VUV Optical Transport to User Lab 1

1. VUV Optical Transport to User Lab 1 Michelle Shinn Director's Review of Proposed Pilot Experiments at the Jlab VUV/FEL May 20, 2011 This work was supported by U.S. DOE Contract No. DE-AC05-84-ER40150, the Air Force Office of Scientific Research, DOE Basic Energy Sciences, the Office of Naval Research, and the Joint Technology Office.

2. Outline • Introduction • The current VUV optical transport system • Proposed enhancements to meet evolving user requirements • Design methodology • Optics required • Design results • Conclusions

3. Introduction • Steve Benson’s just presented details on the UV Demo FEL and our initial characterization of the 10eV output. • This year we have succeeded in transporting pulsed output into User Lab 1 of the FEL Facility. • We also acquired and borrowed some VUV optical diagnostics for future characterization of the output. • I’ll discuss enhancing this beamline. • Users have requested we disperse the raw output to provide only the 3rd harmonic to their experiments. • Joe Gubeli will present addition details of this beamline and provide an estimate to implement this design.

4. Our FEL beamline design methodology lowers risk in implementation • Our optical transport components have grown more sophisticated over time as the requirements have grown more rigorous. • Range from one static, uncooled in-vacuo mirror • To four cooled, actuated, gimbal-mounted mirrors with associated orientation and thermometric transducers. • In-vacuo power-handling to 50 kW • Optical and thermal modeling used to ensure design meets specifications. • The current and proposed optical transport optomechanics are built using proven designs. • It is the optical elements that have unique requirements.

5. Features of the current VUV OTS • The VUV optical transport system (OTS) has much in common with our two other FEL transport systems: • Water-cooled mirrors for transporting high power beam upstairs • Beam viewers to determine the position and mode size of the fundamental at the turning mirror positions. • Measurement of the power • Averaged - several second time constant • “Fast” - over a few msec • Measurement of the spectrum (100 – 500nm) • McPherson 218 with an IRD AUX100 detector • Monochromator would be attached to beam dump at end of experiment.

6. The VUV OTS brings beam from the vault to the users • Beam transported in vault to a position under User Lab 1, then brought upstairs. • Propagation distance from the outcoupler to the lab is ~ 20 m User Lab 1 Vault ~1m OC mirror vessel ~7m Turning mirror ~11m

7. VUV experiments will be in User Lab 1 General Purpose Optics/ Materials Nano/ NASA Dyna- mics Micro fab THz Lab PLD • Current User Facility has 7 Labs • Lab1 General set-ups and prototypes • Lab 2 Materials studies • Lab 3 THz dynamics and imaging • Lab 3a NASA nanofab • Lab 4 Aerospace LMES • Lab 5 PLD • Lab 6 FEL + lasers for dynamics studies

8. Our users have requested enhancements to this beamline • Our users have expressed concern that the fundamental will induce multiphoton interactions that will complicate the experimental results. • To meet their requests, we need to: • Disperse raw output to provide only 3rd harmonic to their experiments. • We’d like to add: • Beam viewers to determine the position and mode size of the 3rd harmonic at various positions in the beamline. • Measurement of the spectrum independent of the experimenter’s equipment state.

9. Proposed new VUV OTS top-level specifications • Beam sizes are for the first two turning mirrors and grating. • Specifications can be met, based on previous experience

10. A schematic view of the new VUV OTS • The optical transport system- • Separates the fundamental from the 3rd harmonic • Harmonic beam is condensed or brought to a focus • Slit at focus for bandwidth control and stray light rejection • “Raw beam” option available • Insertable mirror delivers f-matched pulsed beam through a LiF window to monochromator • Isolating the monochromator from beamline vacuum lowers contaminants

11. A schematic view of the new VUV OTS • The optical transport system- • Separates the fundamental from the 3rd harmonic • Harmonic beam is condensed or brought to a focus • Slit at focus for bandwidth control and stray light rejection • “Raw beam” option available • Insertable mirror delivers f-matched pulsed beam through a LiF window to monochromator • Isolating the monochromator from beamline vacuum lowers contaminants

12. A schematic view of the new VUV OTS • The optical transport system- • Separates the fundamental from the 3rd harmonic • Harmonic beam is condensed or brought to a focus • Slit at focus for bandwidth control and stray light rejection • “Raw beam” option available • Insertable mirror delivers f-matched pulsed beam through a LiF window to monochromator • Isolating the monochromator from beamline vacuum lowers contaminants

13. Optical specifications for the turning and telescope mirrors • The telescope is Keplarian in design • Two 3” diameter spherical mirrors, one with ½ the ROC of the other to reduce beam size by 2x. • In this case, 4m & 2m ROC mirrors separated by 3m. • Provide translation on 1 mirror to set collimation accurately. • We routinely receive silicon substrates polished to 0.5nm microroughness. • Yields <0.5% total integrated scatter per mirror, so not an issue. • A mirror figure of l/30 will be challenging for our usual laser optics vendors, but well within the capabilities of vendors of synchrotron mirrors. • We have the ability to characterize these mirrors. • Wyko RTI4100 laser interferometer • Wyko NT1100 noncontact optical profilometer

14. The grating is a challenging component • The grating must separate a high average power fundamental from the 3rd harmonic, which is ~ 103times weaker. • If users desire a lot of dispersion, we must correct for the effective astigmatism caused by the grating’s linear dispersion. • Angular dispersion acts like a defocusing cylindrical lens • At this time, groove densities up to 300 gr/mm doesn’t require this correction. • Correction would be done by increasing the angle of incidence on the first telescope optic. • Will need to actively cool the grating. • With the anticipated absorbed power, should only require water cooling.

15. Optical modeling tools • Software tools like SRW or SHADOW are still being developed for FELs. • We use two physical optics software packages for optical transport designs • Sciopt “Paraxia Plus” • Runs quickly • Graphical interface • Limited inclusion of aberrations • Doesn’t handle the FEL interaction • A FEL interaction/optical propagation simulator • Genesis/OPC or Medusa/OPC • Perl script describes modes inside and outside of the optical cavity. • Runs more slowly, but aberrations and diffraction are accounted for far more completely.

16. Modeled results for the condensed beam • Goal is to reduce 10eV beam to ½ original size and collimate. • Desired by the ANL and Sandia groups • Use parameters for plane gratings produced for the McPherson 218 • 300 gr/mm, blazed at 124nm • Induces slight ellipticity on beam (~ 85% for 1% bandwidth)

17. Modeled results for the focused beam • Goal, achieve best focus ~2m away from mirror.

18. Estimated power throughput • Assume 100W of fundamental output, or 0.1W of 10eV at the outcoupler: • For the condensed beam, have 2 s-plane reflections, the grating (p-plane) and 3 p-plane bounces. • S-plane reflectivity in the VUV is ~90% • P=plane reflectivity in the VUV is ~75% • Grating efficiency ~ 30% (McPherson catalog) •  = (0.9)(0.9)(0.3)(0.75)(0.75)(0.75) = 0.1 (condensed beam) • For the focused beam we lose the last two p-plane reflections: •  = (0.9)(0.9)(0.3)(0.75) = 0.18 (focused beam) • Resulting intensity: • Condensed beam: 26mW/cm2 • Focused beam: 1.4kW/cm2

19. Discussion and conclusions • We have a beamline based on initial user input. • We’ve designed an enhanced beamline based on subsequent user input. • Cost for the “raw beam” option are estimated at ~$15K • Costs for the enhanced beamline estimated at ~$500k • More detail presented in this afternoon’s talk.