VUV Optical Transport to User Lab 1

Michelle ShinnDirector's Review of Proposed Pilot Experiments at the Jlab

VUV/FELMay 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.