Annual Report of Naka Fusion Research Establishment
from April 1, 2002 to March 31, 2003

Naka Fusion Research Establishment

Japan Atomic Energy Research Institute
Naka-machi, Naka-gun, Ibaraki-ken

(Received October 10, 2003)

This annual report provides an overview of research and development (R&D) activities at Naka Fusion Research Establishment, including those performed in collaboration with other research establishments of JAERI, research institutes, and universities, during the period from 1 April, 2002 to 31 March, 2003. The activities in the Naka Fusion Research Establishment are highlighted by high performance plasma researches in JT-60 and JFT-2M, research and development of fusion reactor technologies towards ITER and fusion power demonstration plants, and activities in support of ITER design and construction.

JT-60 program has continued to produce fruitful knowledge and understanding necessary to achieve reactor relevant performances of tokamak fusion devices. JFT-2M has made contributions in more basic areas of tokamak plasma research and development in pursuit of high performance plasma.

The objectives of JT-60 research have been more shifted to physics R&Ds in support of the International Thermonuclear Experimental Reactor (ITER) and establishment of physics basis for a steady state tokamak fusion reactor like SSTR as a fusion power demonstration plant. Major achievements of JT-60 research program in this fiscal year can be summarized as follows;

1. A real-time control of neoclassical tearing mode by electron cyclotron wave was successfully demonstrated and the improvement in normalized beta was achieved as a result.
   
2. Start up of plasma current by lower hybrid current drive without using a central solenoid the vertical field coils and bootstrap current resulting in a very high performance plasma.
   
3. Transport property of various type of H-mode plasma has been investigated. Property of Internal Transport Barrier (ITB) has also been investigated mainly from the view point of a radial electric field.
   
4. Mechanism of the current hole formation has been investigated and significant understanding has been gained.
   
5. A new type of Alfvén eigenmode was theoretically proposed to explain the observed behavior of the mode in JT-60.
   
6. New understanding on divertor and scrape-off-Layer plasmas has been gained from probe measurements and analyses.
   
7. Detritiation of the vacuum vessel has successfully been simulated by three kinds of discharge cleaning methods using H₂, He and Ar.
   
8. Steady improvements towards long pulse operation have been made in both Neutral Beam injection and Radio Frequency heating systems.

In JFT-2M, the advanced material tokamak experiment program has been carried out to test the low activation ferritic steel for development of the structural material for a fusion reactor. Major achievements in this fiscal year are summarized as follows;

1. The inside wall of JFT-2M except the port openings was fully covered by ferritic steel plates to investigate the compatibility of the ferritic steel as a first wall with the high performance plasmas.
   
2. As a new attractive operation regime, high recycling steady H-mode with ITB was explored.
   
3. A basic study on the edge transport barrier in the H-mode was pursued by utilizing the heavy ion beam probe.
   
4. Operation of the Compact Toroid (CT) was drastically improved by the modification of the injector and a fast density increase due to the CT injection has been measured.

In the area of theories and analyses, significant progress has been made in understanding of the ITB, energy confinement scaling in ITB plasmas, MHD equilibrium in the current hole region, asymmetric feature of divertor plasmas and the divertor detachment. In addition, through the project of numerical experiment on tokamak, the mechanism of the ion temperature gradient mode was clarified by particle simulations. The physics of divertor plasma was also studied by particle simulations.

R&Ds of fusion reactor technologies have been carried out both to further improve technologies necessary for ITER construction, and to accumulate technological database to assure the design of fusion power demonstration plants, which include the development of Blanket Test Modules to be tested by ITER, reduced activation structural materials, and their neutron irradiation facility, now called the International Fusion Materials Irradiation Facility (IFMIF). Major achievements in the area of fusion reactor technologies in this fiscal year are as follows;

1. Superconducting Magnet: The world不 first large coil using the Nb₃Al conductor was successfully operated at 46 kA, 13 T. The Nb3Al conductor was demonstrated to be a promising technology for fusion power demonstration plants that require toroidal field higher than 13 T. A 60-kA High Tc Superconductor current lead was successfully developed.
   
2. Neutral Beam Injection; The voltage holding in the 1-MeV Vacuum Insulated Beam Source (VIBS) was improved by a new stress ring to decrease electric field at the triple junction (interface of metal flange, FRP insulator, and vacuum). The VIBS sustained 1 MV stably for 8500 s and a 100mA-class negative ion beam was accelerated at 900 keV.
   
3. Radio Frequency Heating; In the development of the advanced launcher (remote steering launcher), the transmission efficiency of > 95% at a steering angle of -12 to +12 was obtained by improving the waveguide corrugation.
   
4. Blanket; Development of the breeding Test Blanket Module for ITER with water cooled solid breeder and reduced activation ferritic steel is underway. Fabrication method of Li₂TiO₃ pebbles was improved and the thermo-mechanical properties of the pebble bed were studied. Efforts to develop an advanced neutron multiplier brought us a bright prospect that Be₁₂Ti containing αBe phase can be applied to pebble fabrication.
   
5. Plasma Facing Components; Thermal fatigue experiments of a divertor mock-up with Cu screw cooling tube were conducted to investigate lifetime of the divertor structure as a reduced cost option of the ITER divertor.
   
6. Structural Materials; Neutron irradiation hardening of a low activation material F82H was examined up to 20dpa. This effect was saturated with an increase of radiation dose. Development of post irradiation fracture toughness test equipment has been conducted successfully. The three-year Key Element Technology Phase of the IFMIF activity under the IEA collaboration has been completed in 2002 successfully.
   
7. Tritium Technology; As tritium removal technique from plasma facing components, effectiveness of excimer laser irradiation was verified.
   
8. Fusion Neutronics; The D-T neutron skyshine experiment was carried out. An analysis using the Monte Carlo Neutron Particle transport code agreed well with measured neutron and gamma-ray dose rate distributions within ± 20% uncertainty.
   
9. Vacuum Technology; A new scroll type roughing vacuum pump with transfer coating technique was developed in order to improve the lubrication characteristics.

In the ITER Program, Canada made the first site proposal to host ITER in June 2001 and three additional site offers including Japanese Rokkasho proposal were submitted in June 2002. Fourteen years after the inception of ITER, construction of ITER has come close to a reality. JAERI as the main implementation institute of the ITER program in Japan, has made major technical contributions in preparing the Japanese site proposal and licensing procedures. JAERI has also coordinated scientific and technical activities in support of ITER collaborating with universities and other research institutions in Japan.

Finally, in the area of fusion reactor design studies, major achievements can be summarized as follows;

1. A concept of advanced tokamak without central solenoid coils was proposed.
   
2. A prospect of the current hole plasma as a reactor core was computationally assessed. Good confinement of alpha particles in the core and enhanced fusion output are expected.
   
3. A new mechanism governing the vertical displacement event at the thermal quench was conceptually revealed by axisymmetric MHD simulations.
   
4. An innovative liquid wall divertor utilizing the latent heat of solid grains floating on the surface of the liquid flow was proposed.