electron-cloud instability in the clic damping ring for positrons
DESCRIPTION
Electron-cloud instability in the CLIC damping ring for positrons. H. Bartosik, G. Iadarola , Y. Papaphilippou , G. Rumolo CLIC workshop, 05.02.2014. Introduction. Electron cloud can lead to coherent beam instability Coupled bunch instability feedback - PowerPoint PPT PresentationTRANSCRIPT
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Electron-cloud instability in the CLIC damping ring for positrons
H. Bartosik, G. Iadarola, Y. Papaphilippou, G. Rumolo
CLIC workshop, 05.02.2014
CLIC workshop, H. Bartosik 05.02.2014
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2CLIC workshop, H. Bartosik 05.02.2014
Introduction
o Electron cloud can lead to coherent beam instability• Coupled bunch instability feedback
• Single bunch instability emittance growth faster than radiation damping, feedback difficult due to bandwidth
o Single bunch electron cloud instability depends on• electron density
• optics (beta functions)
• synchrotron tune
• chromaticity
• transverse emittance
• bunch length
o Here: study of single bunch instability for superconducting wigglers
… needs to be studied in detail
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3CLIC workshop, H. Bartosik 05.02.2014
CLIC damping rings
Description Symbol Value
Beam energy E0 [GeV] 2.86
Normalized transverse emittances εn,x,y [nm] 500, 5
Average beta and dispersion functions (Wigglers) bx,y, Dx [m] 4.2, 9.8, 2.6 x 10-5
Bunch length (rms) σz [mm] 1.6
Synchrotron tune Qs 6.5 x 10-3
Wigglers occupy ~ ¼ of the total ring…
C = 427.5 m, Lwigglers = 104 m
Qx=48.38Qy=10.39
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4CLIC workshop, H. Bartosik 05.02.2014
Electron cloud simulations
Multi-bunch beam s
Primary and secondary electron production, chamber properties E-cloud build up
x
y
Noise Equations ofmotion of thebeam particles
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5CLIC workshop, H. Bartosik 05.02.2014
Electron cloud simulations: splitting the problem
Multi-bunch beamOne turn
s
Primary and secondary electron production, chamber properties E-cloud build up
x
yThe build up problem
Single bunchSeveral turns
e.g. with PyECLOUD
see talk of G. Iadarola
used here: HEADTAIL code Noise
The instability problem
Equations ofmotion of thebeam particles
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6CLIC workshop, H. Bartosik 05.02.2014
The HEADTAIL code: simulation principle
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7CLIC workshop, H. Bartosik 05.02.2014
The HEADTAIL code: simulation principle
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8CLIC workshop, H. Bartosik 05.02.2014
The HEADTAIL code: simulation principle
→ The effect of the electron cloud on the beam becomes visible only after many turns
→ The electron cloud is refreshed at every interaction point
→ Slicing is renewed at every turn
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9CLIC workshop, H. Bartosik 05.02.2014
Instability threshold for e-cloud in wigglers
o Simulations with uniform electron density
o Electrons in magnetic dipole field ( no horizontal motion in HEADTAIL)• No (single bunch) instability in horizontal plane
Horizontal centroid motion Vertical centroid motion
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10CLIC workshop, H. Bartosik 05.02.2014
Instability threshold for e-cloud in wigglers
o Simulations with uniform electron density
o Electrons in magnetic dipole field ( no horizontal motion in HEADTAIL)• No (single bunch) instability in horizontal plane
• Strong growth of vertical emittance (εn,y) above threshold
rise time τ ≈ 0.7 msτ ≈ 0
.5 m
s
τ ≈
0.4
ms
⇒ fast growth compared to vertical damping time (2 ms)
⇒ local electron densities of 1.2 x 1013 m-3 in wigglers (25% of circumference) drive beam unstable
Vertical emittance
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11CLIC workshop, H. Bartosik 05.02.2014
Dependence on bunch intensity
o Simulations with uniform electron distributions
o Weak dependence of instability threshold on bunch intensity• Studied in view of re-baselining of CLIC parameters
⇒ Similar threshold electron density for the studied range of positron bunch intensities …
nominal CLIC intensity
Threshold electron density in wigglers
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12CLIC workshop, H. Bartosik 05.02.2014
Using electron distributions from PyECLOUD
o PyECLOUD simulation for generation of macroparticle distribution• 1 ns bunch spacing
⇒ Using distribution (just before bunch passage) with maximum central density = most critical for instability
max. central density
See talk of G. IadarolaSEY = 1.5
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13CLIC workshop, H. Bartosik 05.02.2014
Using electron distributions from PyECLOUD
o PyECLOUD simulation for generation of macroparticle distribution• 1 ns bunch spacing
• Generation of uniform spatial distribution with variable charges/weights
Histogram Electron distribution
beam (±3σ)
SEY = 1.5 (bunch 350)
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14CLIC workshop, H. Bartosik 05.02.2014
Using electron distributions from PyECLOUD
o PyECLOUD simulation for generation of macroparticle distribution• 1 ns bunch spacing
• Generation of uniform spatial distribution with variable charges/weights
o HEADTAIL simulations with distributions from PyECLOUD • Bunch passages with maximum central density along the train
Vertical emittance Maximum central density along train
⇒ Beam unstable as soon as electron build-up saturated
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15CLIC workshop, H. Bartosik 05.02.2014
Mitigation with chromaticity?
o Increasing vertical chromaticity is one of the measures against e-cloud instability
o Simulation for uniform e-cloud distribution • ρe = 4 x 1013 / m3 (equivalent to ρe = 1.6 x 1014 / m3 in the wigglers)
⇒ Even very high chromaticity not sufficient for beam stability (+ incoherent effects)
Vertical emittance
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16CLIC workshop, H. Bartosik 05.02.2014
Summary and conclusions
o Electron cloud instability simulations with HEADTAIL• Decoupled from build-up simulations (computing power)
• Semi-self consistent by using electron distribution from build-up code
• Only single bunch instability
o Simulations for wigglers with uniform e- distribution• Threshold density in wigglers ≈ 1.2 x 1013 / m3
• Emittance growth rate fast compared to damping times
• Little dependence on bunch intensity
• Mitigation by chromaticity not sufficient
o Simulations for wigglers using distribution from PyECLOUD• Beam is unstable for all SEY values above build-up threshold (i.e. SEY>1.4)
• Need to suppress e-cloud build-up!