# Presentation: 671569

## NSLS-II Soft X-ray Undulator Beamlines

**Steve Hulbert**
**NSLS-II User Workshop**
**July 17, 2007**

## 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

## 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**

## NSLS-II source brightness

## Soft x-ray EPU45 source transverse coherence

**Soft x-ray EPU45 source transverse coherence**

## Soft x-ray EPU45 source brightness

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

** ****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.**

**Soft x-ray EPU45 source brightness**

**Soft x-ray mirror reflectivity**

**Higher harmonic interference:** **use QEPU**

## 3D schematic of soft x-ray undulator layout

**3D schematic of soft x-ray undulator layout**

- Side view, single beam mode

- Side view, canted beam mode, fast switchable

## Possible NSLS-II Soft X-ray Undulator Beamlines

**Possible NSLS-II Soft X-ray Undulator Beamlines**

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

**High flux**
**Soft x-ray imaging and coherent scattering**

**XRMS**

**RIXS**

**Imaging**

**Coherent**

- 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.

## Possible High flux soft x-ray undulator endstations/techniques

**Possible**** ****High 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 m on pinhole
- resolving power: ~104
- flux: > 1013 photons/s on sample
**Microscopy** endstation:  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 <10nm on sample; coherent imaging: ~few m on 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)**

## 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)

**Possible**** High resolution**** soft x-ray undulator endstations/techniques**

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

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

**Side View**

- Horizontal Focusing by M4 -- 52:1

- Rapid Switching: change M2 and M4

- Possible planar M1

| Beamline | High resolution | High flux |
| --- | --- | --- |
| Source to M1 | 27m | 30m |
| M1 to M2 | 1m | 1m |
| M2 to M3 | PGM design | PGM design |
| M2 to grating | ~2m | ~2m |
| Grating to exit slit | 20m | 10m |
| Exit slit to M4 | 1.85m | 1.85m |
| M4 to M5 | 0.5m | 0.5m |
| M5 to sample | 1.0m | 1.0m |
| Total | 53.35m | 46.35m |

## Sample spot size

- 100nrad RMS planes
- 0.5rad RMS Elliptical, cylinder meridional

**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**

**Req’d figure error**

**and divergence**

## Calculated flux & resolving power

**High resolution**** beamline**
**10****10****’s @ 10****5 ****rp; 10****12****’s @ 10****4**** rp**

**High flux**** beamline**
**10****12****’s @ 2********10****4**** rp; 10****13****’s @ 5********10****3**** rp**

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

## Spot sizes on sample, dual (canted) undulator beams

**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**

- Near, e.g. LCP, beam

- Far, e.g. RCP, beam

**To ensure complete overlap, defocus and/or**
**trim with baffles**

## Comparisons of selected soft x-ray beamlines in the US

## 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

## Sensitivity to Horizontal Beam Motion: Large Demagnification Helps

| Element | Position From Source |
| --- | --- |
| M1 | 28 m |
| M2 | ~29 m |
| Grating | ~30 m |
| Exit Slit | 50 m |
| M3 | 51.8 m |
| M4 | 52.3 m |
| Sample | 53.3 m  |

**NSLS-II: small divergence**
**	****(~15 ********rad vert., ~20 ********rad 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 ********m (10% of beam size) beam motion (horizontal) ******** 0.06 ********m beam motion on sample**
**Beam spot insensitive to vertical motion (imaging exit slit)**

## Sensitivity to Vertical Source Motion: Effect on Photon Energy Resolution

**Sensitivity to Vertical Source Motion: Effect on Photon Energy Resolution**

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

**High Flux****(HF, 400 l / mm)**

|  | UHR
1000 eV | HF 1000 eV 
(300 l/mm) | UHR
200 eV | HF 200 eV
(300 l/mm) |
| --- | --- | --- | --- | --- |
| Beamline 
Resolution | 11 meV
 | 114 meV | 2 meV | 21 meV |
| Energy Shift
4 m beam motion | 0.89 meV
 | 11 meV | 0.08 meV | 1 meV |

## Sensitivity to Beam Divergence (Angular) Motion 

- At hv = 100 eV:  ******v****’ **~ 40 rad, ******h****’ **~ 45 rad
- 10% values: ~ 4 rad (vert.), ~ 4.5 rad (hor.)
- At hv = 2000 eV:  ******v****’ **~ 10 rad, ******h****’ **~ 20 rad
- 10% values: ~ 1 rad (vert.), ~ 2 rad (hor.)
-  k ~ k * sin  k / k ~ sin 
- Contribution to uncertainty of wavevector from beam angular motion is on order few parts in 106 (i.e. nada)

## Technological issues (1)

- 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)

**Technological issues (2)**

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

- 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.

## NSLS-II Soft X-ray Undulator Beamlines

- 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**