Juichi Irisawa

Simulation of nonlinear coaxial line using ferrite beads

A ferrite sharpener is a nonlinear coaxial line using ferrite beads which produces high-voltage, high-dV/dt pulses. We examined the characteristics of ferrite sharpeners experimentally when the following parameters were changed independently: (1) magnetic field bias, (2) amplitude of input voltage pulse, (3) rise time of input voltage pulse and (4) sharpener length. Then we made a simulation of the ferrite sharpener and compared the predictions with the experimental results in detail to analyze the characteristics of the sharpener. The predictions of the simulation are found to be in good agreement with the experimental results, thus it is confirmed that the simulation is a powerful tool for designing the sharpener.

Characteristics of Multichannel Arc Gap

The characteristics of multichannel arc gap are investigated experimentally. The gap comprises 12 or 24 steel needle electrodes and a brass rod electrode. We examined the characteristics of the number of arc channels when the parameters below were changed: (a) rising rate of voltage pulse applied to the gap (b) length of feeders which transmit voltage pulse from the ferrite sharpener to each needle electrode (c) gap length (d) kind and pressure of inter-gap gas (e) with and without ultraviolet irradiation.

Rail Gap Switch Using Porous Metal as Electrode

For multichannel gap switches, the shorter the formative and statistical time lags of each gap, the larger the number of arc channels. As we examined the breakdown characteristics of a sphere/plane gap using a mirror-finished brass plate and a porous metal as plane electrodes, we confirmed that formative and statistical time lags decrease when porous metals are used as a discharge electrode, so we expect that the number of arc channels increases when porous metal is used as one of the electrodes of a rail gap. We compared experimental results to confirm this. As a result, in the case of a gap length of 1 mm, the number of arc channels increases when porous metal is used as an electrode.

Breakdown Characteristics of Porous Metals

We have investigated the breakdown characteristics of porous metals as discharge electrodes, for applications to multichannel arc gaps. We examined a single-needle/plane gap as a nonuniform electric field gap and a sphere/plane gap as a uniform one using a mirror-finished brass plate and a porous metal as plane electrodes. First, we examined the breakdown characteristics of the single-needle/plane gap. The characteristics we investigated are the DC flashover voltage of the gap, and the formative and statistical time lags of flashover when a flat-top nanosecond pulse was applied to the gap. We predicted that the formative and statistical time lags decrease when porous metals are used as a discharge electrode instead of a mirror-finished electrode. However, contrary to our prediction, a significant difference was not observed in the needle/plane experiments. Then we examined a sphere/plane gap. In this case, a significant difference was observed. Therefore, we confirm that the porous metal is a suitable electrode material for the rail gap.

Surge Impedance of Nonlinear Coaxial Line Using Ferrite Beads

A ferrite sharpener is a nonlinear coaxial line using ferrite beads, which produces high-voltage, high-dV/dt pulses. Although the surge impedance of sharpener in the region where the ferrite is generally considered to be saturated is very important in practical applications, surge impedance and relative permeability in the region have not been clarified so far. Therefore, using three evaluation methods, we attempted to clarify the surge impedance and relative permeability of the ferrite sharpener in the region where the ferrite is generally considered to be saturated when the following parameters are changed: (1) magnetic field bias and (2) terminating resistance. We confirmed that different values of surge impedance and relative permeability were obtained in the experiments using the three evaluation methods. Also, we found that the simulation could reproduce a tendency similar to the experimental results.

Bremsstrahlung Rates in Fully Ionized Gases in a Magnetic Field

The bremsstrahlung rates in fully ionized gases in a magnetic field are theoretically investigated. Here it is assumed that the charged particles have the Maxwell velocity distributions, and that an external electric field does not exists. In the theoretical treatment, the followings are considered: When the Larmor radius is larger than the radius of the collisional cross section due to the bremsstrahlung, the radiation of energy during a short time at an instant of the collision is not affected by the magnetic field. However, according to the gyration, the relative velocity between an electron and a colliding ion during the free path is affected by the magnetic field strength. Numerical calculations show that the bremsstrahlung rate in the presence of the magnetic field is smaller than that in the absence of the magnetic field.

Transformer-type accelerator for inertial confinement fusion

In pulse-power technology, a transformer-type accelerator is a good candidate for a highvoltage pulse generator suitable for repetitive operations. In this article, a reverse-conducting thyristor has been used as a switching element in a primary circuit and good operations have been obtained. A dual-resonant mode and the mode proposed by Reed, preferable to the former in the sense of voltage gain, have been investigated experimentally. A voltage gain of 25% increase compared to that of the dual-resonant mode operation has been obtained. Two different types of transformers, a spiral and a Tesla type with various turn ratios, were constructed and the characteristics of each type when applied to the same circuit have also been investigated in detail. As a result, it has been made clear that the stray capacitance of the secondary winding restricts the upper limit of the turn ratio to get the large voltage multiplication.