Shinichi Moriyama

Neoclassical tearing mode control using electron cyclotron current drive and magnetic island evolution in JT-60U

The results of stabilizing neoclassical tearing modes (NTMs) with electron cyclotron current drive (ECCD) in JT-60U are described with emphasis on the effectiveness of the stabilization. The range of the minimum EC wave power needed for complete stabilization of an m/n = 2/1 NTM was experimentally identified for two regimes using unmodulated ECCD to clarify the NTM behaviours with different plasma parameters: 0.2 < jEC/jBS < 0.4 for Wsat/dEC ~ 3 and Wsat/Wmarg ~ 2, and 0.35 < jEC/jBS < 0.46 for Wsat/dEC ~ 1.5 and Wsat/Wmarg ~ 2. Here, m and n are the poloidal and toroidal mode numbers; jEC and jBS the EC-driven current density and bootstrap current density at the mode rational surface; Wsat, Wmarg and dEC the full island width at saturation, marginal island width and full-width at half maximum of the ECCD deposition profile, respectively. Stabilization of a 2/1 NTM using modulated ECCD synchronized with a mode rotation of about 5 kHz was performed, in which it was found that the stabilization effect degrades when the phase of the modulation deviates from that of the ECCD at the island O-point. The decay time of the magnetic perturbation amplitude due to the ECCD increases by 50% with a phase shift of ±50° from the O-point ECCD, thus revealing the importance of the phasing of modulated ECCD. For near X-point ECCD, the NTM amplitude increases, revealing a destabilization effect. It was also found that modulated ECCD at the island O-point has a stronger stabilization effect than unmodulated ECCD by a factor of more than 2.

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.

Analysis of Optimal Feedback Control of Vertical Plasma Position in a Tokamak System

An optimal control method was used to control the vertical position of a non-circular plasma with negative n-indices in a tokamak system. Taking the time delays in the control devices into consideration was found to improve the optimal feedback control law. Transient-response analysis of the control system in the TRIAM-1M tokamak showed that the optimal control technique can be used to control the plasma position in the stable region and that the ultimate lower limit of the n-index for the stabilization can be close to the limit nc for the fast-growing mode of positional instability. In addition, a pulsewidth-modulated inverter power supply with gate turn-off thyristors is proposed as a feedback power supply. This power supply has enabled the optimal position control of highly-elongated plasmas to be carried out an actual tokamak system.

Observation of spontaneously excited waves in the ion cyclotron frequency range on JT-60U

In this paper, experimental observations of spontaneously excited waves in the ion cyclotron range of frequency (ICRF) on JT-60U are described. The fluctuations in ICRF are driven by the presence of non-thermal ion distribution in magnetically confined plasmas. Two types of magnetic fluctuations are detected: one is due to high-energy D ions from neutral beam (NB) injections and the other is due to fusion products (FPs) of 3He and T ions. These fluctuations have been reported as ion cyclotron emissions (ICEs) in the burning plasma experiments on large tokamaks. This paper describes the first measurement of the spatial structures of the excited modes in the poloidal and toroidal directions. It is confirmed by using ICRF antennas as pickup loops that all modes excited spontaneously have magnetic components. The modes due to D ions have zero or a small toroidal wave number kz. On the other hand, the measurement of finite kz in the modes due to FP ions supports the excitation of the Alfven waves, which is the possible origin of FP-ICEs. It is also observed that the excited modes due to FP ions (3He and T ions) have different characteristics: driven by different NBs and having different parameter dependences. ICE due to T ions has no harmonics and the value of ω/Ωci is smaller than that due to 3He. Both beam-driven ICEs and FP-ICEs are clearly observed and their spatial structures are obtained on JT-60U.

Development of advanced operation scenarios in weak magnetic-shear regime on JT-60U

Fully non-inductive discharge having a relaxed current profile and a high bootstrap current fraction fBS = 0.5 has been realized in the high-βp ELMy H-mode discharge with weak magnetic-shear having q95 = 5.8, qmin = 2.1 and q(0) = 2.4, where qmin and q(0) are the safety factor q at the minimum and the plasma centre, respectively. The rest of the plasma current is externally driven by neutral beams and lower-hybrid waves. The safety factor profile evaluated by the motional Stark effect diagnostic is kept unchanged for 0.7 s at the end of the full current drive (CD) sustainment for 2 s (1.5 times the current relaxation time τR). The loop voltage profile is spatially uniform at 0 V at the end of the sustainment. This demonstration shows, for the first time, that the steady sustainment of full-CD plasma is possible at high fBS = 0.5 and reasonably low q95 = 5.8 regime and is stably controlled by appropriate external current drivers. On the other hand, when the combination of bootstrap current and externally driven current does not match the steady current profile, a slight change in the current profile due to current relaxation resulted in the appearance of magnetohydrodynamic instability, e.g. neo-classical tearing mode, in a high-beta plasma. This discharge clarifies the importance of the demonstration of steady current profile developed here. These discharges contribute to the ITER steady-state operation scenario development.

