| Shanghai synchrotron radiation facility (SSRF) is an advanced third generation synchrotron light source with a 3.5 GeV electron storage ring, a designed beam current of 300mA and a natural emittance of 3.9nm·rad. Front-end, locating between the electron storage ring and beamline, is an important part of synchrotron radiation facility. Front-end is a combination of machinery, ultrahigh vacuum, radiation protection, control interlock, and so on. As critical elements of front-end, photon absorbers are to handle the high heat load from bending magnets or insertion devices and confine beam apertures as beamline expected.A superconducting wiggler, which will be first utilized to generate high energy X-rays for the ultra-hard X-ray applications beamline in SSRF phase-Ⅱ, has an intensive power of over 43.3kW and a peak power density of 45W/mrad2. Overlapping the radiations from upstream and downstream bending magnets, the total power will reach 44.7kW, which is the highest heat load for a front-end in the third generation synchrotron light source up to now. The sharply increased heat load, which is 4 to 64 times higher than the commissioned ones at SSRF, becomes a big challenge to design of ultra-hard X-ray applications front-end and its photon absorbers.Overall design of ultra-hard X-ray applications front-end, including general layout, ray tracing, power allocation and vacuum system, was firstly obtained in the paper. Technical specifications of 4 or 5 photon absorbers in the two kinds of layout, abbreviated as 4A and 5 A respectively, were all determined and to be studied.A dispersion strengthened copper called Glidcop Al-15, which has a high strength and thermal conductivity, is chosen to manufacture the absorbers. Direct water cooling and grazing incidence structures are applied to improve heat-absorbing ability for front-end photon absorbers. The cooling channels were optimized to obtain a high film coefficient and a low pressure drop by Petukhov formula and Darcy-Weisbach formula. The temperature and thermal stress distributions of photon absorbers with different structure parameters were simulated carefully by thermal analysis with ANSYS. The optimized parameters are:diameter of cooling channels is 6mm, distance of absorbing surfaces to cooling channel walls is 9mm, corner radiuses of two adjacent absorbing surfaces are bigger than 2mm, roughness of absorbing surfaces is better than 6.3μm and directions of cooling channels should be parallel to the beam approximately. The simulation results show that photon absorbers in 5A layout exceed the failure criteria, while those in 4A layout are fairly safe even the beam current of electron storage ring were elevated to 500mA.Technology of manufacturing photon absorbers is a key issue for ultra-hard X-ray applications front-end. Increasing the amounts of photon absorbers, which is a common way to handle high heat load and is mostly applied in other facilities, is not workable for SSRF because of the limited space of front-end. A kind of novel photon absorbers, called EBW photon absorbers, which has two sub-absorbers welded to each other by electron beam welding (EBW), was firstly proposed to prolong the absorbers and decline power densities on them. Metallographic microscope and scanning electron microscopy were employed to study the microstructure of brazed joints and EBW joints for Glidcop Al-15. Vacuum sealing tests results indicated that both joints satisfy the vacuum requirements of SSRF front-end. The tensile mechanical properties and fatigue properties were also obtained by tensile tests and fatigue tests at 20℃, 100℃ and 200℃. Compared to brazed joints, EBW joints have higher tensile strength, better ductility and more stable performance. Furthermore, the failure criteria for EBW joints were successfully created and two fixed masks of ultra-hard X-ray applications front-end will be manufactured as EBW photon absorbers.With a higher mechanical strength and a little worse accuracy, medium speed wire-cutting electrical discharge machining (WEDM-MS) becomes another effective scheme to make high heat load photon absorbers and is well suited to long photon shutters. After constantly adjusting of process parameters of WEDM-MS, the photon shutter for ultra-hard X-ray applications front-end has been successfully developed and will be commissioned in the near future, which is the first application of WEDM-MS in synchrotron radiation facility front-end. |
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