Ken Kajiwara

Steady-state operation of 170 GHz–1 MW gyrotron for ITER

A 170?GHz gyrotron has been developed at JAEA, which has achieved operation of 1?MW/800?s and up to 55% efficiency. This is the first demonstration of a gyrotron achieving and even exceeding the ITER operating requirements of 1?MW/500?s and 50% efficiency. In addition the gyrotron demonstrated operation for 1?h with an output power of 0.6?MW. The oscillation was stable with all cooling water temperatures and vacuum pressure reaching equilibrium conditions during the pulse length at either power level. The successful operation was aided by a very low level of stray radiation (~2% of the output power), which contributed to fast conditioning and stable operation. The output power from the gyrotron was coupled into an ITER sized corrugated waveguide ( = 63.5?mm) via a matching optics unit with a total of 0.97?MW transmitted to the dummy load after two miter bends and ~7?m of a waveguide without any trouble. These results give an encouraging outlook for the success of the ITER electron cyclotron heating and current drive system.

Progress of high power 170?GHz gyrotron in JAEA

Recent progress on the high power gyrotron development in JAEA is presented. The gyrotron is featured to have a triode-type magnetron injection gun, a cylindrical resonator working at 170 GHz with TE31,8 mode, a water-cooled diamond window and a depressed collector. After the demonstration of the ITER basic performance, the gyrotron has been operated for 3 years, and recorded ~200 GJ of total output energy. Next, a gyrotron which oscillates in higher order resonator mode, TE31,12, is designed and fabricated to study the long pulse oscillation at greater than 1 MW. In parallel, feasibility studies of a CW-power modulation for neoclassical tearing mode stabilization, a dual frequency gyrotron and a rapid frequency control are carried out. It is shown that these gyrotrons will be available with current technology.

THE EC H&CD TRANSMISSION LINE FOR ITER

Abstract The transmission line (TL) subsystem associated with the ITER electron cyclotron heating and current drive system has reached the conceptual design maturity. At this stage the responsibility of finalizing the design has been transferred from the ITER Organization to the U.S. Domestic Agency. The purpose of the TL is to transmit the microwaves generated by the 170-GHz gyrotrons installed in the radio-frequency building to the launchers located in one equatorial and four upper tokamak ports. Each TL consists of evacuated HE11 waveguides, direct-current breaks, power monitors, mitre bends, polarizers, switches, loads, and pumping sections and will have a typical length that ranges from 100 to 160 m. Overall transmission efficiency could be as high as 92% depending on the specific path between a given gyrotron and launcher. All components are required to be 2-MW compatible, and their layout and organization have been optimized for simplifying the maintenance accessibility and monitoring the primary tritium barrier integrity. Two different TL layouts are at the moment under study, to accommodate the two alternative options for the European sources: four 2-MW units or eight 1-MW units. In this paper the actual design is presented and the technical requirements are discussed.

Development of Dual-Frequency Gyrotron with Triode Magnetron Injection Gun

A high power dual-frequency gyrotron is designed and tested. The design is based on a 170 GHz single-frequency gyrotron with a triode magnetron injection gun (MIG). The triode MIG enables to choose variety of oscillation modes for different frequencies with suitable pitch factor, which is the great advantage for a multi-frequency gyrotron. Another frequency of 137 GHz is selected in order to use a 1.853-mm-thick single-disk output window. Cavity modes are TE31,11 and TE25,9 for 170 and 137 GHz, respectively, which have high mode conversion efficiency to the RF beam mode with similar radiation angles. In short-pulse experiments, the maximum power of more than 1.3 MW is achieved with high-efficiency for both frequencies.

Improved design of an ITER equatorial EC launcher

New design improvements for an ITER equatorial EC launcher are described. A design of the front shield modules was evaluated analytically. The results indicate that these modules can withstand the electromagnetic forces induced by plasma disruption and vertical plasma motion. The steering mirror mock-up including the spiral cooling tubes was fabricated based on the present design with application of the hot isostatic pressing technique to bond the copper alloy mirror body and the embedded stainless steel cooling tubes. The test of water flow was carried out and the expected flow rate was successfully obtained. A quasi-optical (QO) transmission layout has been proposed instead of the waveguide lines. The advantage of this option is possibly to increase the reliability in terms of fabrication and refurbishment. According to the beam propagation analysis, it is verified that transmission loss at the QO region is 1.1%–2.2%, which is comparable to the reference design. The peak heat load on the steering mirror surface could be reduced by 55%, which relaxes the mirror design.

