Construction of a booster linac for the RIKEN heavy-ion linac

A linac consisting of six rf cavities has been constructed as an energy booster of the RIKEN heavy-ion linac. Various heavy ion beams have been accelerated to $5.8\mathrm{MeV}∕\mathrm{u}$ with intensities of more than $1p\mu \mathrm{A}$ (particle microampere) in the continuous-wave mode. The booster cavities are based on a quarter-wavelength resonator of a coaxial structure. The first two cavities are frequency-variable in a range from $36\text{to}76.4\mathrm{MHz}$ , whereas the last four are operated at a fixed frequency of $75.5\mathrm{MHz}$ . The design study started in 1997 and the booster was commissioned in 2001. The total voltage gain of the booster ever achieved is $11\mathrm{MV}$ , which is 70% of the designed value of $16\mathrm{MV}$ . The high-intensity beams from the booster have been successfully applied to a systematic study on the synthesis of superheavy elements. The beam energy from the ring cyclotron has also been increased, owing to the additional velocity gain provided by the booster.