BACKGROUND TO THE INVENTION A number of types of linear electron beam tubes are known for RF signal amplification. These types include klystrons and Inductive Output Tubes (IOT's) as well as travelling wave tubes. Traditionally klystrons have been used to amplify RF signals for broadcast. However, klystrons are relatively inefficient amplifiers and are very expensive to run. In recent years, lOTs have replaced klystrons as they are inherently more efficient and so reduce operating costs. More recently, an improved efficiency version of the IOT has been developed: the ESCIOT (Energy Saving Collector Inductive Output Tube) which uses a multi-stage depressed collector.

It is desirable for an electron beam tube in a transmitter to be able to broadcast both digital and analog television signals. A few years ago it was considered that analog signal transmitters would be phased out by 2006. However, it is now clear that this will not be the case. Analog signals require more power than their digital counterparts and there is therefore a need to improve the efficiency of devices designed with digital transmission in mind, and to minimise heat losses that occur within the device which will be more problematic at higher operating powers.

As well as the requirement for Analog and Digital compatibility there is a general need to increase the efficiency of linear beam tubes to reduce operating costs.

Linear beam tubes are also used in other fields, for example in scientific applications such as synchrotrons, driving superconducting cavities and accelerators SUMMARY OF THE INVENTION The present invention addresses these needs.

According to the invention there is provided an electron beam tube, comprising a vacuum envelope partially defined by an end wall, a DC insulating RF transparent wall attached thereto, and a balance ring arranged between the end wall and the DC insulating wall, characterised in that the balance ring comprises metallised DC insulator material.

Embodiments of the invention have the advantage of reducing thermal stress, heating and electrical stress by reducing the length of the RF path between the pole piece and the flare and eliminating eddy currents while maintaining the same thermal expansion characteristic as the insulator wall. In one embodiment of the invention only surfaces of the DC insulator material of the balance ring which, in use, are on an RF path, are metallised. Alternatively substantially the entire outer surface of the balance ring may be metallised.

Preferably, the insulator is metallised, plated with nickel and overplated with copper.

The vacuum tube may be defined by annular pole pieces and a tubular DC insulating RF transparent wall. The wall may be attached to the ferromagnetic pole pieces at its end by a flare, with a balance ring arranged at each end between the flare and the pole piece. In an electron beam tube, a vacuum tube a metallised insulator material with metallisation applied over at least those surfaces that are on the RF path.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 is a section through a portion of an IOT embodying aspects of the invention; Figure 2 is an enlarged view of a part of a pole piece of the IOT of Figure 1 embodying a first aspect of the invention; Figure 3a is an enlarged view showing the r. f. path at the connection between the pole piece and a ceramic insulator in a known IOT ; and

Figure 3b is a similar view to Figure 3a showing an embodiment of a second aspect of the invention.

DESCRIPTION OF BEST MODE The present invention may be applied to any linear beam tube used for RF amplification, including IOTs, ESCIOTs, Klystrons, TWTs and other devices. The embodiment to be described is applied to a conventional IOT but this is not in any way limiting to the scope of the invention. A linear beam tube embodying the invention is particularly suited for use with broadcast transmitters but may be used in any other environment in which high power RF amplification is required.

An inductive output tube has an electron gun which produces a beam which is focused by a magnetic field. The beam is density modulated by the RF signal to be amplified and RF power extracted from the density modulated beam by a resonant output cavity. The Klystron differs from the IOT in that it uses velocity modulation of the electron beam to amplify the RF input.

Density modulation in an IOT is achieved by a grid arranged in front of the cathode and isolated therefrom by a ceramic insulator such as aluminium oxide. The RF signal enters the tube through the ceramic insulator and is applied to the grid. An anode is arranged at a distance from the cathode and grid and is separated by a further ceramic insulator. The anode is grounded. The further ceramic insulator holds off the full beam voltage, typically of about 30kv.

Figure 1 shows a portion of an electron beam tube embodying the invention.

The device shown is an IOT having an electron gun assembly shown generally at 10.

