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  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 . 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.