電子顕微鏡用電子銃の電極系寸法の最適条件
1973 年 42 巻 4 号 p. 357-367
詳細
1973 年 42 巻 4 号 p. 357-367
Influence of electron microscope gun geometries on the field distribution over a spherical cathode surface is estimated using the axial potential formula given by M. Ploke. Special attention is paid to the field in the immediate vicinity of the cathode surface. The emitting area and cut off bias voltage are analytically calculated including gun geometries.
In order to obtain a sufficiently high brightness with a small emission current, it is found that a gun arrangement functions more efficiently,
1) the smaller the cathode height,
2) the larger the Wehnelt diameter in the case of a positive cathode height,
3) the smaller the Wehnelt diameter in the case of a negative cathode height (with the cathode protruding from the Wehnelt aperture),
4) the smaller the Wehnelt-anode distance,
5) the smaller the radius of the cathode sphere.
In cases of 1), 2) and 4), there exists a suitable compromise from the viewpoint of insulating problems of the bias circuit. This conclusion is also experimentally confirmed. Furthermore, a design example is given to illustrate the use of calculated results.
