An ongoing collaboration among four US Department of Energy (DOE) National Laboratories has demonstrated key technology prototypes and software modeling tools required for new high-coherent flux beamline optical systems. New free electron laser (FEL) and diffraction-limited storage ring (DLSR) light sources demand wavefront preservation from source to sample to achieve and maintain optimal performance. Fine wavefront control was achieved using a novel, roomtemperature cooled mirror system called REAL (resistive element adjustable length) that combines cooling with applied, spatially variable auxiliary heating. Single-grating shearing interferometry (also called Talbot interferometry) and Hartmann wavefront sensors were developed and used for optical characterization and alignment on several beamlines, across a range of photon energies. Demonstrations of non-invasive hard x-ray wavefront sensing were performed using a thin diamond single-crystal as a beamsplitter.
The Linac Coherent Light Source (LCLS) is undergoing an upgrade to a double source setup to provide eight experimental hutches (five existing and three new) with either high-repetition or high-intensity pulses and highly coherent X-ray beams. The photon transportation and distribution to each hutch relies on, among other elements, bendable mirrors. Given the coherence of the LCLS source, and to avoid introducing wavefront distortions beyond workable limits, the mirrors need to have extremely smooth surfaces, with a figure compliant with the nominal profile (usually elliptical). The effectiveness and the accuracy of the bending system and of the actuators over the entire length of the mirror (up to 1.2 m) need to be assessed by an appropriate metrology system. Long Trace Profilometry (LTP) is a suitable technique to characterize a slightly-curved surface mirror profile with very high sensitivity, provided that the optomechanical system implementation enables sensitivity and accuracy values compatible with the mentioned surface quality requirements. In this paper, we show the status and performance of the LTP under development at LCLS. The LTP essentially consists of an advanced optical head that endows a laser beam with sharp interferential features to increase its resolution and detects the optical lever of the beam reflected by the sample, plus a high-precision gantry system (Q-Sys) for accurate scanning of the mirror under test, under impact of its bending mechanics and cooling system. The measured results are compared to the simulated performance of the LTP, and we show the way of the oncoming improvement of the instrument.
The Linac Coherent Light Source (LCLS), a US Department of Energy Office of Science X-ray facility operated by the Stanford University, is being upgraded with a second source to provide eight beamlines (five existing and three under construction) with either high-repetition or high-intensity pulses and highly coherent X-ray beams. The photon transportation and distribution to each beamline relies on, among other elements, elliptically- bendable mirrors, often in Kirkpatrick-Baez (K-B) configuration. One of the crucial tasks in beamline design and performance prediction is the self-consistent simulation of the final point spread function of the complete optical system, simultaneously accounting for diffractive effects, mirror deformations, and surface finishing defects. Rather than using ray-tracing routines, which cannot manage diffractive effects, and rather than employing the first-order scattering theory, which cannot be applied when the optical path differences exceed the radiation wavelength, a wavefront propagation formalism can be used to treat all the aspects at the same time. For example, the WISE code, initially developed for astronomical X-ray mirrors at INAF-OAB, and subsequently used to simulate X-ray reflective systems at the Fermi light source, is now a part of the well-known OASYS simulation package. In this paper, we extend the model to a two-dimensional imaging and show performance simulations of two elliptical mirrors to form a complete Kirkpatrick-Baez system
With the onset of high power XFELs and diffraction limited storage rings, there is a growing demand to maintain sub nanometer mirror figures even under high heat load. This is a difficult issue as the optimum cooling design for an optic is highly dependent on the power footprint on the mirror, which can be highly dynamic. Resistive Element Adjustable Length (REAL) cooling can be utilized to change the cooling parameters during an experiment to adapt for changing beam parameters. A case study of the new soft x-ray monochromator for the LCLS L2SI program is presented that utilizes this new capability to allow the beam to translate across the mirror for different operation modes, greatly simplifying the monochromator mechanics. Metrology of a prototype mirror will also be presented.
The Linac Coherent Light Source (LCLS) of SLAC is upgrading the facility to a more flexible design, permitting both, high energy per pulse mode and High Repetition Rate mode. Two independent “sources” Soft and Hard X-ray will serve five of the existing beamlines and three completely new ones.
We will present here, one of the new beamlines, mainly dedicated to Resonant Inelastic Scattering (RIXS) and Liquid Jet based experiments.
The beamline is designed to deliver the beam to the floor upstairs of the existing experimental area by using a large deflection, grating based monochromator. The monochromator is designed to provide both, very high resolving power (E/ΔE<50,000) and transform limited low-resolution mode. To accommodate those very different operative modes, the footprint of the beam on the gratings is controlled to illuminate the proper amount of lines. An elliptical bendable mirror, in front of the monochromator, will create four different virtual sources, to cover the photon energy range from 250 to 1600 eV in both modes.
After the monochromator, the beamline will serve three experimental stations installed in line. A pair of plan elliptical bendable mirrors, operating in the Kirkpatrick-Baez configuration, will focus the radiation in the proper experimental station. The focal spot size, will be controllable, permitting to adapt it to the need of the experiment. Another major requirement is to preserve the uniformity of the beam, out of focus, with a maximum intensity variation of less than 5%. This implies maintaining the shape error of all the optics to sub-nm levels, even in the presence of heat load. The design principle and performance of the three active mirrors and the impact on the monochromator and spot profile will be presented in details, together with some preliminary tests.
