3D-Printed Beam Window for J-PARC COMET Project

We have developed the beam windows for the COherent Muon to Electron Transition (COMET) project. The COMET project is under construction at the proton beam accelerator facility J-PARC in Japan. Windows are one of key components on the beam line. The beam window should pass the beam of proton or other particles effectively, with robustly separating sections of the beam line. After research and developments for the beam windows especially focused on the shape and structure, we employed a dome shape which has radius curvature(s) on to the beam passing area, instead of a conventional thin and flat shape. Additionally, we employed a thicker structure on the circumference part to give higher mechanical strength. To manufacture cost consciously, the beam windows are fabricated by a three-dimensional printer, additive manufacturing (AM) to realize a hard to cut and machine shape. The detailed dimensions were designed through numerical analysis. There are requirements of high transmission efficiency of the beam, such as the material density must be low, and the thickness must be as thin as possible, while minimizing the nuclear heat generation by beam energy loss. Thus, the material was chosen as titanium alloy, Ti-6Al-4V. After the window shape was formed by AM, the voids inside the window were killed by hot isostatic pressing. Finally the window was polished to be smoothen the surface and tailored its thickness. As the first step to be used for the COMET project, for Phase-α we fabricated domes of 270 and 220 mm to mate with the rotatable flanges. The target values of the dome were the thickness tailored below 0.5 mm, and the pressure withstanding performance over 0.8 MPa for each diameter model. Throughout the tests we confirmed that the windows satisfied the target values, and the windows could withstand 0.9 MPa when pressurized from concave side of the dome.