Patent GB-790017-A (GB)
Title: Improvements in or relating to travelling wave tubes
IPC: H01J23/087
Filed: 1955-10-07
Abstract: 790,017. Travelling wave tubes. WESTERN ELECTRIC CO., Inc. Oct. 7, 1955 [Oct. 22, 1954], No. 28632/55. Class 39(1). [Also in Group XL(b)] A travelling wave tube has a waveguide feeder and a permanent magnet of ferrite within the waveguide feeder arranged to focus the beam over the region of the feeder. The device uses periodic magnetic focusing successive annular permanent ferrox-dure magnets being oppositely poled and being separated by high Á, e.g., permalloy discs which act as pole pieces to concentrate the magnetic field and make it uniform in each field region ; the spacing between the discs is uniform along the length of the tube. Ferrox-dure magnets 12, Fig. 2, are arranged within the feeder waveguides. Waveguide walls 15, 16 are also of high Á to act as polepieces and are coated with copper to minimize high frequency losses. Ferrite magnets 12 are in a position of electric field minimum to minimize wave reflections due to change in characteristic impedance of the guide produced by the magnets, i.e., away from the central axis of the waveguide. Ridge 24 concentrates the electric field in its vicinity, and both magnets 12 and ridge 24 are tapered to further reduce reflection. The effect of magnets 12 is to broaden the frequency band and thus offset the reduced size of the waveguide near the tube which leads to narrow the frequency band. Magnets 12 are relatively lossless, but have a high dielectric constant. Further annular permanent magnets are provided at the ends of the tube so that the periodic focusing magnetic structure extends substantially the entire length of the tube. The tube of Fig. 1 (not shown) is a forward wave amplifier but may be a backward wave amplifier or oscillator in which case it preferably has a ribbon helix. The helix pitch is increased at its ends for matching and ceramic spacers are provided between the input and output couplers on one hand and gun and collector on the other hand. In a modification, Fig. 3 (not shown) magnets 12 are tapered in a direction at right angles to the tapering of ridge 24. In another embodiment, Fig. 4, two rows of ferrox-dure magnets 12, preferably tapered, are positioned inside the waveguide feeder 6 in the vicinity of the travelling wave tube, adjacent ferrite magnets 12 being oppositely poled. Members 41, 15, 16 serve as polepieces. In another embodiment, Fig. 5, the high dielectric constant of the ferrox-dure is utilized by making the low frequency cut-off wavelength of the tapered waveguide portion equal to that for the wider portion 51. All the waveguide walls taper outwardly, and the ferrox-dure magnet 53 is tapered to keep the low frequency cut-off wavelength constant and to minimize reflections. Pole pieces 55, 56 are provided. Resistance material 57 is provided along one side of waveguide 51 and the travelling wave tube is arranged on the side of the waveguide axis away from 57 so that the non-reciprocal properties of the ferrite 53 can be utilized ; the path along one side of the guide axis is of low impedance for a circularly polarized wave travelling in one direction and along the other side a low impedance path is formed for a similar wave travelling in the other direction. Thus a wave travelling along the feeder towards the travelling wave tube has a low impedance on the side of the guide away from resistance 57 but the reflected wave passes along the side of the guide where the resistance material is and is attenuated. Other permanently magnetized ferrites can be used instead of ferrox-dure and the magnets outside the waveguides can be replaced by properly designed " metallic magnets." The ferroxdure used has the formula BaFe<SP>III</SP> 12 O 19  having a hexagonal crystal structure and one axis of lossy magnetization parallel to the hexagonal axis. Specifications 669,475 and 750,208 are referred to.