Conventional materials engineering approaches for polycrystalline ceramic gain media rely on optically isotropic crystals with high equilibrium solubility of luminescent rare-earth (RE) ions. Crystallographic optical symmetry is traditionally relied upon to avoid scattering losses caused by refractive index mismatch at grain boundaries in randomly oriented anisotropic crystals and high-equilibrium RE-solubility is needed to produce sufficient photoluminescence (PL) for amplification and oscillation. These requirements exclude materials such as polycrystalline sapphire/alumina that have significantly superior thermo-mechanical properties (Rs~19,500Wm-1), because it possesses 1) uxiaxial optical properties that at large grain sizes, result in significant grain boundary scattering, and 2) a very low (~10-3%) RE equilibrium solubility that prohibits suitable PL. I present new materials engineering approaches operating far from thermodynamic equilibrium to produce a bulk Nd:Al2O3 medium with optical gain suitable for amplification/lasing. The key insight relies on tailoring the crystallite size to the other important length scales-wavelength of light and interatomic dopant distances and show that fine crystallite sizes result in sufficiently low optical losses and over-equilibrium levels of optically active RE-ions, the combination of which results in gain. The emission bandwidth is broad, ~13THz, a new record for Nd3+ transitions, enabling tuning from ~1050nm-1100nm and/or ultra-short pulses in a host with superior thermal-mechanical figure of merit. Laser grade Nd:Al2O3 opens a pathway for lasers with revolutionary performance.
Along with the demanding requirements for the extreme limit pushing LCLS II project, comes the challenge in metrology work for qualifying the optical and mechanical components. Besides qualifying the components against specifications, it is also crucial to study performance, repeatability and stability of the mirror systems designed for meeting the LCLS II conditions. Therefore a dedicated metrology laboratory has been jointly funded by LCLS II project and LCLS facility.
The laboratory, located close to the experimental hall of LCLS, is currently equipped with a 6” Fizaeau interferometer (Zygo DynaFiz) and a Zygo NewView 8300 white light interferometer. A profilometer, hosting a Long Trace Profiler optic head, an autocollimator (Moller Wedel) and a Shack Hartman head (SHArPer, Imagine Optics), is under assembling.
The combination of these instruments will enable us to measure spatial periods from the µm scale up to 1.5 m. Further implementation in progress are the implementation of a stitching method for the 6” interferometer and reduction of environmental noise.
The results obtained from measuring 1-m long flat mirrors, with sub-nm shape errors, produced by Jtec, show a very high sensitivity of the interferometer. These results, as well as the results obtained in testing the bender prototype and some diffraction gratings, will be presented.
To preserve the full coherence of the FEL, the acceptance of the optics should be at least 2*FWHM of the X-ray beam. The LCLS-II soft X-ray experiments cover a photon energy range from 250 eV to 1300 eV. The photon beam footprint on the flat and KB mirrors varies from 150 mm to 1000 mm. The length of the mirror is chosen as 1 meter. Resistive Element Adjustable Length (REAL) cooling technique has been proposed to minimize the thermal deformation [1] for LCLS-II mirrors when the power FEL is above 200 W. The water cooling of the mirror is applied on the top-up-side [2]. The additional electric heater is adjustable both in length and power density to cope with the variable X-ray beam footprint length. A R&D project including the prototype of this REAL cooling technique is funded by DoE for FY2017 & FY2018.
In this paper, we will present the modeling results of this REAL cooled prototype mirror. The two parameters of the electric heater (length and power density) are optimized for the thermal deformation minimization of the mirror Finite Element Analysis (FEA) with ANSYS. This optimization of two parameters within ANSYS is not straight forward and necessity large number of FEA calculations. SRW software is used for the wavefront propagation simulation to compare the performance of REAL cooled mirror with other frequently used cooling techniques.
1. Zhang L., Cocco D., Kelez N., Morton D.S., Srinivasan V. and Stefan P.M. - Optimizing X-ray mirror thermal performance using matched profile cooling, J. Synchrotron Rad. (2015). 22,1170–1181, doi: 10.1107/S1600577515013090
2. Zhang L. , Barrett R. , Friedrich K. , Glatzel P. , Mairs T. , Marion P. , Monaco G. , Morawe C. , Weng T. - Thermal distortion minimization by geometry optimization for water-cooled white beam mirror or multilayer optics, Journal of Physics : Conference Series 425, 052029-1-052029-4 (2013)
The success of the LCLS led to an interest across a number of disciplines in the scientific community including physics,
chemistry, biology, and material science. Fueled by this success, SLAC National Accelerator Laboratory is developing a
new high repetition rate free electron laser, LCLS-II, a superconducting linear accelerator capable of a repetition rate up
to 1 MHz. Undulators will be optimized for 200 to 1300 eV soft X-rays, and for 1000 to 5000 eV hard X-rays. To
absorb spontaneous radiation, higher harmonic energies and deflect the x-ray beam to various end stations, the transport
and diagnostics system includes grazing incidence plane mirrors on both the soft and Hard X-ray beamline.
To deliver the FEL beam with minimal power loss and wavefront distortion, we need mirrors of height errors below 1nm
rms in operational conditions. We need to mitigate the thermal load effects due to the high repetition rate. The absorbed
thermal profile is highly dependent on the beam divergence, and this is a function of the photon energy. To address this
complexity, we developed a mirror cradle with variable length cooling and first order curve correction. Mirror figure
error is minimized using variable length water-cooling through a gallium-indium eutectic bath. Curve correction is
achieved with an off-axis bender that will be described in details.
We present the design features, mechanical analysis and results from optical and mechanical tests of a prototype
assembly, with particular regards to the figure sensitivity to bender corrections.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.