Field of the invention

This invention relates to an electron beam emitting assembly, such as in an electron beam gun used in electron beam welding.

Background to the invention

Electron beam emitting assemblies are used within electron beam guns to position a heating filament, cathode and anode relative to one another. The filament can be placed in direct physical contact with the cathode to heat the cathode to its electron emission temperature using Joule heating. The filament is small, typically around 1mm diameter and 3mm in length, and holding such filaments in place to an accuracy of microns whilst heating to a temperature of around 1600K is difficult, especially when two electrically isolated connections are required to contact the filament. Problems can be experienced with efficiency of Joule heating due to conducted heat loss into the locating assemblies and overheating of components proximal to the filament.

Summary of the invention

In accordance with the invention, there is provided an electron beam emitting assembly comprising a cylindrical cathode element, typically formed from Lanthanum Hexaboride or Cerium Hexaboride, and a current source, wherein an electrically conductive element connected to the current source is positioned to electrically contact a longitudinal axis of the cathode element and so establish a coaxial electrical connection with the cathode element. This produces a substantially symmetrical current flow through the cathode element. As such the magnetic field associated with current passing through the cathode element is substantially symmetrical about the longitudinal axis which means fewer magnetic distortion effects need to be compensated for.

The electrically conductive element may be a rod inserted into a negative electrode and the electrically conductive element may be moveable axially to ensure it is urged into physical contact with the cathode element. Instead of a rod, the electrically conductive element may be made from Tungsten wire or foil strip, preferably having a cross-sectional area of around 200 pm ² .

Where the electrically conductive element is made from wire or foil strip, it may be secured to one end of the cathode element by a first portion of electrically conductive adhesive so as to be positioned coaxially with the longitudinal axis of the cathode element.

If desired, the first portion of electrically conductive adhesive may be disc-like in shape, being a disc or cylinder of limited height, having a diameter the same as or greater than the cylindrical cathode element.

Preferably the cylindrical cathode element and the electrically conductive element are held in fixed relationship to an electrically insulating casing. This ensures no inadvertent electrical contact with an outer housing providing a return path to the current source connector.

The casing may be formed with a conduit aligned with the longitudinal axis of the cathode element, the electrically conductive element disposed within the conduit so as to align with the longitudinal axis of the cathode element.

A second portion of electrically conductive adhesive may be positioned part-way along the cylindrical cathode element, spaced apart from the first portion of electrically conductive adhesive, the second portion establishing an electrical connection with the current source.

The second portion of electrically conductive adhesive may be annular in shape. Alternatively the second portion of electrically conductive adhesive may be formed as a cup with an apertured base, the cathode element extending through the apertured base. The cup may be located substantially within an electrically insulating casing.

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 schematic diagram of a first embodiment of a cathode arrangement used in such an assembly;

Figure 3 is a close up view of the cathode arrangement of Figure 2;

Figure 4 is a schematic diagram of a second embodiment of a cathode arrangement; and

Figure 5 is a schematic diagram of a third embodiment of a cathode arrangement.

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 or emitter 18 and anode 20. In response to heating by filament 16, cathode 18 generates 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 electron beam welding.

In embodiments of the invention as shown in Figures 2 to 5, electrical current is used to heat the cathode to an electron emission temperature.

Figure 2 shows part of the electron beam gun where a cylindrical cathode element 32, also known as an electron emitter and typically formed from Lanthanum Hexaboride or Cerium Hexaboride, is secured within casing unit 34 and connected to a current source 36 using a current conveying wire 40 or foil strip attached to casing unit 34.

As shown in detail in Figure 3, casing unit 34 comprises an electrically insulating ceramic casing 42 formed with a central channel 44 sealed at one end to create a well within which a lower portion 46 of cathode element 32 locates, an upper portion 48 of cathode element 32 protruding beyond casing unit 34 and beyond lip 49 of housing 50 so as to be proximal to bias voltage electrode or anode 20. A narrow conduit 51 extends downwards from the centre point of well 44 to an outer lower face 52 of ceramic casing 42.

Wire 40 is positioned in conduit 51 to form part of unit 34 and make physical and electrical contact with the centre of origin of circular lower face 54 of emitter 32 and so contact a central or centred longitudinal axis 56 of emitter 32. Wire 40 is secured within conduit 51 by way of electrically conductive adhesive 60 which has a lower electrical conductivity than wire 40 to ensure that current predominantly passes from wire 40 into emitter 32 rather than into adhesive 60. Thus wire 40 connects a negative electrode 58 of current source connector 36 to cathode element 32 without the need for connections to metal posts secured to the cathode element, such connections tending to remove heat from the cathode element and being prone to failure under heating. Typically the wire has a cross-sectional diameter of around 200 pm.

Adhesive 60 is electrically conducting so that any small amounts of adhesive inadvertently positioned between wire 40 and cathode element 32 do not prevent electrical contact being established coaxially along axis 56. Using electrical adhesive also has the advantage that any stray electrical fields from wire 40 are contained within adhesive 60 and directed into cathode element 32 in a symmetrical fashion.

