Electron guns using a gridded thermionic cathode have been used in many accelerator facilities because of their ease of handling. However, due to the lens effect of the electron beam passing through the grid, the gridded cathode has been considered unusable as an electron source for free electron laser, which require a normalized emittance of less than 2 mm mrad. We researched the low-emittance beam generation using a gridded thermionic cathode, focusing on its advantages such as high maintainability and long lifetime. Both numerical analysis and simulation using physical models were used to study the distortion of the electric field and electron beam trajectory near the grid. It was found that there exist conditions of voltage between the cathode and grid under which the electron trajectory passing through the grid are parallel when the distortion of the electric field is at a minimum. Low emittance of less than 2 mm mrad/1 nC can be produced with an electron gun high voltage of 50 kV, obtaining a uniform particle distribution and minimum emittance at the gun exit. The low-emittance beam can be accelerated 500 keV by using the rf cavity immediately after the electron gun to avoid emittance increase due to space-charge effect. We constructed an electron gun system based on the scheme and confirmed that the target performance of less than 2 mm mrad emittance was achieved in the beam test. The emittance dependence due to the electric field distortion of the grid was also obtained in agreement with the simulation.

The performance of superconducting radio-frequency cavities depends on niobium surface conditions. Recently, various surface treatments focused on heat treatment have been investigated to realize the high-Q value and high accelerating gradient cavities. We report the influence of a new heat treatment process called furnace baking on the performance of 1.3 GHz TESLA type superconducting radio-frequency cavities made of niobium. Furnace baking is simpler than the previously reported heat treatment method, therefore it is easy to apply to the cavities. We find that the Q value increases after furnace baking at temperatures ranging from 250 to 400°C and Q behavior to the accelerating gradient are similar to nitrogen doped cavities. Furthermore, we find a significant difference in the surface resistance between baking temperatures ranging from 200 to 800°C.

We have started construction of a small heavy-ion storage ring equipped by an internal target at RIKEN Nishina Center. This is named RUNBA (Recycled-Unstable-Nuclear Beam Accumulator) aiming at R&D study for beam recycling (BRC) technique. RUNBA will be a novel tool for a nuclear reaction study for rarely produced radioactive nuclei and will leads the RI-beam research to the next stage. In this paper we describe ideas for realizing the BRC technique and report the status of RUMBA.

In 1973, National Laboratory for High Energy Physics was established in Tsukuba Science City. Since then, various particle accelerators have been constructed and operated. In this account, we review characteristics of these accelerators.