Patent GB-1238667-A (GB)
IPC: H01J25/44
Filed: 1968-09-16
Abstract: 1,238,667. Travelling-wave tubes. S.F.D. LABORATORIES Inc. 16 Sept., 1968 [29 Jan., 1968], No. 43985/68. Heading H1D. A crossed-field travelling-wave tube amplifier includes an interaction region defined by a cathode and by an anode structure which incorporates a slow-wave structure extending orthogonally with respect to the electric and magnetic fields, and means for injecting a stream of electrons into the interaction region along the direction of the magnetic field to a collector electrode which is disposed at an axial end of the interaction region and at least a part of which is electrically insulated from the slowwave structure and the cathode to permit depressed collector operation. In the re-entrant stream arrangement of Fig. 2 an annular stream of electrons from the cathode 11 immersed in the magnetic field B is injected through an end-cap and accelerating electrode 12 into the interaction region 9 defined by a non-emissive sole electrode 2, which is grooved to reduce secondary emission and a non-re-entrant slow-wave structure 4. With no input signal to the slowwave structure, the annular rotating electron stream is collected by electrode, 14, which is insulated from the sole electrode by an alumina ceramic disc; in the presence of an input signal, a fraction of the stream electrons move out towards the slow-wave structure, and are collected either by the latter or by the further, annular, collector electrode 16. Noise in the electron stream derived from variations of axial velocity components are not coupled to the slow-wave structure, while the stream may be caused to flow through the interaction region sufficiently rapidly to ensure that any portion of it traverses only a fraction of the length of the slow-wave structure, whereby both the coupling of noise derived from circumferential velocity variations, and the build up of space charge in the interaction region, is minimized. In further embodiments, the slow-wave structure is severed into two portions having resistive terminations at the sever (Fig. 5, not shown), and two structures, each similar to that of Fig. 2, are arranged end-to-end (Fig. 4, not shown) with a cathode at each end and a central collector electrode receiving both electron streams. A linear version of the device is described (Figs. 6 and 7, not shown). The two-stage re-entrant stream arrangement of Fig. 11 employs a crossedfield gun formed by the cylindrical emitting surface 71, the accelerating electrode 73, and the repeller electrode 74 all of which are disposed in the magnetic field B. The stream passes in succession past two non-re-entrant slow-wave structures in the form of helices 63 and 64 which of rectangular cross-section and are supported from the envelope by ceramic insulators 79, and is collected by a plurality of cup-shaped electrodes 75 held at different depressed potentials. The signal to be amplified is applied to one end of helix 63, the output is taken from one end of helix 64, the other ends of the two helices being terminated in matched resistive loads. Some of the spokes of space charge produced by the first helix 63 circumferentially traverse the break in that helix, which break is sufficiently long to produce debunching and hence prevent instability in this section of the tube. Other well-defined spokes of space charge from the last half of the first helix drift down past the second helix 64, the break in which is circumferentially displaced from that in the first helix (Fig. 12, not shown) such that these spokes enter near the input end of the second helix to excite and amplify a signal thereon. The anode potentials applied to the first and second helices are, respectively, 4 to 5 and 10 to 25 times the synchronous voltage, the radial spacing of the second helix from the sole electrode 62 also being larger, to maintain the same angular phase velocity in the two sections of the tube. The second helix also has greater axial extent, to increase interactions with the beam. Reference is made to the use of loaded interdigital and strapped-bar slow-wave structures.