GB2277635A - Magnet arrangements - Google Patents

Abstract

Magnet arrangements used in PPM focusing systems, for example, with electron beam tube devices, include a stack of cylindrical magnets 1 having pole pieces 2 interposed between them. The pole pieces are configured so as to have a Curie temperature Tc which is non-uniform in a radial direction. The inner portion 3 of the pole piece discs is of iron having a relatively high Tc and the outer portion 4 is of nickel iron having a lower Curie temperature. At relatively low operating temperatures, the magnetisation of the inner and outer disc pieces remains substantially constant. At higher temperatures, the magnetisation of the outer portion 4 decreases hence causing flux to be concentrated to the inside of the arrangement along the longitudinal axis X-X. With increasing temperature, the strength of the magnet 1 also decreases. These two effects compensate for each other to produce a substantially temperature invariant magnetic field along the axis X-X. In other arrangements, the change in temperature Tc across the pole piece discs is gradual. <IMAGE>

Description

Magnet Arrangements This invention relates to magnet arrangements and more particularly, but not exclusively, to magnet arrangements used to focus electron beams in linear beam tubes such as travelling wave tubes.

Travelling wave tubes and klystrons typically use periodic permanent magnet (PPM) focusing to ensure that an electron beam directed longitudinally through the device is kept focused. In the absence of such magnetic focusing, charge repulsion tends to defocus the electron beam and electrons will collide with the walls of the structure along which the beam is directed.

In a typical PPM arrangement for focusing an electron beam, a plurality of cylindrical permanent magnets magnetised in the axial direction, and having central apertures are arranged as a "stack" along and around the electron beam path with adjacent magnets being of opposite poiarity. Pole pieces of a soft magnetic material, usually ferromagnetic iron, are located between the magnets and comprise apertured discs. The pole pieces shape and confine the magnetic fields of the magnets so as to produce a significant component in the axial direction which varies approximately sinusoidally with axial distance as required to confine the electron beam. For example, the electron beam may be focused within the inner diameter of an RF circuit, such as a helix or coupled cavity slow wave structure.

Although conventional designs of PPM stacks are satisfactory for many applications, variation of magnetic properties with temperature result in variation of the axial focusing field with temperature. This can affect the electron beam so as to increase the proportion of current intercepted on the tunnel wall and to cause deterioration in RF performance. Such temperature variation may determine the limits of temperature over which the tube can operate.

The present invention arose from the consideration of a PPM arrangement suitable for confining an electron beam at relatively high temperatures and over a wide range of operating temperatures compared with the conventional arrangement, but it is envisaged that it may also be useful for other applications where magnet arrangements are required to perform under wide variations in temperature.

According to the invention, there is provided a magnet arrangement comprising permanent magnet means and associated pole piece means for shaping its magnetic field, material of the pole piece means exhibiting a spatially non-uniform change in magnetisation with varying temperature over a range of operating temperatures.

Pole pieces are usually made from ferromagnetic material for which the rate of change of magnetisation with temperature is non-linear and generally increases with increasing temperature. The magnetisation falls to approximately zero when the Curie temperature is reduced, above which the material is not ferromagnetic. The general shape of the reduced magnetisation - temperature curve is similar for all ferromagnetic materials. It follows that at a temperature below the Curie temperature, two ferromagnetic materials with different Curie temperatures will exhibit different rates of change of magnetisation with temperature. The material with the lower Curie temparature will in general exhibit a higher rate than the material with the higher Curie temperature. The Curie temperature is a material characteristic.For iron it is approximately 1000K whereas for nickel-iron having a composition ratio of 1:1, it is approximately 600K.

By employing the invention, the reduction in strength of the magnet means at higher temperatures of the operating range may be compensated for, either partially or substantially entirely, in a volume in which the pole piece means shapes the magnetic field. The distribution of the pole piece materials is arranged such that magnetisation changes in a non-uniform manner with varying temperature so as to alter the proportion of the magnetic flux conducted through different parts of the pole piece means. This enables a greater percentage of the available flux to be confined in the volume by the pole piece means at higher temperatures.The effects of the temperature change on the magnetisation of the pole piece means and also on the magnetic field strength may be arranged to balance out so as to give a substantially constant magnetic field between the pole pieces which is invariant with temperature change over the range of operating temperatures.

This mechanism is further explained with reference to one advantageous embodiment of the invention in which the magnet arrangement is suitable for PPM focusing and is cylindrically symmetrical about an axial aperture along which the pole piece means is arranged to confine the magnetic field. However, the invention may also be usefully employed in magnet arrangements having non-cylindrical geometries and used for purposes other than PPM focusing.The pole piece means in this example consists of two soft magnetic materials having first and second different Curie temperatures Tci and Tc2. One of the materials makes up the inner portion of the pole piece means nearest the axis and is such that its Curie temperature Tci is not approached during normal operating conditions of an apparatus in which the magnet arrangement is included. Therefore its magnetisation remains substantially constant.

