed for 50ns beams (due to margin taken by scrubbing with 25ns beam)

• Few hours reconditioning required for 25ns beams

Summary

• The 2012 Scrubbing Run has lowered the SEY in the arcs to less than 1.45 resulting

in a reduced heat load as well as improved beam quality (lifetime, emittances)

• In spite of the high heat loads, close to the cryogenics limit, the second part of the

Scrubbing Run and the ramps to 4 TeV did not exhibit any clear improvement in

the conditioning state of the arcs (to be investigated with additional simulations

and lab measurements)

• Emittance blow-up ascribable to EC is still observed at injection energy with long

and closely spaced trains

• At 4TeV, no indication of further emittance deterioration driven by EC

• A concentrated scrubbing run will be likely to be insufficient to fully suppress the

EC from the arcs for 25ns beams in future operation.

Thanks for your attention!

electron cloud and scrubbing in the lhc

Documents

effects ec effects

electron cloud ec

ecloud effects

valuch10electron cloud

scrubbing run 450gev25ns

days of scrubbing

scrubbing of cu

beam quality ec