1. Field of the Invention
The present invention relates to an electron gun with band-shaped beams for producing an electron beam with a band-shaped cross-section. It is equipped with a linear cathode and is particularly suitable for low-energy electron beam systems. The electron beam exits into the atmosphere from a beam window, also called Lenard window. Preferred applications are the curing and cross-linkage of thin polymer coatings with a coating thickness of up to several 10 .mu.m on band- as well as slab-shaped coating substrates.
2. Discussion of Background Information
Electron beam devices are known, wherein short, wire-shaped cathodes that are stressed in the running direction of the product are arranged in its electron gun. These cathodes are arranged equidistant and parallel to one another at certain distances and individually suspended under tension by springs. Their number is determined by the respective beam widths and therefore the window length. The cathode length is derived from the window width. Each of these cathodes is concentrically surrounded by a cylindrical grid. In addition another broad grid which stretches over all the grid-cathode arrangements is generally provided. The grids have been assigned different potentials and serve to homogenize the current density laterally to the cathode voltage direction (U.S. Pat. No. 3,863,163).
Since all of these cathodes are wired electrically in parallel, the disadvantage is that very high heating currents can arise--especially with large exposure widths and a correspondingly large number of parallel cathodes. Such cathode systems are used in particular for electron beam systems with acceleration voltages in the range of &gt;180 kV. The width of the beam window is thereby in the range of 200 mm. Another disadvantage of such electron beam systems is that if an outage of single cathodes occurs, the homogeneity of the current density in the window length is significantly disturbed. The change of a cathode is very costly and time-consuming due to the construction.
Electron beam generators with a band-shaped beam cross-section whose beam window corresponds in length to the product width (DE 44 32 983 C1) are known. They have one or more parallel cathodes arranged over the window length and therefore the exposure width. A spring prestressing of the cathodes is necessary in order to limit the cathode sag. This cathode arrangement is surrounded by a generally tube-shaped control electrode, slitted in the beam direction, which serves to form the beam and if necessary to control the beam current.
All the devices have the disadvantage that with increasing window length, and thus also cathode length, a voltage strength is necessary which exceeds the yielding point of the cathode material at operating temperature. The result thereof is that with increased operating time the cathode length increases and the planned constructional oscillation path is reached. A cathode tear or an inadmissibly cathode sag leads to a critical limitation of the cathode's lifespan. The stretching of the cathode material thus limits the applicability of such cathode Systems based on cathode lengths, and therefore beam widths, of about one meter.
In addition, these cathode systems tend to undergo mechanical oscillations. Such oscillations can easily be instigated by external influences. The result of such cathode oscillations can be uncontrollable effects upon the beam formation and thus the homogeneity of the electron beam. The problems resulting from the oscillations also increase with the length of the voltage cathode system.
To limit the disadvantages connected with the voltage cathode system, one or more intermittent suspension devices of the cathode have been suggested. These solutions are of only limited use, since the heating of the cathodes is connected with a considerable change in length and the suspension must tolerate this change in length. To this is added an emission intrusion of the cathode at the place of suspension due to heat dissipation. A further disadvantage is that the suspension device itself has a local effect on the potential distribution near the cathodes, which can also have a negative impact on the homogeneity of the beam current density distribution.