| 摘要 |
PURPOSE:To suppress an increase of crossover under a large current by providing a projection with a cathode material layer on the cathode and providing at its side a plate-like Wehnelt electrode with a small hole. CONSTITUTION:A projection 22 is formed on a cathode 21, and a cathode material layer is coated on the edge of the projection 22. A plate-like Wehnelt electrode 23 with a small hole is arranged on the side of the cathode projection 22, and a voltage which is lower than that of the cathode 21 by several volts to tens volts is applied to the electrode 23. The first, second, third grid electrodes 24, 25, 26 are arranged by being separated from each other; the first grid electrodes 24 is applied with a voltage which is equal to or higher than that of the Wehnelt electrode 23 by tens volts, and the second and the third grid electrode 25, 26 are applied with a normal voltage. An electron beam is modulated by the cathode 21 or the first grid electrode 24. Thereby, the distortion of the electric field is reduced, and an increase of the crossover diameter under a large current can be suppressed. |
