TANDEM ACCELERATION ELECTROSTATIC LENS

PROBLEM TO BE SOLVED: To realize operation of acceleration mode focusing lenses at a lower voltage compared with a conventional lens. SOLUTION: This device is constituted by superposing acceleration mode focusing electrostatic lenses. One acceleration mode focusing electrostatic lens is formed between a first, a second and a third electrodes 4a-4c, and another acceleration mode focusing electrostatic lens is formed between the third, a fourth and a fifth lenses 4c-4e. High energy of the ion beam passing through the positions where focusing lens operation is generated keeps a small chromatic aberration coefficient of the acceleration electrostatic lenses. Increase of the number of the positions where the focusing lens operation is generated enables the acceleration mode focusing at a low lens voltage.