DC systems are often used in conventional line trains, but irregular magnetic field fluctuations constantly occur due to changes in overhead line current during operation hours in the vicinity of these lines, which is a precision device that uses electron beams. May be affected.

Figure 1 shows the positional relationship between the building where the target device is installed and the railway. There is a subway line closest to the building in the southeast direction, about 150 m, and there are several other railway lines in the vicinity. In a DC-type train, the current flowing through the overhead line (track) is constantly changing regardless of running in the vicinity, and this leakage magnetic field affects the surrounding magnetic field environment. The magnetic field effect tends to be attenuated in proportion to the distance (exponentiation), but at a position close to the line, the effect becomes extremely large. [Scanning Electron Microscope] A scanning electron microscope narrows an electron beam to irradiate an object as an electron beam and detects various electrons such as secondary electrons emitted from the object to detect the object. Observe. An object is mainly placed in a high vacuum, and an image is displayed by scanning the sample with an electron beam. Since the electron beam is used, the high resolution device is easily affected by the magnetic field.

Table 1 shows the results of the effect confirmation after installing a magnetic field canceller coil (W:1300 x D:1300 x H:2000 mm) at the target device installation location and performing optimal control for a DC fluctuating magnetic field. When the magnetic field canceller is OFF, the effect of the subway appears remarkably, and irregular and large magnetic field fluctuations occur mainly in the vertical (Z) direction. On the other hand, when the active magnetic field canceller is ON, the level is reduced to 1/100 or less, and it can be seen that a great effect is obtained. Electron microscopes often set a level around 1mGauss as the installation environment condition (* 0.5mGauss in this case), but when the canceller is OFF: maximum 12mGauss, ON: maximum 0.06mGauss. The environment has improved to a level that is fully satisfactory. Thus, when installing a high-precision device using an electron beam, it is necessary to pay attention to not only vibration but also magnetic field environment. Compared with the construction (large-scale construction, cost) of the shield room, the active magnetic field canceller can obtain a great improvement effect only by installing a coil around the target device.