Y. Endo

Development of 28GHz and 77GHz 1MW Gyrotron for ECRH of Magnetically Confined Plasma

Abstract We are developing a new 28GHz 1MW and a 77GHz 1MW gyrotron for ECRH system of tandem mirror GAMMA10 and Large Helical Device (LHD), respectively. The detail design study of 28GHz 1MW gyrotron such as cavity, magnetron injection gun (MIG) has been done. We obtained the oscillation power of 1.37MW and the oscillation efficiency of 42.7% with the pitch factor of 1.2. Two 77GHz 1MW gyrotrons have been fabricated and tested. The maximum output power of 1.1MW was obtained. The pulse width with 0.46MW extended to 5s with the short aging time of only 65 hours. A plasma injection for LHD with MOU output of 0.81MW 3.6s was performed.

Development of gyrotrons for fusion with power exceeding 1 MW over a wide frequency range

Megawatt-class gyrotrons covering a wide frequency range (14 GHz?300 GHz) are in increasing demand for nuclear fusion. Recent electron cyclotron heating and electron cyclotron current drive experiments highlight a requirement of megawatt-scale gyrotrons at a relatively lower frequency (14?35 GHz) range of some plasma devices, like GAMMA 10/PDX of the University of Tsukuba, QUEST of Kyushu University, NSTX-U of Princeton Plasma Physics Laboratory, and Heliotron J of Kyoto University. Collaborative studies for designing a new 28 GHz/35 GHz dual-frequency gyrotron and a 14 GHz gyrotron have commenced. Operation above 1 MW of 28 GHz/35 GHz dual oscillation was demonstrated experimentally. Further in the design of dual-frequency gyrotron, operations with 2 MW 3 s and 0.4 MW CW (continuous wave) at 28 GHz, and power exceeding 1 MW for 3 s at 34.8 GHz have been shown to be feasible. The 14 GHz gyrotron is expected to operate above 1 MW. We are also developing higher frequency gyrotrons (77?300 GHz). The joint program of National Institute for Fusion Science and the University of Tsukuba developed two new 154 GHz gyrotrons for the large helical device after the demonstration of three 77 GHz gyrotrons. The 154 GHz gyrotrons achieved a maximum output power of 1.25 MW and quasi-CW operation of 0.35 MW for 30?min.

Soft x-ray intensity profile measurements of electron cyclotron heated plasmas using semiconductor detector arrays in GAMMA 10 tandem mirrora)

Temporally and spatially resolved soft x-ray analyses of electron cyclotron heated plasmas are carried out by using semiconductor detector arrays in the GAMMA 10 tandem mirror. The detector array has 16-channel for the measurements of plasma x-ray profiles so as to make x-ray tomographic reconstructions. The characteristics of the detector array make it possible to obtain spatially resolved plasma electron temperatures down to a few tens eV and investigate various magnetohydrodynamic activities. High power electron cyclotron heating experiment for the central-cell region in GAMMA 10 has been started in order to reduce the electron drag by increasing the electron temperature.

Experimental Results and Design of Mirror Antenna and MW Gyrotron for Control of High Intermittent Heat Flux in GAMMA 10 Tandem Mirror

Abstract Development of an electron cyclotron heating (ECH) mirror antenna and MW gyrotrons for power modulation experiments in GAMMA 10 has been started in order to generate and control the high heat flux and to make the edge-localized mode (ELM)-like intermittent heat load pattern for divertor simulation studies. A peak heat flux of more than 10 MW/m2 on the GAMMA 10 axis has been obtained during ECH injection within the available power of ECH. This value almost corresponds to the steady-state heat load of the divertor plate of ITER. However, we need a substantial upgrade of the heating power to approach the ITER level ELM energy density. We carry out a design study for a mirror antenna and higher power gyrotrons in order to generate higher intermittent heat flux in GAMMA 10 tandem mirror for future divertor simulation studies.

