San Jose State University
X-ray Correlation Spectroscopy Inst. Leader
Linac Coherent Light Source
United States Department of Energy
Office of Science, Science Undergraduate Laboratory Internship Program
SLAC National Accelerator Laboratory
Menlo Park, California
August 20 th , 2012
Abstract
Inline Spectrometer as Permanent Optics at the X-ray Correlation Spectroscopy Instrument to Support Seeding Operation.
AMBER GRAY (San Jose State University, San Jose, CA 95112) AYMERIC ROBERT (SLAC National Accelerator Laboratory,
Menlo Park, CA 90425)
The X-ray Correlation Spectroscopy instrument (XCS) at the Linac Coherent Light Source (LCLS) is a dedicated instrument using coherent x-ray scattering techniques to investigate dynamics in condensed matter systems. XCS can probe both slow and ultrafast dynamics on length scales of interest. It employs an extensive suite of X-ray instrumentation to tailor the LCLS X-ray beam properties to the experimental requirements. Results demonstrating the full transverse coherence of the LCLS beam are presented.

I. Introduction
The Linac Coherence Light Source (LCLS) is the first hard X-ray Free Electron Laser that produces ultrafast intense
X-ray pulses in order to non-destructively analyze material’s physical properties and crystalline/disordered structures. It provides a unique opportunity to observe dynamical changes of large groups of atoms in condensed matter systems (i.e. looking at what is happening inside a material) over a wide range of time scales using Coherent X-ray Scattering (CXS) in general and X-ray Photon Correlation Spectroscopy (XPCS) in particular. The X-ray Correlation Spectroscopy (XCS) instrument at the LCLS allows the study of equilibrium and non-equilibrium dynamics in disordered or modulated materials [1, 2].
Coherent X-rays are particularly well suited for investigating disordered system dynamics down to nanometer and atomic length scales, using X-ray Photon Correlation Spectroscopy [1]. When coherent light is scattered from a disordered system, the scattering pattern presents a peculiar grainy appearance also known as speckles. These speckles originate from the exact position of all scatterrers within the coherently illuminated volume. XPCS thus characterizes the temporal fluctuations of such speckle patterns from which insights in the dynamic behavior of the system can be revealed [1].
When lasing, due to the spikey nature (intrinsic noise) of the beam in SLAC’s two-mile long accelerator as described by the Self Amplified Stimulated Emission (SASE) process, monochromators and diagnostic tools are necessary means for testing the single shot spectral properties of the beam before probing the dynamics of materials.
II. Methods
The XCS instrument is a hard x-ray instrument that was designed to use the LCLS FEL beam for coherent x-ray scattering techniques such as XPCS. A schematic of the various optical components of the XCS instrument is provided on the next page in Figure 1.

Figure 1. A schematic view of the optical components and diagnostics of the XCS Instrument. The distances from each component to the sample are indicated in meters. XCS can operate various monochromators: a Si(111) or (220) Large Offset
Double Crystal Monochromator (LODCM) and also an artificial Si(511) Channel Cut Monochromator (CCM). The beam can focus to small sizes (typically 5 × 5μm 2 ) with Compound Refractive Lenses (CRL) available for installation at two different locations. XCS can also operate in pink beam mode by translating most of its components in the main LCLS beamline as indicated in red [3].
The detailed characterization of each single x-ray pulse reaching the sample at XCS is required (i.e. intensity and spectral content). A transmissive spectrometer is needed inline just before the sample in order to properly evaluate the spectral content of each single pulse. The requirements of the spectrometer are to capture the full SASE spectrum in the hard x-ray regime on a single shot basis for individual spectral spikes. It should also cover the energy range of operation of XCS.
The design of the spectrometer includes the selection of thin silicon crystal membranes of a given thickness for the desired transmission of hard x-rays. The silicon crystal membranes will be bent to a specific radius of curvature in order to provide the necessary dispersion (i.e. provide an appropriate resolution), and the diffracted beam will then reach the detector’s scintillator screen as seen in Figure 2.

Figure 2. Dispersion geometry of the spectrometer [4].
As the incoming beam hits the bent crystal different parts of the beam are diffracted at different angles satisfying
Bragg’s Law given by Equation 1 𝜆 = 2𝑑 sin 𝜃
𝐵
, (1) where d is the spacing between the lattice planes of a given crystal and orientation and 𝜃
𝐵
is the Bragg angle for a particular wavelength, 𝜆
.
The wavelength dispersion, Δ𝑥 on the detector is related to ΔΕ , the energy increment and is expressed as:
Δ𝑥 = 2 tan 𝜃
𝐵
(
𝑅 sin 𝜃
𝐵
2
+ 𝐿 ′ )
ΔΕ
Ε
, (2) where R is the radius of curvature of the crystal and 𝐿 ′ is the distance from the membrane to the detector. The beam size,
Η, and the radius of curvature of the crystal are determining factors in the spectral range of the bent crystal spectrometer [4].
Equation 3 shows this dependency
ΔΕ 𝑚𝑎𝑥
= cot 𝜃
𝐵
Ε
H
𝑅 sin 𝜃
𝐵
. (3)
After understanding the dispersion geometry in order to meet the physics requirements of the project, from an engineering standpoint it is important to start a feasibility study in the XCS Hutch 4 to see if the space available before the sample is capable of hosting a spectrometer with the appropriate energy range.
A preliminary study was done for a Si(111) crystal to see if the allowable space could accommodate a He-filled plexiglass enclosure holding the crystal with different stages to move about four different axes. It would also hold a detector directly on top of the enclosure rotating independently of the crystal. There are two linear and two rotational stages: x, y, theta and chi respectively. The x-stage is necessary to move the crystal in and out of the beam path as desired in the horizontal plane. The y-stage is to get the crystal in alignment with the beam’s path direction. The y-stage specifically helps to select an appropriate curvature on the crystal. Theta and chi stages will align the crystal and allow the diffracted X-rays to reach the detector. The beam will be diffracted at nearly 90 degrees in the vertical scattering plane to prevent polarization losses.
After taking measurements in the hutch it was determined there is enough space to rotate a detector that would allow an energy range from 7.5 to 10keV. The next step is to choose appropriate stages with the required resolutions and sizes, then determine the desired base and brackets to house the crystal holder set up, and finally begin a technical design for the
XCS inline spectrometer.
III. Materials

