R&D experiments at BNL to address the associated issues in the Cascading HGHG scheme Li Hua Yu for DUV-FEL Team National Synchrotron Light Source Brookhaven National Laboratory FEL2004
Outline The DUVFEL performance and the system parameters Suggestion on some of the experiments that can be carried out at DUVFEL to address the associated issues in the Cascading HGHG scheme
DUV-FEL Configuration DUVFEL using NISUS wiggler Step 1. SASE at 400 nm (2. 2002) Step 2. direct seeding at 266nm (8.2002) Step 3. HGHG 800nm 266 nm(10.2002) After energy upgrade Next Goal 400nm 100 nm
Deep UV Free Electron Laser at SDL Relation of pulse lengths During 2002-2003 operation Cathode driver 4 ps Uncompressed bunch 4-5ps Compressed 1ps Seed 9ps Nisus Wiggler FEL seed at 800 nm Modulator Undulator 177 MeV Normal incidence 77 MeV Ion Pair Imaging Experiment at 88 nm NISUS pop-in monitors FEL Measurements Energy, Spectrum, Synchronization and Pulse Length Measurements at 266 nm Dispersion Magnet RF zerophasing CTR Monitor Trim Chicane Adjustable Chicane Photoinjector 30 mj Ti:Sapphire Amplifier
800 nm HGHG Experiment 266nm MINI Dispersion Section NISUS 88nm HGHG from 800 nm266 nm, Output at 266 nm: ~ 120µJ, e-beam: 300 Amp, 3 mm-mrad, Energy 176MeV Output at 88 nm ~ 1 µj On-going Experiment Application in Chemistry
HGHG power vs. distance (266 nm) E-beam: 300 pc 4.7 m (rms) 1.ps (FWHM) Power measured by kicking beam off axis agree with photodiode measurement along NISUS
Bunch Length Measurement by Zero Phasing (12/2002) Head Left Head Right Projection of energy chirped pulse on energy axis, so the profile is not current but related to current. Ripples are due to small energy modulation, not large density modulation Average current: 300pC/1ps =300 Amp
Slice Emittance Measurement Without Compression Integrated emittance = 4.8 µm, Q=200 pc W.Graves, Data from 3/25/02. 4 ε n (µm) 2 0 0 5 10 Slice Number
Global energy spread measurement result: 5 10-4
Based on measurement of 12/11/2002: L G =0.8m, current = 300 amp, slice emittance n < 4mm-mrad Theory : local energy spread (rms) < 0.05% n 4.5 µrad 4.0 µrad 3.5 µrad 3.0 µrad L G (m) 2.5 µrad /
Two sets of data compared with TDA Simulation 10 2 Pulse Energy (µj) 10 0 (a) 1.8 MW (b) 30 MW TDA 0 2 4 6 8 10 Wiggler Length (m) σ γ /γ= ψ/γ!!" #!$%$&%%'% ψ/γ!!"($%$%%' %ψ/γ
Autocorrelation Pulse Length Measurement (Zilu, 2003) (a) (b) current 300 Amp, energy spread σγ/γ= 1 10 4, dispersion dψ/dγ = 8.7 (a) 12/11/02 Model: P in =1.8MW pulse length 0.6 ps, slice emittance 2.7 mm-mrad (b) 3/19/03 Model: P in =30MW pulse length 1 ps, projected emittance 4.7 mm-mrad Whole bunch contributes to the output
R&D Towards Cascading HGHG: One of the Earlier Calculated Schemes A Soft X-Ray Free-Electron Laser 1-ST STAGE 2-ND STAGE 3-RD STAGE FINAL AMPLIFIER MODULATOR λ w = 11 cm Length = 2 m Lg = 1.6 m AMPLIFIER λ w = 6.5 cm Length = 6 m Lg = 1.3 m MODULATOR λ w = 6.5 cm Length = 2 m Lg = 1.3 m AMPLIFIER λ w = 4.2 cm Length = 8 m Lg = 1.4 m MODULATOR λ w = 4.2 cm Length = 2 m Lg = 1.4 m AMPLIFIER λ w = 2.8 cm Length = 4 m Lg = 1.75 m AMPLIFIER λ w = 2.8 cm Length = 12 m Lg = 1.75 m DISPERSION dψ/dγ = 1 DISPERSION dψ/dγ = 1 DISPERSION dψ/dγ = 0.5 e- e- LASER PULSE e- e- DELAY Fresh Spent DELAY DELAY electrons electrons Fresh Spent electrons electrons FRESH BUNCH CONCEPT 1.7 GW 500 MW 266 nm SEED LASER 400 MW 800 MW 70 MW 5 53.2 nm 5 10.64 nm 5 e-beam 750Amp 1mm-mrad 2.6GeV σ γ /=2 10 4 total L w =36m 2.128 nm
