Stability needs for state-of-the-art user experiments at NSLS-II Garth J Williams 1 Representative experiments We consider two microscopy techniques as examples Differential Phase Contrast (DPC) Physical limits on image contrast are driven by the index of refraction, so phase contrast is often dominant. Normally, a sample is scanned through a focused x-ray beam. Coherent Diffractive Imaging (CDI), including full-field, reciprocalspace Bragg CDI and scanning-point, real-space ptychography
Uses a continuous, normally far-field, diffracted intensity to recover the complex field leaving the sample. The amplitude and phase of the field is then interpreted to discover structure and deformation. This normally requires data sets formed by collections of 2D images. The requirements on the coherence of the x-ray field are very stringent. 2 Representative experiments We consider two microscopy techniques as examples Differential Phase Contrast (DPC) Physical limits on image contrast are driven by the index of refraction, so phase contrast is often dominant. Normally, a sample is scanned through a focused x-ray beam.
Coherent Diffractive Imaging (CDI), including full-field, reciprocalspace Bragg CDI and scanning-point, real-space ptychography Uses a continuous, normally far-field, diffracted intensity to recover the complex field leaving the sample. The amplitude and phase of the field is then interpreted to discover structure and deformation. This normally requires data sets formed by collections of 2D images. The requirements on the coherence of the x-ray field are very stringent. 3 Scanning transmission DPC in detail The intensity is attenuated and shifted This is governed by the index of refraction in the sample
The angular shift is due to phase retardation in the sample The shift is measured by comparing opposing detector elements. 4 Stability in DPC at NSLS-II 1 um Horizontal Phase Gradient Measured at ~0.5 m away from the focus Courtesy Yong Chu Fluorescence imaging measured at the focus
5 Coherent Diffractive Imaging CDI requires highly coherent x-ray fields CDI solves an inverse problem to recover the complex amplitude of the x-ray field leaving the sample. The recovered field is interpreted to gain structural information. We will discuss ptychographya scanning CDI methodand Bragg CDI, which relies on reciprocalspace measurements and recovers material deformation.
doi:10.1038/ nphoton.2012.209 6 Bragg CDI Collect the 3D intensity distribution around a Bragg peak Apply iterative phase retrieval magic Recover shape and deformation information at the nanoscale with 10-4 or better sensitivity Newton et al.
DOI: 10.1038/NMAT2607 7 How does beam stability affect Bragg CDI? When the motion is fast compared to the measurement time, typically 1 sec or less, the beam motion smears the measured signalit is effectively partially coherent radiation! It is possible to accommodate this, but it is vital that the angular distribution be known. Stability should be maintained at the urad level and measured
to the 10s-of-nrad level Whitehead et al., doi: 10.1103/PhysRevLett.103.243902 8 Section summary Often, beamlines perform spatial filtering at a secondary source formed by the optical system. These geometries allow a good degree of isolation from beam motion in the focal plane of the final optics. Out of the final focal plane, beam pointing instabilities are a significant source of measurement error Some techniques (CDI) invoke significant data analysis that can accommodate pointing errors, but more direct techniques (DPC) suffer.
9 Instrument stability Consider the typical conditions for x-ray experiments Present HXN measurements as an example of what can be achieved Highlight limitations 10 Ground Vibrations ~50 nm
~15 nm Nanoprobe Site, 2009 Courtesy, Nick Simos 11 Vibrations at the MLL Microscope Vertical MLL Horizontal MLL ~ 0.25 nm @ 295 Hz
12 HXN Optical Layout X-ray Camera 98 m Mono XBPM V. Foc. H. Foc. H. H. Coll. FE X-ray CRLs mirror mono mirror XBPM Source
67 m 34.1 m 32.6 m 30.4 m 28.4 m 16 m 0m H. demag = 2.3 Horizontal direction Secondary Source
V. demag = 1.9 Vertical direction FXBPM: sensitive to source angle and source position MXBPM: highly sensitive source angle XCAM: sensitive to source angle and position 13 Courtesy Yong Chu Active Feedback for Beam Positioning Horizontal Direction
200 Hz 100 Hz 50 Hz 25 Hz 10 Hz 5 Hz 1 Hz No Feedback Courtesy Yong Chu 14 Active Feedback for Beam Positioning
Current status of HXN Summarize current best results from HXN beam stabilization efforts Present before and after measurements 16 X-ray Angular Stability with active feedback at 100 Hz Vertical: 6 nrad RMS Horizontal: 17 nrad RMS
Courtesy Yong Chu 17 Without Active Feedback DPC XRF image Ptycho: amplitude Ptycho: phase W
19 Ptycho: phase Courtesy Yong Chu Are these solutions universal? Beamline-local feedback Optical components need to be specially designed and modeled. The mechanical systems tend to have resonant frequencies lower than a few hundred Hz. The feedback is typically achieved with a double-crystal monochromator, but not all beamlines have them and feedback with a device designed to take the white beam may be
challenging. Sensors can be problematic. In the case of BCDI, the commonlyused segmented diamond screens will adversely affect the data quality. 20 Talking-point requirements Typically, beamlines are relatively long, with the final optics sitting more than 50 m from the source. This likely drives the reduction of pointing variation over positional variation. 1 microradian FWHM might be regarded as a strong upper limit on pointing stability. State-of-the art experiments will strongly benefit from 100 nanoradian or better stability (or tracking).
Beamline-local feedbacks can help, but will begin to fail to correct motion above 100 Hz. These feedback loops can also be difficult to control for the wide range of beam conditions that an experiment may require. Experiments are already conducted with dwell times below 10 ms and this will decrease to below 1 ms. 21 Supporting: Simple calculation for dependence of diffraction/deformation sensitivity from Braggs law 22
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