| 摘要 |
PURPOSE:To enable an electron beam to be more intensely focused by means of a main lens by alleviating the divergent force of the electron beam by providing the surface of a third grid facing a second grid with a concave and decreasing an electric potential intruding through the electron beam hole of the concave. CONSTITUTION:A semispherical concave 6 is formed on the surface of a third grid 13c facing a second grid 12. An electron beam hole 7 is provided in the center of the concave 6. A medium voltage of around 7-8KV for example is applied to the third grid 13c. It intrudes into the electron beam hole of the second grid 12 and exerts a force which brings an electron beam 5 close to the center axis (Z) in order to give a curvature to an equipotential line. Since the depth and the diameter of the concave 6 can freely be changed, the curvature of the equipotential line can be adjusted optimally. As a result, the electron beam 5 can be focused more intensely by means of a main lens. |
