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HHG based Seed Generation for X-FELs

HHG based Seed Generation for X-FELs. Franz X. Kärtner, William S. Graves and David E. Moncton and WIFEL Team Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology Cambridge, MA, USA. Acknowledgement.

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HHG based Seed Generation for X-FELs

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  1. HHG based Seed Generation for X-FELs Franz X. Kärtner, William S. Graves and David E. Moncton and WIFEL Team Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology Cambridge, MA, USA

  2. Acknowledgement Students: Ch.-J. Lai, A. Benedick, S.-W. Huang, S. Bhardwaj A. Siddiqui, V. Gkortsas B. Putnam, Li-Jin Chen Research Scientists: K.-H. Hong, J. Moses Postdocs and Visitors: G. Cirmi (Politecnico Milano, Rocca Foundation) A. Gordon (Technion, Israel) O. Muecke (Techn. Univ. Vienna) E. Falcao (Pernambuco, Brazil)

  3. Outline • Required Seed Power Levels • Single Pass Efficiencies in High Harmonic Generation • Wavelength Scaling of HHG • Seed Generation for High Repetition Rate FELS A High Average Power HHG Source for 13.5 nmpumped by 515 nm Lasers (SHG of 1030nm), where powerful Yb-doped lasers exist

  4. Required Seed Power Levels Direct Seeding: 100 kW (30fs)  3 nJ Seeding for HGHG: 100 MW (30fs)  3 µJ  Push direct seed wavelength as short as possible. How does efficiency scale? Efficiency determines required drive pulse energy Repetition rate determines drive power: Efficiency: 10-6  Pulse energy 3 mJ // 3 J Rep. Rate 1kHz / 10MHz Power: 3W / 30kW // 3 kW / 30MW

  5. Electric Field, Position Time High Harmonic Generation Trajectories Three-Step Model Ionization Corkum, 1993 Cutoff formula ħωmax = Ip+3.17 Up

  6. Wavelength Scaling of HHG Efficiency Increase intensity Field amplitude Atomic units Decrease frequency (increase wavelength) Ionization potential Drive pulse frequency What is the impact on HHG conversion efficiency? 1.) Single-Atom Response 2.) Gas properties 3.) Phase matching 6

  7. HHG efficiency for N-cycle flat top pulse Cutoff E. L. Falcão et al., Opt. Expr. 17, 11217 (June, 2009).

  8. 400-nm driver (He) 800-nm (Xe) HHG Efficiency into Single Harmonic 800-nm driver (He) • Conversion efficiency very sensitive to drive wavelength and interaction parameters

  9. Experimental HHG Setup Telescope & Beam delivery 800-nm Ti:S amplifier (1 kHz, 7 mJ) Soft-X-ray spectrometer Beam input port Beam transport HHG chamber Pulsed nozzle

  10. HHG spectra generated by 400-nm driver Ar: 0.05 mbar Ne: 0.3 mbar He: 1 bar • Pulse energy of 0.94 mJ for all gases • Peak intensity: ~7.8x1014 W/cm2 (estimation) • Nozzle length: 2 mm

  11. Total HHG efficiency from 400-nm driver Ar: 0.05 bar Ne: 0.3 bar He: 1 bar • Conversion efficiency of up to 2x10-4 from He over Al window • “Good” agreement to analytic theory [1] E. L. Falcão-Filho et al., Opt. Express 17, 11217 (June, 2009).

  12. Efficiency per harmonic from 400-nm driver • 8x10-5 at ~35 eV and 1x10-5 at ~60 eV for He • 6x10-5 at ~27 eV for Ar

  13. HHG spectra generated from 800-nm driver Ne: 0.3 bar Energy: 2 mJ He: 1 bar Energy: 2 mJ Peak intensity: ~1.6x1015 W/cm2

  14. Total HHG efficiency from 800-nm driver Zr window (60-100 eV) Al window (20-70 eV) • Conversion efficiency of up to 2x10-6 from He over Al and Zr window • Efficiency per harmonic is one-to-two-order-of-magnitudes lower.

  15. 400-nm driver (He) 800-nm driver (He) Comparison with previous results • Conversion efficiency very sensitive to the driving wavelength • But predictable from our analytic theory that has shown a good agreement to experimental results studied by 400-nm and 800-nm drivers.

  16. 2-µm drive laser based on cryo-Yb:YAG pump laser Yb:YAG CPA system CFBG, YDFA, Yb:YAG regen amp + Yb:YAG multipass amp, grating stretcher 15 ps, 30 mJ @1kHz l = 1.0 µm Nd:YLF CPA system CFBG, 2 YDFA, Nd:YLF regen amp + 2 Nd:YLF multipass amp, grating stretcher 12 ps, 4 mJ @1kHz 800-nm OPCPA seed 800-nm OPCPA pump DFG MgO:PPLN Ti:Sapphire oscillator l = 2.0 µm 140µJ OPA 1 Si MgO:PPLN 1 mJ 30 mJ 2.5 mJ, 30 fs OPA 2 AOPDF OPA 3 • In final OPA stage: • Yb:YAG pump replaces Nd:YLF • BBO replaces MgO:PPSLT MgO:PPSLT Suprasil BBO