Fresh Bunch Technique Laser pulse Before Shifter Electron bunch After Shifter Technical issues: Time jitter of seed << electron bunch length Reduce number of stages: higher harmonic, smaller local energy spread Shorter seed laser pulse
Some of the experiments that can be carried out at DUVFEL to address the associated issues in the Cascading HGHG scheme 1. Chirped Pulse Amplification study how to generate short pulse in HGHG 2. 8 th harmonic HGHG (800nm100nm) possibility to reduce the number of stages of HGHG study how small the local energy spread is relation of coherence with harmonic number? 3. HGHG with seed shorter than electron bunch length study the jitter and its relation to pulse shape 4. Regenerative synchronization of seed pulse and electron bunch accurate synchronization is essential for cascading 5. Cascading using NISUS+VISA: 400nm100nm50nm. Proof-of-principle experiment for cascading 6. Tuning of HGHG without changing seed (Timur Shaftan) feasibility study at DUVFEL
Preliminary Data of CPA HGHG Spectra (A. Doyuran, et al., 8. 2003) λ γ t Interpretation of 3 sets of measurements: effect of RF curvature; the second matched e-beam chirp to seed chirp
Chirped and Unchirped HGHG spectra SASE spectrum HGHG spectrum no chirp CPA HGHG ~1 MeV CPA HGHG ~2 MeV CPA HGHG ~3 MeV
FWHM Bandwidth (nm) Bandwidth vs Chirp 1/3 BW of seed 1.8nm 1.5nm γ/γ (%) Seed bandwidth is 5.5 nm currently, thus 1.8 nm is expected at third harmonic. Next: measurement of phase distortion Spectrum at γ/γ = 0.62 % CPA: expected pulse length > 50fs (B. Sheehy, Z. Wu) R&D: CPA at 100 nm?
Measuring the spectral phase: SPIDER (Spectral Interferometry for Direct Electric-Field Reconstruction) (Walmsley group, Oxford) 400 nm 266 nm 800 nm
intensity (arb units) radians Spidering a laboratory 266 nm source (B. Sheehy, Z. Wu_) 1 0.8 0.6 0.4 0.2 0-0.05 0 0.05 ω - ω 0 (PHz) 30 20 10 0 Typical Spider Trace Reconstructed phase and amplitude amplitude phase fit chirp phase-chirp Undercompress a 100 femtosecond 800 nm Ti:Sapph chirped-pulse-amplification system Frequency-triple in BBO to 266 nm(spoil phase matching to create an asymmetry in the time profile) Compare scanning multi-shot x-correlation of the 266 nm and a short 800 nm pulse with the average reconstruction, convolved with 250 fsec resolution of the x-correlator intensity (arb units) 1 0.8 0.6 0.4 0.2 Comparison with x-correlation spider cross correlation 900 fsec FWHM -10-0.05 0 0.05 ω - ω 0 (PHz) 0-1 -0.5 0 0.5 1 1.5 time (psec)
8 Preliminary Data of SPIDER for a chirped HGHG Phase and amplitude vs. time pulse shape and phase in time domain 6 4 Phase phase (Radian) (radians) 2 0-2 -4-6 -8-1500 -1000-500 0 500 1000 1500 fs Time (fs)
Preliminary Data of SPIDER for a chirped HGHG Frequency change vs. time (electron beam energy chirp unmatched ) 5 x 10-3 instantaneous frequency 4 3 2 (THz) f-f0 (PHz) 1 0-1 -2 Theory based on measured seed chirp -3-4 -5-2000 -1500-1000 -500 0 500 1000 1500 2000 fs Time (fs)
Possible Step: 8 th Harmonic HGHG 800nm 100 nm MINI Dispersion Section NISUS 33 nm Electron beam energy upgrade to 300 MeV Need to reduce modulator vacuum chamber gap Increase flexibility of wavelength tuning
Power vs. distance for 100 nm in HGHG from 800nm Calculated for the same beam conditions as the present HGHG: current 300 Amp, energy spread σγ/γ= 1 10 4, slice emittance 2.7 mm-mrad input seed with 60 MW Going for higher harmonic and increase dispersion will lower the upper limit of the local energy spread estimate, possibly below 1 10 4: 5 10 5?
