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VUV Optical Transport to User Lab 1. Michelle Shinn Director's Review of Proposed Pilot Experiments at the Jlab VUV/FEL May 20, 2011. - PowerPoint PPT Presentation
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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.
Outline
• Introduction
• The current VUV optical transport system
• Proposed enhancements to meet evolving user requirements
– Design methodology
– Optics required
– Design results
• Conclusions
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.
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.
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 sec
• Measurement of the spectrum (100 – 500nm)
– McPherson 218 with an IRD AUX100 detector
– Monochromator would be attached to beam dump at end of experiment.
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
OC mirror vesselTurning mirror
~11m
~7m
~1m
VaultUser Lab 1
VUV experiments will be in User Lab 1
General Purpose
PLD Microfab
THzLab
Dyna-mics
Nano/NASA
Optics/ Materials
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
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.
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
Parameter Value Spectral range 7-12eV
Vacuum environment ~ 3 x 10-7 torr Translational repeatability <0.2 mm
Angular repeatability <200 rad Power-handling capability (cooled mirrors) 500 W incident 10% absorbed
Input diameter on mirror 1.75 cm
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
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
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
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 /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
The grating is a challenging component
• The grating must separate a high average power fundamental from the 3rd harmonic, which is ~ 103
times 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.
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.
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)
Modeled results for the focused beam
• Goal, achieve best focus ~2m away from mirror.
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
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.