The other material has a lower Curie temperature Tc2 such that, during operation, the magnetisation of that material reduces with increasing temperature over the normal operating temperature range. The behaviour of two suitable materials, iron and nickel iron, is illustrated schematically in Figure 1 (not to scale) which is an explanatory diagram showing the variation of their magnetisation with temperature. The normal range of operating temperatures is from T1 to T2 as shown.

At relatively low temperatures, the permanent magnet means produces a stronger field than at higher temperatures. Flux lines carry the field through the pole pieces to the axis of the arrangement. At lower temperatures, the outer pole piece magnetic material has a higher saturation inductance and thus some of the magnetic field leaks to the outside of the PPM stack. At higher temperatures, the field produced by the permanent magnet means is reduced. However, the magnetisation of the outer pole piece material is also reduced and less field leaks to outside the PPM stack. The two effects may thus be arranged to balance to reduce the temperature coefficient of the axial field compared to a conventional arrangement.

The use of the invention results in there being less confinement of the magnetic field within the volume where the pole pieces shape the field and more loss of flux to the outside of the arrangement at lower temperatures. This decrease in available flux for focusing an electron beam, for example, can be allowed for by using larger permanent magnets to obtain the amount of confinement which would be available with a conventional pole piece design. In many applications, this inconvenience is not significant when set against the ability to efficiently operate the arrangement at high temperatures.

As discussed above, materials having different Curie temperatures may be employed as their magnetisation - temperature curves are different. However, other materials which have the same, or similar, Curie temperature may be used to implement the invention if their magnetisation - temperature curves differ suitably.

In one advantageous embodiment of the invention, the pole piece means is annular and comprises a radially inner portion of a material having one Curie temperature and a radially outer portion of a material having a second lower Curie temperature. In another embodiment, the pole piece means includes a member of material which exhibits a gradual change in Curie temperature in a direction from a region in which the magnetic flux is to be concentrated by the pole piece means to an outer region. The change in Curie temperature may be achieved by mixing two or more different magnetic materials in graded proportions and then sintering them together.This can be used to produce an arrangement which at the inner region is substantially entirely of soft iron and gradually increases its concentration of nickel iron, for example, to the outer diameter at which it substantially wholly consists of nickel iron.

The magnet arrangement in accordance with the invention is particularly advantageously used in an arrangement in which the permanent magnet means provides periodic permanent magnetic focusing suitable for the confinement of a beam of charged particles, for example. In such an arrangement, the outer radius of the pole piece members may be less than that of the permanent magnet members so as to reduce losses to the outside of the arrangement.

The invention may be usefully employed in travelling wave tubes, klystrons and other linear beam tubes.

Some ways in which the invention may be performed are now described by way of example with reference to the accompanying drawings in which: Figure 2 is a schematic longitudinal section of a magnet arrangement in accordance with the invention; Figure 3 is a transverse section through the arrangement of Figure 2 along the line lil-lil; Figure 4 schematically illustrates in longitudinal section another arrangement in accordance with the invention; and Figure 5 is a transverse section through the arrangement shown in Figure 4 along the line V-V.

With reference to Figures 2 and 3, a periodic permanent magnetic focusing arrangement is generally cylindrical in configuration and arranged around a longitudinal axis X-X along which in operation an electron beam is transmitted. The PPM arrangement confines and focuses the electron beam to ensure high efficiency operation and reduce interception of the electron beam current with the walls of the structure through which it travels.

The PPM system includes a plurality of apertured cylindrical magnets 1 (only some of which are shown) arranged about the longitudinal axis X-X. In this arrangement, the magnet material is an alloy of samarium cobalt and there is a substantial component of the magnetic field in a direction parallel to the longitudinal axis X-X, with adjacent magnets 1 having opposite polarities as indicated by the arrows. The magnets 1 are interposed with pole pieces 2. Each pole piece 2 is an apertured disc, having a larger outer diameter and a smaller inner diameter than the magnets 1. The pole pieces 2 direct and confine the magnetic flux produced by the magnets 1 so as to produce an axial magnetic field which confines the electron beam within the inner diameter of the circuit in which the PPM system is included.

Each of the pole piece discs 2 is made up of two different soft magnetic materials. An inner annulus 3, extensive from the internal diameter to a radius r, is of iron which has a relatively high Curie temperature Tc. The outer annul us 4 extends from the radius r to the outer diameter and is of nickel iron which has a relatively low Curie temperature.

