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The high power and long pulse operation of the gyrotron as well as efficient transmission of its output are quite important for achieving improved plasma performances. A 28 GHz 1 MW gyrotron developed for GAMMA 10/PDX achieved an output power of 1.38 MW in 2015 experiment after the power supply was improved. Furthermore, a new 28/35 GHz dual-frequency gyrotron (2 MW 3 s and 0.4 MW CW) for QUEST, NSTX-U, Heliotron J and GAMMA 10/PDX has been fabricated, after the preliminary test of a double-disk sapphire window installed in the gyrotron was performed. In the first experimental test, the oscillation of the main mode was confirmed at a frequency of 28.036 GHz with a Gaussian-like beam and an output power of 1.22 MW. [ABSTRACT FROM AUTHOR]

A computational code for the electromagnetic field in the cavity of high-power gyrotrons is developed. The gyrotron mechanism is described by a self-consistent set of equations taking into account the dependence of the axial structure of microwave oscillations on the relativistic electron beam in the cavity. The parameters of the electron beam are calculated using the EGUN electron trajectory code, and the effects of the space charge and the velocity spread of the electron beams are considered. The computational code includes a time-dependent description of the electromagnetic field and a self-consistent analysis of the electron beams. The equations are solved using a second-order predictor-corrector scheme. The calculations of the present study are carried out for a 1-MW gyrotron developed for the GAMMA10/PDX tandem mirror (80 kV, operating mode: TE94 mode, 35.45 GHz). The calculations reveal that, using a triode-type electron gun, stable excitation of the desired mode can be realized in standard startup scenarios. [ABSTRACT FROM AUTHOR]

To sustain plasmas with higher parameters and with longer pulse duration in LHD, ECH system has been upgraded by introducing newly developed 77 GHz gyrotrons. The designed output power and operation duration time are over 1 MW for several seconds and 0.3 MW for continuous operation, respectively. Owing to the upgrade of gyrotrons and improved power supply operation procedure, total injection power of EC-waves to LHD increased up to 3.7 MW at the last LHD experimental campaign in 2010. Application of the high-power 77 GHz EC-waves of 3.4 MW as focused beams to the center of plasma with low line-average electron density of ∼0.2×1019 m-3 causes highly steep electron temperature profile and the central electron temperature reached up to 20 keV, which highly exceeds the former record of 15 keV. At higher density region of 1×1019 m-3, central electron temperature reached 8.6 keV. Additional electron Bernstein wave heatings, O-X-B and slow X-B heatings, using a 77 GHz ECH system caused clear increase in plasma stored energy even for the high-density plasmas over plasma cutoff (>7.35×1019 m-3) sustained with NBI. For the O-X-B scenario, the 77 GHz EC-wave was obliquely injected from low-field side in O-mode polarization, aiming at the point where high mode-conversion efficiency was expected. For realizing slow X-B scenario, new inner-vessel mirrors were installed in LHD just close to a helical coil, that is, at the high-field side (HFS) region. Using the inner-vessel mirror, X-mode waves were injected from HFS, showing evident increase in plasma stored energy. Oblique injection of long-pulse 0.77 MW/8 s 77 GHz wave with various N∥ clearly demonstrated ECCD in LHD. The EC-driven current changes its direction with the sign of N∥, and the highest EC-driven current reached up to 42 kA. [ABSTRACT FROM AUTHOR]

Electron Cyclotron Resonance Heating (ECRH) has contributed to the achievement of high performance plasma production, high electron temperature plasmas and sustainment of steady-state plasmas in the Large Helical Device (LHD). Our immediate targets of upgrading the ECRH system are 5 MW several seconds and 1 MW longer than one hour power injection into LHD. The improvement will greatly extend the plasma parameter regime. For that purpose, we have been promoting the development and installation of 77 GHz/1–1.5 MW/several seconds and 0.3 MW/CW gyrotrons in collaboration with University of Tsukuba. The transmission lines are re-examined and improved for high and CW power transmission. In the recent experimental campaign, two 77 GHz gyrotrons were operated. One more gyrotron, which was designed for 1.5 MW/2 s output, was constructed and is tested. We have been promoting to improve total ECRH efficiency for efficient gyrotron-power use and efficient plasma heating, e.g. a new waveguide alignment method and mode-content analysis and the feedback control of the injection polarization. In the last experimental campaign, the 77 GHz gyrotrons were used in combination with the existing 84 GHz range and 168 GHz gyrotrons. Multi-frequency ECRH system is more flexible in plasma heating experiments and diagnostics. A lot of experiments have been performed in relation to high electron temperature plasmas by realization of the core electron-root confinement (CERC), electron cyclotron current drive (ECCD), Electron Bernstein Wave heating, and steady-state plasma sustainment. Some of the experimental results are briefly described. [ABSTRACT FROM AUTHOR]