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Photocathode lasers generating long trains of flat-top pulses

Photocathode lasers generating long trains of flat-top pulses. Ingo Will, Guido Klemz Max Born Institute Berlin. 1 m s. 100 ps (10mm glass plate). 800 m s. Optical sampling system for high-resolution measurement of the longitudinal pulse shape. Ingo Will, Guido Klemz

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Photocathode lasers generating long trains of flat-top pulses

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  1. Photocathode lasers generating long trainsof flat-top pulses Ingo Will, Guido Klemz Max Born Institute Berlin 1 ms 100 ps (10mm glass plate) 800 ms I.Will et al., Max Born Institute: Long trains of flat-top laser pulses

  2. Optical sampling system for high-resolution measurement of the longitudinal pulse shape Ingo Will, Guido Klemz Max Born Institute Berlin 100 ps (10mm glass plate) I.Will et al., Max Born Institute: Long trains of flat-top laser pulses

  3. The desired pulse trains and pulse energy(superconducting linac, Cs2Te photocathode) 1 ms Desired parameters (according to the requirements specified by DESY): • Spacing of the pulses: 1 ms • In future: 0.2 ms = 5 MHz (XFEL) and 0.11 ms = 9 MHz (option for FLASH) • Duration of the pulse train: at least 800 ms, variable • Very reliable synchronization • Rectangular envelope of the pulse trains • Energy: • > 100 mJ in the IR (I.e. l = 1047 nm) • corresponds to >100 W power during the pulse train • 15 mJ in the UV (I.e. l = 262 nm) 800 ms

  4. Desired pulse shape • Desired parameters of the micropulses • Wavelength: UV (262 nm) • Edges: < 2 ps (UV) • Noise in the flat-top region: < 10…20 % • Pulse duration t ~ 20 ps • Completely remote-controlled laser system • Very reliable synchronization 20 ps < 2 ps < 2 ps

  5. First completely diode-pumped pulse-train laser operational at PITZ since April 2005 Conclusion: The duration of the train in the amplifiers must be much larger (>1.5 ms) than the length of the output train

  6. Pre-compensation of changes of the pulse shape during amplification and conversion to the UV IR l = 1.047 mm green l = 0.524 mm UV l = 0.262 mm

  7. The MBI setup of a generator of long flat-top pulse trains for the superconducting linac photo- photo- photo- diode diode diode #1 #2 #3 output pulses booster pulse modelocked preamplifier amplifier shaper oscillator wavelength auxiliary main conversion pulse picker pulse picker IR -> UV 100 ps 100 ps

  8. MBI setup of a generator of long flat-top pulse trains for the superconducting linac • Effect of all components of the laser on the micropulsesshould be constant for a duration of 800 ms • Components of the laser should work with 1 MHz rep. rate during the train • High average power during the train of the final amplifiers: Ptrain > 100 W photo- photo- photo- diode diode diode #1 #2 #3 output pulses booster pulse modelocked preamplifier amplifier shaper oscillator wavelength auxiliary main conversion pulse picker pulse picker IR -> UV

  9. Selected pulse shaping techniques: suitability for single pulses and pulse trains

  10. Selected pulse shaping techniques/effects: suitability for single pulses and pulse trains UV output pulses

  11. Simple DST shaper forming flat-top laser pulses • Flat-top laser pulses • generate electron bunches with a flat-top shape in z-direction • -> improved brightness of the electron beam output pulses recorded with a streak camera:

  12. Simple DST shaper forming flat-top laser pulses • Flat-top laser pulses • generate electron bunches with a flat-top shape in z-direction • -> improved brightness of the electron beam output pulses recorded with a streak camera:

  13. Amplification of flat-top pulses from an Yb:YAG oscillator 100 ps (10mm glass plate) • Parameters of the pulses shown: • length of the train: 1.5 ms (1500 pulses) • Energy in the train: 27 mJ • Energy per micropulse: 18 mJ (at 1030 nm) • Streak camera measurement taken with SHG (at 515 nm) • Energy is ~ 4…5 times smaller than in the present Nd:YLF phothocathode laser • Increasing this energy is a major challenge to the laser designer Record of flat-top pulses with a synchroscan streak camera (Optronis, ~3...4 ps resolution) at 515 nm wavelength

  14. The MBI setup of a generator of long flat-top pulse trains for the superconducting linac photo- photo- photo- diode diode diode #1 #2 #3 output pulses booster pulse modelocked preamplifier amplifier shaper oscillator wavelength auxiliary main conversion pulse picker pulse picker IR -> UV 100 ps 100 ps

  15. Some amplification techniques: suitability for single pulses and pulse trains

  16. Part 2: OPCPA stage generating femtosecond pulses • OPCPA: Optical Parametric Chirped-Pulse amplification • Generates femtosecond pulses t 150 fs FWHM • pulse energy available at present : Emicro = 100 mJ (before compressor) Emicro = 50 mJ (behind compressor) • Available wavelength: • l = 790…830 nm • on request: l = 395…415 nm output pulse train which contains 700 micropulses

  17. Scheme of the Pump-Probe laser Pump laser OPCPA stage I.Will et al., Max Born Institute: Long trains of flat-top laser pulses

  18. 1ms (1000 pulses) 20 ms Regenerative amplifiers can be made to work at 1 MHz repetition rate • Specialty in burst mode:Each micropulse can extract only a small fraction (~ 0.2%) of the stored energy • Low stability (2% fluctuation during the train) • Reduced efficiency (~50%) in comparison to single pulses • Failure in the trigger will damage the amplifier • sophisticated software solution, (present DOOCS not save) • NL limiter • Fast repair technology Pulse traveling in the resonator Output pulses 50 ns

