LCLS Laser Heater Review LCLS Laser Heater Review March 1, 2004 March 1, 2004 Zhirong Zhirong Huang, SLAC Huang, SLAC 1 [email protected][email protected]Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Suppression of Microbunching Instability in the LCLS Z. Huang, M. Borland (ANL), P. Emma, J. Wu C. Limborg, G. Stupakov, J. Welch Suppression of Microbunching Instability in the LCLS Z. Huang, M. Borland (ANL), P. Emma, J. Wu C. Limborg, G. Stupakov, J. Welch Motivation CSR/LSC Instability Cure (laser heater) LCLS LCLS
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Linac Coherent Light Source Stanford Synchrotron … · Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center ... PAC03 Local rms
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Linac Coherent Light Source Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center
Microbunching instabilityMicrobunching instability• Initial density modulation induces energy modulation through long. impedance Z(k), converted to more density modulation by a chicane growth of local energy spread/emittance!
Linac Coherent Light Source Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center
SDL RF zero-phasing measurementSDL RF zero-phasing measurement• RF zero-phasing method can be used to extract LSC-induced energy modulation in the linac (Shaftan/Huang)
Linac Coherent Light Source Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center
Space charge oscillationSpace charge oscillationCurrent modulation Energy modulation
• Space charge oscillation in the photoinjector (see C. Limborg’s talk). Initial density modulation can be reduced at the expense of increased energy modulation (which is also bad)
Linac Coherent Light Source Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center
LSC impedanceLSC impedance
• Free-space longitudinal space charge impedance
• At end of photoinjector, beam density modulation freezes and energy modulation accumulates
rb
λ
1/γ
• Evaluate microbunching gain relative to density modulation at injector end, which may over/under estimate gain relative to the drive laser modulation
Linac Coherent Light Source Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center
• High BC1 gain significant energy modulation in Linac-2, which can be temporally smeared in BC2 and becomes effective energy spread ( SC wiggler too late)• Slice energy spread at undulator (14 GeV), seeded with 1% initial density modulation (see J. Wu’s talk)
Growth of energy spreadGrowth of energy spread
20 30 40 50 60Λ0 �Μm�
5
10
15
Σ∆f��10
4�
FEL limitElegantTheory
Need < 0.1% initial density modulation or suppress theBC1 gain effectively
Linac Coherent Light Source Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center
Laser HeaterLaser Heater
2 cm2 cm
10 cm10 cm
10 cm10 cm 50 cm50 cm
~120 cm~120 cm
θθ ≈≈ 5.75.7ºº
• Laser-electron interaction in an undulator induces rapid energy modulation (at 800 nm), to be used as effective energy spread before BC1 (3 keV 40 keV rms)
• Inside a weak chicane for easy laser access, time-coordinate smearing (Emittance growth is completely negligible)
Linac Coherent Light Source Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center
• Large laser spot generates “double-horn” energy distributioin, ineffective at suppressing short wavelength microbunching• Laser spot matched to e-beam size creates better heating
�50 0 50�Γ0mc2 �keV�
0
0.005
0.01
0.015
V��ke
V��
1�
large laser spot
matched laser spot
50 100 150 200Λ0 �Μm�
0
5
10
15
BC1Gain Elegant �matched laser spot�
Theory �matched laser spot�Elegant �large laser spot�Theory �large laser spot�
Linac Coherent Light Source Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center
Control of longitudinal phase spaceControl of longitudinal phase space• Large laser spot yields large total gain, which is still capable to amplify <0.1% initial density modulation• For larger initial density modulation (1%), gain saturates due to longitudinal phase space distortion, but energy spread increase
50 100 150 200Λ0 �Μm�
0
100
200
300
Total
Gain Elegant �matched laser spot�
Theory �matched laser spot�Elegant �large laser spot�Theory �large laser spot�
50 100 150 200Λ0 �Μm�
2
4
6
Σ∆f��10
4�
FEL limitHeater �matched laser spot�Heater �large laser spot�No laser heater