The gun includes a thermionic cathode 12 and a grid 13. The electron beam generated by the cathode is focussed by magnetic coils (not shown) and shaped by pair of ferromagnetic pole pieces, 14,16, which, with a ceramic insulator 18 define a vacuum envelope 20. The ceramic insulator, also known as an RF window or an output ceramic, is transparent to RF but is a DC insulator. It will be appreciated from figure 1 that the pole pieces extend radially beyond the vacuum envelope. Within the envelope is a two-part drift tube 22, the two parts 24,26 being separated by a gap 28. Electrons enter the RF interaction region via a first part 24 of the drift tube

22, which is typically copper and has an annular flange 25, which forms a part of the anode 27. Between the first portion 24 of the drift tube 22 and a second part 26 of the drift tube 22 is a gap 28 at which point the RF modulated beam is inductively coupled to an output cavity 30 to provide an output signal. Only a portion of the output cavity 30 is shown in Figure 2. The drift tubes are so called as both portions 24 and 26 are at DC ground potential and there is no acceleration of the electric beam within them. The ceramic insulator 18 is a cylindrical tube, preferably made of Alumina which is transparent to RF. The ends of the insulator are attached to the magnetic pole pieces 14,16 by an arrangement shown in more detail in Figure 2.

The construction described is well known and embodied, for example, in the IOTD2100 available from e2v Technologies Ltd of Chelmsford UK.

The second portion 26 of the drift tube is flared and has a serrated inside surface. The electron beam passes through the drift tube, through an aperture in the second magnetic pole piece 16 and into a collector 29 a portion only of which is shown. The purpose of the collector is to slow down the electron beam after RF amplification. The collector may be a conventional collector or a multistage depressed collector. The design of the collector is outside the scope of the present invention.

The ends 31,32 of the two drift tube portions 24,26 may be made of molybdenum.

The ferromagnetic pole pieces 14,16 are essential for correct shaping of the electron beam. Each comprises an annulus of ferromagnetic material having a central aperture through which the beam passes. The pole pieces are typically Nickel or Iron. The magnetic field is provided by an external device such as a pair of magnetic solenoid coils (not shown), and the pole pieces acting together to generate a linear magnetic flux in the vacuum envelope defined by the ceramic insulating tube 18 and the pole pieces 14,16. The size of the centre holes in the annular pole pieces determines the shape of themagnetic field, and therefore, the electron beam.

The pole pieces complete a DC magnetic circuit.

It will be appreciated from Figure 1 that the ferromagnetic pole pieces have surfaces that, when the device is in use, are RF visible, and that the pole pieces partially form a wall of an RF cavity, namely the vacuum envelope. A ferromagnetic

material such as iron or nickel is an undesirable material for such a cavity as it is RF lossy ; it is not a good conductor at RF frequencies as it has a poor skin depth. This leads to a loss in efficiency and generation of unwanted heat. In order to improve RF performance, an embodiment of the invention coats the ferromagnetic pole pieces with a good RF conductor such as copper. It is preferred to coat the entire pole piece as this is the most convenient way of applying a coating. However, it is only necessary to coat the RF visible surfaces of the pole pieces. Although it is preferred that at least the RF visible surfaces of both pole pieces are coated, benefit is obtained by coating at least the RF visible surfaces of only one of the pole pieces.

Figure 2 shows a portion of the RF resonant cavity in more. detail. The ferromagnetic pole piece 14 is connected to the Alumina insulator sleeve by a pair of flares 34,36. The outer flare 34 is brazed to the pole piece 14 and the inner flare 36 is brazed to the end of the insulator sleeve. The free ends of the two flares are welded together to join the sleeve to the pole piece. The flares are typically copper coated nickel. A shim or balance ring 38 is arranged between the cylindrical RF window and the ferromagnetic pole piece. The shim 38 is brazed to the underside of the inner flare 36. The RF window, flare and balance ring assembly acts as a means of sealing the vacuum envelope and relieving thermal stresses. A similar arrangement is used to seal the second ferromagnetic pole piece 16 to the ceramic RF window 18.

Figure 2 also shows the anode 27 and the first portion 24 of the drift tube.

This element is typically made of copper and is a good RF conductor. Dashed line 40 shows the RF path that includes the circumferential face 42 of the ferromagnetic pole piece and an outer annular portion 44 of the inner face 46 of the pole piece. It is these portions that are coated with a layer of a good RD conductor such as copper.

The copper coating is shown at 48 and also covers a small outer annulus of the outer face 50 which may also lie on the RF path depending on the geometry of the tube. It will be appreciated that the coating is required not only on the surface of the pole pieces that is within the vacuum envelope but also on surfaces outside the vacuum envelope that are on the RF path.

The coating may be applied to the pole pieces by any convenient method, including but not limited to: plating, cladding, coating or sandwiching. Although

copper is presently preferred, other good RF conductors such as silver may be used.

