• H. Ao, K. Hasegawa, K. Hirano, T. Morishita, A. Ueno
  JAEA/LINAC, Ibaraki-ken
• H. Asano
  JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
• M. Ikegami, F. Naito
  KEK, Ibaraki
• V.V. Paramonov
  RAS/INR, Moscow
• Y. Yamazaki
  J-PARC, KEK & JAEA, Ibaraki-ken

The ACS (Annular-ring Coupled Structure) cavities were under development for the J-PARC Linac from 190 MeV to 400 MeV. We have fixed the cavity specification, taking into account the results of the high-power conditioning and the fabrication experience. The mass production of the ACS with a tight time schedule is now an issue, since the user community strongly requests the beam power upgrade as early as possible. Therefore, the design and the fabrication process of the ACS cavity have been reexamined on the basis of the experience, stored during the course of the fabrication and the tuning of the prototype ACS tanks. Here, we also discussed about the key issues on the mass production with a manufacturer. The cavity shape, that required complicated machining, was simplified to some extent, while the frequency tuning strategy was reconsidered to reduce the production period. The paper describes these recent activities on the ACS development.