Extension of high T _e regime with upgraded electron cyclotron resonance heating system in the Large Helical Device

Enhancement of the output power per gyrotron has been planned in the Large Helical Device (LHD). Three 77-GHz gyrotrons with an output power of more than 1 MW have been operated. In addition, a high power gyrotron with the frequency of 154 GHz (1 MW/5 s, 0.5 MW/CW) was newly installed in 2012, and the total injection power of Electron cyclotron resonance heating (ECRH) reached 4.6 MW. The operational regime of ECRH plasma on the LHD has been extended due to the upgraded ECRH system such as the central electron temperature of 13.5 keV with the line-averaged electron density n_e-fir = 1 × 10¹⁹ m^-3. The electron thermal confinement clearly improved inside the electron internal transport barrier, and the electron thermal diffusivity reached neoclassical level. The global energy confinement time increased with increase of n_e-fir. The plasma stored energy of 530 kJ with n _e-fir = 3.2 × 10¹⁹ m^-3, which is 1.7 times larger than the previous record in the ECRH plasma in the LHD, has been successfully achieved.