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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.

The upcoming operation of Wendelstein 7-X (W7-X) will be supported by an Electron Cyclotron Resonance Heating (ECRH) system working at 140 GHz in second harmonic at the nominal magnetic field of 2.5T. Because the optimization of the plasma breakdown is crucial to ensure a successful plasma build-up, dedicated plasma start-up experiments were performed on three stellarator/heliotron devices: Heliotron J, LHD and WEGA. Start-up behavior and dependencies on ECRH injected power, neutral gas pressure and rotational transform were obtained in X2 heating. Plasma start-up delay time decreases with the increase in ECRH input power. However, this behavior saturates when low pre-fill neutral gas pressure conditions are met. Both the delay time and the electron density are an increasing function of the gas pressure. On Heliotron J and WEGA devices, the higher the rotational transform is, the faster the start-up and the higher the plasma density are. Analysis of the temporal evolution of the plasma start-up shows that plasma start-up on stellarators is a two-step process. In addition, off-axis heating experiments are characterized by a longer plasma start-up duration compared to on-axis heating discharges. Third harmonic in X-mode has been attempted on LHD for different neutral gas puffing settings but no plasma breakdown was achieved. This multi-machine study was useful to define ECRH start-up scenarios for W7-X.

In a high power Electron Cyclotron Resonance Heating (ECRH) system, a long-distance and low-loss transmission system is required to realize effective heating of nuclear fusion-relevant plasmas. A millimeter-wave beam position and profile monitor, which can be used in a high-power, evacuated, and cooled transmission line, is proposed, designed, manufactured, and tested. The beam monitor consists of a reflector, Peltier-device array and a heat-sink. It was tested using simulated electric heater power or gyrotron output power. The data obtained from the monitor were well agreed with the heat source position and profile. The methods of data analysis and mode-content analysis of a propagating millimeter-wave in the corrugated wave-guide are proposed.

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.