The present invention relates to an electron gun for use in an apparatus having a body along which a beam of electrons emitted by the electron gun travels during use.

Background

Various electronic apparatuses, amongst them traveling wave tubes (TWT’s) and klystrons, require an electron beam to travel through an evacuated body housing an interaction structure. The present invention is concerned with a gun that can be employed in any such apparatuses to generate the required electron beam.

Electron guns have a cathode assembly from which electrons are emitted and an anode. The cathode assembly may typically comprise a cathode, a heater to heat the cathode and a beam forming electrode to help converge the electrons emitted thermionically from the wider surface of the cathode into a beam. In use, a potential difference is applied across the cathode assembly and the anode to accelerate electrons emitted from the cathode. The accelerated electrons pass through a hole in the anode and enter the evacuated body of the apparatus. The electrons then travel through the body of the apparatus towards a collector arranged at the opposite end.

The function served by the electron beam during passage through the body will depend on the nature of the interaction structure within the body. In the case of some designs of TWT, the electron beam serves to amplify an RF signal passing through a helical coil surrounding the beam, but this is mentioned only as an example.

Summary of the invention

According to the present invention, there is provided an electron gun for use in an apparatus having a body along which a beam of electrons emitted by the electron gun travels during use, the electron gun comprising a housing for forming a vacuum seal with the body of the apparatus, a cathode disposed within the housing for generating electrons, an anode towards which electrons emitted from the cathode are accelerated and a hole in the anode to permit the accelerated electrons to pass into the body of the apparatus, wherein the cathode is formed as a thermionic cathode assembly comprising a cathode electrode and a heater for heating the cathode electrode and in that the anode of the electron gun is constituted by the housing. Conventionally, in the type of apparatus to which the present invention relates, all the components of an electron gun are mounted within, and electrically isolated from, the body of the apparatus. There has previously been proposed in US2003/0052612 a microminiature electron source for use in medical applications, where the entire electron source is sized to fit on the end of a coaxial cable. The anode in the latter reference is constituted by a cylindrical cap connected to the outer conductor of the coaxial cable, while the central conductor of the coaxial cable serves as the cathode. The output power of such a device is limited by its size and on account of the fact that, because the coaxial cable has only one central conductor, it can only employ a cold cathode.

In the invention, because the anode of the electron gun having a thermionic cathode assembly forms part of the evacuated body of the apparatus, the number of fabrication steps required to feed connection wires into the interior of the evacuated body is reduced, as it allows the wires to pass through a component that is made of metal rather than of a ceramic material.

In some embodiments, the housing comprises a tubular side wall, an end wall at one axial end of the side wall formed with the hole through which accelerated electrons are emitted into the apparatus, an end plate at the opposite axial end of the side wall, and connection wires leading to the cathode electrode and the heater vacuum sealed within the end plate.

The cathode assembly may further comprise a beam forming electrode for shaping the electrons emitted from the cathode electrode into a beam, and a connection wire vacuum sealed within the end plate and leading to the beam forming electrode.

To avoid the risk of the cathode assembly coming into contact with the anode, as a result of mechanical shock or vibration, the electron gun may in some embodiments further comprise an insulating lining overlying the inner surface of the tubular side wall. If desired, an insulating lining may additionally or alternatively overlie the inner surface of the end wall formed with the hole through which accelerated electrons are emitted into the apparatus.

In some embodiments, the housing is provided with a screw thread for engaging with a complementary screw thread of the apparatus, in order to secure the housing to the apparatus and form a vacuum seal. In such embodiments, the screw-thread for engaging with the body of the apparatus may suitably be concentric with the electron beam emitted by the gun during operation. Such a construction allows the electron gun to be removed, for example for repair, and replaced with affecting the alignment of the electron beam.

Brief description of the drawings

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

Figure 1 shows three different components that are assembled with one another to form a travelling wave tube,

Figure 2 is a section through an electron gun shown in Figure 1,

Figure 3 is a similar section to that of Figure 2 with the cathode assembly and the feedthrough connectors removed to reveal the structure of the anode more clearly, and

Figure 4 is a similar view to Figure 3 showing an alternative embodiment in which the anode is formed of three initially separate parts that are screw threaded to one another.

Detailed description of the drawings

The components shown in Figure 1 are an electron gun 100, a barrel 200 housing an interaction structure, and a collector 300. These are assembled to one another and the interior of the assembly of Figure 1 is evacuated to form a TWT. The present invention is concerned primarily with the electron gun 100 and this component alone will be described in detail below with reference to the remaining drawings. The barrel 200 and the collector 300 are shown to provide an example of an apparatus in which an electron gun of the invention may be employed and will be described only briefly.

