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Until 2009, three high-power, over 1MW each, 77GHz gyrotrons have been installed and applied to the LHD experiment. In addition, a 154GHz gyrotron of 1MW was installed in 2012. The 77GHz gyrotrons suffer gradual increases of internal pressure during long-pulse operation delivering power to LHD. To mitigate the problem, quasi-steady operation by combination of on-off operations of the 77GHz gyrotrons was performed. Applying two 77GHz gyrotrons alternately with intervals of two minutes and an 84GHz gyrotron continuously, a 30 min. long-pulse discharge with the line average electron density ne_ave of 0.7×1019m−3 and the central electron temperature Te0 of 1.7keV was achieved by the time average injection power Pinj of 260kW in 2012, showing significant progress in sustained density from the former 65 min. discharge with ne_ave of 0.15×1019m−3 and Te0 of 1.7keV by Pinj of 110kW of 84GHz wave. In 2013, one of the 77GHz gyrotron was improved to furnish a sub-window to remove stray radiation inside the tube. And the new 154GHz gyrotron was applied to the long-pulse discharge experiment. Using three gyrotrons: 154, the improved and an existing 77GHz with Pinj of 340kW in total, higher temperature plasma having steep temperature gradient typical for internal transport barrier, with ne_ave of 1.1×1019m−3 and Te0 of 3.5keV was quite stably sustained for 325s.

In the large helical device (LHD), fundamental electron cyclotron resonance heating (ECRH) by the electron Bernstein wave (EBW) excited via the ordinary-extraordinary–EBW (O-X-B) mode conversion process was performed with high power (~1MW) launching. Profiles of increase of the electron temperature (Te) and the soft X-ray signals during the power injection suggest power absorption in the core region. Effects of the local modification of the rotational transform l/2π(=1/q) by electron cyclotron current drive (ECCD) on the formation and sustainment of the electron internal transport barrier (e-ITB) was investigated for the first time. Co ECCD raised l/2π close to 0.5 in the core region and caused the flattening of the Te profile. Additional ECRH power is required to form the e-ITB. On the contrary, counter (cntr.) ECCD separates l/2π from 0.5 in the core region and avoids the flattening of the Te profile. The e-ITB can be formed and sustained without additional ECRH. Analysis of the heat pulse transport with use of the modulation ECRH (MECH) shows the good confinement region extends to the l/2π =0.5 rational surface in the case of cntr. ECCD.

The central bulk electron temperature of more than 20 keV is achieved in LHD as a result of increasing the injection power and the lowering the electron density near 2 × 1018 m−3. Such collision-less regime is important from the aspect of the neoclassical transport and also the potential structure formation. The presences of appreciable amount of high energy electrons are indicated from hard X-ray PHA, and the discrepancy between the stored energy and kinetic energy estimated from Thomson scattering. ECE spectrum are also sensitive to the presence of high energy electrons and discussed by solving the radiation transfer equation. The ECRH power absorption to the bulk and the high energy electrons are dramatically affected by the acceleration and the confinement of high energy electrons. The heating mechanisms and the acceleration process of high energy electrons are discussed by comparing the experimental results and the ray tracing calculation under assumed various density and mean energy of high energy electrons.