ERL07 May17 07

Information about ERL07 May17 07

Published on November 21, 2007

Author: Laurie

Source: authorstream.com

Content

Drive lasers for photo injectors:  Drive lasers for photo injectors Ingo Will, Guido Klemz Max-Born-Institute Berlin, Germany in cooperation with DESY and FZD Driving a photo injector with a solid-state laser:  Driving a photo injector with a solid-state laser l = 523 nm l = 262 nm l = 1047 nm l = 1047 nm wavelength Converters IR -> UV (sent into the linear accelerator) Photocathode laser RF gun main selonoid bucking coil waveguide 1.3 GHz Cs2Te photo cathode ultraviolet laser pulses high-brightness (low emittance) electron bunches Driving a photo injector with a solid-state laser:  Driving a photo injector with a solid-state laser Roadmap:  Roadmap Photocathodes Solid-state photocathode lasers Picosecond Neodymium-doped lasers (Nd:YLF, Nd:YVO4, Nd:YAG) Single pulses and pulse trains Ti:Sa lasers for shorter pulses Pulse shaping Lasers with novel Ytterbium-doped lasers (Yb:YAG, Yb:KGW …) Made possible by the development of high-brightness pump diodes Can generate trains of shaped pulses Laser beamline Roadmap:  Roadmap Photocathodes Solid-state photocathode lasers Picosecond Neodymium-doped lasers (Nd:YLF, Nd:YVO4, Nd:YAG) Ti:Sa lasers for shorter pulses Lasers with novel Ytterbium-doped lasers (Yb:YAG, Yb:KGW …) Laser beamline Three most common Photocathode materials, desired laser wavelength and pulse energy:  Three most common Photocathode materials, desired laser wavelength and pulse energy Roadmap:  Roadmap Photocathodes Solid-state photocathode lasers Picosecond Neodymium-doped lasers (Nd:YLF, Nd:YVO4, Nd:YAG) Ti:Sa lasers for shorter pulses Lasers with novel Ytterbium-doped lasers (Yb:YAG, Yb:KGW …) Laser beamlines Laser class #1: Neodymium-doped laser crystals:  Laser class #1: Neodymium-doped laser crystals Typical crystals in use: Nd:YAG, Nd:YLF and Nd:YVO4 Transition: from flashlamp-pumping to diode pumps Diode-pumped lasers reach a very high stability (up to 1 %) and are more simple to maintain Used at ASTeC, FLASH, PITZ, CLIC, and in many other labs Suitable for single pulses and for pulse trains Example of a high repetition rate system: Nd:YLF laser for ELBE (FZ Dresden-Rossendorf) Examples for a system generating pulse trains: Nd:YLF laser driving the RF gun at FLASH (DESY Hamburg) and PITZ (DESY Zeuthen) 500 kHz Nd:YLF laser under development for the FZD (Rossendorf) :  500 kHz Nd:YLF laser under development for the FZD (Rossendorf) Pulse parameters of this system Max frequency: f  500 kHz Pulse shape: Gaussian pulse duration (UV): t = 16 ps Average power: In the IR : P = 5…10W In the UV (l = 263 nm): P ~ 0.5 W Bottleneck: Conversion to the UV in LBO+BBO crystals limits the average power to ~ 0.5…1 W. Present work concentrated on: Replacement of the vacuum tubes in the Pockels drivers by semiconductors (MOSFETs) Control computer RF and synchronisation system Slide10:  Nd:YLF photocathode laser for pulse trains in use at FLASH (DESY) and PITZ Slide11:  Nd:YLF photocathode laser for pulse trains in use at FLASH (DESY) and PITZ Nd:YLF photocathode laser for pulse trains in use at FLASH (DESY) and PITZ :  Nd:YLF photocathode laser for pulse trains in use at FLASH (DESY) and PITZ Nd:YLF photocathode laser for pulse trains in use at FLASH (DESY) and PITZ :  Nd:YLF photocathode laser for pulse trains in use at FLASH (DESY) and PITZ Pulse trains and pulse energy (superconducting linac, Cs2Te photocathode):  Pulse trains and pulse energy (superconducting linac, Cs2Te photocathode) 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 1 ms Desired parameters (according to the requirements specified by DESY): New requirement: pulse shaping to reduce the emittance of the electron beam:  New requirement: pulse shaping to