Fusion Studies in Japan

A new strategic energy plan decided by the Japanese Cabinet in 2014 strongly supports the steady promotion of nuclear fusion development activities, including the ITER project and the Broader Approach activities from the long-term viewpoint. Atomic Energy Commission (AEC) in Japan formulated the Third Phase Basic Program so as to promote an experimental fusion reactor project. In 2005 AEC has reviewed this Program, and discussed on selection and concentration among many projects of fusion reactor development. In addition to the promotion of ITER project, advanced tokamak research by JT-60SA, helical plasma experiment by LHD, FIREX project in laser fusion research and fusion engineering by IFMIF were highly prioritized. Although the basic concept is quite different between tokamak, helical and laser fusion researches, there exist a lot of common features such as plasma physics on 3-D magnetic geometry, high power heat load on plasma facing component and so on. Therefore, a synergetic scenario on fusion reactor development among various plasma confinement concepts would be important.


Introduction
Japanese government is promoting fusion research and development based on so-called Phase Basic Program formulated by Atomic Energy Commission (AEC). Currently the third phase basic program started in 1992 has been on progress. This program sets forth key elements of research and development for a DEMO reactor, and is aiming at achieving necessary conditions for the self-ignition and at realizing the long pulse burning plasmas in a tokamak experimental reactor.
Based on this principal policy, the ITER has been assigned as a core device in the third phase basic program. While, just before the start of ITER project, this third phase basic program has been reviewed, and report entitled by Future Fusion Research and Development Strategy in the Third Phase Basic Program of Fusion Research and Development was issued in 2005 [1]. In this report action items of recent research and development including ITER project have been described.
In 2011 we had a Great East Japan Earthquake, and quite serious nuclear accidents have taken place in Fukushima nuclear power plants. Therefore, in Japan the government has reconsidered energy strategy, and the Cabinet has decided a new strategic energy plan in 2014. In this new strategic energy plan, the Government declares that although nuclear power is expected to be a baseload power supply, Japan will minimize its dependency on nuclear power. While, concerning to nuclear fusion, this report declares that the Japanese government steadily promotes nuclear fusion development activities, including the ITER project, which is being implemented through international cooperation, and the Broader Approach Activities from the long-term viewpoint [2].

Japanese Policy on Fusion Energy Development
The report compiled by AEC in 2005 describes the role of fusion energy as an important option in solving energy and environmental problems in addition to energy security viewpoints, and following key points are listed up.
(1) Reduction of CO 2 If highly economic fusion energy is soon introduced in addition to energy-saving and use of renewable energy, possible significant reduction of carbon dioxide concentration in the end of this century might be feasible. Early realization of fusion energy is important from the viewpoint of solving the global environmental problem. Tokimatsu et al. have analyzed the feasibility on the introduction of fusion energy in this 21 st century, and concluded that fusion energy might have a potentiality to supply electric power demand of 10 ~ 20 % in the world at 2100, if the first fusion reactor could be introduced around 2050 with a reasonable cost [3].
(2) Necessity of introduction to developing countries The increase of energy consumption is coming from developing counties in the near future. Active introduction to developing countries is also expected, since fusion energy has less resource localization and less restriction in introduction to society. Japan should take a leadership in early realization of fusion energy toward resolution of energy and environmental problems, because Japan is a responsible country to actively contribute to the world prosperity and promotion of international cooperation as a nation based on the creativity of science and technology.

