M. Oyaizu

Test results of 18 GHz ECR charge breeder for KEK–JAERI RNB facility

Some of the test results of KEK ECR charge breeder made for KEK–JAERI RNB facility are presented and discussed. The charge breeding efficiencies for singly charged ions of noble gases have been measured; typically 13.5% of Ar9+, 10.4% of Kr12+, and 6.8% of Xe20+ with a global efficiency of about 50%. By injecting pulsed Xe1+ ion beam into the charge breeder, the time necessary for charge breeding has been also measured and found to be about 60 ms for Xe21+ ions. As an effort to reduce the residual gas ions incidentally existing in the plasma chamber of the charge breeder a nonevaporable getter has been tested, and the preliminary results are discussed.

KEKCB electron cyclotron resonance charge breeder at TRIACa)

The KEKCB is an electron cyclotron resonance (ECR) ion source for converting singly charged ions to multicharged ones at Tokai Radioactive Ion Accelerator Complex. By using the KEKCB, singly charged gaseous and nongaseous ions were converted to multicharged ones of A/q approximately 7 with efficiencies of 7% and 2%, respectively. The conversion efficiency was found to be independent of the lifetime of the radioactive nuclei having lifetimes of the order of one second. Three collimators located at the entrance and the exit of the KEKCB defined the beam axis and facilitated beam injection. Grinding and washing the surfaces of aluminum electrode and plasma chamber dramatically reduced impurities originating from the ECR plasma of the KEKCB.

The KEK–JAERI joint RNB project

A new ISOL-based radioactive nuclear beam (RNB) facility has been constructed as a joint project of the High Energy Accelerator Research Organization (KEK) and the Japan Atomic Energy Research Institute (JAERI) at the Tandem accelerator of the Tokai site of JAERI. It uses heavy-ion linacs and related equipment of the presently closed KEK-RNB facility. Fission fragments with energy from 0.1 to 8 MeV/u will be available at this facility.

The development of an ECR charge breeder for KEK-JAERI joint RNB project

For the construction of an electron cyclotron resonance (ECR) charge breeder (CB) at KEK-JAERI radioactive nuclear beam facility, we have made a pilot charge breeding system consisting of two compact-sized ECR ion sources. In order to find the necessary conditions for an ECRCB with high efficiency, the charge breeding efficiency for Ar ions in nitrogen plasma has been measured and compared with the results from the simulation of the trajectories of the ions in the plasma. By explicitly taking into account the cumulative small angle scattering of charged ions as well as the confinement magnetic field of the pilot CB, the simulation predicts the stopping efficiency. This is defined as a probability that the injected ions are still within the plasma volume when their initial directional motions have become randomized. Incorporating the ionization efficiency for the ions, the overall breeding efficiency has been estimated. Finally, the characteristics of the ECRCB recently installed for tests at KEK is described.

Beam bunching of the radioactive nuclear beam in a 6.4 GHz electron cyclotron resonance ion source

A single stage 6.4 GHz ECR ion source for an isotope separator on-line based radioactive nuclear beam facility at KEK-Tanashi, is under an on-line test for the effective production of 19Ne2+ ions by using alpha beams from a cyclotron. Synchronized with the pulsed operation of heavy ion linacs, the millisecond beam bunching is now being performed to reduce the loss of radioactive ions in acceleration by using a pulsed-gating potential method. The highest beam intensity at 100 Hz and 20% duty factor reaches 60% of that obtained in a continuous-wave mode.

GEM-MSTPC: An active-target type detector in low-pressure He/CO2 mixed gas

We developed an active-target type gas counter operating with low pressure He/CO2(10%) detector gas for application in studying low-energy nuclear reactions using radioactive nuclear beams (RNBs). A 400-?m-thick gas electron multiplier (THGEM) was used as the proportional counter for high injection rate capability. We examined the gas gain stability and the influence of ion feedback on particle tracks at high beam injection rates of up to 105 particles per second (pps) using a low-energy 12C beam. By means of modifications to the THGEM, the shift in the relative pulse height of the output signal normalized to that measured at the 12C injection rate of several hundred pps improved to within 2% at the rate of 105 pps. The position distortion induced by ion feedback was suppressed to within 1.5 mm because of the relatively high drift field value of 1 kV/cm?atm and the use of the double GEM configuration. The THGEM was found to be applicable for our active target at high injection rates of up to 105 pps.

Stopping efficiency of an ECR charge breeder for axially injected ions

We have simulated the trajectories of ions injected into an ECR plasma, by taking into account explicitly the cumulative small angle scattering of charged ions as well as the confinement magnetic field of the ECR source. Assuming homogeneous and uniform plasma in a given volume, the stopping efficiency, defined as a probability that the injected ions are alive in the plasma volume until their initial directional motions become at random, has been extracted and discussed.

The development of a charge breeding system with ECR ion sources of permanent magnets

For the construction of an electron cyclotron resonance (ECR) charge breeder at the KEK-JAERI radioactive nuclear beam facility, we have made and tested a pilot charge breeding system consisting of two compact-sized ECR ion sources. Using a simulation code, we have investigated the geometrical beam acceptance of the present system for the external injection of ions of interest into ECR plasma. Following discussions on the ECR plasma influenced by a deceleration potential for the external injection, the charge breeding efficiency for Ar ions is presented and discussed.

Ion source development for isotope separator on-line based radioactive nuclear beam facility at KEK-Tanashi

For an isotope separator on-line based radioactive nuclear beam facility at KEK-Tanashi, two different types of ion sources are currently employed: a single stage 6.4 GHz electron cyclotron resonance ion source for 18Ne2+ and 19Ne2+ ions and a surface ionization-type ion source for 8Li1+ ions. The production target for Ne radioisotopes is LiF powder. Enclosed in a water-cooled Cu target cell, the target can sustain proton beam power of 120 W for a long term. The methods to suppress unwanted isotopes, like 19F in 19Ne and 18O in 18Ne, are discussed. For the production of 8Li1+ with the surface ionization-type ion source, a recoil-catcher method is adopted. The feasibility is discussed, comparing to a thick target method.

Design of a small electron cyclotron resonance ion source with partially pulsed magnetic field

A small size electron cyclotron resonance ion source to be operating in a pulsed mode has been designed. The main static magnetic field for the confinement is formed by permanent magnets. In addition to these magnets, two small coils are introduced for controlling the magnetic field with a repetition rate synchronized with the pulsed operation of microwave power feeding. One is installed in the middle of the plasma chamber, the other is near the anode electrode.