The barrel 200 has a body 210 with flanges 212 and 214 at its opposite ends for attachment to the electron gun 100 and the collector 300, respectively. The body 210 contains an interaction structure through which an RF signal is transmitted. The interaction structure may, for example, have the form of a double corrugated waveguide. Ports 216 and 218 shown in Figure 1 are the input and output port of the helical waveguide. The interaction between the RF signal and the electron beam that passes axially through the barrel 200 results in amplification of the RF signal. After traversing the barrel 200, the electron beam emitted by the electron gun 100 strikes the collector 300 which is cooled to dissipate the generated heat, for example by means of fins 310. In the TWT shown in Figure 1, a tube 320 for connecting the interior chamber of the TWT to a vacuum pump forms part of the collector 300, but it may be placed in a different location.

Conventionally, the anode, cathode and beam forming electrode would be enclosed within an evacuated housing 210, but in the embodiment of the invention shown in Figure 2 and Figure 3 the anode 110 of the electron gun 100 is in the form of a housing that has an end flange 112 in screw threaded engagement with the flange 212 of the body 210 of the barrel 200. Therefore, rather than being located within an evacuated chamber, as is conventional, the anode 110 in the present invention forms part of the housing defining the evacuated chamber. The anode 110 is internally insulated and houses the cathode assembly. Conductors 114 leading to the cathode assembly pass through an end wall of the anode 110 and are electrically isolated from the anode 110 by insulating cylinders 116.

The interior of an embodiment of the electron gun 100 will now be described by reference to Figures 2 and 3. Figure 2 shows all the components that make up the electron gun 100, whereas Figure 3 shows only the anode 110 of a variant of the electron gun shown in Figure 2.

The anode 110 in Figure 3 is in the form of a canister having a cylindrical side wall 120 formed integrally with an end wall 122 having a central opening 124 through the electron beams exist the electron gun 100. The flange 112 that is shown in Figure 1 is formed at the same end of the side wall 120 as the end wall 122 and is externally threaded, to engage with an internal thread on the flange 212 that forms part of the body 210 of the barrel 200. A sealing ring 126 is clamped between the two flanges 112 and 212 to prevent ingress of air and maintain vacuum. The opposite end of the cylindrical side wall 120 is formed with an internally threaded flange 128 that engages with an external thread on a metallic end plate 130 which acts as part of the envelope of evacuated chamber 132. The end plate 130 is formed with three cylindrical sleeves 134 for the passage of the conductors 114 leading to the cathode assembly and with two internal part-cylindrical upstands 136, that serve as a mechanical support and joint for the cathode assembly.

The interior of the cylindrical side wall 120 is lined with an electrically insulating sleeve 138, made from a ceramic material. A circular disc 140, made of the same insulating material and having a central hole 142, lies against the end wall 122.

In the illustrated embodiment, a metal tube 150 is connected to the side wall 120 for attachment of a vacuum pump, in addition to the tube 320 shown in Figure 1. Figure 2 shows a cathode assembly 160 that includes a ceramic cylinder 162 supported on the upstands 136. The internal construction of the thermionic cathode assembly is well known in the context of TWT’s and is not of critical importance to the present invention. It suffices to know that it includes a cathode, a heater and a convergence, or beam forming, electrode and that the electrical connection to these elements is effected by means of the conductors 114. A vacuum seal is formed between the sleeves 134 of the end plate 130 and the ceramic cylinders 116 surrounding the conductors 114 in a conventional manner.

To assemble the electron gun of Figures 2 and 3, the conductors 114 are first assembled to the end plate 130. The cathode assembly 160 is next electrically connected to the conductors 114 and it is supported on the upstands 136. With the insulating lining 138 and disc 140 in place inside the anode 110, its cylindrical side wall 120 is next screwed to the end plate 130, trapping the cathode assembly 160 between the end wall 122 and the end plate 130 of the anode 110 and aligning the cathode with the holes 124 and 142. The anode 110 may next be screwed to the flange 212 to form a vacuum seal and a vacuum pump may be connected to the tube 150 to complete the assembly of the TWT. Sealing rings 126 may suffice during prototyping and testing while the apparatus remains connected to a vacuum pump. For permanent sealing of the apparatus, the side wall 120 of the anode may be soldered, welded, or brazed to the flange 212 and to the end plate 130. Thereafter, the tube 150 may be sealed by cold welding and disconnected from the vacuum pump.

An important advantage of the above-described embodiment of the invention is that it allows the electron gun 100 to be removed, dismantled, and reassembled without disturbing the alignment of the electron beam. The screw threaded engagement between the anode 100 and the barrel 200 ensures alignment of the electron beam with the barrel, while the upstands 136 and the screw threaded connection between the end plate 130 and the side wall 120 ensure correct alignment and positioning of the cathode assembly in relation to the anode.

The embodiment of the anode shown in Figure 4 differs from that of Figure 3 only in that the end wall cylindrical side wall 120’ and the end wall 122’ are not formed integrally with one another but as separate components that are connected to one another by a further screw threaded coupling. In this case, the entire screw threaded coupling lies within the evacuated chamber and the two parts do not therefore need to be sealed relative to one another. This embodiment does not differ in its operation from that previously described but simplifies the manufacture of the anode, as it can now be produced by machining metal tubing and flat discs.