Whilst only a small amount of adhesive 60 is required to secure wire 40 in place, if desired adhesive 60 can be formed as a disc-like structure, such as a thin disc or thin cylinder, extending over the entirety of lower face 54 so as to increase the area of contact. This improves the robustness of the connection between wire 40 and cathode element 32 and also ensures stray current is applied symmetrically to base 54 of cathode element 32.

Electrically conductive adhesive 60 includes graphite particles within its adhesive matrix and is able to resist the high temperatures generated within the electron gun and so is typically stable up to temperatures of 2985°C.

A second portion of electrically conductive adhesive 62 is affixed to upper face 64 of casing 42 so as to contact cathode element 32 partway along its length and to provide an electrical return path to positive electrode 66 of current source connector 36. Typically second portion 62 is formed as an annular structure extending over the entire upper surface of casing 42 so as to surround and physically and electrically contact cathode element 32 where it emerges from casing 42.

The combined weight of casing unit 34 with cathode element 32 is less than lOOmg and wire 40 is chosen to have sufficient rigidity to support casing unit 34 from beneath, typically by selecting a Tungsten wire of around 200pm diameter. The rigidity of the wire enables casing unit 34 to be held close to lip 49 of housing 50, the positioning allowing for some upwards expansion during operation of the electron gun so as to prevent cracking of ceramic case 42 against lip 49.

The direction of current flow is shown by arrows with current travelling from negative electrode 58 to reach cathode element 32 and current returning by way of electrically conductive adhesive 62 and conductive housing 50 to reach positive electrode 66. Negative electrode 58 is insulated from the remainder of the conductive housing 50 by insulator 70.

By establishing electrical connection along longitudinal axis 56, the electrical current is provided coaxially with cathode element 32 and current flow through cathode element 32 acts to provide resistive heating, resulting in emission of electrons. The magnetic field 72 associated with current passing through cathode element 32 is substantially symmetrical about longitudinal axis 56 which means there are fewer magnetic distortion effects to compensate for within electron gun 10, and the magnetic field is substantially symmetric around the electron beam as it emerges from cathode element 32.

To ensure that emitter 32 can readily be replaced as required, casing unit 34 and emitter 32 are assembled together before being introduced into housing 50 and connected to current source connection 36. Typically this is achieved by casting alumina ceramic paste into a PTFE mould to form ceramic casing 42 and then securing wire 40 and cathode element 32 in electrical contact along longitudinal axis 56 using first adhesive portion 60 and then applying second adhesive portion 62. Casing unit 34 with cathode element 32 secured in position can then be easily mounted into housing 50.

Figure 4 shows an alternative embodiment where coaxial electrical contact along axis 56 of emitter 32 is provided by a first portion of electrically conductive adhesive 80 adhered to outer lower face 52 of electrically insulating casing 82, wire 40 being held within adhesive 80. Part-way along cathode element 32 and within casing 82 a second portion 84 of electrically conductive adhesive is formed as an inverted cup 86 with a central aperture 88 through which cathode element 32 extends so as to physically contact portion 84. The side walls of cup 84 extend just beyond electrically insulating casing 82 so that a second Tungsten wire 89 can be secured into second adhesive portion 84 and used to connect to positive electrode 66 of current source 36 as shown by the arrows representing current flow. In this arrangement both positive electrode 66 and negative electrode 58 are insulated from housing 50 by insulators 70.

As with the other embodiments, both portions of electrically conductive adhesive 80, 84 are coaxial with longitudinal axis 56 of cathode element 32 and as such magnetic fields around cathode element 32 generated by the current flow are symmetrical about axis 56.

As with the embodiment shown in Figure 2, the casing unit is assembled with cathode element 32 before being introduced into housing 50 and connected to current source connection 36. Typically this is achieved by casting alumina ceramic paste into a PTFE mould to form ceramic casing 82 with integral cup-shaped channels 90, positioning cathode element 32 within central channel 92, introducing electrically conductive adhesive into channels 90 with retaining rings positioned to ensure the edge of cup 84 extends beyond the surface of casing 82, securing wire 89 into the adhesive, then securing wire 40 and cathode element in electrical contact using electrically conductive adhesive portion 80 to secure wire 40 to the central origin and thus axis 56 of cathode element 32. If desired adhesive portion 80 can be applied as a disc so as to cover the entire end face of cathode element 32. Casing unit 34 with cathode element 32 secured in position can be easily mounted into housing 50. Figure 5 shows a third embodiment where a contact rod 94, typically made of Tungsten or other refractory material, is used. Electrically insulating casing 98 is tubular with rod 94 fixed within and slotted into negative terminal 104, rod 94 electrically contacting cathode element 32 at central longitudinal axis 56. Electrode 104 is configured to move axially, typically by being mounted on an extendible support means such as formed by nested rings 105, 105’. Spring 96 urges upwards against shoulder 99 of terminal 104 to ensure rod 94 is moveable axially to contact the base of cathode element 32. Electrically conductive adhesive portion 100 is adhered to upper face 102 of casing 98 and can optionally be surmounted by a thin Tungsten or Tantalum disc to further enhance electrical connection between cathode 32 and housing 108. Current flow is shown by the arrows, current returning to positive electrodes 106 by way of conductive housing 108. Negative electrode 104 is insulated from housing 108 by insulator 110.