the microbunching instability in the lcls-ii linac
DESCRIPTION
The Microbunching Instability in the LCLS-II Linac. LCLS-II Planning Meeting October 23, 2013 A. Marinelli and Z. Huang. Microbunching Instability. Microbunching instability. Modulation induced by self-fields: -Longitudinal space- charge (Coulomb) - Wakefields - PowerPoint PPT PresentationTRANSCRIPT
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The Microbunching Instability in the LCLS-II Linac
LCLS-II Planning MeetingOctober 23, 2013
A. Marinelli and Z. Huang
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Microbunching Instability
Microbunching instability
Modulation induced by self-fields:-Longitudinal space- charge (Coulomb)-Wakefields-coherent Synchrotron radiation -broad-band effect
-can start from shot-noise
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Microbunching Instability
Microbunching instability
Modulation induced by self-fields:-Longitudinal space- charge (Coulomb)-Wakefields-coherent Synchrotron radiation -broad-band effect
-can start from shot-noise
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Microbunching in LCLS-1
Example:
Recent X-TCAV measurement with FEL off.
Strong microbunching due to 2-stage compression and high-current operation.
Microbunches in phase-space ~ vertical:SATURATION!
Ratner, Marinelli, Beherens, Ding, Turner
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Analytical Model
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Analytical Model
Energy modulation induced by space-charge
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Analytical Model
Chicane Dispersion
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Analytical Model
Fourier-transform of the energy distribution:
INCREASE ENERGY SPREAD TO SUPPRESS THE INSTABILITY
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Final Energy Spread
Microbunching gain is not the most meaningful quantity since it does not directly affect the FEL performance (at least for SASE and Self-Seeding).
What really matters is the energy-spread induced by the instability.
Simplified model: 1) Track microbuching up to the last bunch compressor
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Final Energy Spread
Microbunching gain is not the most meaningful quantity since it does not directly affect the FEL performance (at least for SASE and Self-Seeding).
What really matters is the energy-spread induced by the instability.
Simplified model:2) Compute energy-spread induced by SC acting on the microbunched beam in the rest of the accelerator/transport
(neglects spread induced in the early stages of the gain process)
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Final Energy Spread
Microbunching gain is not the most meaningful quantity since it does not directly affect the FEL performance (at least for SASE and Self-Seeding).
What really matters is the energy-spread induced by the instability.
Simplified model:
SPACE-CHARGE IS THE LARGEST CONTRIBUTION TO ENERGY-SPREAD
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Induced Energy Spread from Shot-Noise
Integrate induced energy spread in the frequency domain starting from shot-noise…
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Example
LCLS1 parameters. Final peak current:Ipk = 3kA
Finite mismatch between laser heater and electron beam:
sr/sx = 2
Final spread computed as sum of three contributions:
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Example
LCLS1 parameters. Final peak current:Ipk = 3kA
Finite mismatch between laser heater and electron beam:
sr/sx = 2
Final spread computed as sum of three contributions:
Heater induced spread x compression
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Example
LCLS1 parameters. Final peak current:Ipk = 3kA
Finite mismatch between laser heater and electron beam:
sr/sx = 2
Final spread computed as sum of three contributions:
Initial gaussian spread x compression
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Example
LCLS1 parameters. Final peak current:Ipk = 3kA
Finite mismatch between laser heater and electron beam:
sr/sx = 2
Final spread computed as sum of three contributions:
Energy-spread induced by LSC
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Comparison with Recent X-TCAV Measurements
Experimental result consistent with theory: optimum at~12-14 keV heater induced spread
Ratner, Marinelli, Beherens, Ding, Turner, Decker
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LCLS-2 Microbunching Gain (NO HEATER)
300 eV1000 eV2000 eV
Gain estimateassuming initial Gaussian spread
G
l(m) (initial)
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LCLS II BC2 at 1.6 GeV
BC2 at 1.6 GeV
LCLS2 parameters. Final peak current:Ipk = 1kAStarting from ~12 A
Finite mismatch between laser heater and electron beam:
sr/sx = 2
Compression factor= 5 x 16
Energy-spread minimized at 5keV heater induced spread
Assumes ~ 2500 m of transport after linac
Final spread ~ 0.5 MeV
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20LCLS-II Planning Meeting, Oct 9-11, 2013
LCLS II 25 A Initial Current
BC2 at 1.6 GeV
LCLS2 parameters. Final peak current:Ipk = 1kAStarting from ~25 A
Finite mismatch between laser heater and electron beam:
sr/sx = 2
Compression factor= 4 x 10
Energy-spread minimized at 5keV heater induced spread
Assumes ~ 2500 m of transport after linac
Final spread ~ 0.5 MeV
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BC2 at 800 MeV
LCLS2 parameters. Final peak current:Ipk = 1kAStarting from ~12 A
Finite mismatch between laser heater and electron beam:
sr/sx = 2
Compression factor= 5 x 16
Energy-spread minimized at 5keV heater induced spread
Assumes ~ 2500 m of transport after linac
Final spread ~ 0.5 MeV @ 5GeV
BC2 @ 800MeV
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Effect of Plasma Oscillations
Long drift section between linac and undulators.
For the lower energy cases (2-3 GeV):Ldrift ~ ½ PlasmaPeriod.
Integrated impedance is effectively smaller since the collective field oscillates in time
For certain frequencies
Sin(kp L) ~ 0Overall spread reduced
k (rad/m)
Leff(m)
effective Drift-length VS wavenumber
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23LCLS-II Planning Meeting, Oct 9-11, 2013
Conclusions
-MBI is the largest source of energy-spread for LCLS1-2 linacs.
-Microbunching instability is weaker in LCLS-2 than we are used to for LCLS1.
-Heater level around ~5 keV needed to minimize energy spread.
-Long drift between linac and undulators is a source of increased energy-spread but self-consistent electron response comes to our aid!
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End
LCLS-II Planning MeetingOctober 9, 2013
Thanks for your attention…