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Many steel pipes are made with planar steel plates and are used at oil and gas plants. Steels could easily corrode due to anode and cathode reactions causing electrochemical redox. Defects or corrosions are formed on both surfaces of a planar steel but it is difficult to perceive them formed on the other invisible side of plate from one surface with conventional non-destructive testing methods such as eddy current inspection, multi-finger caliper measurement, electromagnetic/acoustic casing thickness measurement, etc. Because of very high conductivity of steel materials, the penetration of electric current for measurement from the surface would be limited to a thin range. In this research we propose a new non-destructive testing method using magnetic field to detect defects on the invisible side, i.e., the reverse side from the measurement, of steel materials. We conducted numerical analysis using an FDTD method for showing that we could find the defects on the rear side of steel material. It is confirmed that the spatial variation of the amplitude of magnetic flux gives a change at the location of a defect on the other side when applying magnetic field to the material. We verified that the change comes from the magnetic flux leakage caused by the circumvention of magnetic flux around the defect. The phase of magnetic component normal to the surface showed an amplitude reversal at the center of a defect on the other side of the plate. We conclude that detects on the other side of a plate could be detected in the measurement on one side.

To investigate characteristics of helicon plasma with a very small diameter, we have developed the Small Helicon Device (SHD) and measured the electron density under various conditions over a wide range of radio frequencies. Using a tube with inner diameter of 20 mm, an electron density ne of ~ 10 13 cm near the antenna region was obtained with an excitation frequency f of 7 MHz, rf power Prf of ~ 1000 W, and axial magnetic field in the magnet coil region B of 560 G. In the case of an inner diameter of 5 (10) mm under conditions of f = 12 (7) MHz, Prf ~ 1000 W, and B = 280 G, ne of ~ 10 11 cm was succesfully achieved even away from the antenna region.