The next generation heavy ion accelerator facility, such as the RIKEN radio isotope (RI) beam factory, requires an intense beam of high charged heavy ions. In the past decade, performance of the electron cyclotron resonance (ECR) ion sources has been dramatically improved with increasing the magnetic field and rf frequency to enhance the density and confinement time of plasma. Furthermore, the effects of the key parameters (magnetic field configuration, gas pressure, etc.) on the ECR plasma have been revealed. Such basic studies give us how to optimize the ion source structure. Based on these studies and modern superconducting (SC) technology, we successfully constructed the new 28 GHz SC-ECRIS, which has a flexible magnetic field configuration to enlarge the ECR zone and to optimize the field gradient at ECR point. Using it, we investigated the effect of ECR zone size, magnetic field configuration, and biased disk on the beam intensity of the highly charged heavy ions with 18 GHz microwaves. In this article, we present the structure of the ion source and first experimental results with 18 GHz microwave in detail.

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 5.8MeV∕u with intensities of more than 1pμA (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 36to76.4MHz, whereas the last four are operated at a fixed frequency of 75.5MHz. The design study started in 1997 and the booster was commissioned in 2001. The total voltage gain of the booster ever achieved is 11MV, which is 70% of the designed value of 16MV. 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.

No abstract

The charge state distributions of krypton ( 86 Kr) with an energy of 2.7 MeV=nucleon were measured using hydrogen (H 2 ) and helium (He) gas charge strippers. A differential pumping system was constructed to confine H 2 and He gases to a thickness sufficient for the charge state distributions to attain equilibrium. The mean charge states of 86 Kr in H 2 and He gases attained equilibrium at 25.1 and 23.2, respectively, whereas the mean charge state in N 2 gas at equilibrium was estimated to be less than 20. The charge distributions are successfully reproduced by the cross sections of ionization and electron capture processes optimized by a fitting procedure.

A new type of variable-frequency radio-frequency quadrupole (RFQ) linac has been constructed as a new preinjector for the RIKEN heavy-ion linac (RILAC). The RFQ resonator, based on a folded-coaxial structure with a movable shorting plate, is compact even in a low frequency region below 20 MHz. It accelerates ions with mass-to-charge ratios of 5.3 to 26.4 in the energy range up to 450 keV per charge, by varying its resonant frequency from 17.4 to 39.0 MHz. Moreover, the power loss is small in the low frequency region; the rf power consumption in cw mode is 7 kW at 17.4 MHz, and it increases to 30 kW at 39.0 MHz at the maximum intervane voltage of 36.8 kV. We initiated this project in 1991, and installed the new preinjector consisting of an electron cyclotron resonance ion source of 18 GHz and this RFQ in 1996. Since the installation, the beam intensity has become larger by more than 1 order of magnitude than that formerly obtained with the Cockcroft–Walton preinjector. Recently, the maximum power of the beam extracted from the RILAC and that from the ring cyclotron has reached 560 W and 2 kW, respectively.