Optics for VUV and soft x-ray FEL Oscillators

1. Optics for VUV and soft x-ray FEL Oscillators Michelle Shinn & Steve Benson Future Light Sources Jefferson Lab March 5, 2012 Work supported by the U.S. Dept. of Energy under contract DE-AC05-06OR23177 and the Commonwealth of Virginia

2. Introduction • Past DOE Basic Energy Science (DOE-BES) workshops have stressed the desire to have a soft x-ray FEL operating at 1keV (1.24nm) . • While great progress is being made to achieve this energy, it remains an elusive goal. • This presentation proposes a path forward to achieve this energy • Use an FEL oscillator’s harmonics to provide the harmonics, and modulation to seed a radiator undulator. • Must consider the challenging environment for the cavity optics.

3. The flat HR optical resonator • Last year we published a design* for an FEL oscillator operating in the VUV (12-150nm). • In the process of designing the optical resonator, we came up with an entirely new architecture dubbed, “flat HR” • This high gain, very low Q resonator makes use of the low divergence beam to avoid diffraction at the wiggler. • Strong optical guiding permits this architecture, even though the cold cavity is unstable. • Such a cavity architecture should also work at higher energies, where the 3rd or 5th harmonic output can be used as a seed for a radiator, producing coherent output at ~ 1keV. • Goal is to produce fundamental output in the GW level, so 3rd & 5th harmonic’s powers are of order 1MW. • To keep the resonator length reasonable, rep rate will be in the range 2.5 – 5 MHz. * Benson et al JMO 58 p1431 (2011)

4. Flat HR FELO schematic and transverse profiles @ 12.4nm

5. Wiggler and e beam parameters • Cold cavity parameters calculated using Paraxia-Plus (Sciopt Inc) • 3D FEL simulations were done using Genesis/OPC

6. VUV FEL output • Very high gain g = 6 @ 12.4nm, g=21 @ 20nm • Saturated gains > 1, imply intracavity power much lower than output.

7. Advantages • Relative to a two curved-mirror resonator, the flat HR resonator • Does not have evidence of mode-hopping or shift in position in an attempt to avoid the hole. • Is less sensitive to mirror steering (e.g. vibration). • To changes in mirror figure • It’s easier to maintain flatness of the HR. • Loading on OC is minimized due to the majority of power transported through the hole.

8. VUV FELO @ 200eV • Goal is to seed downstream radiator with 5th harmonic • Must not induce too much energy spread on the exhaust e beam. • 2GeV, 100 pC e beam • Minimum rep rate of 4.68 MHz • Genesis/OPC single slice *G. Neil private communication Based on discussions with FOM-IPP staff

9. Output characteristics at 200eV • Lasing efficiency = 0.225%  P = 2.1 kW (!) E = 0.45mJ Far-field output net gain ~ 100% sat gain ~ 13 Wiggler input Wiggler exit HR mirror

10. SXFELO @ 340eV • Goal is to seed downstream radiator with 3rd harmonic • Relative to 200eV case: • Same e beam parameters • Wiggler gap larger • Outcoupling hole ½ as large • Reflectivity lower • Genesis/OPC single slice * C. Montcalm et al Appl. Opt. 35 (1996)

11. Output characteristics at 340eV • Lasing efficiency = 0.076%  P = 714W E = 0.15mJ Far-field output net gain ~ 30% sat gain ~ 10 Wiggler exit HR mirror Wiggler input

12. Optics considerations • Scattering • While scattering does not distort optics, it competes with the reflectivity, so it must be considered. • Hole quality • Requires ion milling to achieve desired shape and smoothness. • Laser damage • A survey of the literature shows that laser-induced damage is dominantly thermal in nature. • Damage most likely to occur at the HR mirror. • Fluence estimated to be 480mJ/cm2 @ 200eV and 160mJ/cm2 @ 340eV • For comparison, Ethr = 45mJ/cm2 @ 92eV* & many 100s/mJ/cm2 @ 830eV** • Suggests we need to consider a longer resonator. • Cryogenically cool the mirrors – known to raise damage threshold. * A.R. Khorsand et al Opt. Ex. 18 (2010) ** S. P. Hau-Riege et al Opt. Ex. 18 (2010)

13. Conclusions • We’ve presented conceptual designs for FEL oscillators at 200eV and 340eV. • Based on the flat HR architecture. • Lasing induces modulation at harmonic’s frequencies and the harmonics seed a radiator to produce output in the soft x-ray region (~ 1keV) • Mirror reflectivity's at higher energies continue falling, and become too low for energies above ~ 500eV. • Appears to be a viable alternative to laser-seeded amplifiers in the 0.1 – 2keV energy range. • Laser damage must be managed – • Resonator length will do this, of order 60m. • Future plans • Optimize oscillator e.g., OC ROC, hole size, etc. • Use 4D Genesis/OPC and Medusa/OPC to predict performance of complete system  oscillator + radiator

14. Acknowledgements • Anne Watson (JLab/NC State) & Peter van der Slot • Genesis/OPC software development and discussions. • Gwyn Williams – font of information on VUV (and beyond) optical properties. • George Neil – for goading us to think beyond near-concentric resonators.