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NSLS-II Soft X-ray Undulator Beamlines. Steve Hulbert NSLS-II User Workshop July 17, 2007. NSLS-II Soft X-ray Undulator Beamlines.

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Nsls ii soft x ray undulator beamlines l.jpg

NSLS-II Soft X-ray Undulator Beamlines

Steve Hulbert

NSLS-II User Workshop

July 17, 2007


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NSLS-II Soft X-ray Undulator Beamlines

The extremely high brightness of NSLS-II in the soft x-ray range will enable the design and implementation of the next generation of soft x-ray beamlines. These beamlines will have

(resolving power)  flux  (spot size)-1

product considerably greater than is available presently (2007), as well as fast (kHz) polarization switching capability. These beamlines can be optimally matched to a broad array of soft x-ray measurement techniques.

Steve Hulberta, Dario Arenaa, Cecilia Sánchez-Hankea, and Ruben Reiningerb

aBrookhaven National Laboratory bScientific Answers & Solutions, Madison, WI


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NSLS-II Soft X-ray Undulator Beamlines

Outline

  • Source properties

  • Example soft x-ray scientific programs & measurement techniques

  • Beamline design, layout, and calculated performance

  • Comparison with other US soft x-ray beamlines

  • Outstanding technical issues

  • Summary



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Soft x-ray EPU45 source transverse coherence

Vertical, horizontal, andtotal number of transverse coherent modes contained in the EPU45 beam as a function of photon energy.


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Soft x-ray EPU45 source brightness

Source polarization: linear (horiz., vert. or any angle in between), circular, or elliptical

Higher harmonic interference:use QEPU

Soft x-ray mirror reflectivity

  • s-polarized reflectivity of 30nm-thick candidate mirror/grating coatings.

  • p-polarized reflectivities are similar at these small grazing angles.

  • Si substrate

  • assumed RMS surface roughness 0.2 nm.


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3D schematic of soft x-ray undulator layout

Experimental

Station

Beam Chopper

Plane Mirror

Exit Slit

Elliptical cylinder

KB refocusing mirrors

Plane Grating

Cylindrical Collimating Mirror

EPU

EPU

Side view, single beam mode

Side view, canted beam mode, fast switchable


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Possible NSLS-II Soft X-ray Undulator Beamlines

High flux

Soft x-ray imaging and coherent scattering

High resolution

Soft x-ray resonant magnetic and inelastic scattering (XRMS, RIXS)

Coherent

XRMS

Imaging

RIXS

Source for each beamline: Two 2m-long EPU45’s located in a 5m NSLS-II straight section.

Operating modes: (1) uncanted and phased as a single 4m-long EPU45, or (2) two 2m-long EPU45’s, canted by ~0.25 mrad, for fast-switching polarization capability.


Slide9 l.jpg

Magnification

Energy and angle selection

Acceleration, focusing

and magnification

PossibleHigh flux soft x-ray undulator endstations/techniques

  • Coherent imaging endstation: coherent scattering, diffraction, coherent SAXS, diffraction imaging, holography

  • Scientific themes / drivers: mesoscopic (few x 10nm resolution) imaging, e.g. large cells, and magnetic domains; time correlation spectroscopy of fluctuations in materials; 3D imaging of granular materials (with grains around 30 nm)

  • Beamline requirements:

  • beam size:~few mon pinhole

  • resolving power: ~104

  • flux:> 1013photons/s on sample

  • Microscopyendstation: STXM, phase contrast scope, full field scope, PEEM, spectro-scopy

  • Scientific themes / drivers: simultaneous spectroscopic (electronic) and microscopic (structural) measurements on nano- and mesoscopic length scales, e.g. of single nano-elements , nano-contacts, or nano-magnets, to understand isolated behavior differences between boundaries/interfaces and bulk

  • Beamline requirements:

  • beam size: STXM: fill 250m Fresnel zone plate (full transverse coherence), <10nm x <10nmon sample;coherent imaging: ~few mon pinhole; PEEM: ~few m on sample

  • resolving power: ~104

  • flux:STXM: > 1012 photons/s on sample, coherent imaging: > few x 1013 photons/s through pinhole, PEEM: > few x 1013 photons/s on sample

Eisebit & Luening, Nature 432, 885-888 (2004)

http://xraysweb.lbl.gov/peem2/PEEM2-02.html


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Possible High resolution soft x-ray undulator endstations/techniques

  • XRS, XRMS(x-ray resonant [magnetic] scattering) endstation

  • Scientific themes / drivers: ordering phenomena in correlated electron systems, magnetic & spintronic materials/devices, nanomagnetism, “soft” materials (C, N, O)

  • Beamline requirements:

  • - small spot size (small crystals, grazing incidence geometry)

  • - high energy resolution (select different multiplet states)

  • - high flux (detect weak, diffuse features in scattered beam)

  • RIXS(resonant inelastic x-ray scattering) endstation

  • Scientific themes / drivers: correlated electron materials (low energy excitations, phase transitions, bulk sensitivity), organic molecules (fullerenes, nanotubes)

  • Beamline requirements:

  • - small spot size (collect large solid angle, possibly no entrance slit)

  • - very good energy resolution (precise selection of initial state)

  • - highest flux possible (very low inelastic cross section)


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Soft x-ray beamline design:VLS PGM illuminated by collimated light