The available space for the spectrometer is not large. When looking in the z-plane (beam direction plane) from instrument component to component there is only 30 cm to fit a base for the spectrometer box. MORE WILL BE ADDED HERE
IV. Results
Initially a test spectrometer was set up by mounting a Si(333) on the XCS’s sample holder diffractometer. Fixed 32.2 cm directly above the crystal was a detector. The X-ray Free-Electron Laser (XFEL) hits the bent Si(333) crystal and diffracts at
90 degrees vertically to the scintillator on the detector screen as seen in Figure 3.
Figure 3. Representation of spectrometer test set up when the Si(333) crystal took the FEL beam to show proof of concept that a spectrometer can read seeded beam in the XCS.
The test set up for the spectrometer on the sample diffractometer was tedious and took many times to get in the correct position above the crystal. Next the crystal is set up in air, which absorbs more than half of the X-rays. Even though it worked well as a proof of concept that a spectrometer in the XCS could interpret seeded beam, a more robust and permanent solution was necessary for a permanent spectrometer.
The conceptual design for a permanent inline spectrometer in the XCS comprised of a helium plexiglass box with a curved top. The detector for the spectrometer will rotate about an arc of available radius within the available space between

the sample diffractometer and components upstream of the future spectrometer. Figure 4 shows the spectrometer design concept.
Figure 4. (place holder figure before I make another) Conceptual design of future XCS spectrometer. A plexiglass box filled with helium houses the thin bent Si(111) crystal that diffracts the FEL beam to a detector nearly 90 degrees. The detector rotates across an arc independently of the rotational axis of the crystal. The two boxes holding the chi and theta stages control the rotational axes and act as a goniometer lining the crystal with the beam and the beam with the detector, as does the y stage just below theta. The x stage moves the crystal in and out of the beam path.
A base that acts as a vibration oscillation table will be mounted to a bread board already attached to upstream components to reduce any vibrations on the crystal. Next a bracket will mount the base holding the plexiglass box that houses the crystal and stages that the detector rotates on top of.
V. Conclusion
The test spectrometer proved that seeded beam could be recorded in the XCS. The open-air test that borrowed the sample diffractometer’s goniometer, however, does not provide a permanent solution.
After a feasibility study showed there is the allowable space for a spectrometer using energies from 7.5 to 10 keV, a conceptual design was started. The concept is to house the Si(111) crystal that will diffract the LCLS FEL beam 90 degrees to a detector mounted on a curved helium filled plexiglass box. The detector will rotate about an arc using the allowable space between the sample diffractometer and other upstream components of the XCS instrument. In order to diffract the beam as needed four stages will be mounted with encoded motors underneath the crystal in order to control the crystal’s movement.
The next step is to begin technical design of the spectrometer which includes picking components of the spectrometer and beginning technical drawings.
Acknowledgements

I would like to thank my mentor Aymeric Robert for his time and support in my learning of the project and inclusion in many aspects of the XCS instrument including a trip to Argonne National Laboratory for detector testing. I would also like to thank
Yiping Feng, Venkat Srinivasan, Marcin Sikorski, and Daniel Flath for their time and assistance in learning all the aspects necessary for a successful project design. I would like to thank the Department of Energy for funding this program and for everyone at the SLAC National Laboratory for being welcoming and helpful. Thank you to Stephen Rock, Maria Mastrokyriakos and Anita Piercy for all of the work they put into making the summer go smoothly.
References
[1] Grübel G, Madsen A and Robert A 2008 X-ray Photon Correlation Spectroscopy in Soft Matter Characterization , ed R Borsali and
R Pecora (Heidelberg: Springer) chapter 18 pp. 954-995
[2] Stephenson G B, Grübel G and Robert A 2010 Nature Materials 8 702-703
[3] Robert A, Curtis R, Flath D, Gray A, Sikorski M, Song S, Srinivasan V, Stefanescu D 2012 To be published
[4] Zhu D et al. 2012 A Single-Shot Transmissive Spectrometer for Hard X-ray Free Electron Lasers