measurements, ideas, curiosities

F. RuggieroUniv. “La Sapienza”, Rome, 20–24 March

2006CERN

Measurements, ideas, curiosities

fundamental limitations to the ultimate performance of high-luminosity colliders

Complement to the lecture on Electron Cloud and Beam-Beam Effects

F. Ruggiero Electron Cloud and Beam-Beam EffectsCERN

Electron Cloud build-up and Le Châtelier’s principle

• Henri Louis Le Châtelier (1850-1936) was a French industrial chemist. In 1888 he made a remarkable observation:

Henri Le Châtelier A man of Principle…

• In modern accelerators we want to accumulate more and more intense, positive beams.

• Nature reacts by the fast build-up of negative electron clouds that tend to neutralize the system!

"If a dynamic equilibrium is disturbed by changing some conditions, the position of equilibrium moves to counteract the change."

3

blue: e-cloud effect observedred: planned accelerators

longer fewer more intense bunches

more ‘ultimate’bunches

Scaling of electron cloud effects

experienceat severalstorage ringssuggests thatthe e-cloudthreshold scales as Nb~tsep

possible LHCupgrades considereithersmaller tsep

with constantNb, or theyincrease tsep

in proportionto Nb


Electron-cloud build-up in the LHC

• Beam synchrotron radiation is important• provides source of photo-electrons

• Secondary emission yield (SEY) (E) is important • characterized by peak value max

• determines overall e– density

• e– reflectivity (0) is important• determines survival time of e–

• Bunch intensity Nb and beam fill pattern are important

• Main concern: power deposition by electrons on the cold beam screen


Secondary Electron Yieldfor normal electron incidence

R. Cimino et al., Phys. Rev. Lett. 93, 014801 (2004)


Electron density [m-3] during the passage of an LHC bunch in a field-free region

HEADTAIL simulation, courtesy E. Benedetto


Electron density [m-3] during the passage of an LHC bunch in an SPS dipole (top view)

HEADTAIL simulation, courtesy E. Benedetto


Electron flux measured in an SPS dipole (strip detector) with LHC beam at 26 GeV

courtesy G. Arduini and M. Jimenez


LHC bunch train at injection in the SPS

Evolution of bunch length and bunch population for the first and the last bunch in an LHC bunch train of 72 bunches. SPS measurements with electron cloud in Aug 2004. Courtesy G. Rumolo, G. Arduini, and F. Roncarolo.

rms

bunch

leng

th (

ns)

bunch

inte

nsi

ty (

au)

time (min) time (min)

Qx=26.135

Qy=26.185

ξx=0.15

ξy=0.1

VRF ~ 3 MV

dampers on coupling:

0.008


4pb

HO

rN 2)/(

2

sep

HOparLR dn

/ IP no. of IP’s Qbbtotal

SPS 0.005 3 0.015

Tevatron (pbar) 0.01-0.02 2 0.02-0.04

RHIC 0.002 4 ~0.008

LHC (nominal) 0.0034 3 ~0.01

tune shift from head-on collisions (primary IP’s)

tune shift from long-range collisionsnpar parasitic collisions around each IP

conservative value for total tune spread based on SPScollider experience

Beam-Beam tune spread for round beams

increases for closer bunches or reduced crossing angle

limit on limits Nb/(

*

ccsep

)(

s

sd relative beam-beam separation for full crossing angle c

high-lumi in IP1 and IP5 (ATLAS and CMS), halo collisions in IP2 (ALICE) and low-lumi in IP8 (LHC-b)


Beam-Beam tune footprints

Comparison of tune footprints, corresponding to betatron amplitudes extending from 0 to 6 , for LHC nominal (red-dotted), ultimate (green-dashed), and “large Piwinski parameter” configuration (blue-solid) with alternating H-V crossing only in IP1 and IP5. (Courtesy H. Grote)


• Effects of long-range beam-beam interaction observable at RHIC injection energy (24.3 GeV) with a single proton bunch per ring

• Bunch intensity Nb~1-21011

Long-Range Beam-Beam Experiment in RHIC, 28 April 2005, Wolfram Fischer et al.


collision at s=10.65m

Long-Range Beam-Beam Experiment in RHIC, 28 April 2005, Wolfram Fischer et al.

Beam losses increase when the beam-beam separation at a single parasitic collision point is reduced below ~5 .

1 bunch per ring

Blue beam moved vertically

Tunes: B

(0.739,0.727) Y (0.727,0.738)


Electron Cloud: recent links & referenceshttp://ab-abp-rlc.web.cern.ch/ab-abp-rlc-ecloud/(web site on Electron Cloud Effects in the LHC)

http://icfa-ecloud04.web.cern.ch/icfa-ecloud04/(ECLOUD’04 workshop, Napa, California, 19–23 April 2004)

http://www-project.slac.stanford.edu/ilc/testfac/ecloud/elec_cloud.html(web site of ILC Damping Ring Task Force 6)

“Can low energy electrons affect high energy physics accelerators?”, R. Cimino, I.R. Collins, M.A. Furmann, M. Pivi, F. Ruggiero, G. Rumolo, and F. Zimmermann, Phys. Rev. Lett. 93, 014801 (2004)

“Simulation study of electron cloud induced instabilities and emittance growth for the CERN Large Hadron Collider proton beam”, E. Benedetto, D. Schulte, F. Zimmermann, and G. Rumolo, Phys. Rev. ST Accel. Beams 8, 124402 (2005)

“Update on electron-cloud power deposition for the Large Hadron Collider arc dipoles”, M. Furmann and V.H. Chaplin, LBNL-59062/CBP Note 723/LARP-doc-157, March 2006, to appear in Phys. Rev. ST Accel. Beams


Beam-Beam: recent links & referenceshttp://wwwslap.cern.ch/collective/zwe/lhcbb/(web site of LHC Beam-Beam studies)

http://www.agsrhichome.bnl.gov/AP/BeamBeam/Workshop03/(Beam-Beam Workshop, Montauk, Long Island, 19–23 May 2003)

“Coherent beam-beam modes in the LHC for multiple bunches, different collision schemes and machine symmetries”, W. Herr and T. Pieloni, Proc. CARE HHH-2004 Workshop, CERN, 8–11 November 2004

“Progress of Beam-Beam Compensation Schemes”, F. Zimmermann and U. Dorda, Proc. LHC-LUMI-05 Workshop, Arcidosso, 31 Aug–3 Sept 2005

“Beam-beam effects in the Tevatron”, V. Shiltsev, Y. Alexahin, V. Lebedev, P. Lebrun, R.S. Moore, T. Sen, A. Tollestrup, A. Valishev, and X.L. Zhang, Phys. Rev. ST Accel. Beams 8, 101001 (2005)

“The effects of solenoids and dipole magnets of LHC experiments”, W. Herr, LHC Project Workshop, Chamonix XV, 23-27 January 2006


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