Possible planar M1

  • Rapid Switching: change M2 and M4

Side View

  • Horizontal Focusing by M4 -- 52:1


Sample spot size l.jpg
Sample spot size

and divergence

h=200eV: h=2.3m, v=1.1m

h=1000eV: h=2.1m, v=1.1m

h=200eV: h’=1.1mrad, v’=0.44mrad

h=1000eV: h’=0.8mrad, v’=0.3mrad

  • 100nrad RMS planes

  • 0.5rad RMS Elliptical, cylinder meridional

Req’d figure error


Calculated flux resolving power l.jpg
Calculated flux & resolving power

High resolution beamline

1010’s @ 105 rp; 1012’s @ 104 rp

High flux beamline

1012’s @ 2104 rp; 1013’s @ 5103 rp

Each soft x-ray undulator beamline would be equipped with at least 3 gratings, interchangeable under computer control


Slide14 l.jpg

Spot sizes on sample, dual (canted) undulator beams

Near, e.g. LCP, beam

Far, e.g. RCP, beam

h=200eV: h=2.2m, v=1.1m

h=1000eV: h=2.4m, v=1.1m

h=200eV: h=2.6m, v=1.1m

h=1000eV: h=2.0m, v=1.1m

To ensure complete overlap, defocus and/or

trim with baffles



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Sensitivity to electron beam motion

Overall conclusions for soft x-ray beamlines:

  • Spectroscopy and diffraction experiments in the soft x-ray range are very forgiving: 10% criterion (in both position and angle) is sufficient for spectroscopy and scattering

  • Imaging is another story (c.f. Chris Jacobsen)

    • STXMs/nanoprobes are very demanding on beam stability.

    • EPU45 beam is ~7 modes horiz. by ~1 mode vert. The horiz. overfilling provides some tolerance to beam motion. The lack of vert. overfilling will require more stability than is now needed at NSLS and even APS, ALS.

    • Must have very stable flux on timescales of 0.01-100,000 Hz, in order to match faster scan rates possible with high brightness beam

    • On that timescale, must have stability in flux to 1:104 or better for transmission experiments (conclusion: need to use appropriately-designed I0 normalization)

  • Switched EPU with mechanical chopper: systematic differences between fixed sources and very stringent beamline alignment tolerances; hard to match source profiles– may end up using switched ID with subsequent stability requirements


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Sensitivity to Horizontal Beam Motion: Large Demagnification Helps

  • NSLS-II: small divergence

  • (~15 mrad vert., ~20 mrad hor. @1000 eV)

  • CDR concept: no horizontal focusing until after exit slit (M3)

  • Effective horizontal demagnification: ~50:1

  • Sample position: 4- spot size ~ 4mm (v) x 8 mm (h)

  • 3 mm (10% of beam size) beam motion (horizontal)  0.06 mm beam motion on sample

  • Beam spot insensitive to vertical motion (imaging exit slit)


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Sensitivity to Vertical Source Motion: Effect on Photon Energy Resolution

Ultra-High Resolution(UHR, 3600 l / mm)

High Flux(HF, 400 l / mm)


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Sensitivity to Beam Divergence (Angular) Motion Energy Resolution

  • At hv = 100 eV: sv’ ~ 40 mrad, sh’ ~ 45 mrad

    • 10% values: ~ 4 mrad (vert.), ~ 4.5 mrad (hor.)

  • At hv = 2000 eV: sv’ ~ 10 mrad, sh’ ~ 20 mrad

    • 10% values: ~ 1 mrad (vert.), ~ 2 mrad (hor.)

  • Dk ~ k * sin q ; Dk / k ~ sin q

  • Contribution to uncertainty of wavevector from beam angular motion is on order few parts in 106 (i.e. nada)


Technological issues 1 l.jpg
Technological issues (1) Energy Resolution

  • For resolution and spot size, need better than 2007 state-of-the-art mirror (and grating blank) figure, as well as extremely good finish

    • Require 100 nrad figure error on planar optics (M3, VLS grating)

    • Require 0.5 rad meridional figure error on cylinders (M2) and plane ellipticals (M4, M5)

  • For ultra-high resolution spectroscopy, need better than 2007 state-of-the-art in VLS PGM motion precision/accuracy/repeatability

  • For optimum performance, alignment of optical elements, exit slit, and polarization chopper must be performed more accurately than has been required to date


Technological issues 2 l.jpg
Technological issues (2) Energy Resolution

  • Thermal loading on M2 and/or M3 may be too high to maintain req’d figure error; solution is a combination of:

    • Insertion of planar first mirror (M1) at more grazing angle of incidence than M2

    • LN2 cooling of M3

Power density absorbed by M3 when the ID is tuned to 200 eV in linear polarization mode and the mirror is tuned to the same energy with the VHR grating.

Power density absorbed by M2 when the 4m-long EPU45 is tuned to 200 eV in linear polarization mode.


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NSLS-II Soft X-ray Undulator Beamlines Energy Resolution

  • Ultra-high-brightness NSLS-II accelerator design is well matched to the high-resolution, high-flux, polarization-switchable, soft x-ray beamline design presented here.

  • These beamlines will have

    (resolving power)  flux  (spot size)-1

    product considerably greater than is available presently (2007), plus fast (kHz) polarization switching capability.

  • Example soft x-ray scientific programs are well matched to this beamline design: XRMS, RIXS, imaging, coherent scattering.

  • User groups (BATs) would be expected to define the mission of the NSLS-II soft x-ray beamlines, and carry out their detailed design, construction and operation.

  • Successful implementation requires R+D in one major area:

    Next generation improvement in mirror figure and finish