Masanori Akazaki

A method for prediction of gaseous discharge threshold voltage in the presence of a conducting particle

Abstract In this paper, calculations are presented of the field strength near a small conducting sphere floating in a parallel plane gap, and also the force acting on the sphere when the sphere is placed at an arbitrary position in a parallel plane gap. A method is proposed for the prediction of the gaseous discharge threshold voltage on the basis of the calculations. It was found that the breakdown mechanisms in a non-uniform field gap with a free conducting sphere can be classified into four kinds with regard to the particle movement and the occurring position of breakdown. Moreover, the breakdown voltage for each of the four mechanisms has a different pressure dependence. The predicted values were confirmed by using a concentric hemispherical gap and steel spheres, and are in fairly good agreement with experimental values.

Energy Dependence of Angular Distributions of Sputtered Particles by Ion-Beam Bombardment at Normal Incidence

The angular distributions of sputtered Fe-atoms were measured using the laser fluorescence technique during Ar-ion bombardment for energies of 0.6, 1, 2 and 3 keV at normal incidence. The measured cosine distribution at 0.6 keV progressively deviated to an over-cosine distribution at higher energies, and at 3 keV the angular distribution was an over-cosine distribution of about 20%. The experimental results agree qualitatively with calculations by a recent computer simulation code, ACAT. The results are explained by the competition between surface scattering and the effects of primary knock-on atoms, which tend to make the angular distributions over-cosine and under-cosine, respectively.

Applications of the Fraunhofer-diffraction method for plasma-wave measurements

The improved theoretical analysis of Fraunhofer-diffraction method as a new means to measure the frequency, wavenumber, intensity and even spatial position and propagating direction of plasma waves is presented. It is further, experimentally verified and the measuring accuracies are ascertained by measurements in a microwave region, of ion-acoustic waves in a DC discharge. Successes of introducing the practically important conditions into theoretical analysis have made it possible to establish this method as a standard and powerful diagnostic technique for measurements of plasma waves and turbulences in various plasmas such as in Tokamaks.

Effects of Mechanicl Stresses on the Dielectric Breakdown Strengths of PET and FRP

Organic materials to be used for the electrical insulation of superconducting magnet coilsin large fusion reactors must withstand electrical stresses under high mechanical loads, extreme temperatures, and intense nuclear radiation. In this paper, measurements at room temperature are reported on the effects of mechanical compressive and tensile stresses on the dielectric strengths of bi-stretched polyethylene terephthalate (PET) and epoxy glass cloth (FRP, G-10) which are widely used for the insulation of superconducting magnet coils. For PET, the breakdown strength increases with increase of compressive stress in the region of elastic deformation but decreases in the region of inelastic deformation. The value of compressive stress showing maximum dielectric strength is independent of film thickness and is approximately 100 MPa, smaller than the stress appearing in large fusion reactors. In the case of FRP, the behavior of breakdown characteristics under compressive stress is nearly the same as that of PET but the relationship between characteristics of electrical breakdown strength and mechanical properties of FRP isnotclear because it is a composite of two materials: epoxy resin and glass filler. The effect of tensile stress on the dielectric strength is small for PET, but significant for FRP.

Measurement of low-frequency ultrasonic waves by Fraunhofer diffraction

The application of the Fraunhofer diffraction method, which has been developed as a new means to identify long-wavelength plasma waves or fluctuations appearing in plasma nuclear fusion research machines, to the measurement of low-frequency ultrasonic waves propagating in air was theoretically and experimentally examined. It is experimentally demonstrated that a wave with a wave number of about 0.1 to 0.74 mm-1 can be detected by using a visible laser. Such a wave number corresponds to a diffraction angle of 0.6 to 4.3 mdeg, which is much smaller than the standard value measured by the conventional diffraction method. Furthermore, it is shown that if a diverging probe beam is adopted, the wave position can be identified by measuring the spatial phase distribution of the diffracted wave.