Regenerative synchronization of seed pulse and electron bunch (L.H.Yu, FEL2003) HGHG output 266 nm, 1ps stretcher 266 nm, 6 ps compressor delay 12 6 11 266 nm, 0.2ps 7 10 9 8 6 3 3 5 3 3 4 4 4 11 2 1 The jitter between the seed pulse and the electron bunch will be reduced by the compression ratio It is possible to reduce jitter to below 50fs: an unprecedented precision. Important for multi-stage cascading to X-ray FEL Can be tested without significant cost under present conditions Energy fluctuation is also reduced by the same ratio
Cascading HGHG Using Fresh Bunches Under the present e-beam Conditions 400nm 2MW 200fs 100nm 20MW 200fs 50nm 180MW 200fs MINI NISUS NISUS VISA 6 m 0.8 m 3m e-beam 1ps 300Amp 2.7mm-mrad 288MeV σ /=1 10-4 γ
!" Measuring timing jitter between electron beam and laser is important in externally seeded FEL schemes like HGHG Single shot measurement decouples jitter from pulse length Estimated jitter from this electro-optic measurement between electron beam and seed laser is rms 170 fs Jitter between low-level RF and Ti:Sapphire oscillator is 200 fs Energy jitter obtained in dipole spectrometer corresponds to 1 ps RF amplitude/phase jitter
Tomography of electron bunch distribution by Henrik Loos 12/2/03 Reconstruction Uncompressed Bunch Reconstruction compressed Bunch 50 200 Energy (kev) 25 0 Energy (kev) 100 0-25 -100 E (kev) Current (A) -50-5 0 5 Time (ps) 20 0-20 -5-2.5 0 2.5 5 50 0-5 -2.5 0 2.5 5 E (kev) Current (A) -200 100 0-100 400 200 0-1 -0.5 0 0.5 1 Time (ps) -1-0.5 0 0.5 1-1 -0.5 0 0.5 1 Intensity 0.1 0-10 -8-6 -4-2 0 2 4 6 8 10 Time (ps) Intensity 0.1 0-10 -8-6 -4-2 0 2 4 6 8 10 Time (ps)
Illustration on how to use the electron bunch twice during cascading of HGHG 400 Current (A) 200 0-1 -0.5 0 0.5 1 Time (ps) 100 nm 50nm 400 nm 100nm Time (ps)
Cascading HGHG from 400nm to 100 nm, then to 50 nm, input 20MW, dispersion=3.6 If we improve beam current from 300 Amp to 600 Amp, the saturation is reached before 2 m in VISA
HGHG with variable wavelength (Timur Shaftan, 2004) There is a small compression of the electron bunch in the dispersion section, hence the wavelength is also slightly compressed.
Single-Shot spectra for different chirps for the same seed wavelength showed different output wavelengths (Timur, Brian, Henrik, Zilu 2004) Possible experiment in near future: increase dispersion increased tuning range
Summary Analysis of the present HGHG experiment provides estimates for the operating parameter range Based on the roughly determined beam parameters, we suggest some of the experiments that can be carried out at DUVFEL to address the associated issues in the cascading HGHG scheme: 1. Chirped Pulse Amplification 2. 8 th harmonic HGHG (800nm100nm) 3. HGHG with seed shorter than electron bunch length 4. Regenerative synchronization of seed pulse and electron bunch 5. Cascading using NISUS+VISA: 400nm100nm50nm. 6. Tuning of HGHG without changing seed (Timur Shaftan)