  19. Emicro = 3 mJ First regen Compensation of the drop by the drive current of the pump diodes, but the „pumping“ of the beam diamter remains! Emicro = 15 mJ Second regen 2ms (2000 pulses) Two-stage regenerative amplifier concept • Thermal lens in the power regen leads to a drop of the intensity to 50% during 2000 pulses • The two- or three-stage regen concept may enable us to apply advanced amplifier techniques (i.e. thin-disk amplifiers) Yb:KGW oscillator Yb:YAG regen Emicro = 3 mJ First regen DST shaper Drop due to thermal lensing Yb:YAG power regen Emicro = 15 mJ Second regen 2ms (2000 pulses)

  20. Amplification of flat-top pulses from an Yb:YAG oscillator 100 ps (10mm glass plate) • Parameters of the pulses shown: • length of the train: 1.5 ms (1500 pulses) • Energy in the train: 27 mJ • Energy per micropulse: 18 mJ (at 1030 nm) • Streak camera measurement taken with SHG (at 515 nm) • Energy is ~ 4…5 times smaller than in the present Nd:YLF phothocathode laser • Increasing this energy is a major challenge to the laser designer Record of flat-top pulses with a synchroscan streak camera (Optronis, ~3...4 ps resolution) at 515 nm wavelength

  21. Without compensationby pump current with compensationby pump current 5 ms(5000 pulses) 5 ms(5000 pulses) Amplification of long pulse trains for the cold linac by an Yb:YAG booster • Energy per micropulse: ~ 15 mJ • Amplification (two stages): G = 5...8 • Stable pulse train: control of the ramp of the current of the pump diodes • Stable beam diameter: beam-shaping aperture at the output • Can the beam-shaping aperture in the beamline play this role? • Technology for lossless stabilisation of the beam diameter: fast deformable mirror Emicro = 3 mJ Emicro = 15 mJ

  22. Shortest pulses and bandwidth of this amplifier combination Emicro = 3 mJ 2ps • Output pulses of the KGW oscillator: t = 0.5 ps • Output pulses of the regen combination: t = 1.8 ps • Can pulses of this duration efficiently be transferred to the UV(forth harmonics, l = 258 nm) ? Yb:KGW oscillator Yb:YAG regen d = 1.2mm 12ps Emicro = 2x0.1 mJ Yb:YAG power regen Emicro = 2x7 mJ

  23. Shortest pulses and bandwidth of this amplifier combination Emicro = 3 mJ • Output pulses of the KGW oscillator: t = 0.5 ps • Output pulses of the regen combination: t = 1.8 ps • Can pulses of this duration efficiently be transferred to the UV(forth harmonics, l = 258 nm) ? Yb:KGW oscillator 2ps Yb:YAG regen 12ps Emicro = 2x0.3 mJ Yb:YAG power regen Emicro = 2x7 mJ

  24. Broadband pulse: UV output pulse from Yb:KGW laser - sharp edges - sharp edges - E beam m - E > 20 J micro micro = 20 - stop l l - = 260 nm m J =1038 nm to photocathode BBO crystal beam stop Narrowband pulse from Nd:YLF laser - slow edges J m = 100 micro - E = 349 nm l - Two-channel mixing scheme: reduced energy requirements to the broadband laser amplifier • 75% of the total laser energy delivered by the Nd:YLF long-pulse system • only 25% need to be delivered by broadband channel

  25. Pre-shaping the beam of the photocathode laser may significantly reduce losses in the beamline present scheme pre-shapingby an aspherical Lens pair

  26. Summary: Long trains of flat-top laser pulses for the superconducting linac • Pulse shaping techniques: Only minor limitations • Most linear pulse-shaping techniques (gratings, filters etc) work for long trains • Techniques based on travelling acoustic waves (i.e. DAZZLER) cannot be used • Nonlinear techniques: limited duration of the pulse train (100...500 micropules) (only for nonlinear techniques using bulk materials) • Amplifiers: • Optical-parametric amplifiers (OPA): work without restrictions, but large pump laser required • For laser amplifiers: The broadband laser materials require regenerative amplifiers in the first stages. These regens can work at 1 MHz for Yb:YAG, Yb:KGW: • With somewhat reduced stability and with slightly less less efficiency (~ 50%) than for single pulses Reason: low saturation during each micropulse (0.2% energy extraction per pulse) • Linear power amplifiers: work well with pulse trains • Some problems arise from the thermal lens, that drifts during the train Solution: Dynamic correction with fast deformable mirrors • Ti:Saphire: No solutions for long, intense pulse trains available • We have made the correct choice for the laser material: Diode-pumped Yb:KGW and Yb:YAG instead of Ti:Saphire

  27. Amplification of flat-top pulses from an Yb:YAG oscillator 100 ps (10mm glass plate) • Parameters of the pulses shown: • length of the train: 1.5 ms (1500 pulses) • Energy in the train: 27 mJ • Energy per micropulse: 18 mJ (at 1030 nm) • Streak camera measurement taken with SHG (at 515 nm) • Energy is ~ 4…5 times smaller than in the present Nd:YLF phothocathode laser • Increasing this energy is a major challenge to the laser designer Record of flat-top pulses with a synchroscan streak camera (Optronis, ~3...4 ps resolution) at 515 nm wavelength

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