reduce the emittance of the electron beam 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 The goal: Limits of pulse shaping in a Nd:YLF laser: (example: PITZ photocathode laser):  Limits of pulse shaping in a Nd:YLF laser: (example: PITZ photocathode laser) Micropulses have flat-top shape: duration: 15…25 ps (configurable) But: fluctuation during the flat-top: s = 3…8% edges: t ~ 5 ps, (desired for optimum emittance: t < 2 ps) Reason: limited fluorescense bandwidth of Nd:YLF Streak camera record of the UV output pulses of the PITZ photocathode laser Laser class #1: Neodymium-doped laser crystals:  Laser class #1: Neodymium-doped laser crystals Typical crystals in use: Nd:YAG, Nd:YLF and Nd:YVO4 Transition: from flashlamp-pumping to diode pumps Diode-pumped lasers reach a very high stability (up to 1 %) and are more simple to maintain Used at ASTeC, FLASH, PITZ, CLIC, and in many other labs Suitable for single pulses and for pulse trains (macropulses) Examples for a pulse train: Nd:YLF laser driving the RF gun at FLASH (DESY Hamburg) and PITZ (DESY Zeuthen) Example of a high repetition rate system: Nd:YLF laser for ELBE (FZ Dresden-Rossendorf) Limits: Laser power: P < 20 W (IR), P < 2 W (UV) Pulse duration: s > 4 ps Limited pulse shaping capabilities: rising/falling edges of shaped pulses > 4 ps Roadmap:  Roadmap Photocathodes Solid-state photocathode lasers Picosecond Neodymium-doped lasers (Nd:YLF, Nd:YVO4, Nd:YAG) Ti:Sa lasers for shorter pulses Well suited for pulse shaping Lasers with novel Ytterbium-doped lasers (Yb:YAG, Yb:KGW …) Laser beamline Laser class #2: Titanium Saphire (Ti:Sa) lasers:  Laser class #2: Titanium Saphire (Ti:Sa) lasers More complicated than Nd-doped lasers Ti:Sa systems contain sophisticated pump lasers Very suitable for development/optimisation of RF guns, Very short laser pulses can be generated (down to s ~ 5 fs) (or pulses with short edges) Very good pulse-shaping capabilities Example of a Ti:Sa system: photocathode laser at SPARC:  Example of a Ti:Sa system: photocathode laser at SPARC Taken from C.Vicario: Laser pulse shaping for high-brightness photoinjector CARE meeting, LNF, Nov15, 2006 Ti:Sa system allows for impressive pulse shaping: Example: photocathode laser at SPARC:  Ti:Sa system allows for impressive pulse shaping: Example: photocathode laser at SPARC Taken from C.Vicario: Laser pulse shaping for high-brightness photoinjector CARE meeting, LNF, Nov15, 2006 1. With a DAZZLER: Time distribution at oscillator level C. Vicario et al, EPAC04 Time distribution after the UV conversion H. Loos et al, PAC05 UV cross-correlation with 0.5 ps IR probe 2. With gratings: Laser class #2: Titanium Saphire (Ti:Sa) lasers:  Laser class #2: Titanium Saphire (Ti:Sa) lasers More complicated than Nd-doped lasers Ti:Sa systems contain sophisticated pump lasers Very suitable for development/optimisation of RF guns, Very short laser pulses can be generated (down to s ~ 5 fs) (or pulses with short edges) Very good pulse-shaping capabilities Day-to-day stable operation requires a high effort in maintenance Currently only suitable for single pulses, (no technical solution for pulse trains (macropulses) available at present) Roadmap:  Roadmap Photocathodes Solid-state photocathode lasers Picosecond Neodymium-doped lasers (Nd:YLF, Nd:YVO4, Nd:YAG) Ti:Sa lasers for shorter pulses Lasers with novel Ytterbium-doped lasers (Yb:YAG, Yb:KGW …) Laser beamline Laser class #3: Ytterbium-doped lasers under development:  Laser class #3: Ytterbium-doped lasers under development Crystals: Yb:YAG, Yb:KGW, Yb:SYS and many others Directly pumped by semiconductor diodes -> no need of sophisticated pump lasers (as used in Ti:Sa systems) Relatively simple to maintain Allow for short pulses (down to s = 0.1 ps) or shaped flat-top pulses with short edges Pulse