Electric heating – Metal heating – By arc
Patent
1981-12-18
1983-12-06
Albritton, C. L.
Electric heating
Metal heating
By arc
B23K 1500
Patent
active
044195613
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an electron beam apparatus, comprising an electron source which comprises a metal wire which is displaceable in the longitudinal direction and which is to be locally heated by an energy beam, and also comprising a beam-limiting diaphragm which is disposed near the wire and which carries a positive potential with respect to the wire.
An electron beam apparatus of this kind is known from U.S. Pat. No. 3,745,342. In an apparatus described therein, a metal wire which is heated, for example, by means of an electron beam or a laser beam, is used as a cathode for the apparatus. In order to ensure an adequate service life of the source, the metal wire is displaced in the longitudinal direction during operation.
An electron current emerging from an extremely narrow line of such a metal wire exhibits a comparatively large spread in the emission direction in the case of a wire having a circular cross-section of small dimension. Because a lens for further processing of the electron beam inherently has a spherical aberration which is comparatively large with respect to the line thickness, a beam-limiting diaphragm is required in order to form a target spot having a small dimension in at least one direction.
Due to such large directional spread, however, a diaphragm of this kind is likely to transmit an insufficient part of the beam current. In the known electron beam apparatus, therefore, in order to obtain an acceptable service life of the source, limitations must be imposed which adversely affect aspects of the target spot quality such as maximum current density and minimum spot size.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an electron beam apparatus in which these limitations are substantially reduced and in which, consequently, a target spot of higher quality can be realized whilst an acceptable service life of the source is maintained. To this end, steps are taken to reduce the directional spread in an electron beam to be emitted by the metal wire.
In order to reduce the directional spread of the electron beam, use can be made of a strong focusing field, which is to be applied near the metal wire. For the application of an electrostatic field, the wire can then be mounted between two plates which are arranged in parallel and which exhibit a large potential difference with respect to each other. For example, the plates can include a first plate which is strongly negative with respect to the wire and a second plate, having an aperture, which is strongly positive with respect to the wire. However, it has been found that the potential differences become unpractically high for suitable operation. This drawback can be eliminated by utilizing a magnetic quadrupole field for the realignment, but this solution necessitates extremely accurate positioning of the wire in the magnetic field, so that the stability of the source is affected. The required strength of the lens near the cathode is also an unfavourable factor in both said solutions.
In a preferred embodiment of an electron beam apparatus in accordance with the invention, the transverse profile of the wire is shaped so that the electron beam emitted exhibits a smaller directional spread upon emission. It has been found that a strip-shaped, commercially available metal wire is not stable during the strong local heating, so that the position and the shape of the emissive surface can vary in an uncontrolled manner with respect to the beam-limiting diaphragm. The field strength near the surface, necessary for carrying off the emitted electrons, then becomes comparatively low. As is known, the field strength on a surface in a field space is greatly dependent of the radius of curvature of the surface. A preferred embodiment of the metal wire is produced by flattening the outer surface of a round metal wire at the area where the electron emission is to occur. This can be accomplished, for example, by the rolling of a circular wire. The degree of flattening enables a favourable compromise t
REFERENCES:
patent: 3466487 (1969-09-01), Davis et al.
patent: 3745342 (1973-07-01), Le Poole
patent: 4151422 (1979-04-01), Goto et al.
Frima Heico J.
Le Poole Jan B.
Albritton C. L.
Kraus Robert J.
U.S. Philips Corporation
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