Tadamitsu Kaneko

Thermal-bubble initiated breakdown characteristics of liquid helium and nitrogen at atmospheric pressure

Experiments show that electric field promotes to form a vapor locking in a gap with a hot spot and in turn the formation of locked vapor produces the serious reduction of breakdown voltage. Especially it is argued that the dielectric strength of liquid helium under serious hot spot conditions decreases below that for saturated gas.

Electrical breakdown mechanism of liquid nitrogen in the presence of thermally induced bubbles

Abstract An investigation of the electrical breakdown behaviour of liquid nitrogen in the presence of a local hot spot on an electrode has been performed to determine the breakdown mechanism of liquid coolant in cryoresistive or superconducting apparatus under quenching conditions. It has been found that the vapour bubbles are generated at a hot spot and the manner of the vapour generation and the vapour structure depend strongly on the ambient pressure, the heat transfer rate at a hot spot and the applied voltage. Electrical breakdown took place through various series of composites of liquid and gas phases, depending on the pressure and the heating power applied to a hot spot. Therefore, the breakdown characteristic of liquid nitrogen under hot spot conditions depended to a marked degree on these factors.

Breakdown characteristics of cryogenic gaseous nitrogen and estimation of its electrical insulation properties

The electrical properties of liquid nitrogen and its vaporized gas have become of great interest since the discovery of high-temperature superconducting materials. The authors present the breakdown characteristics of gaseous nitrogen from room temperature down to 74 K. With uniform field gap geometry, the validity of Paschens law is confirmed even under the saturated gas condition at 74 K. In the case of a nonuniform field gap, there is a remarkable a difference in the effects of positive and negative polarities on corona stabilization. A wider gap length is necessary for stable negative corona formation than for positive corona at temperatures >

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.

D.c. electrical breakdown of saturated liquid helium at 0.1 MPa in the presence of thermally induced bubbles

Abstract Breakdown phenomena initiated by thermally generated bubbles in liquid helium are investigated experimentally. The process is considered to be of importance for the design of electrical insulation of superconducting devices. The results of the investigations show that the boiling processes are significantly affected by the application of an electrostatic field and that the bubble shape varies complicatedly with the magnitude of electric field, heater power for generation of bubble and the arrangement of electrode system. Thus the breakdown voltage of liquid helium in a parallel plane gap depends on those experimental conditions. It is argued that the breakdown strength of saturated liquid helium at atmospheric pressure in the presence of thermal bubbles becomes lower than that of saturated helium vapour at the same pressure.

Effects of Voltage on Dielectric Breakdown Characteristics in Gaps Partially Immersed in Liquid Nitrogen

It is well known that abnormally high voltages with various voltage waveforms occur during a quench and cause the electrical insulation in superconducting magnet coils to deteriorate. No data have been reported for the effects of voltage waveforms on dielectric breakdown characteristics in non-uniform field gaps partially immersed in liquid coolant. In the present study, effects of voltage on dielectric breakdown characteristics in non-uniform field gaps, partially immersed in liquid nitrogen coolant (PI condition), are examined with ac and impulse voltages. Furthermore, the dielectric breakdown characteristics of non-uniform gaps completely immersed in liquid nitrogen or wholly in cryogenic nitrogen gas are also investigated and compared with those under the PI condition for ac and impulse voltages. The obtained results are compared with data already reported for dc voltages. It was found that the Sumoto effect appears for ac voltage, but does not appear for impulse voltage. Breakdown characteristics of nonuniform field gaps under the PI condition are strongly influenced by the appearance of the Sumoto effect which depends on the voltage waveforms.