Inline Spectrometer as Permanent Optics at the X-ray Correlation Spectroscopy Instrument to Support Seeding Operation

Transcription

1 Inline Spectrometer as Permanent Optics at the X-ray Correlation Spectroscopy Instrument to Support Seeding Operation Amber L. Gray Office of Science, Science Undergraduate Laboratory Internship (SULI) San Jose State University Stanford Linear Accelerator Center Stanford, CA August 24 th, 2012 Prepared in partial fulfillment of the requirements of the Office of Science, Department of Energy s Science Undergraduate Laboratory Internship under the direction of Aymeric Robert at the Linac Coherent Light Source (LCLS), Stanford Linear Accelerator Center. Participant: Signature Research Advisor: Signature

2 TABLE OF CONTENTS Abstract ii Introduction 1 Methods 2 Materials 4 Results 5 Conclusion 8 Acknowledgements 9 References 9

3 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 (XCS) Instrument at the Linac Coherent Light Source (LCLS) investigates the dynamics of condensed matter systems by using coherent x- ray scattering techniques. The XCS has the option to probe both slow and ultrafast dynamics on desired length scales. It employs an extensive suite of X- ray instrumentation to tailor the LCLS X- ray beam properties to the experimental requirements. One of the peculiar structures of the LCLS beam is its spectrum, which presents numerous spikes that jitter from one shot to another. Therefore, a dedicated diagnostic is needed to characterize each spectral detail for single shots. Diagnostics are realized with an inline spectrometer, allowing measurement of such quantity, while transmitting most of the LCLS beam to perform an experiment. ii

4 INTRODUCTION The Linac Coherence Light Source (LCLS) is the first hard X- ray Free Electron Laser (FEL) that produces ultrafast intense X- ray pulses in order to non- destructively analyze a material s physical properties and crystalline or 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] by means of these techniques. Coherent X- rays are particularly well suited for investigating disordered system dynamics down to nanometer and atomic length scales, using XPCS [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 scatterers 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, spiky spectral features appear in the LCLS beam as a result of the SASE (Self- Amplified Spontaneous Emission) process as shown on in Figure 1. Shot to shot, the spikes seen in Figure 1, are never in the same place. Therefore, monochromators that select specific wavelengths of the beam, and diagnostic tools are necessary means for characterizing the single shot spectral properties of the beam while probing the dynamics of materials. An inline spectrometer directly in front of the experiment will therefore be the last crucial diagnostic before the FEL beam hits the sample. Figure 1. The figure shows a spectral representation of the Linac beam. The spikey spectral features of the beam are a natural part of the chaotic SASE (Self Amplified Spontaneous Emission) process. The spikes are never the same from shot to shot. Therefore, it is essential that the XCS Instrument both control and measure the characteristics of single shot spectral properties by using monochromators and diagnostic tools. Using monochromators to control and diagnostics to measure beam characteristics help scientists understand the difference between the dynamic material properties of an experiment and fluctuations in the beam s SASE process. Image Source: SLAC Today, August 13,

5 METHODS The XCS Instrument is a hard X- ray instrument 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 in Figure 2. Figure 2. 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 future spectrometer location is indicated with the arrow between a set of slits/diagnostics and the diffractometer. The beam can focus to small sizes (typically 5 5 μm 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 is required (i.e. intensity and spectral content). As one can see from Figure 2, a transmissive spectrometer is needed inline just before the sample at 0.8 m before the diffractometer in order to properly evaluate the spectral content of each single pulse. It will therefore be located directly upstream of the sample s diffractometer location. The requirements of the spectrometer are to capture the full SASE spectrum in the hard X- ray regime (5-10 kev) on a single shot basis (up to the maximum repetition rate of the LCLS, i.e., 120 Hz), with a resolution sufficient to resolve individual spikes. 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. Crystal thickness determines the percentage of X- rays that are transmitted through the crystal itself, while others are reflected to the detector. The silicon crystal membranes will be bent to a specific radius of curvature in order to provide the 2

6 necessary dispersion (i.e., provide an appropriate resolution), and the diffracted beam will then reach the detector s scintillator screen across x, as seen in Figure 3. Figure 3. Dispersion geometry of the spectrometer [4]. L is the length from the center of the crystal to the detector screen as can also be seen from the test spectrometer set up in Figure 5. H is the size of the FEL beam, which can be reduced by the slits illustrated in Figure 2. And Δx represents the diffracted beam s wavelength spectrum across the scintillator screen. Relationships between the dispersion geometry variables are defined below. 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!, (1) where d is the d- spacing between the lattice planes of a given crystal and orientation and θ! is the Bragg angle for a particular wavelength, λ. The wavelength dispersion, Δx on the detector is related to ΔΕ, the photon energy increment and is expressed as: Δx = 2 tan θ!!!"#!!! + L!!"!, (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 3

7 crystal are determining factors in the spectral range of the bent crystal spectrometer [4]. Equation 3 shows this dependency!"!"#! = cot θ!!!!"#!!. (3) A preliminary study was done for a Si(111) crystal to see if the allowable space could accommodate a Helium filled plexiglass enclosure holding the crystal with different stages to move about four different axes. The Helium filled and enclosed plexiglass box will hold a detector directly on top rotating independently of the crystal. There will be two linear and two rotational stages: x, y, θ and χ 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. The θ- and χ- stages will align the angles of 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. Once the physics requirements of the project are met the next step is to start a conceptual report. From an engineering standpoint a conceptual report begins by conducting a feasibility study in the XCS Hutch to see if the space available upstream of the sample is capable of hosting a spectrometer with the appropriate energy range as indicated for the spectrometer requirements. After taking measurements in the hutch it was determined there will be enough space to rotate a detector allowing an energy range from 7.5 to 10keV. The energy range corresponds to the available space for the spectrometer. The next step is to come up with a technical design based on this concept. This involves choosing appropriate stages with the required resolutions and sizes, then determining the desired base and brackets to house the crystal holder set up. This will lead to the procurement and engineering of the required components, together with an assembly, and test plan. MATERIALS The available space for the spectrometer is not large. It will be located upstream of the diffractometer as seen in Figure 2. When looking in the z- plane (beam direction plane) of XCS Instrument, component to component, there is only 30 cm to fit a base for the spectrometer box. 4

