J-PARC provides 3 GeV proton beam to Material and Life Science Experimental Facility (MLF) and produce intense neutron beam and muon beam. It also provides 30 GeV proton beam to hadron experimental facility (HEF) and neutrino facility. In HEF, proton, secondary pion and kaon, or tertiary muon are used for particle and nuclear physics experiments. At neutrino facility, muon neutrino beam is produced for neutrino experiments. Research fields with those beams range very widely from industrial applications to elementary particle physics. Overviews of those research with J-PARC beams are described.

The J-PARC accelerator has been in operation for 18 years, starting with the linac beam commissioning in 2006. Currently, the MLF is in continuous operation with a beam power of 1 MW, and the linac and RCS have achieved the nominal performance. R&D for 1.5 MW operation for future plans has also been started. The MR has increased its beam power through a major upgrade works in 2021, currently achieving 80 kW for HD and 800 kW for NU. In addition, MR is aiming to achieve 100 kW for HD and 1.3 MW for NU in 2026 and 2028, respectively.

The J-PARC linac has started beam operation to users since 2008. With energy and current upgrade at 2013 and 2014 respectively, the linac provides a nominal peak current of 50 mA beam at an energy of 400 MeV to the following 3 GeV rapid-cycling synchrotron (RCS) since Oct. 2018. The linac is currently operating at the design macro-pulse length of 0.5 ms, which corresponds to 1 MW output from the RCS. Beam studies have been continued to improve beam quality to reduce beam loss. Further beam current/duty upgrades of the linac are currently being considered to achieve beam output beyond 1 MW at RCS and to provide a 400 MeV beam to the test facility through 50 Hz operation of the linac.

J-PARC 3 GeV Rapid Cycling Synchrotron (RCS) had demonstrated its potential to accelerate a 1-MW beam in 2015 with a single shot trial. After that, we have continued the beam studies and hardware improvements to establish a 1-MW continuous beam condition. Thanks to those efforts, we achieved the 1-MW user operation in April 2024. This report summarizes the major topics of those studies and improvement works. We also introduce the plan to accelerate beyond 1-MW beam.

The main ring (MR) is the final stage accelerator of J-PARC to provide high-intensity proton beams to experiments for the elementary particle and nuclear physics. We have been increasing the beam power since the start of the beam commissioning in 2008, because the success of the experiments depends largely on the beam power. MR has two operation modes: the fast extraction (FX) mode for the neutrino experimental facility and the slow extraction (SX) mode for the hadron experimental facility. For the FX mode, the beam power of 500 kW was achieved in 2019 and operated until the summer of 2021 with the cycle time of 2.48 s. For the SX mode, the beam power of 65 kW was achieved in 2021 with the cycle time of 5.2 s and the extraction efficiency of 99.5%. The accelerator components have been upgraded to achieve higher beam power with the faster cycling scheme in 2021 and 2022. The FX operation resumed in Apr. 2023 with the cycle time of 1.36 s. We have achieved the beam power of 760 kW in Dec. 2023 and 800 kW in June 2024, exceeding the original design of 750 kW. The faster-cycle SX operation was started in Apr. 2024 and the beam power of 81 kW was achieved with the cycle time of 4.24 s. We plan to upgrade for the beam power of 1.3 MW for the FX mode and more than 100 kW for the SX mode.

High-intensity proton beams generate large electromagnetic fields, the effects of which are expressed as the wake fields W(t) in the time domain and the beam coupling impedances Z(ω) in the frequency domain. The fields affect the beam energy (beam loading etc.), the collective beam instabilities in longitudinal and transverse (horizontal or vertical or both) directions, and component heating. The coupling impedances were estimated in advance to the construction and beam instabilities were calculated (impedance budget). The ceramic ducts in the magnets of RCS (Rapid Cycling Synchrotron), kickers in RCS and MR (Main Ring Synchrotron), and stainless-steel vacuum ducts in MR were considered as large impedance sources. Stretched wire methods were also performed for the ceramic ducts and kickers. Transverse instabilities were observed both in RCS and MR under low chromaticity operation. The transverse instabilities in RCS mainly driven by the FX kickers are controlled by chromaticity and betatron tune. Recently kicker impedance reduction by diode and resister units is also implemented. The transverse instabilities in MR mainly driven by the resistive wall of the vacuum ducts and the kickers are controlled by chromaticity and an intra-bunch feedback system. Electron clouds have also been investigated in RCS and MR.

Magnetic Alloy, Finemet^®, is a material developed in Japan. We have been developing Magnetic Alloy-loaded cavities for approximately 30 years. Although the developments aimed to develop RF system to accelerate very high intensity proton beam, fruitful spin-outs were born through the researches. We will summarize the history of the cavity developments, successful status of beam acceleration of J-PARC synchrotrons, and future prospect of the cavities.

For the study of material damage under the beam irradiation circumstance of accelerator-driven systems (ADS), the Japan Atomic Energy Agency (JAEA) had planned to construct a Transmutation Experimental Facility-Target Facility (TEF-T) using J-PARC Linac 400 MeV protons beams and the Lead-Bismuth Eutectic spallation target. The task force for evaluating partitioning and transmutation technology in the Ministry of Education, Culture, Sports, Science and Technology (MEXT) recommended that the facility be considered to maximize the advantages of using Linac to meet users’ various needs. The proton irradiation facility, a successor of TEF-T, is planned to be constructed for 1) Material irradiation examinations, 2) Semiconductor soft-error research using spallation neutrons, 3) Medical RI production, and 4) Proton beam applications for space use. A user community was established in 2022 to incorporate user input as a more attractive facility. In this paper, the present design status of the facility is described.

Rationale are presented for the choice of J-PARC accelerator scheme and parameters.

The 32nd International Linear Accelerator Conference (LINAC2024) was held in downtown Chicago, U.S., from August 25 to 30, 2024. The conference was co-hosted by Argonne National Laboratory and Fermi National Accelerator Laboratory at the historic Hilton Chicago. LINAC is the major biyearly gathering for the world-wide community of linear accelerator experts. The conference provided a unique opportunity to hear about the latest progresses in researches and developments on linacs and their applications.