Development of Mega-Watt Gyrotrons for Fusion Research

Abstract At the Plasma Research Center (PRC) in University of Tsukuba, development of Mega-Watt Gyrotrons is performed for fusion research. We are developing a new 28 GHz 1 MW and a 77 GHz 1.5 MW gyrotron for ECRH system of tandem mirror GAMMA10 and Large Helical Device (LHD), respectively. In the short pulse test of 77 GHz gyrotron, the maximum output power of 1.6 MW and the maximum total efficiency of 49.4% with CPD were obtained. In the long pulse test, the pulse length extended to 5 sec. with 1 MW and 4500 sec. with 0.2 MW. The design study of 154 GHz 1MW gyrotron for LHD has been started. For each cavity oscillation mode, TE28,8, TE28,12 and TE31,8, cavity, electron gun (MIG), mode converter, collector and SCM design are being examined.

Optimization of 28 GHz Gyrotron Output Performance for ECRH Experiment of the GAMMA 10

A new 28GHz 500kW gyrotron was developed for the electron cyclotron resonance heating system of tandem mirror GAMMA10. The maximum output power of 570 kW was obtained by experimental optimization. The efficiency of the gyrotron is 40˜50% in the power range of 300˜500 kW output with the optimization in spite of the restriction of the present magnet. The non-desirable influences caused by the leakage field of the magnet system of GAMM10 were minimized by the adjustment of gyrotron coi l parameters and the minor change of the gyrotron and magnetic shield design.

Measurement of effect of electron cyclotron heating in a tandem mirror plasma using a semiconductor detector array and an electrostatic energy analyzer

Temporally and spatially resolved soft x-ray and end-loss-electron analyses of the electron cyclotron heated plasmas are carried out by using a semiconductor detector array and an electrostatic energy analyzer in the GAMMA 10 tandem mirror. The flux and the energy spectrum of the end loss electrons are measured by a multi-grid energy analyzer. Recently, the electron cyclotron heating power modulation experiments have been started in order to generate and control the high heat flux and to make the edge localized mode-like intermittent heat load pattern for the divertor simulation studies by the use of these detectors for electron properties.

Development of 28 GHz Gyrotron for Cooperative ECH Study

Abstract At the Plasma Research Center at University of Tsukuba, development of megawatt gyrotrons is being performed as a collaborative electron cyclotron heating (ECH) study with some research organizations. A 28 GHz 1 MW 1 s gyrotron has been developed to upgrade the GAMMA 10/PDX ECH systems. To improve the oscillation efficiency in high current regions, the magnetron injection gun (MIG) of the 28 GHz gyrotron has been modified. Output power of 1.25 MW has been achieved with this gyrotron. For the first step of the collaborative research between Tsukuba University and Kyushu University, the Tsukuba 28 GHz gyrotron was adapted to the Q-shu University Experiment with Steady-State Spherical Tokamak (QUEST) ECH system, and the plasma heating and current drive effect were demonstrated. We obtained successful results, including an electron cyclotron–driven plasma current of 66 kA in the QUEST plasma experiment. For the next step of the collaborative research, the design targets of a 2 MW 3 s and 0.4 MW continuous wave have been achieved in a design study of a new 28 GHz gyrotron.

Development of 28/35 GHz dual-frequency gyrotron for ECH study

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.

Development of new mirror antenna for generation of high intermittent heat flux in GAMMA 10 tandem mirror

Electron cyclotron heating (ECH) power modulation experiments in GAMMA 10 tandem mirror have been carried out in order to generate and control the high heat flux and to make the ELM (edge localized mode) like intermittent heat load pattern for divertor simulation studies. The maximum energy density obtained by the use of present mirror antenna systems is about 0.08 MJ/m2 and is still far lower than that of ITER ELM. To achieve the generation of higher heat flux, the development of a new mirror antenna has carried out in order to concentrate the heating power on the axis. The calculated e-folding radius of power density has been improved from 63 mm for the present mirror to 40 mm for the new mirror antenna.