  17. Theoretical Prediction • 2.2-mm drive wavelength extends HHG cutoff to 500 eV • Conversion efficiency of 10-7-10-8 • Best current water-window experimental result: • 300 eV cutoff, h ~ 5x10-8, using multi-mJ 1.6-mm drive pulses • E. J. Takahashiet al., PRL 101, 253901 (2008). Simulation parameters: Gaussian pulse, tFWHM = 6 cycles Ne gas, p = 3 bar, L = 2.5 mm, w0 = 50 mm, E ~ 1 mJ

  18. High-flux, High Repetition Rate 13.5-nm (~93 eV) EUV source • With 515-nm drive pulses generated from SGH of powerful 1µm lasers  Efficiency into single harmonic: ~ 10-5

  19. Use enhancement cavity to scale efficiency to ~ 10-2 High Intensity Femtosecond Enhancement Cavities for High Repetition Rate FELs 19

  20. High-Power Enhancement Cavity • Requirements: optical beam access, high-intensity in interaction region, and low loss • 1-MW intracavity power, 10 mJ, ~100 fs pulses circulating • Cavity Finesse > 3000 patterned dielectric mirror 0.1 TW/cm2 1000 TW/cm2 2.6 mm 15 cm Confocal cavity for high-intensity Bessel-Gauss beams – Cavity shown enables 1000 TW/cm2 20

  21. Preliminary Cavity Demonstration First demonstration of cavity operation is carried out with CW laser. Also, axicon coupling optics excluded. Instead, collimated beam is used allowing measurement of intrinsic suppression of higher modes. Single-mode HeNe source Polarizer λ/2 Beam Expander Pellicle R=99% R=91% Photodiode 42 kHz 20μm Piezo 2μm Piezo PI LPF Lock-in Amp CCD 21

  22. Cavity Results With One Patterned Mirror Transverse profiles at cavity center • First cavity experiments done with single patterned mirror • Asymmetric modes seen, showing general structure of desired modes, but differing transverse profiles Pellicle (loss<1%) R = 99% R = 91% or 99% R=91% R=99% CCD 22

  23. Cavity Results With One Patterned Mirror One Patterned Mirror Two Patterned Mirrors Loss Loss only 2 modes (superposition modes in each direction) with <1% loss, next higher mode >5% loss ~30 modes with <1% loss Mode Mode 23

  24. Needs large average power Yb-doped Lasers! Thank You 24

  25. Analytical Bessel-Gauss Form of Modes The cavity modes have been analyzed numerically with custom paraxial wave optics software package. They can also be understood from an analytical perspective as Bessel-Gauss beams. Tilted Gaussian Beam Bessel-Gauss beam is a superposition of tilted Gaussian beams with wavevectors lying along the surface of a cone, 25

  26. Analytical Bessel-Gauss Form of Modes Bessel-Gauss beams traversing paraxial optical systems transform with an ABCD matrix similar to a Gaussian beam. Bessel-Gauss beams can then be shown to be modes of the confocal resonator, and the dominant modes of our special cavity. Numerically computed mode Field profile at focus: numerical versus analytical solution Analytical Bessel-Gauss mode Bessel-Gauss Modified Bessel-Gauss 26

  27. (b) Yb:YAG regenerative amplifier Yb:YAG crystal >40 W Yb:YAG crystals Fiber-coupled pump laser LN2 Dewar Fiber-coupled LD DM L2 DM L1 L1 L2 PC TFP l/4 Telescope FI l/4 Telescope TFP TFP Telescope TFP Regen output 5 mJ@2 kHz seed fs, Yb-fiber oscillator Telescope >60 mJ@2 kHz FI Telescope PBS l/4 1 mW 400 ps F1029 10 ps, >50 mJ@2 kHz l/2 l/4 CFBG stretcher (d) Multi-layer dielectric grating compressor 30-mW Yb-fiber preamplifier (1030 nm) Pump laser upgrade > 50 mJ, 2 kHz, 10 ps (c) Yb:YAG 4-pass amplifier (a) Fiber seed 27

  28. Summary • kW-class cryogenically cooled Yb:YAG ps-lasers are ideal for • Inverse Compton Scattering Sources (direct use) -> 2nd generation synchrotron like laboratory sources with exceptional beam properties • micron sized source ideal for phase contrast imaging • fs-pulse durations ideal for time resolved x-ray diffraction • Pumping of few cycle OPCPAs covering the visible to MID IR range • Analytic HHG efficiency formulas and wavelength scaling • Development of few-cycle 2-mm OPCPA (200 mJ) • Initial results on 800 nm OPCPA

  29. For a 5-cycle-driver-pulse, • Dk = 0, L=5 mm at 1 bar. (b) Same as (a) including plasma and neutral atom phase mismatching.

  30. Efficiency Measurement using Calibrated XUV Photodiode At 40eV, Al transmission = 30%, photodiode response = 4 electrons/photon photodiode response

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