Speaker
Description
Beam windows used in MW-class proton accelerator facilities are subjected to severe thermal shock, cyclic stress, and radiation damage under intense pulsed beam operation. In the J-PARC neutrino beamline, a 0.4-mm-thick double-dome Ti-6Al-4V (Ti-64) beam window structure with He gas cooling between the two layers has been successfully operated; however, future higher-power operation toward the Hyper-Kamiokande era will require improved irradiation tolerance and structural reliability[1].
To address this issue, metastable β-Ti alloys are being investigated as candidate beam window materials. In particular, Ti-15V-3Cr-3Sn-3Al (Ti-15-3) alloy with a specially designed two-step aging treatment (ST2A) has shown promising indications of reduced irradiation hardening and enhanced defect recovery behavior under high-energy proton irradiation compared with conventional Ti-64 alloys. These characteristics are considered to be related to the unique defect dynamics and lattice instability inherent in metastable β-Ti systems[2].
Based on these findings, a practical beam window upgrade concept for future MW-class operation is being studied using a hybrid structure consisting of Ti-15-3 ST2A on the accelerator side and Ti-64 on the downstream target-vessel side, in order to reduce the fracture risk of the upstream window region, which could lead to severe contamination of the accelerator vacuum system.
Post-irradiation examination (PIE) of actual irradiated Ti-64 beam window materials and Ti-15-3 OTR beam monitor foils recovered from the J-PARC neutrino facility is also planned, including small punch testing and gas production measurements such as thermal desorption spectroscopy (TDS). Furthermore, dual-ion irradiation experiments at Japanese facilities (HIT/TIARA) are planned to investigate spallation-induced He production effects, together with meso-scale ultrasonic fatigue testing using the proton cyclotron facility at the University of Birmingham.
This presentation will discuss current irradiation issues of high-power proton beam windows at J-PARC, recent progress in β-Ti alloy development, and future prospects toward radiation-tolerant beam-intercepting components for next-generation accelerator facilities.
[1] T. Ishida, E.Wakai, S.Makimura et al., JPS Conf. Proc., 28, 041001 (2020).
[2] T.Ishida, S.Kano, E.Wakai et al., J. Alloys Compd., 995, 174701 (2024).