The nitrogen infusion, the recently discovered surface processing method for high-gradient and high-quality factor superconducting cavities, attracts much attention in the particle accelerator community in particular in the context of the International Linear Collider project. In this paper, we briefly review the basics of superconducting cavity, the causes of performance limitation, and the three modern surface processing methods including the nitrogen infusion and resultant cavity performances. The present understanding on physics behind these surface treatments are also discussed.

We are facing a serious confusion about the proton size, referred to as “Proton Radius Puzzle.” The proton charge radius determined by electron scattering and hydrogen spectroscopy, and that determined by muonic hydrogen spectroscopy disagree seriously, and the reason is not yet clear. It is a big surprise to realize that we do not yet fully pin down the most basic feature of the proton. In this article, we discuss on an on-going project at the Research Center for Electron-Photon Science (ELPH), Tohoku University to determine the proton charge radius by low-energy electron scattering under the lowest-ever momentum transfer, which is crucial to extract the radius reliably from electron scattering data. It is worth emphasizing that ELPH is the only facility in the world, where such a low-energy electron scattering experiment is possible.

At the J-PARC muon science facility, a muon linac for a muon g-2/EDM experiment is being developed. In this experiment, Ultra-Slow Muons (USMs), which are generated by resonantly ionizing the thermal muonium evaporated from a silica aerogel, will be accelerated to 212 MeV. The momentum spread of the accelerated muon beam is 0.1%, and the normalized transverse emittance is approximately 1.5π mm mrad. Proof of the slow muon acceleration scheme is an essential step toward realizing the world’s first muon linac. In October 2017, we succeeded in accelerating slow negative muoniums generated using a simpler muonium source, even though they are not USMs. Negative muonium atoms (Mu⁻), which are bound states of positive muons (µ⁺) and two electrons, are generated from µ⁺’s through the electron capture process in an aluminum degrader. Generated Mu⁻’s are accelerated to 89 keV by a radio frequency quadrupole linac. In this paper, the present design of the muon linac for the g-2/EDM experiment and the result of the world first demonstration of the muon acceleration are described.

JSNS hopes to build compact and medium-sized accelerator-driven neutron sources and to construct regional bases for neutron research. To construct these neutron facilities, JSNS is calling for collaboration with PASJ.