N. Fukumoto

Effects of ambient humidity and temperature on partial discharge characteristics of conventional and nanocomposite enameled magnet wires

Effects of ambient humidity and temperature on partial discharge (PD) characteristics in twisted pairs with conventional and nanocomposite enameled magnet wires have been investigated. Measurements of the PD inception voltage (PDIV) and the apparent PD charge were performed by applying an AC sinusoidal waveform voltage of 60 Hz to the twisted pair. The observed PDIV in this study can be classified into the following three behaviors. First, the PDIV in both wires decreases with increasing relative humidity (RH) at low temperature. Secondly, the PDIV in both wires slightly increases when increasing the RH at high temperature. Thus, there is a certain value of the absolute humidity to determine the lowest PDIV in both cases with the conventional and nanocomposite enameled wires. Thirdly, the PDIV in the conventional enameled wire significantly decreases at the RH of 95%, while the decrease rate of that in the nanocomposite enameled wire is relatively small. The observed PD phenomena are discussed in terms of material characterization of the enameled wires such as changes in permittivity due to moisture absorption and wettabilitiy under voltage application.

The internal magnetic field structures and current density profiles in the Helicity Injected Spherical Torus plasma driven by coaxial helicity injection

In the Helicity Injected Spherical Torus device [Nagata et al., Proceedings of the 17th International Atomic Energy Agency Fusion Energy Conference, Yokohama, 1998 (International Atomic Energy Agency, Vienna, 1998) CD-ROM, EXP4/10], internal magnetic field and current density structures of spherical torus (ST) plasmas sustained by coaxial helicity injection (CHI) have been revealed via intensive internal magnetic measurements. The internal magnetic configuration of the ST plasma formed by CHI is in good agreement with the results of numerical equilibrium fitting calculations. The generation of closed poloidal flux of ST has been verified by varying the external toroidal field strength in the same device. Interestingly, the transition of the current profile from hollow to peaked has been observed during the sustainment phase, which could be explained by inductive effects of currents on open field lines winding the central conductor.

Experimental studies of the dynamics of compact toroid injected into the JFT-2M tokamak

We present the first results from recent compact toroid (CT) injection experiments in the JFT-2M tokamak using the improved CT injector and diagnostics with fast time resolution. We have observed that the core line density increases rapidly at a maximum rate of ~1.3 × 1022 m−3 s−1 after a delay of 100–200 µs. This increment rate of the core density is about several times larger than that obtained so far. Interferometry measurement along the peripheral line chord of R = 1.1 m in the inboard side indicates that CT plasma reaches a region near the plasma core beyond the separatrix. Time-frequency and space distribution analyses of edge magnetic probe signals show that the magnetic fluctuation induced by the CT has the spectral peak at 250–350 kHz and propagates in the toroidal direction at the Alfven speed of the order of 106 m s−1. These results indicate the excitation of Alfven wave by CT injection. We have observed that the fluctuation level of the ion saturation current in the divertor and the Dα spectral line intensity decrease significantly after CT injection. Corresponding increase in the soft x-ray signals in the core region may suggest that the CT causes a transition to H-mode-like discharges.

An experimental investigation of the propagation of a compact toroid along curved drift tubes

Compact toroid (CT) injection is a viable technology for fuelling large tokamak reactors in the future. Experimental demonstration of CT injection has thus far been conducted using horizontal injection in the midplane of tokamak devices. However, recent analyses indicate adverse effects of the toroidal magnetic field on CT injection. In order to avoid these adverse effects, the CT would need to be injectable in any direction. We have therefore devised a curved drift tube to change the direction of CT propagation and have experimentally demonstrated its efficacy. It has been observed that a CT can be transported smoothly through curved drift tubes with 45° and 90° bends without any appreciable change in the CT parameters. The magnetic field, electron density and speed of CTs transported through both 45° and 90° bends are similar to those observed in a linear drift tube.

Investigation of partial discharge with twisted enameled wires in atmospheric humid air by optical emission spectroscopy

We investigated the characteristics of the partial discharge (PD) with twisted enameled wires by optical emission spectroscopy (OES) in atmospheric humid air. The electric field strength in the gap of the twisted enameled wires during the discharges was estimated from intensity ratio of the spectrum bands of nitrogen molecules obtained by the OES. It was found that the electric field strength was increased when increasing the relative humidity, and the increasing rate was different depending on the temperature. The increase of the electric field strength in the gap of the twisted pair could be related to a reduction of PD inception voltage (PDIV) observed in the experiment. The reduction of the PDIV could be determined by not only dielectric properties of enameled wires such as permittivity and surface conductivity but also changes of the discharge phenomena in the humid air. In this study, effects of the increase of the permittivity of the enameled wire due to the moisture absorption into the insulating layer were quantitatively discussed as one of the candidates to explain the reduction of the PDIV in the humid conditions. On the other hand, the discharge current amplitude was decreased with the increase of relative humidity at high temperature. It was considered that the discharge could be quenched by dissociative electron attachment to the large number of water molecules. The observed PD phenomena with respect to the change of the humidity are very important for quality management of low voltage inverter-fed random wound motors.

