Shigeo Nagao

Space-charge limited current in insulating oil

In order to analyse the pre-breakdown phenomena in insulating oil, a hot tungsten filament emitting an abundance of electrons in a vacuum was employed as cathode. The gap between the electrodes was therefore only partly occupied by the liquid. Currents measured with various voltages and depths of oil were explained on the basis of space-charge limitation in the oil, and the mobility of the negative ion in transformer oil formed by the attachment of electrons to oil molecules was found to be 1 × 10-4 cm/s per V/cm.

Recrystallization of Silicon Film on Nitride/Oxide Double Insulating Structure by CW Laser Irradiation

Double layer structure of thin nitride on thick oxide is used as an insulating material in order to get a good quality of silicon layer on insulator by laser recrystallization. This structure makes deposited polycrystalline silicon (polysilicon) more resistant to stripping off at high power laser irradiation and results in large grains of silicon layer with random crystal orientations. Laser scanning steps of 20 to 30 µm/step are obtained as optimum values for the largest grain size and smooth surface by investigation of grain growth and surface features. A fully oxide isolated bipolar transistor is fabricated in recrystallized silicon film on the substrate with the double insulating structure, and it shows following characteristics; typical values of hFE, BVCBO, BVCEO, and BVEBO are 29, 73 V, 30 V and 5.2 V, respectively. This is the first demonstration of the bipolar transistor in silicon layer on insulator. Quite similar results in grain sizes after laser irradiation and electrical characteristics of transistors are obtained between two kinds ofpolysilicon by conventional LPCVD furnace and an epitaxial reactor at relatively high temperatures, while original grain sizes of the latter are apparently larger than that of the former.

On the Minimization of Displacement of Plasma Column in a Toroidal Magnetic Field with a Non-Planar Axis

The system with a toroidal helical magnetic axis is re-examined numerically to obtain the minimum displacement of the plasma column in an infinitely conducting chamber wall. The results of the numerical calculation show that in order to minimize the displacement of the plasma column in the toroidal helical system, we should have a device with a large number of pitch n , for example n =32, and in such a device the β value of the equilibrium state is permitted up to 100 percent. A rule which determines the optimum condition for the minimization of the displacement is shown by the figure.

A Helical Field Configuration for Plasma Confinement with a Very Deep Magnetic Well

It has been found that in a stellerator field configuration with helical stabilizing fields of l=2, l=3 and l=4 and with a helical magnetic axis, the well depth of 50% can be achieved at the separatrix surface. Thus the m.h.d. stable confinement of high beta plasma in a toroidal system may become promising.

PIG Discharges in Various Field Configurations

PIG discharges with uniform magnetic field configuration and Divertor type field configuration are studied. Low frequency fluctuations in the plasma column are observed by a Langmuir probe and two photo-transisters. In the case of uniform field configuration, the frequencies of fluctuation agree well with the theory by Hoh and Simon. In the case of Divertor type field configuration, the amplitude of fluctuation is small compared with that for uniform field configuration. Plasma parameters (space potential, particle density and electron temperature) are measured for both magnetic field configurations. The most remarkable difference between two types of field configurations exists in radial electric field intensity. In a uniform configuration, the radial electric field E r is about 3–4 V/cm. Whereas in a Divertor type field configuration, E r is only 0.2 V/cm or less. This small E r results in a rather quiescent PIG discharge in a Divertor type field configuration.

Toroidal Effect on Magnetic Line of Force in Non-Planar Helical Magnetic Axis System (Asperator NP-3)

The behavior of magnetic line of force in a stellarator-type toroidal device with three dimensional helical magnetic axis (Asperator NP-3) is investigated both numerically and analytically, especially taking into account of the toroidal perturbation. To avoid or decrease the influence of the crucial 2/3 resonance, the absolute value of ellipticity of magnetic surface e should be over 1/3, or both e and triangularity Q should be near zero simultaneously. The swing δ employed in the helical windings is revealed to be very effective to the structure of magnetic surface.

Collisional Damping of the Whistler Mode in a Dense Plasma

The effect of Coulumb collisions on the transverse wave along an external magnetic field in a fully ionized dense plasma is investigated by means of the Fokker-Planck equation. Expressions for the dispersion relation and the spatial damping constants of the whistler mode are obtained. The spatial damping constants, to first order, are given by κ_i=\frac{2}{3\sqrt{\pi}}(\frac{k\nu_{c}}{\omega_{c}-\omega})[1+\frac{7}{5}(\frac{kv_{0}}{\omega-\omega_{c}})^2], κ_e=\frac{14}{15\sqrt{2\pi}}(\frac{k\nu_{c}}{\omega_{c}-\omega})(\frac{kv_{0}}{\omega-\omega_{c}})^2, where νc is the electron collision frequency, v0 is the electron thermal velocity, k is the wave number and ωc is the electron cyclotron frequency. Finally, their applications to the experimental results are briefly discussed.

Current Equilibrium in a Toroidal Device with a Non-Planar Magnetic Axis (Asperator NP-3)

The results of the experiment on plasma equilibrium in Asperator NP-3, a toroidal device with a three-dimensional magnetic axis, are described. It is shown that the equilibrium of a current column in this system is mainly determined by the vacuum masnetic surfaces produced by solenoids with a spatial axis. By the application of an l =1 transverse field, a method for controlling the current position by the external field is confirmed experimentally.

Effects of Toroidal Magnetic Field on an Annular Plasma

An experimental investigation on some effects of the toroidal magnetic field on an annular plasma produced by a modified hard-core θ-pinch is described. The length of the annular plasma could by varied by a pair of end coils. A high speed camera, a floating double probe and a magnetic probe were used for the investigation. The experimental results show that the toroidal magnetic field has some effects to suppress the macroscopic instabilities, such as the positional instability, and the axial contraction of the annular plasma, where the former was observed in the case with the shorter annular plasma and the latter in the case with the longer one when the toroidal field was not applied.

A New Type of Plasma Source

The construction and the performance of a new type of plasma source, designed by applying a magnetic field similar to that in the divertor of the stellarator, are described. The plasma parameters are measured by Langmuir probes. A hollow plasma of He with a density of the order of 10 10 particles/cm 3 is produced along the separatrix line of force in this apparatus. The fluctuation of the plasma density is rather small. Since this new type plasma source has no electrode on the axis, it is possible to introduce it for some stationary torus machine.