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Transcript of 1 Limits of Beam-Beam Interactions Ji Qiang Lawrence Berkeley National Laboratory Joint EIC2006 &...
1
Limits of Beam-Beam Interactions
Ji Qiang Lawrence Berkeley National Laboratory
Joint EIC2006 & Hot QCD Workshop, BNL, July 17 - 22
2
Outline
• Introduction• Experimental observations• Physical mechanisms• Computational models• Validation of computer codes• Beam-beam issues in linac-ring colliders• Summary
3
Beam Blow-Up during the Collision
4
Beam-Beam Interactions
– Limit the peak luminosity– Reduce the beam lifetime– Cause extra background– Large number of particles loss may quench
superconducting machine
5
Luminosity and Beam-Beam Parameter
22
21
22
21
21
2 yyxx
cfNNL
)(2 2221
1201
yxy
yy
Nr
yy
x
y IEL
)1(
6
• First beam-beam limit
• Saturation of beam-beam parameter• Luminosity scales linearly with current
• Second beam-beam limit• Ultimate limit of luminosity• Loss of particles and reduce of beam
lifetime
Beam-Beam Limits
7
& Luminosity vs. Current for e+e- Rings(J. Seeman, 1983)
SPEAR
CESR PETRA
PEP
1st beam-beam limit (max. ) 2ndb-b limit due to tails!
8
Background Noise and Scraper Location vs. Current (J. Seeman, 1983)
9
Luminosity vs. Current Square at PEP-II (J. Seeman et al, 2001)
10
Transverse Size vs. Current Square at PEP-II (J. Seeman et al, 2001)
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Observation of Flip-Flop at PEP-II: Transverse Beam vs. Bunch Number (R. Holtzapple, et al (2002)
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Observation of Flip-Flop at PEP-II: Luminosity vs. Bunch Number (R. Holtzapple, et al, 2002)
13
Observation of Flip-Flop at PEP-II:Horizontal Width vs. Snap Picture Number at LER
R. Holtzapple, et al (2002)
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Lepton Beam-Beam Tune shift vs. Proton Current at HERA (F. Willeke 2002)
e coh. bb tuneshift vs p Bunch current
-0.025
-0.02
-0.015
-0.01
-0.005
0
0.005
0 100 200 300 400 500 600
Ibp/mA
Df x
,y/k
Hz
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0.15 0.2 0.25 0.30.15
0.2
0.25
0.3
Explored Tune Region0.35
.15
q y i
R p q xi N( )
R m q xi N( )
R x q xi
R y q xi
.350.15 q xi
0.15 0.2 0.25 0.30.15
0.2
0.25
0.3
Explored Tune Region0.35
.15
q y i
R p q xi N( )
R m q xi N( )
R x q xi
R y q xi
.350.15 q xi
2Qx +2Q
y4Qx
3Qy
e+ Beam-Beam Limit at HERA tune footprint appears to be limited by 3rd & 4th order
resonances
beams separated in South IP
luminosity in the North increases
H1 and Zeus spec. lumi vs time
16
RHIC Working Point and Background (W. Fischer, 2003)
Deuteron-gold collisions, / IP 0.001, 4 head-on collisions
Lowest order resonances are oforder 9 between0.2 and 0.25
High background ratesnear 9th order resonaces
Low background rates near 13th resonances
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7th 5th12th 12th 5th
Contour Plots of Background Halo Rates for Protons and Antiprotons at Tevatron (V. Shiltsev et al 2005)
18
Beam-Beam Parameters in Hadron Accelerators (W. Fischer, 2003)
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beamenergy[GeV]
tune shiftper IP
total tuneshift
dampingdecrementper IP
LEP 100 0.083 0.33 1.6x10-2
KEKB 8, 3.5 0.05-0.095
0.05-0.095
2x10-4
PEP-II 3.1/9 0.048-0.075
0.048-0.075
10-4
DAFNE 0.51 0.03 0.03 10-5
Beam-Beam Parameters in Lepton Accelerators(F. Zimmermann, 2003)
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Physical Mechanisms• Collective/Coherent Resonance (Keil 1981, Dikansk and
Pestrikov 1982, Chao and Ruth 1985, Hirata 1987, Krishnagopal and Siemann, 1991, Shi and Yao, 2000)
– Dipole mode instability– Quadrupole model instability
• Flip-flop• Period n oscillation higher mode instability• Blow-up
– Higher order modes
21tune
Stability Region of Coherent Dipole Mode (Keil, Chao, Hirata)
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Max Mode =2
