This invention relates to travelling wave tube arrangements and more specifically to travelling wave tube arrangements including periodic permanent magnet focusing systems. A section through part of a typically known travelling wave tube arrangement including a periodic permanent magnet focusing system is shown in Figure 1 of the accompanying drawings.

Referring to Figure 1 , the helix of the travelling wave tube is represented at 1. The helix 1 is supported by support rods 2 in a helix support tube 3« Surrounding the helix support tube 3 is the periodic permanent magnet focusing system 4. This system consists of a series of annular pole pieces 5 _» each of which is provided with a hub 6, through which the tube 3 passes. Between adjacent ones of the pole pieces 5 are shaped pole piece spacers 7 ^a nd annular magnets 8 of the "Alnico" type, (that is to say, formed of the alloy Co Fe Ni Al Cu & Ti). ^■»

The assembly is mounted on a heat conduction place 9. What is known as the "cell length" is represented at 1.

This is the total length of one magnetic unit (i.e. one pole piece and one magnet) .

The purpose of providing t .e hubs 6 formed in the pol pieces 5 n accordance with established practice is to establish a near true" sinusoidal magnetic field pattern along the axis of the travelling wave tube.

It is known that rare earth magneto, of the Samarium/ Cobalt type for example, provide a significant increase in magnetic field per unit volume compared to magnets of the Alnico type generally used in travelling wave tube practic However attractive is the possibility of using a rare earth magnet in extending the range of existing and futur designs of travelling wave tubes focusing systems, this ha been avoided since rare earth magnets are substantially more expensive per unit volume than magnets of the Alnico type and to replace existing Alnico type magnets in a periodic permanent magnetic focusing system has been re ard as being unduly expensive and would provide an excessive amount of magnetic material or a given cell length. One object of the present invention is to provide an improved travelling wave tube arrangement utilising a periodic permanent magnetic focusing system in which the above disadvantages are reduced.

According to this invention a travelling wave tube arrangement utilising a periodic permanent magnet focusin

system is provided wherein said periodic permanent magnet focusing system comprises a series of annular rare earth

_* magnets interposed between annular pole pieces which are substantially hubless. The present invention 'resides in the discovery or realisation that in contradiction to previous belief, it is not necessary for the magnetic pole pieces to have hubs to establish a near true sinusoidal magnetic field pattern along the axis of the travelling wave tube if rare earth magnets are provided. Thus, the manufacture of both the pole pieces, and any pole piece spacers which are provided, is substantially simplified and for a given cell length thicker pole pieces, and hence thinner magnets than would normally be the case in previous designs, may be used. Not only does this reduce or even eliminate the cost disadvantage of utilising rare earth magnets in the periodic permanent magnet focusing system of the travelling wave tube, but at the same time an increase in field may be obtained.

Furthermore, in examples of the invention in which the pole pieces are made thicker than would usually have been the case in previous designs, less distortion is likely xo be experienced during processing of the travelling wave tube at elevated temperatures.

Furthermore, where, as will normally be the case, pole piece spacers are used, these may be of simpler cross section,

since these do not have to accommodate the hubs of con¬ ventional pole pieces and not, only may mating tolerances be reduced or eliminated, but also the absence of hubs tend to make assembly somewhat simpler. Preferably said pole pieces are annular plates of uniform cross section throughout.

Preferably said magnets are annular plates of uniform cross section throughout.

Preferably said pole pieces are separated by pole piec spacers which are in the form of square section annular rin Normally for each magnetic cell one pole piece spacer and one magnet will be provided, the pole piece spacer being nearer the helix of the travelling wave tube than is the magnet. Preferably said rare earth magnets are Samarium/Cobalt magnets.

The invention is further described with reference to Figure 2 of the accompanying drawings, which is a section through part of a travelling wave tube arrangement utilisi a periodic permanent magnetic focusing system in accordanc ".■ ^r dth the present invention. The section of Figure 2 does, in fact, correspond to the section of the known arrangement shown in Figure 1 and corresponding reference numerals are utilised for corresponding parts, but those reference numerals relating to modified parts are identified by the suffix a.

^" 3URE4

In the arrangement shown in Figure 2, it will be seen that the pole pieces 5 are hubless and in fact consist of annular plates of uniform thickness throughout. The overall thickness of the pole pieces _._. in Figure 2 is considerably greater than the thickness of the major part of the pole pieces 5 of Figure 1 and in fact the thickness of the pole pieces 5a in Figure 2 corresponds approximately to the axial length of the hubs 6 in Figure 1.

The annular magnets 8a of Figure 2 are of Samarium/ Cobalt and are correspondingly thinner than the annular magnets 8 of Figure 1.

Tbe pole piece spacers 7a of Figure 2 are in the form of simple rectangularly sectioned annular rings.

It will be noted that compared to the arrangement of Figure 1 , the thermal path provided by the pole pieces 5a of Figure 2 from the helix support tube 3 to the outer heat conduction plate 9 s improved due to the increased overall thickness of the pole pieces a.

OMPI