High power millimeter wave experiment of ITER relevant electron cyclotron heating and current drive system

High power, long pulse millimeter (mm) wave experiments of the RF test stand (RFTS) of Japan Atomic Energy Agency (JAEA) were performed. The system consists of a 1 MW/170 GHz gyrotron, a long and short distance transmission line (TL), and an equatorial launcher (EL) mock-up. The RFTS has an ITER-relevant configuration, i.e., consisted by a 1 MW-170 GHz gyrotron, a mm wave TL, and an EL mock-up. The TL is composed of a matching optics unit, evacuated circular corrugated waveguides, 6-miter bends, an in-line waveguide switch, and an isolation valve. The EL-mock-up is fabricated according to the current design of the ITER launcher. The Gaussian-like beam radiation with the steering capability of 20°-40° from the EL mock-up was also successfully proved. The high power, long pulse power transmission test was conducted with the metallic load replaced by the EL mock-up, and the transmission of 1 MW/800 s and 0.5 MW/1000 s was successfully demonstrated with no arcing and no damages. The transmission efficiency of the TL was 96%. The results prove the feasibility of the ITER electron cyclotron heating and current drive system.

Development and achievements on the high power ECRF system in JT-60U

An output power of 1.5 MW for 1 s was achieved at 110 GHz in a recent gyrotron development using the JT-60U ECRF system. It is the worlds highest power oscillation for a pulse duration of at least 1 s. The achievement was enabled by, in addition to the carefully designed cavity and collector, necessary because of thermal stress, an RF shield for the adjustment bellows and a low-dielectric-loss dc break. The way the power was modulated was improved upon by controlling the anode voltage, with high modulation frequency of 5 kHz being achieved in NTM stabilization experiments. Moreover, as a developmental step to realizing a reliable ECRF system for use in future fusion experiments, a long pulse demonstration of 0.4 MW and a 30 s injection into the plasma was achieved with real time control of the anode/cathode-heater. Confirmation was made that the temperature of the cooled components had been saturated with no evidence of any damage being discovered in the waveguides and antenna without forced cooling. An innovative antenna with a relatively wide range of beam steering capabilities utilizing a linearly moving-mirror concept was also designed for use as an active cooling antenna with longer pulses in the future, e.g. for JT-60SA. The beam profile and mechanical strength analyses proved the feasibility of the antenna.

In-Tube Shock Wave Driven by Atmospheric Millimeter-Wave Plasma

A shock wave in a tube supported by atmospheric millimeter-wave plasma is discussed. After atmospheric breakdown, the shock wave supported by the millimeter wave propagates at a constant velocity in the tube. In this study, a driving model of the millimeter-wave shock wave is proposed. The model consists of a normal shock wave supported by a propagating heat-supply area in which an ionization front is located. The flow properties predicted by the model show good agreement with the measured properties of the shock wave generated in the tube using a 170 GHz millimeter wave beam. The shock propagation velocity Ushock is identical to the propagation velocity of the ionization front Uioniz when Uioniz is supersonic. Then the pressure increment at the tube end is independent of the power density.

Investigation of Transmission Characteristic in Corrugated Waveguide Transmission Lines for Fusion Application

A 170 GHz, high power and long pulse RF experiments of the ITER relevant transmission line consisting of 63.5 mm circular corrugated waveguides, bends, an in-line switch and etc... was, for the first time, carried out. The transmission efficiency of 92% from the inlet of the transmission line to the end, which is comparable to the specification value was successfully obtained. The anomalous incremental temperature between the bends or after/before the bend were observed. This result suggests that the bends generated the higher order modes and the modes are possibly trapped between them and reflect back to the direction propagating toward them.

Gyrotron beam coupling method into corrugated waveguide

A coupling method of a gyrotron beam into a corrugated waveguide is proposed to achieve high HE11 mode purity in ITERs ECH&CD transmission line. A matching optical unit (MOU) with two phase correcting mirrors is used. Each mirror is mounted on a goniometer stage for angle control. The mode content in the waveguide is measured for several sets of mirror angles to maximize the mode purity. The mode content is deduced using RF field profiles radiated from the waveguide outlet. A demonstrative procedure of the beam coupling achieved 93% of HE11 mode in the transmission power.