(3) Energy security
It is also important to ensure the energy security of Japan by increasing its self-sufficiency presently in low level in energy resources.
This report expects that if a switchover to the DEMO phase is done in the early 2020s followed by prompt construction of DEMO, it will be foreseeable to start test program and improvement using DEMO from 2030s with the objective of demonstrating continuous power generation, safety, economical viability and operation reliability. If so, it is foreseeable to expect commercialization by the mid-21 st century.
A schematic drawing of strategy for fusion reactor development was compiled in the report, as shown in Fig. 1. Now third phase basic program is strongly promoted, and ITER is a core device. In the previous 2nd phase a large tokamak JT-60 device was constructed, and in the 4th phase program construction of DEMO reactor is expected. In addition, along with the strong promotion of ITER project, necessity of complemental projects is described in this report, as well.
ITER will advance burning physics, while deep understanding and further improvement of core plasma might be necessary. For that purpose promotion of tokamak plasma experiments is proposed, by modifying JT-60 device to JT-60SA. Of course, the promotion of fusion engineering is indispensable toward DEMO. For example, material development and blanket/divertor technologies in addition to safety research. Now advanced tokamak research and fusion engineering development are promoted in the framework of Broader Approach (BA) activities.
A first candidate of DEMO is a tokamak at present, because plasma performance is better than other approaches. While, there are other attractive approaches for fusion reactors, as well. In Japan helical and laser fusion have been prioritized as other attractive approaches, compared with other basic researches. Therefore, LHD project in helical and FIREX project in laser have been strongly supported by the Government.

Research Activities on magnetic fusion
The Government has been strongly supporting ITER project from the start in 1988, and Japan is playing an important role in ITER project with other Parties. The main contribution is an in-kind As described in the previous section, for the DEMO reactor development, promotion of further development of key technologies for core plasmas and fusion engineering field is indispensable in order to establish a technological basis for DEMO as well as to play supporting and complementary roles for the ITER Project. For this purpose Broader Approach activities have been initiated as a joint project between Japan and EU, and BA agreement was effectuated in 2007. Main sites are located in Japan, i.e., Rokkasho in Aomori prefecture and Naka in Ibaraki prefecture. The total cost is evenly shared by JA and EU. This is a 10-years project, and remains in force thereafter unless terminated by either Party [5].
Rokkasho is a new site established for this BA activities. Two projects are promoted in Rokkasho site; one is a so-called IFMIF/EVEDA project and another is IFERC project. For the development of fusion reactor materials the strong 14 MeV neutron source such as the International Fusion Materials Irradiation Facility; i.e., IFMIMF, might be indispensable. In the framework of BA activities, therefore, R&D for IFMIF has been promoted, as Engineering Validation and Engineering Design Activities; i.e., EVEDA. In IFMIF/EVEDA project, the prototype accelerator with 9 MeV and 125mA beam is under development. At present, as the first step of the validation test of the prototype accelerator, the injector and its peripheral systems have been introduced from EU, and installed in Rokkasho site. The beam commissioning was started in November 2014. Another project in Rokkasho site is International Fusion Energy Research Center, so-called IFERC, project. The purposes of this project are to coordinate DEMO design and R&D, and to manage computational simulation center and ITER Remote Experimentation Center.
In Naka site a satellite tokamak program is ongoing; i.e., the JT-60 tokamak is upgraded to JT-60SA, which is a superconductor device largest in the world except ITER. The plasma experiment will start in 2019. Nine vacuum vessel sectors of 10 have been installed sequentially on the cryostat base in 2015 and they have been welded with each other, remaining an opening of 20 degree sector. Since the toroidal field coil is linked with the vacuum vessel in the tokamak device, the next step is an installation of the toroidal field coil through this opening sector region. The toroidal field coil is just under fabrication in Europe, and the first TF coil will be delivered to Japan in 2016.
Along with tokamak research activities, helical plasma research is strongly promoted in Japan, because the helical plasma has an attractive feature; i.e., steady-state operation is feasible. One of the largest helical device LHD was constructed in NIFS. Since 1998 LHD has conducted 18th experimental campaign, and remarkable progress has been demonstrated. That is, about 10 keV temperature plasma has been achieved, and long pulse operation during about 50 minutes has been demonstrated [6].
Since helical plasma should be considered 3-D geometrical effects into plasma physics, quite comprehensive 3-D theory and simulation codes have been developed in helical plasma society. Recently, even in tokamak plasmas 3-D effect seems to play an important role. For example, in ITER in order to control ELM burst, the in-vessel coil will be installed inside the vacuum vessel. Since this in-vessel coil produces the magnetic island and/or stochastic field, 3-D simulation code is indispensable. Therefore, collaboration between tokamak and helical is strongly enhanced in the magnetic fusion community. This is a good example of synergetic effect between tokamak and helical plasmas on the scenario of fusion reactor development.

