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We have constructed four high performance electron cyclotron resonance (ECR) ion sources in RIKEN and produced a variety of intense beams of heavy ions (e.g., 2 emA of Ar8+, 0.6 emA of Kr13+, 0.3 emA of Xe20+). During the improvement of their performance, we found that several key parameters play essential role on increasing the beam intensity. The parameters are plasma electrode position, magnetic field configuration, property of the chamber wall material and position of a biased disk. To investigate how the parameters influence on the beam intensity, we made a systematic study using the laser ablation method. In these experiments, we observed that Bmin influences the electron density and confinement time of ECR plasma.

The buildup of a plasma produced by short-pulse (0.05–1.2 μs), high-power (60–100 kW) microwaves is studied in a pressure range of 10 mTorr–10 Torr, by measurements of the temporal variation of the current and the optical intensity. The plasma is produced in a cylindrical tube and confined by a minimum-B field. The buildup of the electron current and the optical intensity are found to continue beyond the end of the pulse, for a few to tens of μs depending upon the pressure, and a minimum in their peak values and buildup times occur around 1 Torr. Increase in microwave pulse duration increases the buildup rate and peak current, whereas the pulse repetition frequency (10–500 Hz) has only a weak influence. The results are discussed from the growth of electron temperature during the pulse, and the following plasma evolution after the end of pulse. Collisional wave absorption, electron cyclotron heating, and diffusion are found to play important roles in plasma production and maintenance over the pressure range.

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.

The trim coil system of a superconducting ring cyclotron currently under design at RIKEN is composed of five sets of superconducting and twenty sets of normal-conducting coils. A major correction for isochronization of the magnetic field is accomplished by the superconducting coils, and a fine correction by the normal-conducting coils. For an optimal design of the system, a numerical procedure was developed to generate isochronous fields for design nuclei and consequently current settings on the main and trim coils, employing an iteration of a least-squares fitting to minimize orbital frequency errors. The main and normal-conducting coils can also be used to correct unwanted harmonic fields with sector dependent and/or radially localized excitations. The correction schemes were conceived for presumed error fields, alignment tolerances being estimated based upon beam optics requirements. On the other hand, a new trim coil design was envisaged, which comprises only superconducting trim coils. The subcoils in the present five-set design can be independently excited to replace the normal-conducting coils. This option may be more economical, but complications on the superconducting coil system raise a reliability concern.

As required by the research group of super heavy elements, highly intense beams of 58Fe and 64Ni ions were produced from RIKEN 18 GHz electron cyclotron resonance ion source [Nakagawa et al., Nucl. Instrum. Methods Phys. Res. A 396, 9 (1997)] using the MIVOC method. Maximum intensities of the supplied ion beams were 20 eμA at the extraction voltage of 15.9 kV for 58Fe13+ and 90 eμA at the extraction voltage of 17.5 kV for 64Ni13+. Consumption rate was about 1.2 mg/h for 58Fe and 64Ni, respectively. For the Radio Isotope Beam Factory [Yano, Proceedings of the 15th International Conference on Cyclotron and Their Application [(IOP, Bristol, 1999)] and various requests from users groups, we constructed a double beam injector system for the two ion sources as an external ion source of the Riken Heavy Ion Linear Accelerator [Odera et al., Nucl. Instrum. Methods Phys. Res. A 221, 187 (1984)]. Two ion sources; RAMSES [Arai et al., Nucl. Instrum. Methods Phys. Res. A 491, 9 (2002)] and 18 GHz ECRIS, will be placed...

In order to investigate the ion-optical parameters of the RIKEN 18 GHz electron cyclotron resonance ion source injection line into the RIKEN RFQ linac, we measured the beam emittance for a wide mass range of the ions. First results indicate a strong extraction current dependence of the normalized emittance. When using the insertion method to produce metallic ions from solid rod with high melting point, we observed that the beam emittance of metallic ions was very large compared to the beam emittance of ions from gaseous element. It may be due to the plasma instability caused by inserting the rod into the plasma deeply.