Unstable Max Mode =4
Unstable
0 1
0
0.10
0.20
Stability Diagram for Coherent Resonance up to 2 and 4(Chao and Ruth, 1985)
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IBSTouschek
background scattering
beam-beam bremsstrahlung
orbit noise
tune fluctuation
quantumnexcitation
Arnolddiffusion
resonance overlap
resonancetrapping
collision randomfluctuation
nonlinearresonance
diffusion
particle loss
Particle Loss due to Incoherent Diffusion
24
Resonance Traping: Particle Transport by Slow Phase Space Topology Change (A. Chao 1979)
25
Resonance overlap: Phase Space Evolution vs. Increasing Beam-Beam Tune Shift (J. Tennyson 1979)
26
Computational Models
• Weak-Strong– One beam (weak beam) is subject to the
electromagnetic fields of the other beam (strong beam) while the effects of weak beam on strong beam are neglected
• Strong-Strong– The electromagnetic fields from both
oppositely rotating beams are included
27
Weak-Strong Model• Advantages
– Only one electromagnetic field calculation is needed. This model is fast and many macroparticles can be used in tracking studies.
– The model is useful for halo/lifetime calculations or some quick machine parameter scan
• Disadvantages– Sensitive only to incoherent effects– Not self-consistent
28
Strong-Strong Model• Advantages
– Sensitive to both incoherent and coherent effects– Self-consistently modeling of beam-beam interaction
• Disadvantages– Electromagnetic fields from each beam have to be
calculated at each collision– Computational expensive– Need advanced algorithms and computers
29
Strong-Strong Model• Soft-Gaussian model
– The particle distribution is assumed as a Gaussian distribution with 1st and 2nd moments updated after each collision
• Self-consistent model– PIC: electromagnetic fields are calculated at each
collision point based on the charge distribution on a grid from macroparticle deposition
– Direct numerical Vlasov-Poisson solver
30
Particle-In-Cell (PIC) Simulation
Advance momenta using radiation damping and quantum excitation map
Advance momenta using Hbeam-beam forces
Field solution on grid
Charge deposition on grid
Field interpolation at particle positions
Setup for solving Poisson equation
Initializeparticles
(optional)diagnostics
Advance positions & momenta using external transfer map
31
Finite Difference Solution of Poisson’s Equation(S. Krishnagopal, 1996, Y. Cai, et al., 2001)
• Five point stencil with Fourier analysis by cyclic reduction (FACR)
• Reduced grid:– Before solving the Poisson equation, the potential on
the reduced grid boundary is determined by a Green’s function method
– Poisson solver uses FFT and cyclic reduction (FACR)• Computational complexity:
– Scales as N2log(N) within the domain: N – grid number in each dimension
– Needs 4N3 to find the boundary condition
32
Hybrid Fast Multipole Solution of Poisson’s Equation(W. Herr, M. P. Zorzano, F. Jones, 2001)
• Divided the solution domain into a grid and a halo area• Charge deposition with the grid• Multipole expansions of the field are computed for each grid
point as well as for every halo particle• Computation complexity:
– Scales as PN2 or PNp
33
Green Function Solution of Poisson’s Equation(K. Yokoya, K. Oide, E. Kikutani, 1990, E. Anderson et. al. 1999, K. Ohmi, et. al. 2000,
J. Shi, et. al, 2000, J. Qiang, et. al. 2002, A. Kabel, 2003)
; r = (x, y) ')'()',()( drrrrGr
(ri) h G(rii '1
N
ri' )(ri' )
)log(21),( 22 yxyxG
Direct summation of the convolution scales as N4 !!!!N – grid number in each dimension
34
Green Function Solution of Poisson’s Equation (cont’d)
F(r) Gs(r,r')(r')dr'Gs(r,r') G(r rs,r')
c(ri) h Gc(rii '1
2N
ri' )c(ri' )
(ri) c(ri) for i = 1, N
Hockney’s Algorithm:- scales as (2N)2log(2N)- Ref: Hockney and Easwood, Computer Simulation using Particles, McGraw-Hill Book Company, New York, 1985.
