福本 直之

Energy transfer processes from ELM-like pulsed helium (He) plasmas with a pulse duration of ∼0.1 ms to aluminum (Al) and tungsten (W) surfaces were experimentally investigated by the use of a magnetized coaxial plasma gun device. The surface absorbed energy density of the He pulsed plasma on the W surface measured with a calorimeter was ∼0.44 MJ m-2, whereas it was ∼0.15 MJ m-2 on the Al surface. A vapor layer in front of the Al surface exposed to the He pulsed plasma was clearly identified by Al neutral emission line (Al i) measured with a high time resolution spectrometer, and fast imaging with a high-speed visible camera filtered around the Al i emission line. On the other hand, no clear evaporation in front of the W surface exposed to the He pulsed plasma was observed in the present condition. Discussions on the reduction in the surface absorbed energy density on the Al surface are provided by considering the latent heat of vaporization and radiation cooling due to the Al vapor cloud.

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 15 min at a relative humidity of 90% and a temperature of 30 degrees 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. (C) 2011 The Japan Society of Applied Physics

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.

A new concept of particle fueling method is proposed, and its feasibility is studied by a Monte Carlo simulation; by using a compact torus (CT) injection technique and subsequent neutralization of an accelerated CT plasma, ultrahigh-speed neutral-particle flow can be generated. The charge-exchange reaction between ions in a CT plasma and neutral particles in the neutralizer is found to work efficiently to neutralize the CT plasma within a 1-m long neutralizer in a case that the neutral gas exceeds 5 x 10(-2) torr.

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 R.H 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 wettability under voltage application.

A key feature of the 'high recycling steady' (HRS) H-mode regimes is the reduction of the transient heat load to the divertor target due to large ELMs. The heat flux to the divertor targets measured by means of a Langmuir probe in the scrape-off layer on the target plate is about ∼0.3 MW m-2 in the typical HRS plasmas, which is very small in comparison with the transient heat load in the ELMy H-mode discharge. In addition, the effects of the plasma shaping on the access to and improved plasma performance of HRS H-mode regimes have been investigated. Furthermore, simultaneous measurements of both density fluctuations and potential profiles at the plasma edge region using a heavy ion beam probe are also presented. © 2006 IAEA, Vienna.

Two-fluid flowing equilibria of a helicity-driven spherical torus (HD-ST) are numerically investigated by using the combination of the finite difference and the boundary element methods. It is shown from the numerical results that electron fluids near the central conductor are tied to an external toroidal field and ion fluids are not. The magnetic configurations change from the high-q HD-ST (q>1) with paramagnetic toroidal field and low-β (volume average β value, <β>≈2 %) through the helicity-driven spheromak and reversed-field pinch to the ultra low-q HD-ST (0<q<1) with diamagnetic toroidal field and high-β (<β>≈18 %) as the vacuum toroidal field at the inner edge regions decreases and reverses sign. The effects of the toroidal ion flow on the magnetic configurations are discussed.

Low activation ferritic steel plates were installed in the JFT-2M tokamak to fully cover the inside wall of the vacuum vessel (ferritic inside wall FIW) as a simulation of the blanket wall of the fusion demonstration reactor. After the physics and engineering design concerns with the ripple reduction, the electro-magnetic force and so on, FIW was installed with a tolerance of a few mm. The ripple reduction was confirmed from the detailed measurement. Tokamak discharges were obtained without changing control system, as predicted theoretically. The reduction of ripple loss of fast ions was clearly demonstrated by both experiments and calculations. Compatibility of FIW with high normalized beta plasma up to 3.3 was demonstrated at relatively far wall position. Preliminary investigation showed that the slowdown of the growth rate occurred with closer wall position, which presumably corresponds to wall stabilization effect. © 2004 Elsevier B.V. All rights reserved.

The effect of a ferromagnetic wall on low β tearing modes in the Japan Atomic Energy Research Institute Fusion Torus-2 Modified was studied. To study the effect of the ferromagnetism on the tearing mode stability in tokamaks, the tearing mode equation was solved in presence of a ferromagnetic resistive wall. The results showed that a ferromagnetic wall has a weak effect on the surface tearing modes depending on the current profile.