Patent GB-787271-A (GB)
Title: Improvements in or relating to velocity modulation tubes
IPC: H01J25/10
Filed: 1955-05-09
Abstract: 787,271. Velocity-modulated tubes. COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. May 9, 1955 [May 10, 1954 ; Feb. 22, 1955], No. 13355/55. Class 39(1). A velocity-modulated tube, operating on mm. wavelengths comprises a cathode having a length of the order of the operating wavelength emitting a beam of electrons which is velocity modulated in a microwave circuit and is then passed through a magnetic field which curves the electron trajectories and produces a transverse density modulation of the beam which induces an ultrahigh frequency field in a further microwave circuit. The amplifier tube shown in Fig. 5 comprises a cathode 1, having a filament 9 fed through conductors 10, mounted in an evacuated metal envelope 14. An electron lens 8 focuses the electron beam in a direction perpendicular to the cathode. A modulating space 2 bounded by grids 3 and 4 is separated by an electrode 31 from a collecting space 5 bounded by grids 6 and 7. The electrons are collected by an electrode 18 which may be replaced by a number of plates each positioned in line with a voltage node in the collector space 5 and maintained at a positive potential less than that of the grids 6, 7 by a separate battery. The grids 3, 4, 6 and 7 are maintained at a positive potential with respect to the cathode 1 by a source 33. The envelope 14 is disposed between two pole pieces which create a magnetic field B perpendicular to the plane of the drawing. A water jacket 15 having an inlet 16 and an outlet 17 is provided for cooling purposes. Sections of waveguide 11 and 12 sealed by windows 13 and 13<SP>1</SP> terminate respectively in the spaces 2 and 5. A standing wave is established in the modulating space 2 and the length of this space governs the operating wavelength. The transverse density modulated electron beam induces a standing wave of the same wavelength in the collecting space 5. The grids 3, 4 and 6, 7 are mounted across re-entrant portions of two metallic prisms. A frequency doubler is obtained by replacing the grid 6 by a surface having a number of apertures spaced apart so as to produce in the collecting space 5 a standing wave of twice the frequency of the modulating wave. It is also possible to design tubes with plate cathodes and circuits having a dimension of the order of the operating wavelength in the direction perpendicular to the plane of the drawing, i.e., parallel to the lines of force of the magnetic field B. Fig. 17 shows a modulating cavity resonator 2 employed in such tubes. The grids 3, 4 are disposed in front of the anti-nodes of the exciting wave. The collecting resonator is identical to the resonator 2. An oscillator tube is obtained by replacing the prisms by two cavity resonators which are coupled together by a coaxial line and providing a single waveguide output from the collecting resonator. Tuning may be effected by deforming the cavity resonators. In an alternative oscillator tube. Fig. 11, the circuits 2 and 5 are two portions of waveguide coupled by means of a waveguide of smaller dimensions formed by means of a block 23 the length and height of which governs the phase and amplitude of the feedback. The block 23 may be adjustably mounted in the wall of the waveguide and connected to a rod passing through the envelope 14 and a bellows to a mechanical tuning control. Alternatively the block 23 may be replaced by a further waveguide section passing through the envelope 14 and provided with a tuning piston. The velocity modulation may be effected by a travelling-wave. Fig. 15 shows an amplifier tube comprising a long waveguide 27 to which a travelling wave is fed at 11. A number of identical cathodes 1 are disposed an integral number of wavelengths apart along the waveguide. The travelling wave velocity modulates the beam emitted by a cathode and is then amplified by interaction with the same beam which is density modulated transversely at the end of its semi-circular trajectory. ' A wave which has undergone several successive amplifications is received at 12. A localized attenuation 28 prevents oscillation. Provided that the cathode 1 is close to the grids 3 and 4, Fig. 5, the effect of the magnetic field B on the electrons in the space between electrodes 1 and 4 may be neglected and the electrons leave the grid 3 at right angles and follow semi-circular paths between circuits 2 and 5 with constant transit time. To avoid breakdown due to the potential difference V between the cathode 1 and the grids 3 and 4 it may be necessary to space the electrodes farther apart, in which case the electrons no longer leave the grid 3 at rightangles and the trajectories are no longer semicircles. However, the valves of B and V may be adjusted to obtain an optimum value of the mean transit time for which the bunching phenomenon are substantially the same as when the electron trajectories are semi-circles. Other tubes are described in which the grids 6 and 7 are disposed in planes parallel to those of the grids 3 and 4 but nearer to or farther away from the plane of the cathode 1. Alternatively the grids 3, 6 and 4, 7 may lie on curved surfaces which may be either concave or convex towards the cathode 1. In these tubes the valves of B and V may again be so chosen that the transit time modulation due to the grids being in different planes compensates the effect due to the spacing between the cathode 1 and the grids 3 and 4, and the electron trajectories are once more semi-circles, but the electrons do not leave the grid 3 at right angles. Circular embodiments are described in which a number of cathodes are arranged in a circle either inside or outside a circular waveguide. Fig. 24 shows a circular oscillator arrangement having cathodes 1 arranged outside a waveguide 27. A short circuit 29 fixes the position of a voltage node of the standing field produced in the guide. A guide 12 serves as the output. In addition to the magnetic field B, a radial electrostatic field is produced by a cylindrical electrode 42 connected to a potentiometer 43 by means of which the direction of the field can be controlled. By adjusting the potentiometer the transit-times of the electrons can be made independent of their initial velocity and operation is as though the trajectories were semi-circles. Alternatively the transit times can be controlled by providing a non-uniform magnetic field. In the circular embodiments the pole pieces may be concave, convex or annular and in the linear arrangements may be curved so as to converge or diverge away from the cathode. Figs. 30 and 31 show a circular oscillator arrangement provided with non-uniform magnetic and electrostatic fields. The axial metal rod 42 is connected to an alternating supply 44 which sets up an alternating magnetic field, and a potentiometer 43 which maintains the rod at a positive or negative potential with respect to the envelope 14 and sets up a radial electrostatic field. The cathodes 1 are disposed at the apices of a regular polygon around guides 27 which are coupled by apertures 45. Each guide comprises a modulating circuit 2 and two collector circuits 5, 5<SP>1</SP> which are used alternately depending on the direction of the alternating magnetic field. The cathodes are unblocked by pulses from a source 46 which is synchronized with the source 44 and supplies a negative pulse for each positive or negative. peak of the voltage of the source 44. The ultra-high frequency fields of all the guides are in phase and energy is extracted through a single output 12.