Recent Results in Large Helical Device

摘要

This paper presents recent experimental results in the Large Helical Device (LHD), which is the world largest helical system (R_ax~3.6 m, a_av~0.6 m, B_ax~2.75 T) and has capability of steady state operation because of the superconducting magnets. LHD confines the plasma with T_io~2 keV and n_e ~0.8times1019 m-3 for 1905 sec using the ICRF heating. In this case, the total plasma heating energy was ~1.3 GJ. This indicates that high energy ions generated by ICRF is well confined. Large magnetic island in the vacuum field due to the error field is vanished in plasma with higher temperature or with higher beta, so the error field is not an issue in helical systems. The obtained averaged beta is more than 4.3 % in LHD. Since the beta is not limited by the MHD events or magnetic island but the beta is limited by the lack of heating power, further improvement of beta can be expected. Internal and external transport barrier have been also observed, and the electron temperature and the ion temperature exceed 10 keV in the improved confinement mode in LHD. Besides the potential of the disruption free steady stated operation, LHD experiment accumulates evidences so that fusion reactor becomes more realistic