Status of high power gyrotron development in JAEA

Recent results of a multi-frequency gyrotron and a high frequency power modulation experiment using 170 GHz gyrotron are reported. Three frequency oscillations and their Gaussian beam outputs are demonstrated at 170 GHz, 137 GHz and 104 GHz. At present, 0.9 MW/ 70 sec, 1.1 MW/ 5 sec were obtained at 170 GHz. An experiment of 203 GHz oscillation will be conducted from April 2013 in addition to further power increase and pulse extension. In parallel, 5 kHz full power modulation was demonstrated by an anode voltage modulation using a double anode switching.

High-voltage test of feedthroughs for a high-power ICRF antenna

A feedthrough is one of the most important parts of an ion cyclotron range of frequencies antenna for plasma heating in nuclear fusion devices. It must transmit RF power while keeping the antenna in high vacuum, and support antenna elements against electromagnetic forces and thermal stress. The standoff voltage, a key property for high-power transmission, is experimentally studied for six different types of feedthroughs. The Princeton Plasma Physics Laboratory (PPPL), Massachusetts Institute of Technology (MIT), Japan Atomic Energy Research Institute and Oak Ridge National Laboratory types show a standoff voltage of 40-52 kV with the pulse length 1 s, which indicates the possibility of power injection 1-1.6 MW in the present 50-/spl Omega/ systems. If a 30-/spl Omega/ feedthrough designed for the International Thermonuclear Experimental Reactor antenna, has a similar standoff voltage, 3-4 MW can be injected. Here, the antenna loading resistance is assumed as 3 /spl Omega/. In particular, the PPPL and MIT types show higher standoff voltages, which is inferred to be due to the design of the base flange covering the ceramic end. Multipactoring is observed in the voltage range less than 0.9-1.6 kV as predicted, but it is not a crucial problem. Therefore, a key point in designing a high-voltage standoff feedthrough is the optimization of the base flange and the ceramic ends.

A Mock-Up Test of a Ceramic-Free Antenna Feeder of the Ion Cyclotron Heating System for Next-Generation Tokamaks

We propose a ceramic-free antenna feeder employing a ridged waveguide as a local support in an ion cyclotron range of frequency (ICRF) antenna for next-generation tokamaks. A one-quarter scale mock-up model of the all metal waveguide designed for the ICRF system in the international thermonuclear experimental reactor (ITER) is fabricated, and electrical characteristics of the model, including the coaxial line-to-waveguide converters are measured. Power reflection coefficient of the model including the coax-waveguide converter at the input coaxial line is estimated to be less than 15% when below the cut-off frequency of 107 MHz and less than 3% when above the cut-off frequency. It is found that this ceramic-free antenna support employing a ridged waveguide is suitable for the ICRF antenna for next-generation tokamaks.

Design and Operation of a 2-MW CW RF Load for Gyrotrons

The development is complete on a load to dissipate 2-MW continuous wave of RF power at frequencies above 100 GHz and transmitted in the HE11 waveguide. A rotating launcher distributes the RF power inside the load and prevents excessive localized heating from constructive interference. The load design also minimizes reflected power. Two loads are currently in operation in Japan, and the performance results are presented. A new version is in development for the ITER facility in France.

Real-time Impedance Matching by Frequency Feedback Control in Ion Cyclotron Range of Frequency Heating Experiments in JT-60U and Future Tokamaks.

A frequency feedback control (FFC) system was designed and fabricated for, and utilized in, an ion cyclotron range of frequency (ICRF) heating system in JT-60U. The FFC was confirmed to be quite effective in maintaining the impedance matching between the antenna and the transmission line of the ICRF heating system, to couple high RF power continuously to a plasma with variable parameters in nuclear fusion experiments.