Research Activities on fast ignition laser fusion and reactor designs
In 1972 Central ignition scenario was proposed by Nuckolls, and major facilities such as NIF and LMJ have been constructed to explore this central ignition scenario. While in 1983 a new idea so-called fast ignition was proposed by Prof. Tatsuhiko Yamanaka and other colleagues. Sometimes, the central ignition is likened to diesel engine, and the fast ignition is to gasoline engine. The idea of the fast ignition seems to be quite elegant, and direct-drive fast-ignition could provide potentially high energy gain with small driver, but the development of PW-level high power laser is indispensable. Osaka University is strongly promoting a fast ignition campaign [7]. Design and construction of LFEX laser has started in 2003, and in 2008 target irradiation with high-power beam started. The LFEX laser system consist of 4 beams, and the first one beam was constructed in 2009, and the preliminary experiments for irradiation of fast ignition has started. In 2010 the second beam was fabricated, and the fast ignition experiment with two beams was conducted in 2010. However, due to the great earthquake in 2011, the facility of the company for fabricating LFEX laser components has been damaged. For that reason, the construction of LFEX laser system was delayed by 2 years. In 2014 four beams have been completed, and plasma mirror has been introduced, as well. In 2015 the fast ignition campaign with 4 full beam system has been conducted, especially paying attention to high pulse contrast. The maximum beam energy is, at present, 2 kJ with a pulse length of 1 -1.5 psec [8]. Preliminary experiments on fast ignition have been conducted, and it has been found that efficient heating is achievable by controlling generation and transport of laser-produced electron beam. For this purpose, the guide of the electron beam by introducing strong magnetic field has been proposed in addition to the elegant tailoring of the laser pulse form.
Concerning to laser fusion reactor, KOYO-F was designed in Osaka University, based on fast ignition concept [9]. The cooled ytterbium:YAG ceramic laser of 16 Hz is employed, where 1.1 MJ of the laser energy is for compression and 100kJ for heating. An energy gain of 160 and fusion yield of 200 MJ are expected. One of the critical issues in laser fusion reactor is the chamber wall, where a quite large thermal load should be protected. In this design a flowing liquid LiPb is employed. This is a quite elegant design, but technical barrier seems to be quite high.
To overcome this difficulty, a dry wall laser fusion reactor FALCON-D has been designed in CRIEPI and the University of Tokyo [10]. The design concept of FALCON-D reactor is based on fast ignition scenario, dry wall chamber and high repetition laser. In fast ignition scenario it might be available to achieve a relatively high gain with low laser energy. For example, about 0.3-0.4 MJ might be sufficient for fast ignition. This is an advantage of fast ignition scenario. Therefore, in FALCON-D reactor design, the fusion yield in one pulse is decreased and chamber radius is increased, so as to survive the first wall heat load. While the repetition frequency is increased up to 20 -40 Hz.
Nevertheless, the gap between present experimental results and future DEMO reactor seems to be quite large. Therefore, to mitigate technical gap between single shot present experiments and future commercial plant, the experimental plant LIFT was proposed in 2015 [11]. In LIFT laser-diode pumped Yb:YAG ceramic laser operated at 225K is adopted. The laser energy is 500kJ for compression and 150kJ for heating, and the repetition frequency is 4 Hz with the efficiency of 12%. Since the fusion gain of around 100 could be expected, the fusion yield of 40 -80 MJ per shot can be achieved.
The interesting feature of this LIFT project is that the project will be promoted step by step according to the progress of the experimental research, by utilizing the same laser facility. The mission of the Phase I is the repeated fusion burn, and blanket is not installed. In addition, repetition around 100 shots is planned in one experiment series. The Phase II is a power generation, and the solid blanket will be installed in this Phase. From the viewpoint of power generation, the slightly long time operation is indispensable. Therefore, in this Phase II, operation during one week is expected. Finally, in the Phase III continuous operation during one month or more is expected. In addition, the solid blanket will be replaced with liquid one, so as to explore the future commercial reactor. Since the self-supply of tritium is required, the tritium breeding function is indispensable. In these three phases the common laser system will be utilized, and replacement of chamber room is sufficient for advancing to the next phase . Since replacement of the chamber core region seems to be relatively easy compared with magnetic fusion systems such as tokamak and helical, the stepwise development might be feasible for laser fusion reactor research.
Next let me briefly review the necessity of the technical development of laser facility for laser fusion reactor. In general, a high repetition high power laser around 30 Hz is indispensable in a laser fusion reactor, while LFEX laser facility in Osaka university can be operated in 3 shots per day. That means the repetition frequency of 9x10 -5 Hz. This is quite low level, because LFEX laser facility consists of Nd-doped glass laser with flash lamp pumped, and air cooling operation at room temperature. To achieve the repetition frequency of 30 Hz from 9x10 -5 Hz, several advanced technologies should be introduced such as laser diode, ceramic window, Yb-dopant, 200 K operation and liquid cooling [12]. By taking these advanced technologies into account, we can expect the remarkable increase of the laser repetition frequency from the present low level to 30 Hz level.

