This invention relates to electron beam emitting assemblies, such as used in electron beam guns used in electron beam welding.

Background to the invention

Electron beam emitting assemblies are used within electron beam guns to position an emitter, cathode and anode relative to one other. Precise placement of the filament, cathode and anode is necessary to ensure the electron beam generated has appropriate characteristics for welding.

The filament and cathode need replacing on a regular basis which is a complicated procedure requiring skilled personnel to modify the position and orientation of the emitter, cathode and anode relative to each other. Set up procedures to ensure correct beam characteristics take many hours.

It is an aim of the present invention to provide an electron emitting assembly which reduces the complexity of replacing elements such as cathodes and emitters.

Summary of the invention

In accordance with the present invention, there is provided an electron beam emitting assembly comprising an emitter section and a cathode section, wherein the cathode section comprises a cathode and a cathode shield held in a fixed relationship to each other and the cathode section is separable from the emitter section whilst the cathode and cathode shield remain in a fixed relationship to each other. By having a detachable cathode section, the emitter element can be accessed for replacement without disturbing the position of the cathode relative to the cathode shield. This reduces set-up time required to make the electron beam emitting assembly operational after replacement of the emitter element. The cathode section may further comprise a clamping portion to secure the cathode and cathode shield to each other. Preferably the clamping portion comprises an annular disc.

Clamping of the annular disc to the cathode may be achieved by co-operating channels within the disc and cathode enabling attachment of the disc to the cathode shield by securing elements such as screws.

Preferably the clamping portion is disposed on an outer face of the cathode so that when assembled the clamping portion is positioned between the cathode shield and emitter section.

The invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of an electron beam gun incorporating an electron beam emitting assembly;

Figure 2 is a partial section through a known electron beam gun emitting assembly; Figure 3 shows a partial section of the known electron beam gun emitting assembly in exploded view to illustrate assembly of individual elements;

Figure 4 is a partial section through an electron beam gun emitting assembly according to the invention;

Figure 5 is an exploded view showing an emitter section and cathode shield section before assembly; and

Figure 6 shows an emitter element for use in the present invention. Description of the drawings

A schematic diagram of an electron beam gun 10 is shown in Figure 1 for explanatory purposes. Electron beam assembly 12 from which electrons are generated is located in evacuatable housing 14, with assembly 12 comprising filament 16, cathode 18 and anode 20. Filament 16 is inductively heated to emit electrons which are accelerated towards cathode 18 to generate an electron beam which is accelerated through anode 20 to pass into a second evacuatable housing or chamber 22 in which are disposed focussing coils 24, alignment coils 26 and beam deflection coils 28 so as to produce a high energy focussed electron beam 30 for welding.

A prior art electron emitter assembly 40, disclosed in EP 1018137, is shown in Figures 2 and 3 and comprises a split coupling loop base 42 connected to an inductive coupling loop 44, generally by being machined from a single metal block such as a Copper block. A ceramic insulator collar 46 surrounds loop base 42 to insulate it from housing 50. Heatable filament 52 is positioned centrally within loop base 42 and comprises an elongate mount 54 on which is mounted an emitter disc 56 made of any suitable heat conductive ceramic such as Lanthanum Hexaboride or refractive metal such as Tungsten or Tantalum. Inductive coupling loop 44 surrounds primary emitter 56 and is connected to an RF electrical supply, not shown, so as to inductively heat emitter disc 56 to generate electrons. Cathode 60 is mounted on a hollow frusto- conical support 62 and is secured within housing 50 by spacing collar 64 such that emitter 56 and cathode 60 are positioned along a common axis and spaced apart by a small distance. Cylindrical cathode shield lobe 70 is located around securing collar 64.

When filament disc 56 needs replacing, assembly 40 needs to be dismantled by removing lobe 70, then removing securing collar 64, then removing any additional annular spacing shims 66 and cathode 60 to reach filament disc 56. To replace filament disc 56 can often take at least a day as the filament needs to be replaced, then cathode 60, securing collar 64 and cathode shield lobe 70 positioned in turn, these elements clamped against shoulder 72 of housing 50, and finally measurements and setting of the beam undertaken. If any of the individual elements are loose or have different dimensions from the original components due to engineering tolerances, then it will be difficult to achieve a good quality electron beam and set up will take a long time. Similarly replacing cathode 60 is also time consuming.

In an embodiment of the invention as shown in Figures 4 and 5 cathode 60 and cathode shield lobe 70 are provided as a pre-assembled combined unit that can be slotted into housing 50. In the electron beam assembly 80 of Figures 4 and 5, combined cathode shield and cathode unit 82 comprises cathode shield 84 in the form of a lobe, securing collar 86, cathode 60 and annular securing disc 90. An end face 98 of cathode shield lobe 84 is formed with channels 92 for receiving grub screws or similar extending through channels or apertures 94 in disc 90. Disc 90 is typically made of Copper chrome to provide good electrical conductivity and thermal stability. Lobe 84 has a stepped entrance lip 96 which matches the edge profile of securing collar 86, collar 86 seating in lip 96 so that outer face 98 of lobe 84 and an outer face 99 of collar 86 form a combined outer surface against which disc 90 secures using a plurality of grub screws 100. Cathode 60 is typically a Lanthanum hexaboride disc mounted on a spring- biased frusto-conical clip.

As cathode 60 and collar 86 are clamped together against lobe 84 using disc 90, accuracy and repeatability is improved as the collar and any spacing shims disposed between collar 86 and disc 90 are held flat and any residual spring is substantially eliminated. By moving the clamping point to the cathode shield 84, it is possible to apply a greater pressure to secure cathode 60.

By using securing disc 90 as a clamp, cathode 60 and cathode shield lobe 84 can be fixed in position relative to each other prior to insertion into emitter section 81 and in particular prior to positioning proximal to filament element 56. By having multiple clamping points provided by grub screws 100, any shims disposed between cathode 60 and disc 80 are held flat, counteracting any residual resilience or spring. By having a combined cathode and cathode shield unit, the combined unit can be easily removed to allow access to and replacement of filament 96 and then replaced knowing that the relative positions of the cathode shield and cathode are fixed, so substantially reducing set-up time when a filament is replaced.

Further the combined cathode and cathode shield unit can be assembled prior to use and matched to the set-up of an existing combined unit. Thus when a cathode needs replacing, a replacement combined unit is already available which again reduces the time to set-up the beam for welding. To assist with placing the combined unit into the main body, housing 50 is formed to match edge profile of ceramic insulator 46, leaving a central channel within which the combined cathode shield and cathode unit 80 can be inserted by a push fit. This ensures that the cathode shield and cathode can be positioned in a fixed relationship to each other prior to placing the cathode over the filament.

A detailed view of heatable filament 52 is shown in Figure 6. Filament 52 comprises a substantially cylindrical support 102 made of Molybdenum so as to have a higher melting point and have improved resistance to heat damage. Support 102 narrows at one end to form a shoulder 104 and primary emitter disc 106 supported on a Molybdenum pin 108 with an enlarged head section 109 at one end extending through the cylindrical holder and being secured in position by grub screws (not shown) within lateral channels 110. To assemble filament 52, disc 106 is threaded onto pin 108, pin 108 pushed into support 102 and disc 106 held between shoulder 104 and head 109, with grub screws securing pin 108. Replacement of disc 109 is straightforward, simply requiring release of the screws and extraction of pin 108, and can be undertaken in less than thirty minutes.