Effects of Temperature and Voltage on Dielectric Breakdown Strengths of PET and FRP under Mechanical Stresses

The electric insulators for superconducting magnet coils to be used in a large fusion reactor are subjected to large compressive and tensile stresses to a maximum level of about 400 MPa, and these stresses should pose a significant electrical insulating problem in practice. In this paper, dc and ac dielectric breakdown characteristics of PET and FRP under mechanical stress at liquid nitrogen (LN2) temperature are presented as part of a comprehensive study for electrical insulating design of superconducting magnet coils to be used in a large fusion reactor. The results presented are complementary to those already reported for ac at room temperature. The dielectric breakdwon strength (DBS) of PET and FRP under compressive stress at 77 K shows similar variations to those obtained with ac at room temperature: the DBS increases at first, reaches a maximum value, and falls thereafter. The maximum DBS is little influenced by the temperature variation, but the compressive stress showing the maximum DBS depends strongly on the temperature regardless of voltage sources used. Under tensile stress the DBS of PET and FRP at 77 K is higher than at room temperature at a given tensile stress.

Measurements of density, surface recombination coefficient, and diffusion coefficient of hydrogen atoms by Lα laser fluorescence spectroscopy

The technique of laser fluorescence spectroscopy at the Lyman‐alpha (Lα) wavelength in hydrogen was used to measure the temporally and spatially resolved atomic density, the surface recombination coefficient γ, and the diffusion coefficient DH,H2 of the atomic hydrogen in molecular hydrogen. At the same time, it revealed a temporal change of γ, probably due to the change of surface number density of trapped hydrogen atoms. The present technique provides a new and powerful means for determinations of values associated with hydrogen atoms by absolute and local measurements of the atoms at the detection limit of extremely low concentrations.

Effect of Heat Treatment on Dielectric Strength of Polyethylene Terephthalate under Compressive Stress

Measurements of dc dielectric strength of polyethylene terephthalate (PET) film at room temperature are reported when it is heat set in silicone oil over a range of temperature between 60 to 200°C for durations of 1 and 60 min in order to control the degree of crystallinity of the sample. It is found that the dc dielectric strength under no mechanical stress increases with an increase in the degree of crystallinity, but the maximum dielectric strength under compressive stress at first increases with the degree of crystallinity, then reaches a maximum value, and decreases thereafter. For the samples heat treated near the glass transition temperature, which is about 70°C, the characteristics of dielectric strength under compressive stress are also varied significantly with heat-treated temperature and time although the crystallinity is constant.

Investigation of the thermalization of sputtered atoms in a magnetron discharge using laser‐induced fluorescence

The thermalization of sputtered atoms in a magnetron discharge was investigated using laser‐induced fluorescence. The results clearly indicated for the first time that the velocity distribution agrees well with the Thompson formula [M. W. Thompson, Philos. Mag. 18, 377 (1968)] for d/λ≪1, while a thermalized component becomes appreciable for d/λ≳1, where d is the distance from the cathode surface and λ is the mean free path of sputtered atoms for collisions with filling gas atoms.

Analysis of microdischarge threshold conditions between a conducting sphere and plane

Abstract Geometrical coefficients for the determination of the potential difference across the grounded plane and an arbitrarily charged conducting sphere in the presence/absence of an electric field are analysed and calculated numerically. An estimation method of microdischarge thesshold conditions in a gap between the free sphere and the plane using the Paschen curve and the coefficients is proposed and is applied to two cases: a free conducting sphere placed in a parallel plane gap in SF6 gas; water drops dripping from a high voltage conductor and approaching the grounded plane. It is found from the applications that the influence of gas pressure is less pronounced in the case of microdischarge threshold field strength in SF6 as compared with the breakdown field stregth of the main gap, and the critical radius of the sphere for reducing the breakdown field strength of the main gap is nearly the same as that for the microdischarge threshold. Moreover, the experimental minimum charge on the water drop to produce microdischarge near the grounded plane agrees well with the present estimated value.