Recent activities toward DEMO
In order to reinforce strategy to DEMO reactor development, a special team called Joint-Core Team was organized in the fusion community under the Government, and intensively discussed on critical issues for DEMO reactor. Concerning to the strategy to DEMO development, the Joint-Core team has summarized a report on the basic concept of DEMO and structure of technological issues in 2014 [13]. This Joint-Core team has sorted out tasks regarding the development of the design of DEMO, and research and development programs to resolve the key issues as follows; (i) Superconducting coils, (ii) Blanket, (iii) Divertor, (iv) Heating and current drive systems, (v) Theory and numerical simulation research, (vi) Reactor plasma research, (vii) Fuel systems, (viii) Material development and establishment of codes and standard, (ix) Safety of DEMO and safety research, (x) Availability and maintainability, (xi) Diagnostics and control systems and (xii) Newly required facilities and platforms.
The Joint-Core team has compiled this report based on tokamak DEMO reactor. However, some of these critical issues are common or similar between magnetic and inertial fusion reactors. For example, blanket might be common. Therefore, the technology of the blanket developed by tokamak DEMO might be applicable to inertial fusion reactor. In addition, fuel systems; i.e., tritium circulation systems might be common, except for the fabrication of pellet itself. Of course, material development and establishment of codes and standards might be common issue. Safety is also common issue. While, in laser fusion reactor it is not necessary to develop superconducting coils and heating and current drive systems. Instead, there are several specific R&D issues for laser fusion reactor. That is, high repetition laser, final optics and chamber wall should be developed. Of course, Ignition physics should be developed in addition to pellet fabrication and tracking technologies.
By the way, in the inertial fusion reactor the divertor is not necessary. However, the divertor heat load problem might be quite similar to the chamber wall issue in the inertial fusion reactor. F. Najmabadi has compared heat load conditions between tokamak divertor with IFE armor, and pointed out that although the pulse duration is quite different between IFE and Type-I ELM, the deposition energy and frequency are quite similar in both cases [14].
Although the basic concept is quite different between different confinement concepts, we can see a lot of common features among tokamak, helical and laser fusion reactors. Therefore, it should be emphasized that we have to consider a synergetic scenario on fusion reactor development among various plasma confinement concepts. It is quite important so as to promote fusion reactor development efficiently and smartly.