Two-Stage Chirped-Beam SASE-FEL for High Power Femtosecond X-Ray Pulse Generation

Transcription

1 Two-Stage Chirped-Beam SASE-FEL for High ower Femtosecond X-Ray ulse Generation C. Schroeder*, J. Arthur^,. Emma^, S. Reiche*, and C. ellegrini* ^ Stanford Linear Accelerator Center * UCLA LCLS-TAC 1

2 Schematic of SASE X-ray FEL: electron beam SASE FEL Undulator output X-ray optics experimental stations electron beam dump LCLS output parameters: Radiation wavelength 1.5 Å Mean peak power 7.7 GW Bandwidth Intensity fluctuations 6 % ulse duration, rms 67 fs Disadvantages of standard SASE FEL configuration: Shot-to-shot fluctuations of power after monochromator will increase with increasing photon energy resolution. Conventional x-ray optical elements (monochromator) may suffer damage due to the high output power. Large shot-to-shot fluctuations in mean electron beam energy (0.1%) results in shotto-shot fluctuations of resonant frequency LCLS-TAC 2

3 Two-stage chirped pulse seeding for short pulse production: energychirped electron beam electron beam bypass SASE FEL FEL Amplifier monochromator 1 st Undulator frequencychirped input 2 nd Undulator Resonant condition: ω 2ω u = 2 γ γ 2 output ulse duration selection: (monchromator bandwidth and amount of chirping define pulse duration) δω ω δγ γ Stabilize central frequency: (shot-to-shot jitter in mean electron beam energy) LCLS-TAC 3 (meters) σ = z 36 µm (meters)

4 Two-stage LCLS FEL arameters: LCLS FEL arameters: Radiation wavelength 1.5 Å FEL parameter Undulator type planar Undulator period 3 cm eak magnetic field 1.32 T Undulator strength parameter 3.71 Repetition rate 120 Hz Electron Beam arameters: Electron energy 14.3 GeV Norm. beam emittance 1.1 mm mrad Average beta function 18 m Undulator 1: (L 1 = m) eak current 3.4 ka Bunch duration, rms 120 fs Uncorrelated energy spread, rms % Correlated energy chirp (FWHM) 0.5 % Undulator 2: (L 2 = m) eak current 4.0 ka Bunch duration, rms 103 fs Uncorrelated energy spread, rms % LCLS-TAC 4

5 First Undulator: (energy-chirped) electron beam SASE FEL 1 st Undulator (frequency-chirped) Input Electron Beam arameters: eak current 3.4 ka Bunch duration, rms 120 fs Energy spread, rms % Energy chirp (FWHM) 0.5 % L = 43.2 m < L 1 SASE sat Output Radiation arameters: Mean peak power 13 MW Frequency chirp 1 % FEL bandwidth Rayleigh range 40 m Require: << (1) out sat 1. Reduce damage to optical elements of monochromator. 2. Energy spread of electron beam after the first undulator will satisfy (1) out σ γ ρ < sat ρ L 1 (meters) LCLS-TAC 5 Simulation results obtained using GENESIS 1.3 (1) out (1) out sat 10-3

6 Monochromator: femtosecond pulse generation λ λ chirp 10-2 T m = 41 % monochromator (1) out T m L FWHM 8.7 fs σ λ λ (mono) rms 1.3x % energy chirp FWHM pulse duration (fs) 0.5% energy chirp 1.0% energy chirp Monochromator bandwidth, rms LCLS-TAC 6

7 Monochromator: Bandwidth selection by Bragg diffraction in crystals [ e.g., Ge(111) ]: crystal crystal crystal d crystal σ λ λ (mono) rms 1.3x10-4 path delay = L = 2d tan θ Monochromator arameters: Ge(111) Nominal photon energy Reflection angle 8.3 kev 0.24 rad Monochromator bandwidth, rms 1.3x10-4 ower transmission (0.8/reflection) T m = 41 % Tunability hoton beam path delay kev 5 mm LCLS-TAC 7

8 Electron Beam Bypass: meters β y β x meters Bypass arameters: Total Length R 56 ath delay Maximum displacement Deflection angle Bend magnetic field Bend magnet length Quadrupole strength (max) Quadrupole length 32.4 m 3.6 mm 5.0 mm 20.5 cm 1.68 deg 0.4 T 3.5 m 82 T/m 50 cm Schematic of non-isochronous achromatic chicane for electron beam bypass: dipole dipole doublet doublet dipole dipole LCLS-TAC 8

9 Second Undulator: Input Electron Beam arameters: eak current 4.0 ka Bunch duration, rms 103 fs Energy spread, rms % Input Radiation arameters: Mean peak power 3.2 MW ulse duration, FWHM 8.7 fs Bandwidth, rms Intensity Fluctuations 30 % electron beam femtosecond x-ray pulse L 2 = < L FEL Amplifier 2 nd Undulator SASE sat = out sat output Require: (1) << shot in = T m T diff Radiation power from monochromator dominates over the shot noise, such that the FEL will amplify the input signal (with bandwidth compared to SASE FEL bandwidth). out shot 3 ~ 10 (2) in out (meters) LCLS-TAC 9 L Simulation results obtained using GENESIS 1.3

10 Shot-to-shot fluctuations: Radiation robability Distribution after Monochromator: Negative Binomial Distribution 1 + Γ( + M ) p Γ( + 1) Γ( M ) 1 + Standard deviation of power into second undulator: in ( ) = M σ = 1 M 2πσ L p c M in M Log 10 [ out (W)] Relative rms fluctuations of output power (1) (3) Dependence of output power on input power for FEL amplifier log 10 (1) (3) ( ) in L 2 = L 2 = L 2 = m m m σ Shot-to-shot fluctuations of output power are reduced by operating the FEL amplifier in the non-linear regime. L 2 (meters) LCLS-TAC 10

11 Output Radiation arameters for Two-Stage LCLS: Two-Stage FEL Output Radiation: Mean eak Radiation ower: Radiation wavelength 1.5 Å Bandwidth, FWHM Undulator 1 Bypass + Monochromator ulse duration, FWHM Mean peak power 8.7 fs 23 GW ower fluctuations, rms 2 % RMS spot size 31 µm RMS angular divergence 0.5 µrad Undulator 2 meters LCLS-TAC 11 Simulation results obtained using GENESIS 1.3

12 Monochromator: short pulse limit Output pulse fwhm (fs) Effect of uncertainty principle (E=8 kev) Effect of slicing (1% chirp on 120fs rms pulse) Monochromator resolution δω/ω (rmsx10-5 ) Monochromator with smaller bandwidth slices out shorter pulse δt out = δt in x δω mono /δω chirp But uncertainty principle gives a limit δω mono x δt out 1/2 Note that if the uncertainty principle dominates, then the output pulse has complete longitudinal coherence For LCLS at 8 kev with 1% chirp, the minimum pulse length is about 3.5 fs fwhm, using a monochromator resolution of 3.3x10-5 rms. Some practical monochromator options: Crystal reflection Ge(111) Si(111) Si(220) rms resolution 14x x x10-5 Output pulse fwhm 9 fs 4.1 fs 4.1 fs LCLS-TAC 12

13 Conclusions The Two-Stage Chirped-Beam SASE-FEL offers: 1. An attractive way to produce high intensity X- ray pulses in the 10 to 20 fs range 2. Improved stability of the central frequency 3. Reduced load on optical elements 4. It can be built as an upgrade to present LCLS design LCLS-TAC 13