Patent GB-697550-A (GB)
Title: Improvements in or relating to amplifying space discharge devices such as travellingwave tubes
IPC: H01J25/38
Filed: 1951-06-15
Abstract: 697,550. Travelling-wave tubes. WESTERN ELECTRIC CO., Inc. June 15, 1951 [June 15, 1950], No. 14221/51. Class 39(i) In a travelling-wave amplifier the wave transmission means comprises successively in the direction of electron flow, a region of negligible attenuation, a region of distributed attenuation in which the attenuation per unit length is at least several times greater at the upstream end than at the downstream end, and another region of negligible attenuation, the length of the region of distributed attenuation being greater than the combined lengths of the regions of negligible attenuation. The loss may be concentrated mostly in a short section near the input with an abrupt change to a relatively long region of moderate loss, or may decrease gradually from a maximum value near the input end to a minimum value near the output end, Figs. 5A, 5B (not shown). These distributions are compared with known distributions in which the loss is uniformly distributed over a short or long intermediate section, Figs. 8A, 8B (not shown). The design of the loss section is described in detail and illustrated graphically. The loss material may be a coating of carbon or other conducting material on the helix support rods 35. For example colloidal graphite can be sprayed on or carbon deposited pyrolitically. The travelling- wave tube has indirectly heated cathode 22 with a concave emissive surface, beam forming electrode 26 with a surface formed by two truncated cones of different angles, and anode 31 formed with a conical portion on its left hand flanged end. One end of the cathode heating coil 23 is connected to cathode cylinder 24 and also, via plate 29, to beam forming electrode 26. Two sets of three screws 30, 33 hold the electrodes against ceramic spacers 27, 32 and insulating spacer 28, the potential of anode 31 being supplied via one or more of screws 33 which are insulated from electrode 26. Helix 34 is supported and spaced from the envelope 21 by four ceramic rods 35 as described in Specifications 669,473 and 669,475, and its end turns are of progressively increasing pitch for impedance matching as described in Specification 669,474. The rods 35 are held in notches in collars 36, 39 and the ends of the helix 34 are attached to antennae projections 37, 40 of the collars. Ceramic rings 38, 41 space the collars from the anode 31 and conical collector 42 respectively. The collector lead 43 includes within the tube a coil serving as a choke and the helix 34 is preferably kept slightly or considerably positive with respect to the collector by lead 44 connected to collar 39. Input and output waveguides 45, 46 of rectangular section have their closed ends spaced #/5 from strips 37, 40 which extend about half way across the guides. The electrons are focused by a magnetic field generated by solenoid 48 and the field is extended to the input and output regions by two pairs of permanent bar magnets 58, 68 extending between soft steel plates 55, 52; 53, 67. Soft steel plates 60, 61, 62, 64 serve to correct defects in the shape of the magnetic fields, and magnetic shields 56, 57, 50, 54 and non-magnetic collars 59, 49, 63 complete the magnet structure. Cavity resonators 65, 66 serve as chokes. Correcting plates 60, 61, 62, 64 may be cupped or bent as shown in Figs. 11 A, 11B. The magnetic system may use only permanent magnets e.g. four long ones equally spaced around the tube periphery, a large number of correcting plates being used. The plates may be nearer together at the ends of the tube than at the middle and the flanges serve as shunts. Field uniformity may also be obtained by control of magnetization along the length of the magnets or by using magnets longer than the tube length. To reduce variation in axial velocity across the beam cross-section due to the space charge lowering of potential along the axis, the gun, which is shielded from the magnetic focusing field by members 55, 56, 57, projects a converging beam which is abruptly subjected to the magnetic field at the point of minimum beam diameter. The axial velocity is thus made uniform while the rotational velocity is proportional to the distance from the axis. The attenuation may be made frequency selective e.g. by coating all sides of the rods 35 except those in contact with the helix; as the field strength decreases more rapidly radially at higher than at lower frequencies there is less attenuation at the operating frequency than at a lower frequency which may give maximum gain; in this way stability is also assured at the lower frequency. Secondary emission from the collector 42 which would tend to cause regeneration or even oscillation is rendered harmless by placing a small piece of iron at one side of the collector to distort the magnetic field and so deflect the secondaries away from the helix. A resistance 69 in the connection to accelerating anode 31 prevents 'flash arcing '. To prevent the beam from falling out of synchronism with the wave as it gives up energy thereto it may be projected initially at a higher velocity than the synchronous velocity. Specification 652,155 also is referred to.