Shifted Green function Algorithm:- Ref: J. Qiang, M. Furman, R. Ryne, PRST-AB, vol. 5, 104402 (2002).
35
Green Function Solution of Poisson’s Equation
c(ri) Gi(rii '1
2N
ri' )c(ri' )
Gi(r,r') Gs(r,r') dr'
Integrated Green function Algorithm for large aspect ratio:- Ref: K. Ohmi, Phys. Rev. E, vol. 62, 7287 (2000). J. Qiang, M. Furman, R. Ryne, J. Comp. Phys., vol. 198, 278 (2004).
x (sigma)
Ey
36
Needs for High Performance Computers• Number of particles per bunch:
– 1010 – 1011
• Number of turns:– 109 - 1010
• Number of bunches per beam:– 1 - 1000
37
Scaling on seaborg using strong-strong model (100Mp, 512x512x32 grid, 4 slices)
# of processors
Execution time/turn (sec)
16 18864 49256 14.771024 7.18
During the development of BeamBeam3D, several parallelization strategies were tested. The large amount of particle movement between collisions gives the standard approach (domain decomposition, bottom curve) poor scalability for the strong-strong model. A hybrid decomposition approach (top curve) has the best scalability.
We are now able to perform 100M particle strong-strong simulation on 1024 processors
38Stern and Valishev et. al. SciDAC2006 poster
Synchrobetaron Mode Tunes vs. Beam-Beam Parameter Measurement vs Simulation (BeamBeam3D)
39
Specific Luminosity vs. beta* at HERA (J. Shi et al, 2003)
40
Luminosity of a Routine Operation of PEP-II: Measurement vs Simulation (Y. Cai et. al. 2001)
41
Linac-Ring Beam-Beam Interaction
• Electron beam is re-injected from linac after each turn. This avoids the beam-beam tune shift limit or e-cloud limit to electron intensity inherent in storage ring.
• Issues:– Beam-beam head-tail instability– Electron disruption
42
Schematic Plot of Synchrobetatron ModesE. Perevedentsev and A. Valishev, PRSTAB, 4, 024403 (2001)
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Synchrobetatron Mode Increments vs. Beam-Beam Parameter (Zero Chromaticity)
44
Threshold Value of D_+/s vs. Disruption Parameter (R. Li et al, 2001)
45
Synchrobetatron Mode Increments vs. Beam-Beam Parameter(Finite Chromaticity 0.409) (E. Perevedentsev and A. Valishev)
46
Summary• Beam-beam limit has been improved by fine tuning
of machine, lepton ~ 0.1, hadron ~ 0.01 per IP.• Theoretical models provide a lot of insights to
understand beam-beam limits. • Computer codes can reasonably reproduce coherent
spectrum and luminosity. However, prediction of beam lifetime is still a challenge.
• Linac-ring collider looks promising but detailed study of beam-beam limits including chromaticity and full 3d nonlinearity is needed.
47
Acknowledgements
A. Chao, Y. Cai, W. Fischer, M. Furman, W. Herr, K. Hirata, R. Holtzapple, V. Lebedev, R. Li, L. Merminga, K. Ohmi, E. Perevedentsev, R. Ryne, J. Seeman, J. Shi, C. Siegerist, V. Shiltsev, E. Stern, J. Tennyson, A. Valishev, F. Willeke, F. Zimmermann