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Microbunching Instability and Slice Energy Spread
D. Ratner Oct. 12, 2015
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Microbunching and slice energy spread
MBI & SES Microbunching and slice energy spread Study goals: use the XTCAV to measure MBI characteristics MBI dependence on laser heater Final slice energy spread (SES) at the FEL
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Microbunching Instability
MBI Model l z Current Gain Space Charge z Energy R56 Z. Huang
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Microbunching Instability
MBI Equations Metric: N particles in beam Gain: Impedance: wavenumber k, compression C, energy g, espread d, current I0, beam size sr
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Microbunching Instability
Measuring MBI Metric: COTR: Dispersion e- Space Charge OTR Screen
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Microbunching Instability
Measuring MBI Metric: Energy Position (z) Dispersion e- Space Charge
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Microbunching Instability
Measuring MBI Bunching Current Phase space L1 12 m MeV BC1 45 mm XTCAV BC2 15-50 mm L2 360 m GeV DL2 -0.15 mm 100 m DL1 -7 mm L0, 9m 0-135 MeV Trans. 140 m SXRSS Chicane 0-0.6 mm 130 m Laser Heater 8 mm L3/Trans. 820 m 5-4.3 GeV
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Microbunching Instability
Resolution Limits Resolve structures to 0.5um (but bk could be suppressed) Bunching factor vs. XTCAV voltage Resolution limit below 1um at 44MV
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Microbunching Instability
MBI vs. Laser Heater LH OFF
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Microbunching Instability
MBI wavelength Cutoff: at BC2 MBI grows d before BC2 (compare to 3keV before BC1 and factor of 8 compression 24 keV) But most growth after BC2 BC1 45 mm XTCAV BC2 25 mm DL2 -0.15 mm DL1 -7 mm SXRSS 0-0.6 mm Laser Heater 8 mm
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Microbunching Instability
Peak MBI gain vs. Laser Heater Wavelength gets shorter?!? BC1 45 mm XTCAV BC2 25 mm DL2 -0.15 mm DL1 -7 mm SXRSS 0-0.6 mm Laser Heater 8 mm
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Microbunching Instability
Peak MBI gain vs. Laser Heater Landau damping BC1 45 mm XTCAV BC2 25 mm DL2 -0.15 mm DL1 -7 mm SXRSS 0-0.6 mm Laser Heater 8 mm
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Microbunching Instability
How does BC2 R56 affect bunching? Larger R56 More gain Larger R56 More damping More damping Suppress short wavelength Less gain BC1 45 mm XTCAV BC2 25 mm DL2 -0.15 mm DL1 -7 mm SXRSS 0-0.6 mm Laser Heater 8 mm
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Microbunching Instability
BC2 scan 15mm 25mm 35mm 50mm
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Microbunching Instability
Correlations Sample shot, 500 A Projected current Looks a bit like longitudinal correlations…
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Microbunching Instability
Correlations Compare to simulations Shot noise FFT IFFT X gain Measured gain
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Microbunching Instability
Correlations Simulation: modulate shot noise with measured gain Projected current (that’s just how 50% bandwidth bunching looks)
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Microbunching Instability
Correlations Define autocorrelation: Upshot: No correlations (phases are random) 100% shot-to-shot fluctuations
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Microbunching Instability
Limitations of Metric: varies by day LH OFF June July
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Microbunching Instability
Limitations of Metric LH OFF 9.07 keV 11.9 keV 17.8 keV
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Measuring “slice” energy spread
Select middle of beam
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Measuring “slice” energy spread
Select middle of beam Calculate slice energy spread (linear interpolation because few pixels)
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Slice Energy Spread SES vs. Laser Heater
Solid lines show contribution from laser heater Peak current (kA) 0.5 1 1.6 SES (MeV) 0.9 1.2 Minimum SES: Dominated by MBI Dominated by heater
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Slice Energy Spread SES vs. Laser Heater LH OFF 9.07 keV 11.9 keV
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Conclusions MBI: SES: MBI + SES Strong MBI gain even by BC2
Energy spread dominated by L3 Increasing BC2 R56 reduces MBI Detailed MBI behavior is confusing! SES: Even at optimal LH setting, MBI dominates SES Need more effective way of suppressing MBI! Peak current (kA) 0.5 1 1.6 SES (MeV) 0.9 1.2
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Slice Energy Spread Questions?
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Slice Energy Spread MBI: Methodology
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Microbunching Instability
MBI Analysis: Method 1 Projection Choose integer # of wavelengths, calculate bunching factor Remove bunch shape
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Microbunching Instability
MBI Analysis: Method 2 crop Remove energy correlation Remove bunch shape (current variation across bunch) Take single pixel slices and calculate average bunching Longitudinal Position Current
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Microbunching Instability
MBI Analysis: Comparison Method 1 Method 2
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Slice Energy Spread Measuring SES Two metrics: FWHM or second moment
Choose FWHM in center of beam 2nd moment *1.18 FWHM e- dist
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Slice energy spread MBI Analysis: Comparison
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Microbunching Instability
MBI Analysis: Comparison
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Slice Energy Spread Panofsky-Wenzel Effect Fit of the form: 1.7kA 2kA
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Slice Energy Spread Panofsky-Wenzel Effect Fit of the form: 1.7kA 1kA
Apw=0.045 MeV/MV Apw=0.041 MeV/MV Apw=0.033 MeV/MV 0.5kA
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Slice Energy Spread SES vs. Laser Heater Fit of the form:
With correction for Panofsky-Wenzel
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Slice Energy Spread Panofsky-Wenzel Effect Fit of the form:
0.3 MeV/(mm MV) offset ~ 1.5MeV at 30MV, 150um beam Reduce slope by factor of 4 as directed by Henrik
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Slice Energy Spread SES vs. Laser Heater Fit of the form:
With correction for Panofsky-Wenzel
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Slice Energy Spread MBI: Questions
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Microbunching Instability
MBI Analysis Question for Yuantao: 500A, LH OFF 500A, LH=7.2uJ And why does it disappear when LH Off? What is this?
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Microbunching Instability
MBI Analysis Question for Ago and Zhirong: And what’s the deal with the bi-modal distribution? Can this be explained by multiple stages with different gain curves? 500A 1kA
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Microbunching Instability
Does DL2 Affect Bunching? As expected, BC2 dominates BC1 45 mm XTCAV BC2 25 mm DL2 -0.15 mm DL1 -7 mm SXRSS 0-0.6 mm Laser Heater 8 mm
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Microbunching Instability
Complex behavior? Landau damping BC1 45 mm XTCAV BC2 25 mm DL2 -0.15 mm DL1 -7 mm SXRSS 0-0.6 mm Laser Heater 8 mm
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