Behaviour of compact toroid injected into an external magnetic field

The interactions of a compact toroid (CT) plasma with an external magnetic field and a tokamak plasma have been studied experimentally on the FACT and JFT-2M devices. Fast framing camera and soft X ray emission profile measurements indicate shift and/or reflection motions of the CT plasma. New electrostatic probe measurements indicate that the CT plasma reaches at least up to the separatrix for discharges with toroidal field strengths of 1.0-1.4 T and that there exists a trailing plasma behind the CT. A large amplitude fluctuation on the ion saturation current and magnetic coil signals is observed. Power spectrum analysis suggests that this fluctuation is related to magnetic reconnection between the CT plasmoid and the toroidal field. The CT, including much of the trailing plasma, may be able to move across the external magnetic field more easily in the drift region of the injector owing to the Hall effect.

Design of spheromak injector using conical accelerator for Large Helical Device

Abstract Optimization of CT injector for LHD has been carried out and conical electrode for adiabatic CT compression is adopted in the design. Point-model of CT acceleration in a co-axial electrode is solved to optimize the electrode geometry and the power supplies. The condition to attain the large acceleration efficiency is researched and it is shown to be a function of the ratio of the electrode inductance to the external inductance. Acceleration efficiency of 34% is to be obtained with 3.2 m long conical accelerator and 40 kV–42 kJ power supply. The machine overview of a CT injector named SPICA Mk.I (SPheromak Injector using Conical Accelerator) consisting of 0.8 m conical accelerator is also given.

Characteristics of Magnetized Coaxial Plasma Gun for Simulation Experiment of Thermal Transient Events in ITER

A compact-sized magnetized coaxial plasma gun (MCPG) with a single discharge stage is developed in order to simulate transient heat loads such as type I edge-localized modes (ELMs) in ITER. The MCPG produces hydrogen plasmas with an electron density of 1- 4 × 1021 m-3, an ion energy of 15 eV and a pulsewidth of 0.5 ms. It is found that the absorbed energy density measured by a calorimeter reaches 0.9 MJ/m2 which is comparable with a transient heat load of type I ELMs expected in ITER. Erosion of tungsten due to the plasma irradiation produced by the MCPG is confirmed only at the edge parts of the specimen in this experiment.

Characteristics of partial discharge and time to breakdown of nanocomposite enameled wire

The insulation properties of nanocomposite enameled wire, in which nano-sized inorganic material such as layered silicate is used as an additive component, have been investigated. An extraction of SiO2 at the surface of the nanocomposite enameled wire was observed after the voltage life test by using Fourier transform infrared spectroscopy. This result suggests that the inorganic material remains after evaporation of the organic material due to partial discharge (PD) at the surface. It could be considered that the inorganic material protects the enameled wire from physical sputtering of PD charges. A technical improvement of the twisted pair configuration was proposed to prevent the concentration of PD activity at the edge of the twisted pair when the applying voltage is around the PD inception voltage. It was found that PD charge in the case of nanocomposite enameled wire is reduced when the applying voltage is the same in comparison with the conventional one. It could be considered that the SiO2 layer on the insulating material and the decreased PD charge could play a major role for the superior insulating properties of the nanocomposite enameled wire.

Effects of Environmental Humidity and Temperature on Sterilization Efficiency of Dielectric Barrier Discharge Plasmas in Atmospheric Pressure Air

The inactivation of Bacillus atrophaeus spores by a dielectric barrier discharge (DBD) plasma in atmospheric humid air was investigated in order to develop a low-temperature, low-cost, and high-speed plasma sterilization technique. The biological indicators covered with a Tyvek sheet were set just outside the DBD plasma region, where air temperature and humidity as a discharge gas were precisely controlled by an environmental test chamber. The results show that the inactivation of B. atrophaeus spores was found to be dependent strongly on humidity, and was completed within 15min at a relative humidity of 90% and a temperature of 30 C. The treatment time for sterilization is shorter than those of conventional sterilization methods using ethylene oxide gas and dry heat treatment. The inactivation rates depend on not only relative humidity but also temperature, so that water content in air could determine the generation of reactive species such as hydroxyl radicals that are effective for the inactivation of B. atrophaeus spores. # 2011 The Japan Society of Applied Physics