The invention relates to a method of controlling an electron beam in an electron accel¬ erator in which electrons emerging from a source of electrons are passed through a grid window into an acceleration space and from the acceleration space through a primary window provided with cooling ribs out of the electron accelerator, and in which the electrons are accelerated by an acceleration voltage generated between the grid window and the primary windo .

The invention is also concerned with an electron accelerator comprising a source of electrons, a grid window and a primary window provided with cooling ribs, whereby electrons emerging from the source of electrons are passed through the grid window into an acceleration space and from the acceleration space through the primary window out of the electron accelerator, and the electrons are accelerated by an acceleration voltage generated between the grid window and the primary window. Electron accelerators are used to produce electrons with an energy of 100 to 800 keV typically for various electron beam applications. Such applica¬ tions include the hardening of coatings by polymer¬ ization technique, purification of flue gases, sterilization, etc. In order to obtain adequate effect with sufficiently even results, industrial equipment usually require an even electron beam, which is directed to the surface of a moving material web or to a gas flowing in a gas flue. In practice, the efficiency of an electron beam is in the range

from 10 to 100 k . As the formation of electrons takes place in a vacuum and electrons are passed out of the inner vacuum spaces of the accelerators through thin windows having a thickness of about 0.01 mm, a considerable proportion, about 3 to 15%, of the efficiency of the electrons is dissipated in the windows when the electrons pass through them. The windows are typically long and narrow and usually made of titanium. The purpose of the long narrow window openings is to cause the emerging electrons to form a curtain-like electron beam.

In electron beam equipments presently in use there is typically provided a cooling grate or cool¬ ing ribs of copper behind the window inside the equipment, which grate or ribs are cooled internally by means of water. In this way, the thin window, in which plenty of heat energy is developed, can be kept at a sufficiently low temperature. When electrons are accelerated with the widely used technique from one opening to another, and cooling ribs are provided at the bottom of the opening in front of the window, some electrons always strike against the cooling ribs at least in proportion to the ratio of the areas of the ribs and the window. Weak lines of force further emphasize this effect, even though the lines of force of the acceleration voltage are positioned on the edges of the opening. The proportion of electrons striking the cooling ribs of the total power of the equipment is about 25 to 35%, which is rather a high proportion. As many electron beam applications consume considerable amounts of energy, the loss of nearly one third of the energy is very significant in view of the application and requires unreasonably large and efficient cooling machineries and rises operating costs considerably.

GB Patent 1 179 277 discloses one typical solution of the prior art, in which cooling ribs are provided against the primary window and electrons strike against the ribs as described above. US Patent 4,491,756, in turn, discloses how the striking of electrons against the cooling ribs can be reduced by suitable shaping of the ribs. In the solution of this publication, however, some electrons still strike against the cooling ribs, thus causing the above- described problems.

DE Offenlegungsschrift 30 20 809 discloses a solution in which various lens structures are used in an attempt to pass the electrons between the cooling ribs so as to prevent the striking of the electrons against the ribs. The solution is very complicated and difficult to realize especially in cases where several components dependent on each other, such as a cathode, a first and a second perforated plate, etc., have to be dimensioned and positioned accurately. In practice, the solution is very difficult and expensive to realize. In addition, lens structures focus electron radiation, as a result of which the central portion of the windows is liable to over¬ heating. The object of the present invention is to provide a method and an equipment which avoid the above disadvantages and by means of which the power required for cooling the window of an electron accelerator can be decreased. The method of the invention is characterized in that an electric charge reversely symmetrical with respect to an imaginary plane of symmetry extending midway between the grid window and the primary window is formed between the grid window and the primary window, an even distribu- tion on the grid window being reflected as an even

distribution on the primary window.

The basic idea of the invention is that the window leading into the acceleration space, that is, the grid window is provided with means having a mirror image relationship to the cooling ribs of the primary window, so that a mirror symmetry prevails between the cooling ribs and their mirror images and passes the electrons directly from the grid window to a corresponding point in the primary window, whereby no electrons strike against the cooling ribs on approaching the primary window.

The equipment of the invention is characterized in that electric field control means are provided on the surface of the grid window facing towards the primary window, the electric field control means and the cooling ribs of the primary window being reversely symmetrical in shape with respect to an imaginary plane of symmetry extending midway between the grid window and the primary window. The basic idea of the equipment is to provide electric field control means on the side of the grid window, the control means and the cooling ribs of the main window being reversely symmetrical, so that a mirror symmetry is formed between the cooling ribs and the control means, which passes the electrons directly from the grid window to the primary window.

The invention will be described in greater detail in the attached drawings, in which

Figure 1 is a sectional view of an electron accelerator of the invention in the longitudinal direction of the grid window; and

Figure 2 is a sectional view of the electron accelerator of the invention in the longitudinal direction of a row of grid windows. Figures 1 and 2 show an electron accelerator

comprising an outer wall 1 of a vacuum chamber, an electron generating device 2, which may be, as shown in the figure, a long tubular space in the middle of which a glow filament 2a is tightened. Electrons 3 emerging from the electron generating device are passed to a grid window 4 parallel with the glow filament 2a, and after having passed through the grid window they enter an acceleration space. A support structure 5 usually made of copper is positioned below the grid window. Such a support structure is known per se and therefore will not be described in greater detail. The support structure comprises an opening for the passage of the electrons. In the in¬ vention, electric field control means 5a are provided below the grid window 4 and preferably attached to the support structure 5. The electric field control means 5a divide the single opening below the grid window into several transverse window openings 4a which may extend from one edge of the grid window to the other either in parallel with the glow filament or transversely, straight or obliquely with respect to it. After having been passed between the electric field control means 5a, the electrons further pass on towards the primary window 6, above which a support structure 7 usually made of copper is positioned. The structure is known per se and therefore will not be described in greater detail. The support structure 7 comprises cooling ribs 7a. In shape, the cooling ribs 7a and the electric field control means 5a are mirror images of each other with respect to an imaginary plane S extending midway between the grid window 4 and the primary window 6, so that an electric field 8 is created between them by means of the mirror symmetry. As the electrons pass through the grid window and between the adjacent electric field

control means 5a, they pass through the primary window precisely reversely symmetrically as compared with their passage through the grid window, that is, the paths of the incoming and outgoing electrons are reversely symmetrical similarly as the electric field control means 5a and the cooling ribs 7a. For practical reasons, the grid windows and correspond¬ ingly the primary windows are often obliquely posi¬ tioned, so that the cooling ribs also have to be positioned obliquely transversely across the window. This, however, is of no practical importance if the electric field control means 5a and the cooling ribs are positioned reversely symmetrically with respect to the plane S between the grid window 4 and the primary window 6.

The invention has been described above and in the attached drawings by way of example and is in no way restricted to the example. The invention can be applied in electron accelerators provided with all kinds of electron generating devices, such as a low- voltage electron gun from which electrons emerge as a wide beam to the grid window, or a narrow electron beam is passed continuously across the surface of the grid window. The cooling ribs and the electric field control means can be of practically any shape, provided that they are reversely symmetrically positioned with respect to the plane extending in parallel with and between the grid window and the primary window. By suitably choosing the height c of the cooling ribs, the height a of the electric field control means and a distance b between them, the lines of force can be made to extend substantially straight between the grid windows, whereby they do not act as lenses and do not focus or disperse the

electron beam before it strike against the surface of the primary window.