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The 4GLS VUV-FEL

The 4GLS VUV-FEL. Neil Thompson & David Dunning, MaRS Group , ASTeC Brian M c Neil , University of Strathclyde VUV-FEL Design + Jaap Karssenberg & Peter van der Slot, Twente University 3D Optics Code (OPC). Contents.

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The 4GLS VUV-FEL

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  1. The 4GLS VUV-FEL Neil Thompson & David Dunning, MaRS Group, ASTeC Brian McNeil, University of Strathclyde VUV-FEL Design + Jaap Karssenberg & Peter van der Slot, Twente University3D Optics Code (OPC) 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  2. Contents • NOT ‘How does an FEL work?’ but ‘What sort of FEL is the VUV-FEL?’ and ‘Why?’ and ‘What does it produce?’ • Types of existing FEL • Low Gain High-Q Oscillator • High Gain SASE • Output requirements for 4GLS VUV-FEL • ...and how they can’t be met by existing types of FEL • 4GLS VUV-FEL • Design and characterisation via thought and simulation 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  3. Random electron phase: incoherent emission Electrons bunched at radiation wavelength: coherent emission Types of FEL in operation 1. Low gain High-Q Oscillator electron bunch output pulse optical pulse 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  4. Examples of Oscillator FELs • CLIO, FELIX, JLAB, EUFELE • Common Features • Gain in 10’s of % • Short Undulator (a few m) • Modest Peak Current (10’s of A) • High Reflectivity Optics: > 95% • Small outcoupling fraction (~5-15%) • Pulse to pulse stability (long damping time of High-Q cavity) • Temporal coherence • Radiation transverse profile defined by resonator 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  5. Electrons bunched at radiation wavelength: coherent emission and saturation Random electron phaseincoherent emission Electrons bunching: coherence growing Types of FEL in operation 2. High Gain SASE FEL optical pulse 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  6. Examples of SASE FELs • FLASH, XFEL, LCLS • Common Features • High Gain • Long Undulator (10’s of m) • High Peak Current (kA) • No Optics • Pulse to pulse instability • Start from noise • Poor Temporal coherence • Start from noise • Radiation transverse profile defined by electron beam 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  7. Requirements for 4GLS VUV-FEL • 3-10eV photons • Rules out High-Q oscillator: no broadband optics with high reflectivity • Temporal coherence and pulse-to-pulse stability (variations < 10%) • Rules out SASE FEL • Solution: a ‘middle road’... • A medium length undulator ( ~10m ) with medium peak current (100’s A) • Gain high enough that low reflectivity optics can improve coherence and stability over SASE via small amount of feedback • Radiation transverse profile defined by electron beam and cavity • A Self-Seeding Amplifier FEL(also known as a Regenerative Amplifier FEL or RAFEL) 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  8. Design Parameters 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  9. G=4 • Assuming G = 4, higher reflectivity gives higher max output power, but more sensitivity to outcoupling fraction What Gain/Reflectivity Required?First analysis of cavity FEL via 1D simulations G=4πρNw • For G > 4 output intensity becomes relatively insensitive to reflectivity Choose electron beam and undulator parameters to give G = 4, accept R = 60% (feasible with coated Al), outcoupling = 75% (stability) 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  10. Hole radius set to outcouple ~75% of SR on first pass. Rayleigh length set to give ~75% outcoupling for TEM00. Mirror ROCs set to give cold-cavity mode (TEM00) fundamental waist at end of 1st undulator module (z=12m), maximising overlap over 1st and 2nd modules Optical CavityGeometry 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  11. 4GLS Layout 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  12. VUV-FEL Output 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  13. Cavity detuning curves:10eV Planar Polarisation Peak Power (MW) Pulse length (fs) Bandwidth Pulse Energy (μJ) 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  14. Stability: rms Variation10eV Planar Polarisation ‘spec’ Peak Power Pulse length ‘spec’ ‘spec’ Pulse Energy Bandwidth ‘spec’ 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  15. Time-bandwidth Product transform limited gaussian Coherence Analysis Pulse length Spectral Width 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  16. Intensity Cross SectionsCavity round trip 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  17. Outcoupling Evolution 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  18. OUTPUT POWER (W) CDR Cavity Optimisation: Output Powervs Hole radius and Reflectivity Scans have been done of hole radius, mirror reflectivity and cavity geometry (changing mirror ROC to adjust waist radius and position of fundamental cold cavity TEM00 mode). Aims are To determine sensitivity of performance to changes in CDR parameters To further optimise the cavity geometry • Output power relatively INSENSITIVE to reflectivity • Reflectivity REDUCTION gives small power INCREASE • Consistent with 1D simulations 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  19. The End 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  20. Absorbed Power Fitted ROC = 70m Thermal Loading FEA Analysis of Outcoupler • Average absorbed power = 24W (Doesn’t sound much ~ a light bulb) • Radiative cooling only: ΔT~700K! • Forced cooling: ΔT~80K • ROC change over 1mm strip around hole: 22.75m to 70m! 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

  21. Thermal Loading Possible Solutions • Adaptive optics • a deformable outcoupler allowing adjustable ROC • Cryo-cooling outcoupler • At -149 °C coefficient of thermal expansion for silicon is zero • Pinch electron beam near end of undulator • reduced source size gives stronger diffraction hence lower power density on mirror • Compensate for expected distortion by making anti-deformed mirror • ????... 4GLS VUV-FEL 4 ‘All hands on deck’ 29/03/07

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