The material used should have a better conductivity at RF frequencies than the ferromagnetic material. Both copper and silver have a greater skin depth at RF and so are less lossy. The material used should have an RF loss characteristic that is less than the RF loss characteristic of the ferromagnetic material.

Although the embodiment described is applicable to any linear beam tube, it has particular advantage with lOTs and ESCIOTs in which the currents circulating in the resonant cavity can be tens or even hundreds of amps. Surface losses from the RF exposed parts of the ferromagnetic pole pieces can lead to surface losses and undesired heating. This can be a particular problem when operating lOTs at the high powers required for Analog broadcast transmission. ESCIOTs tend to use iron as the pole piece as iron has a higher magnetic saturation, (permeability) but a higher surface resistivity to UHF currents. Iron performs better at higher temperatures and a thicker first portion of the drift tube can remove some of the heat. In addition, the multistage depressed collector used in ESCIOTs can give rise to an additional source of heating caused by returning electrons.

Referring back to Figure 2, the balance ring 38 is typically made of an the same ceramic as the insulator sleeve. Although a copper balance ring is RF conductive and can reduce heat losses, it is undesirable as it has different expansion properties from the sleeve insulator. It is desirable therefore to use a ceramic material as the balance ring, preferably using the same material as the insulator sleeve. Figure 3a shows how this gives rise to high losses, caused partially by eddy currents and partially by a lengthening of the RF path. In Figure 3a, the RF path is shown as a dashed line 50. It extends over the outside of the outer and inner flares and then loops around between the inner flare and the balance ring ; along the inner surface of the outer flare and along the surface of the substrate pole piece 14. Eddy currents will be generated in the space between the two flares.

Eddy currents are eliminated, and the RF path shortened in an embodiment of the second aspect of the invention shown in Figure 3b. The balance ring is ceramic, again preferably the same ceramic is the insulator sleeve, but it has a copper coating 52. The effect that this has on the electrical path can be seen from the dashed line 54 that shows the RF path as extending only over the outer surfaces

of the inner and outer flares. It will be appreciated that as the purpose of metallisation of the ceramic balance ring (in addition to providing a means of brazing the balance ring to the flare 36), is to short the RF path, so it is not essential to metallise all surfaces of the ring. For example, the outer face of the ring opposite the outer flare need not be metallised and the lower surface, which contacts the pole piece need only be metallised to the extent that an electrical connection is made between the pole piece and the balance ring. In practice it is preferred, and convenient, to metallise all of the surfaces of the balance ring.

In the embodiment shown the balances rings are arranged on the ferromagnetic pole pieces and connected thereto by the flares. Other designs are known in which balance rings attach to a separate wall, typically copper, with the pole pieces being separate from the vacuum envelope. Metallisation of the balance ring is also advantageous for this configuration.

The effect of metallising the ceramic ring is to reduce heat losses and to reduce thermal stresses that can lead to cracking of the insulator sleeve or the balance ring when the flares are brazed into place. The same expansion is achieved in the balance ring as the insulator sleeve as the same material is used.

The balance ring may be metallised using known techniques. For example a powdered molybdenum manganese alloy and binder is fused to the surface of the Alumina balance ring. The binder is lost in processing, leaving a surface which is then nickel plated and over-plated with copper to reduce loss further. Other materials could be used, for example silver is suitable as it has good RF conductivity.

Thus, the embodiment described fully metallises the ceramic balance ring to enable RF losses to be reduced and to enable stresses associated with thermal processing and the operation to be relieved.

The embodiments of the two aspects of the invention described have been described with reference to the pole piece 14 to which the first drift tube portion including the anode is attached. A similar construction of flares and a balance ring is used to attach the second end of the insulating sleeve to the second pole piece. It is preferred that the surfaces of the second pole piece are also coated with a good r. f. conductor and that the second balance ring is also metallised in accordance with the embodiments of the first and second aspects of the invention described above.

It will be appreciated that the embodiments described both have the advantage of reducing RF losses and consequently improving the efficiency of the tube. This contributes to tubes being able tooperate at higher power, which is desirable for analog signal broadcasting, and to reduce operating energy requirements. Although particularly suited to ESCIOTs and conventional IOTs, the embodiments described as also applicable to all other high power linear beam tubes including Klystrons.

Various modifications may be made to the embodiments described without departing from the scope of the invention, which is defined by the following claims.