Intense beams of heavy ions with medium charge states (1.3 mA of Ar8+, 200 μA of Xe20+) have been produced from a RIKEN 18 GHz electron cyclotron resonance ion source (ECRIS) using various kinds of techniques, e.g., utilization of an aluminum cylinder and a biased disk, and optimization of the plasma electrode position. Furthermore, we have recently constructed two superconducting ECRISs (operational frequencies of 14 and 18 Hz) which have unique characteristics, i.e., they do not need liquid He to obtain the superconductivity of solenoid coils and utilize the G–M refrigerator instead. These sources are suitable to generate intense beams of heavy ions with very high charge states. We obtained 10 μA of Xe30+, 5 μA of Xe33+, and 1.5 μA of Xe36+ at a radio frequency power of 700 W (14 GHz microwave) without using the gas mixing method. Through various experiments, we confirmed that not only the magnetic field strength and microwave power but also the characteristics of the plasma chamber surface play the ess...

The effect of the surface of Al2O3 on the beam intensity of highly charged Ar ions is described. To investigate this effect clearly, we directly plated the surface with 20 μm Al2O3 of the Al cylinder (Al2O3 plating method). This cylinder was inserted into the plasma chamber and it covered its inner wall of. We then measured the beam intensity of highly charged Ar ions carefully. By comparing the beam intensity using the Al2O3 plating method and the Al cylinder, it is confirmed that the Al2O3 surface is very effective in increasing the beam intensity of highly charged heavy ions. The best result of 290 eμA of Ar11+ was obtained by using this method.

To investigate the effect of the plasma electrode position on the beam intensity, we measured the beam intensity of Ar ions, with various charge states as a function of the electrode position. In this experiment, we observed that the beam intensity of highly charged Ar ions is strongly dependent on the electrode position and that there is the suitable position to maximize the beam intensity. The optimum position varies with the charge state of ions. The beam intensity of 1.3 mA for Ar8+ was obtained at the optimum plasma electrode position.

Microwave power absorption and intense collimated beam production is studied in the pulsed high-power microwave ion source at RIKEN by varying the radius of the plasma chamber (circular waveguide) in integer multiples of a quarter wavelength of the wave (nλ/4), where n is an integer (n=2–5) and λ is the vacuum wavelength of microwaves. Optimum wave coupling and power absorption due to a change in the microwave power density and the electric field distribution are investigated from measurements of the total achievable current density and the optical intensity, emitted by the pulsed argon plasma in the visible region at a wavelength of 404.3 nm. Results indicate a nonlinear absorption of microwave power. Depending upon the pressure, a decrease in the chamber radius leads to an enhancement of the electron temperature. Favorable beam conditions, i.e., highest density and temperature are obtained for the chamber with radius ≃λ.

One of the most known ways to obtain higher beam intensities in electron cyclotron resonance (ECR) ion sources is to install an electrode (usually disk) into the plasma chamber. We found that a majority of the groups observed the beam intensity improvement by supplying a suitable biased voltage to the electrode and an electron current was injected into the plasma. A few groups observed the enhancement, however, when the electrode operated at floating potential—without being an electron donor. In spite of the great success of the “biased disk” method, the mechanism is still not completely clear. In this contribution, as a step toward of understanding, we examined the above mentioned two modes. The experiments were performed at the 18 GHz RIKEN and at the 14.5 GHz ATOMKI ECR ion sources.

We successfully produced intense beams of highly charged metallic ions from various kinds of organic metallic compounds by the metal ions from volatile compounds method and solid materials in RIKEN 18 GHz electron cyclotron resonance ion source (ECRIS) (e.g., 135 eμA of Fe11+, 80 eμA of Fe13+, 10 eμA of Os29+, and 1 eμA of Os37+). Using this ECRIS the ion confinement time of highly charged ions was determined under the pulsed mode operation.

