The invention relates to an electron beam source with an electron exit window which is connected thereto via a window flange and which essentially consists of the window frame, of a support grid, optionally provided with cooling lines, for the foil permeable to electrons, and of the foil permeable to electrons.
Electron exit windows for electron beam sources are supported for cooling and load relief by rectangular bridges which are in good heat contact with their surroundings. When an electron beam passes through the thin metal foil of the window, which is permeable to electrons, a part of the beam power is absorbed in the metal foil and is removed from there as heat via the bridges. Moreover, the bridges serve to support the metal foil which acts as a sealing foil between the vacuum and atmospheric pressure. The foil is pressed onto the bridges of the support grid by the exterior air pressure. Support grids of this type are of particular importance in the case of electron exit windows with a large surface area, such as are used for electron beam sources with a low dose rate, for example those which operate trouble-free for hundreds of hours at accelerating voltages of 150 - 280 kV. Electron beam sources with exit windows of this type are predominatly used for irradiating thin layers for the purpose of crosslinking or for sterilising surfaces in the packaging industry.
The construction of the support grids, which is relatively involved and hence expensive to manufacture, has proved a disadvantage in all these known electron exit windows. For example, German Published Specification 1,589,773 has disclosed support grids in which the bridges are located in preformed grooves of the flange. If optimum heat transfer from the bridges to the flange is to be achieved, the bridges must be brazed into the flange, and this leads to difficulties and high costs in the industrial manufacture of the electron beam exit windows having dimensions of 130 cm and more in length and 2.5 - 10 cm in width.
Even greater difficulties arise in another embodiment described in that Specification, wherein the individual bridges are soldered to tubes through which the coolant is passed. This soldering requires such an expense that electron beam exit windows of this type cannot be manufactured economically.
Milling the bridges from a solid material also leads to difficulties. For example, in the case of bridge widths of customarily 0.5 or 1 mm, the bridges which are to be milled free are pressed away by the milling cutter towards the preceding gap already milled. Furthermore, since the milling cutter is round, the slots do not have an edge at the transition to the flange but are given the radius of a milling cutter. The circular saw blade cannot be driven completely through the slot.
As can be seen from the above statements, it is thus absolutely necessary, for the successful use of electron beam sources, to develop electron beam exit windows with support grids which can be manufactured by machine in large numbers without great expense. Furthermore, the electron beam exit windows should be constructed in such a way that, for changing the thin foil which is permeable to electrons and which represents a wearing part, it is not necessary to separate the entire window flange from the instrument, such as is still the case with the electron beam exit windows of the state of the art. Moreover, it is an object of the invention to provide support grids of a type which eliminates the necessity of changing the entire support grid - as is still customary nowadays - when individual bridges have burned through.