trains (macropulses) can be generated Laser based on burst-mode regenerative amplifiers being developed for FLASH:  Laser based on burst-mode regenerative amplifiers being developed for FLASH Thermal lens in the power regen leads to a drop of the intensity to 50% during 2000 pulses, which needs to be compensated by an appropriate ramp of the drive current First regen Second regen Emicro = 15 mJ Emicro = 3 mJ 2ms (2000 pulses) Yb:KGW oscillator Yb:YAG regen Yb:YAG power regen DST shaper Formation of flat-top laser pulses:  Formation of flat-top laser pulses output pulses recorded with a streak camera: Flat-top laser pulses generate electron bunches with a flat-top shape in z-direction -> improved brightness of the electron beam Formation of flat-top laser pulses:  Formation of flat-top laser pulses output pulses recorded with a streak camera: Flat-top laser pulses generate electron bunches with a flat-top shape in z-direction -> improved brightness of the electron beam Pulse shaping capabilites of Ytterbium lasers (Yb:KGW/Yb:YAG system being developed for FLASH):  Pulse shaping capabilites of Ytterbium lasers (Yb:KGW/Yb:YAG system being developed for FLASH) Record of flat-top pulses with a synchroscan streak camera (~3...4 ps resolution) Present status: Energy is ~ 4…5 times smaller than in the energy delivered by Nd-doped lasers Increasing this energy is a major challenge 100 ps (10mm glass plate for calibration) Roadmap:  Roadmap Photocathodes Solid-state photocathode lasers Picosecond Neodymium-doped lasers (Nd:YLF, Nd:YVO4, Nd:YAG) Ti:Sa lasers for shorter pulses Lasers with novel Ytterbium-doped lasers (Yb:YAG, Yb:KGW …) Laser beamline Optical laser beamline imaging a beam-shaping aperture onto the photocathode :  Optical laser beamline imaging a beam-shaping aperture onto the photocathode Transverse flat-top laser profile at PITZ Proposal to reduce the losses in the optical beamline:  present scheme: losses ~ 80% Proposal to reduce the losses in the optical beamline only 20% transmission! Summary:  Summary Solid-state lasers driving the RF gun are a relatively small, but important component of the linacs The Laser pulses have strong impact on important parameters of the electron bunches generated Bunch length (duration) Bunch shape Timing Emittance -> very stable operation of the photocathode lasers is strictly required Laser pulse parameters (pulse energy, wavelength) is determined by Bunch charge Q to be produced Quantum efficiency of the photocathode Losses at the optical beamline Average power in the UV is limited to ~ 1 W by frequency conversion crystals (~ 1 mA average beam current for Cs2Te photocathode) Three classes of lasers to drive RF guns:  Three classes of lasers to drive RF guns For a bunch length s > 4 ps: Neodymium-doped laser media (Nd:YLF, Nd:YVO4 or Nd:YAG lasers) Pumped with semiconductor diodes: stability better than 1% Widely used, laser versions for both single pulses and pulse trains exist pulse-shaping capabilities too limited Lasers based on Ytterbium-doped media (Yb:YAG, Yb:KGW) Suitable for short pulses ( ~ 0.1 ps) or shaped pulses with short edges Yb:YAG/Yb:KGW lasers for trains of shaped pulses under development At present: relatively low pulse energy (efficient cathode required) For shorter pulses or shaped pulses with short edges: Ti:Sa lasers Very good possibilities for pulse-shaping Good for development/optimisation of RF guns More difficult to maintain than Nd-doped lasers At present: Only for single pulses, macropulses or pulse trains Several other, special laser types are being used to drive RF guns Direct pumping by semiconductor lasers Fiber lasers (Yb:glass) Driving a photo injector with a solid-state laser:  Driving a photo injector with a solid-state laser

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