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.

For the RIKEN radio isotope factory (RIBF) project, we produced the multicharged uranium beam with two methods. To produce lower charge state U ion beams (14+-20+) we used the UF(6) gas as an ionized gas. The typical beam intensity of U(14+-20+) was 2-1 particle microA at the extraction voltage of 14 kV. To produce higher charge state U ion beam (U(35+)), we chose the sputtering method. The beam intensity was 70 particle nA at the extraction voltage of 5.4 kV. Using this method, we successfully produced multicharged U beam continuously for one month without break for RIBF commissioning.

We have developed a new analytical system that consists of an electron cyclotron resonance ion source (RIKEN 18 GHz ECRIS) and a RIKEN heavy ion linear accelerator (RILAC). This system is called trace element analysis using electron cyclotron resonance ion source and RILAC (ECRIS-RILAC-TEA). ECRIS-RILAC-TEA has several advantages as described in the work of Kidera et al. [AIP Conf. Proc. 749, 85 (2005)]. However, many experimental results during the last several years revealed a few problems: (1) large background contamination in the ECRIS, particularly at the surface of the plasma chamber wall, (2) high counting of the ionization chamber and the data taking system that is monitored by the direct beam from the accelerator, and (3) difficulty in the selection of the pilot sample and pilot beam production from the ECRIS for the purpose of normalization. In order to overcome these problems, we conducted several test experiments over the past year. In this article, we report the experimental results in detail and future plans for improving this system.

Beam intensities of highly charged Ar ions (Ar11+,12+) were measured as a function of plasma electrode position. We observed that the beam intensity of Ar11+,12+ increased when putting the electrode far from the electron-cyclotron-resonance zone. On the other hand, lower charged heavy ions (Ar8+,7+) dramatically decreased. We observed that the intense beam extraction strongly affects the plasma condition. It may be due to the ion pumping effect.

We measured the beam oscillation of RIKEN 18 GHz electron cyclotron resonance ion source for various conditions. In this experiment, the frequency of the beam oscillation increases with increasing the gas pressure and decreases with increasing rf power. The oscillation frequency is strongly dependent on the negative voltage of biased disk, i.e., the frequency increases with increasing the negative voltage. We observed that the frequency is strongly dependent on the mass of ionized gas.

We measured the main plasma parameters (density of electron, temperature of electrons, and ion confinement time) as a function of Bmin and Binj with laser ablation technique. We observed that the Bmin mainly affects the temperature and density of electrons and all of the three parameters increase with increasing the Binj. We also observed that the gas pressure of the plasma chamber at the rf injection side became minimum at the optimum value for Bmin at fixed gas flow. This result indicates that the ionization efficiency becomes maximum at optimum value for Bmin. From these results, it is concluded that the plasma production is strongly dependent on the Bmin (plasma generator). We also observed that the Binj affects the ion confinement time, temperature, and density of electrons. All of the three parameters increase with increasing Binj.

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.

We have started a new project, in which we have utilized an electron cyclotron resonance ion source (ECRIS) and a heavy ion linac for trace element analysis. This combination is expected to identify many kinds of trace elements simultaneously with very low background [very low detection limits (sub-pg/g) were attained by this method] without the pretreatment of a sample. This system is also applicable to fast neutron radiography, because it is possible to produce intense fast neutron beams. When we produce 100 μA of a Li beam from the ECRIS and use the nuclear reaction of H(Li,n)B at ∼2 MeV/u for production of neutrons, we can easily produce an intense focused fast neutron beam (∼2 MeV, 1010 particles/s). It has great advantages for neutron radiography. (1) We can minimize the background of the neutrons and gamma rays produced by the neutron-induced reaction, because the neutrons are emitted just at a forward angle. (2) Intense fast neutron beams can be easily produced. This kind of beam is very useful fo...