Devices for influencing electron beams include, inter alia, beam focussing lenses, beam deflector assemblies and other such units which are incorporated in equipment generating electron beams and which serve for focussing, deflecting or otherwise influencing a beam along its path within the equipment. Devices of this kind are frequently operated by electrical energy which may give rise to a considerable output of heat, the dissipation of which can be problematic in the confines of electron beam equipment. If excessive heat is not removed it can have a relatively significant effect, particularly due to component displacement resulting from expansion and contraction, on sensitive devices influencing beam position in fine tolerance ranges. For example, in the case of a beam deflecting assembly with coils carried by a coil former, increase in the coil wire temperature and consequent increase in the coil former body temperature lead to expansion of the coils and the body, which in turn disturbs the electromagnetic fields produced by the coils and has a deleterious effect on the beam position. This problem is exacerbated by the higher electron accelerating voltage employed in some equipment, as stronger electromagnetic fields are then required for beam deflection and thus greater operating current for the coils and increased heat output.
Attempts to cool beam deflecting and other beam influencing assemblies by use of gaseous or liquid coolants have not met with conspicuous success, primarily because the cooling systems themselves were often a source of vibration or other disturbance. Direct immersion of coils in an inert liquid coolant proved susceptible to pressure fluctuations liable to deform or displace the coil unit of the assembly. An integrated cooling system of this kind, albeit applied to electromagnetic lenses, is disclosed in DE 36 100 73 C2, where an excitation winding is accommodated in a annular housing partly filled with refrigerant in which the winding is immersed. Refrigerant vaporised by the heat of the winding is condensed by heat exchange with refrigerant flowing in heat-exchange tubes similarly accommodated in the housing and located above the level of the refrigerant bath in which the winding is immersed. The cooling system, inclusive of refrigerant feeds and valves, is bulky and space-consuming and is composed of a significant number of additional components which can themselves function as heat sinks and consequently sources of contraction and expansion prejudicing the maintenance of an accurate beam position.
A more compact integrated cooling system is disclosed in U.S. Pat. No. 5,629,526, in which cooling water is conducted by internal and external ducts to a cooling chamber directly adjoining a coil of an electromagnetic lens, the coil having a polyimide coating. The cooling is directed specifically to the coil and, apart from some influence on a surrounding core through which the ducts pass, the cooling effect is highly localised. The system is consequently ineffective for cooling any regions spaced from the coil. A potentially significant problem, which may also affect the system of DE 36 100 73 C2, is separation of the liquid in which the coil is immersed from other sensitive components. This is achieved in U.S. Pat. No. 5,629,526 by O-ring seals, but the harsh temperature environment of electron beam equipment is liable to cause early deterioration of seals and thus create a risk of leakage.
An analogous approach to the problem of cooling is embodied in the system described in DE 197 38 009 A. The cathode of an electron beam gun is indirectly heated by a coil inductor consisting of several windings of an electrically highly conductive hollow conductor. The hollow conductor is filled with a coolant such as liquid nitrogen, hydrogen or helium. The coolant serves exclusively to cool the coil windings through which it flows and accordingly the cooling system has no application beyond the specific construction for which it has been designed.
A different approach is provided by U.S. Pat. No. 6,486,605, which relates to a multibeam electron tube. Multiple beams generated by a corresponding number of cathodes and an anode are conducted via a series of drift tubes and cavities in a body to a beam collector. The collector is cooled, and possibly also the body, by an external electrically insulated cooling device in the form of a plastic duct conducting, for example, deionised water; the specific disposition of the duct and the operation and effect of the cooling device are not discussed. The described device serves to cool components at a point before extraction of microwave energy via a waveguide and the cooling device does not need to take into account—and no account is taken—of detrimental influence on electron beam position by the disposition and operation of the cooling device.