WAVEFRONT DISTORTION MEASUREMENT AT SSRF

The Synchrotron Radiation Monitor (SRM) system has been designed and constructed, at the Shanghai synchrotron radiation Facility (SSRF), for several years and runs good [1]. However, the monitor extraction mirror deformation is quite common at different facilities and other reflecting mirrors in the optic path also have surface error and angle error. As we decide to upgrade the SR monitor system at SSRF, this issue is also one of the most import thing what we should overcome. In order to verify the feasibility and evaluate the accuracy, simulations based on SRW code have been done. In this simulation, a dedicated algorithm was developed to reconstruct wavefront. The result and the algorithm is very useful for our experiment and upgrade program. In this paper, the algorithm and the experiments based on Shark-Hartmann wavefront sensor will be presented detailed. INTRODUCTION There are two diagnostic beam lines in the storage ring of SSRF, one of them adopt interferometer to measure the transverse electron beam size of storage ring. One beryllium mirror with water-cooling was used to transmit X-ray and reflect visible light to satisfy the measurement request. Several reflecting mirrors were used to transmit the visible part of synchrotron radiation to the laboratory [1]. Layout of optical path of SR interferometer is as shown in Figure 1. Figure 1: Layout of synchrotron radiation interferometer at SSRF. The surface of Be mirror was deformed because of Xray heat. A set of Hartmann Screen test was used to measure the Be mirror deformation at SSRF [2]. However, other reflecting mirrors also have surface error and angle error due to equipment. The impact of all mirrors in the optic path to the wavefront transport before interferometer need to be measured and corrected. However, at SSRF or other facilities, Hartmann masks have been used to measure the deformation of the Be mirror, but the impact from all mirrors to wavefront has never been measured yet. In our experiment a Shark-Hartmann wavefront sensor was used, before interferometer, to measure wavefront deformation through the whole optic path. The result of wavefront distortion measurement can be used to correct the measurement of beam size by interferometer. The Beryllium Mirror Designed at SSRF The Be mirror was special designed to reflect visible light from vacuum and let x-ray pass through. Part of the energy of x-ray was absorbed by the Be mirror and the heat cause deformation of the mirror [2]. The Be mirror shape used at SSRF is shown as Figure 2. The power of our Synchrotron Radiation light is about 531.4W. The thinnest part of our Be mirror is about 0.254mm. Even though Beryllium is the best metal to conduct x-ray, the power absorbed by our Be mirror is about 200W. The impact of the deformation on Be mirror cannot be ignored. Figure 2: Be mirror with water-cooling used at SRM of SSRF. SRW "Synchrotron Radiation Workshop" (SRW) is a physical optics computer code for calculation of detailed characteristics of Synchrotron Radiation (SR) generated by relativistic electrons in magnetic fields of arbitrary configuration and for simulation of the radiation wavefront propagation through optical systems of beam lines. Frequency-domain near-field methods are used for the SR calculation, and the Fourier-optics based approach is generally used for the wavefront propagation simulation [3]. Hartmann Method Hartmann method wavefront sensor is a very common method to measure the wavefront distortion. The Hartman ____________________________________________ * Work supported by National Natural Science Foundation of China (No.11375255) gaobo@sinap.ac. cn lengyongbing@sinap.ac. cn MOPAB090 Proceedings of IPAC2017, Copenhagen, Denmark ISBN 978-3-95450-182-3 332 Co py rig ht © 20 17 CC -B Y3. 0 an d by th er es pe ct iv ea ut ho rs 06 Beam Instrumentation, Controls, Feedback and Operational Aspects T03 Beam Diagnostics and Instrumentation test, invented by Hartmann (1900, 1904a, b and c) to test the Great Refractor at Postdam, has its antecedent in the measurements of eye refractive defects using a screen with two holes in front of the eye as described by Tscherning (1894). It uses a screen with an array of holes placed close to the entrance or exit pupil of the system under test [4]. SIMULATION To verify the feasibility of the method and design a good wavefront reconstruct algorithm, we used Synchrotron Radiation Workshop (SRW) code for the source description and wavefront transport, while algorithm based on python is used to analyse its outputs. This algorithm can be used for similar Hartmann based wavefront metrology such as Shark-Hartmann to calculate the wavefront aberration. Source Description and Wavefront Transport We use SRW code to generate a Gaussian beam. Beam size is same as the regular model of SSRF: σHor. = 52 μm; σVer. = 20 μm. The optic path and the visible part of the SR light is shown as Figure 3.