At lower temperatures, the magnetisation of the inner annulus 3 and outer annul us 4 remain substantially unchanged. At higher operating temperatures, whilst the magnetisation of the iron inner annul us 3 remains substantially the same, the magnetisation of the outer annul us 4 reduces with increasing temperature . This results in less flux being conducted to the outside of the arrangement and hence the proportion of the total available flux carried by the inner annulus 3 increases compared to the lower temperature operation. At the higher temperature, the strength of the magnets 1 decreases and hence the resultant magnetic field along the axis X-X remains substantially constant for both the low and high temperature regimes.

The PPM system illustrated in Figures 2 and 3 is such that the boundary between the high and low Tc materials is discrete and located at the same radius around the circumference of each pole piece disc 2. Figures 4 and 5 illustrate another arrangement in accordance with the invention in which the Curie temperature Tc changes gradually from the inner diameter of the pole piece discs to the outer diameter.

The samarium cobalt magnets 5 are similar to those of the arrangement in Figures 2 and 3 but in this case the interposed pole piece discs 6 are formed of a mixture of materials which are sintered together so as to give a gradual change in Tc with radial distance. In some applications it may be desirable to arrange that the Curie temperature Tc is angularly uniform. However, in this arrangement, the change in Tc is profiled as illustrated particularly in Figure 4, where the dotted lines indicate contours of constant Tc. The line Tca defines a region of a particular Curie temperature Tc which is greater than that defined by the line shown as TCb.

In the arrangement shown in Figures 3 and 4, the outer diameter of the pole pieces 6 is smaller than that of the magnets 5 to reduce losses to the outside of the arrangement.

The pole piece discs of the arrangement of Figures 2 and 4 are of uniform composition in the direction of the longitudinal axis. However, they could consist of, for example, a plurality of thin laminated sheets arranged in a plane transverse to the longitudinal axis of the focusing system. In such an arrangement one or more, or all, of the laminated sheets may exhibit the non-uniform change in magnetisation with varying temperature as required by the invention. The overall performance of the pole pieces when the sheets are laminated together is arranged to give the change in magnetisation with temperature required to provide the consistent magnet focusing along the axis of a stack.

Claims (16)

1. A magnet arrangement comprising permanent magnet means and associated pole piece means for shaping its magnetic field, material of the pole piece means exhibiting a spatially non-uniform change in magnetisation with varying temperature over a range of operating temperatures.

2. An arrangement as claimed in claim 1 wherein material of the pole piece means has a spatially non-uniform Curie temperature such that said spatial non-uniform change in magnetisation is obtained.

3. An arrangement as claimed in claim 1 or 2 wherein changes in strength of the magnet means with temperature are substantially compensated for in the volume in which the pole piece means shapes the magnetic field by the change in magnetisation of the pole piece means material to produce a substantially constant field strength in the volume with changing temperature.

4. An arrangement as claimed in claim 3 wherein part of the pole piece means nearer the said volume has a higher Curie temperature than part which is more remote therefrom.

5. An arrangement as claimed in claim 1, 2,3 or 4 wherein the pole piece means and permanent magnet means are substantially cylindrical in configuration, defining an aperture along the longitudinal axis.

6. An arrangement as claimed in claim 5 wherein the Curie temperature of the pole piece material varies spatially in dependence on radial distance.

7. An arrangement as claimed in claim 6 wherein the Curie temperature of the pole piece material is dependent on radial distance and angularly uniform.

8. An arrangement as claimed in claim 6 wherein the Curie temperature of the pole piece material is dependent on radial distance and angularly non-uniform.

9. An arrangement as claimed in claim 5, 6, 7 or 8 wherein the pole piece means comprises a planar annular member arranged substantially transverse to the longitudinal axis.

10. An arrangement as claimed in claim 5, 6, 7, 8 or 9 wherein the pole piece means comprises an annular member having a radially inner portion of a first material and a radially outer portion of a second material, the first and second materials having different magnetisation - temperature curves.

11. An arrangement as claimed in claim 10 wherein the first material has a first Curie temperature and the second material a second, lower, Curie temperature.

12. An arrangement as claimed in any one of claims 1 to 9 wherein the pole piece means includes a member of material having a gradual change in its magnetisation temperature curve in a direction away from a volume in which the magnetic field is shaped by the pole piece means.

13. An arrangement as claimed in claim 12 wherein the member is of material having a gradual change in its Curie temperature in a direction away from said volume.

14. An arrangement as claimed in any preceding claim wherein the permanent magnet means comprises a plurality of cylindrical annular permanent magnet members distributed along a longitudinal axis having pole piece members located between them to provide magnetic focusing along the axis.

15. Linear electron beam apparatus comprising a magnetic arrangement as claimed in any preceding claim arranged to focus an electron beam.

16. A magnet arrangement substantially as illustrated in and described with reference to the accompanying drawings.