Technical issues for the realization of a fusion reactor and the prospects for their resolution through spherical tokamak research

Fusion energy is expected as an infinite clean energy resource, but many technical issues remains before the realization. Some of them are as follows: enormous amount of fusion neutrons causing severe damage on the vessel wall; the plasma disruption; the confinement of plasma in the steady state; expensive construction cost as a realistic problem. Recently, a concept of spherical tokamak (ST), which is a tokamak with an extremely low aspect ratio, attracts the attention of people. Since ST has the high beta (the beta is the plasma pressure normalized by the magnetic pressure) and large apertures, we can expect the low construction cost, the high bootstrap current without disruption and the easy exchange of damaged wall. The Technical Committee of Spherical Tokamak has been investigating, whether ST can provide resolution of technical issues for the realization of a fusion reactor; what problems ST causes newly; how we can construct Japanese ST research system in order to contribute to the world ST community. This technical report presents the above investigations in eight sections as follows: '1. Introduction', '2. Reactor technology in ST', '3. Plasma wall interactions in ST', '4. Control of ST', '5. Plasma physics in ST', '6. Research activities in Japanese ST community', '7. Present status of ST research in the world', '8. Conclusion'. (author)