8 Figure 4 shows a rough shape, from a side view, for the conceptual design of the Helium plexiglass box. Since the detector, which will rotate on top of the plexiglass box, is 44 cm long and there is 60 cm of available space in the x- direction, the box s longest dimension will be in the x- direction. Figure 7 shows a front view of the plexiglass conceptual design that includes a schematic of how it will work and the components necessary for the box to house the crystal. A vibration oscillation breadboard and supporting bracket will hold the box assembly in place. The bracket will be attached to another breadboard in the y- plane fixed to a granite block holding components upstream of the diffractometer as seen in Figure 7. y x z Figure 4. Rough sketch of the conceptual design for the Helium filled plexiglass box. This breadbox shape plexiglass enclsoure will house the crystal and stages to move the crystal. The z- direction is in line with the LCLS beam path. The box will house stages that move the crystal in alignment with the beam and with the spectrometer detector. The detector will rotate along the arc at the top of the box for specific energies. RESULTS Initially a test spectrometer was set up by mounting a Si(333) on the XCS s diffractometer. Directly above the crystal was a detector at 32.2 cm, the L dimension from Figure 3. The LCLS laser beam hits the bent Si(333) crystal and 5

9 diffracts at 90 degrees vertically to the scintillator on the detector screen as seen in Figure 5. Detector FEL Beam diffracted 90 degrees (in pink) Si(333) Crystal FEL Beam Diffractometer Figure 5. Representation of spectrometer test set up. The Si(333) crystal took the FEL beam to show proof of concept that a spectrometer can read seeded beam in the XCS. The purpose for the test was to show the XCS could use a modified Linac for seeding and measure its spectral properties. An example of a single seeded shot is shown in Figure 6. In contrast to Figure 1, seeded beam shows one spike, i.e. a specific wavelength without the noise of the SASE spectrum. The test successfully supported measurement of seeding as machine development continues to optimize modifying the beam for one spike. However, the 6

10 set up for the test spectrometer on the sample diffractometer was time consuming and took many attempts to get the correct position above the crystal. In addition, the set up was in open- air, which therefore absorbs more than half of the X- rays. Figure 6. Spectral representation of seeding. The image illustrates how SLAC s Linac is being developed for a specific and well- defined wavelength width in contrast to the chaotic SASE spectrum. To support machine development of seeding the XCS will use a permanent spectrometer to record and verify single spikes across a detector. Image Source: SLAC Today, August 13, 2012 Even though it worked well as a proof of concept that a spectrometer in the XCS could provide reliable information with the currently developed seeded beam, a more robust and permanent solution is definitely necessary for a permanent spectrometer. The conceptual design for a permanent inline spectrometer at XCS is comprised of a Helium filled plexiglass box with a curved top. The detector for the spectrometer will rotate about an arc of a given radius within the available space between the diffractometer and the existing X- ray components located upstream. Figure 7 shows the spectrometer design concept. 7

11 Figure 7. Conceptual design of the XCS spectrometer. A plexiglass box filled with Helium houses the thin bent Si(111) crystal that diffracts the FEL beam to a detector at nearly 90 degrees. The use of Helium, considerably reduces the X- ray absorption, as compared to air in the energy range of spectrometer operation. The detector rotates across an arc independent of the rotational axis of the crystal, which center coincides with the rotation axis θ of the crystal. The two boxes holding the χ- and θ- stages control the rotational axes and act as a goniometer aligning the crystal with the beam and the beam with the detector, as does the y- stage just below θ. The x- stage moves the crystal in and out of the beam path. A base will be mounted to a breadboard already attached to upstream component granite block to reduce any vibrations on the crystal. Next a bracket will mount the base holding the plexiglass box that houses the crystal and stages on top of what the detector rotates. CONCLUSION The XCS spectrometer test proved that reliable information could be obtained on the performances of the currently developed seeded beam scheme. The in- air test that borrowed the XCS diffractometer, however, does not provide a permanent nor a reliable and flexible solution supporting the operation of a spectrometer while providing user operation. Therefore, a permanent and robust transmissive spectrometer is necessary to record and verify the wavelength of the beam upstream of a sample. An engineering study showed there is the sufficient space for a spectrometer operating at energies from 7.5 to 10 kev. A conceptual design was realized. The 8

12 concept is to house the Si(111) crystal that will diffract the LCLS FEL beam at nearly 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 motors underneath the crystal in order to control the crystal s movement. The next step is to begin the detailed technical design of the spectrometer, which includes selecting components (i.e., size, range, resolution, weight, cost, lead time, interfaces) of the spectrometer as well as begin 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 the Advanced Photon Source (Argonne National Laboratory, IL) 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 [2] Stephenson G B, Grübel G and Robert A 2010 Nature Materials [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 A Single-Shot Transmissive Spectrometer for Hard X-ray Free Electron Lasers 9