| 摘要 |
PURPOSE:To improve the withstand voltage characteristic under the normal operating condition by providing a projection on an outside grid electrode at the position facing one end of an inside grid electrode. CONSTITUTION:A cathode electrode K and the first-sixth electrodes 1-6 are provided almost coaxially, and the first-fifth electrodes are held by an insulating support bar 7 and insulated from each other. A circular body 16 is arranged and fixed inside the electrode 6 to face the end section 5a of the electrode 5 and constitutes a projection facing the end section 5a. The portion of the circular body 16 facing the end section 5a has a diameter nearly equal to that of the end section 5a, and the distance D1 is set to the distance D2 between the electrode 6 and the cylinder section of the electrode 5 or less. The projection is provided over the whole periphery of the end section 5a, thereby no distortion is generated on the electrostatic lens formed between the electrode 5 and the electrode 6, and it has an effect to concentrate the electric field at the time of knocking over the whole periphery of the end section 5a. Accordingly, a spark can be prevented from occurring during the normal operation, and the good withstand voltage characteristic can be obtained with the large-diameter lens maintained.
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