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.