Patent GB-729676-A (GB)
Title: Electromagnetic wave generator
IPC: H01J25/00
Filed: 1953-01-09
Abstract: 729,676. High-frequency discharge apparatus. STANDARD TELEPHONES & CABLES, Ltd. Jan. 9, 1953 [Feb. 5, 1952], No. 718/53. Class 39 (1). [Also in Group XL (b)] An electromagnetic generator of millimetre waves comprises a section of hollow waveguide or a hollow resonator, shaped to sustain standing electromagnetic waves having a given mode of oscillation, means for providing within the guide or resonator a unidirectional magnetic field perpendicular to the electric vector in the said mode, means for causing electrons to travel in the said guide or resonator in the same or opposite direction to the said magnetic field with a superimposed angular motion and at a density such that the plasma frequency of the said electrons lies within the band of frequencies transmitted or sustained, respectively, by the waveguide or resonator, and means for extracting the electromagnetic wave energy generated by oscillation of the said electrons at the said plasma frequency. In one embodiment, Fig. 1, a permanent magnet 2, or alternatively an electromagnet, establishes a substantially uniform unidirectional magnetic field across a rectangular waveguide section 1 closed at one end 1a. An electric discharge is established between electrodes 3 and 4 which are mounted in the walls of the waveguide and insulated therefrom at 3a and 4a. Alternatively, if suitable windows are provided in the waveguide walls, the electrodes may be part of the pole faces 2a and 2b of the magnet, in which case an alternating voltage must be applied to excite the discharge. The centres of the electrodes 3 and 4 may be one or an odd multiple of quarter wavelengths from the closed end 1a of the waveguide or may be other distances therefrom for loadmatching purposes. The interaction volume, i.e. the portion of waveguide 1 between the magnet pole faces 2a and 2b, is sealed off by walls 5 and 6 of dielectric material such as glass and is filled with a gas such as helium or a mixture of neon and argon at a pressure in the range of 10-<SP>3</SP> to 10 mm. of mercury. The discharge between the electrodes 3 and 4 establishes a dense plasma, the electrons of which tend to oscillate at the plasma frequency fp, and due to the presence of the magnetic field oscillate also at the gyromagnetic frequency fH. When the frequency fH as determined by the intensity of the magnetic field is substantially equal to the plasma frequency fp, the electron oscillations are exceptionally strong and electromagnetic energy at substantially the gyromagnetic frequency and harmonics thereof is radiated into the wave-guide portion 1b to which further waveguide sections and filters can be added to select the desired frequency. To ensure that electrons traversing the wave-guide between electrodes 3 and 4 will not just follow a straight line path but will have some component of oscillating angular motion, auxiliary electrodes 7 and 8 may be provided to create an electric field perpendicular to the electron path and the magnetic lines of force. In this case a strong output is obtained, whether or not there is resonance between fp and fH, and the gas filling may be omitted and an electron-emissive cathode utilized to provide an electron beam in vacuum between electrodes 3 and 4. The energy radiated into the wave-guide section 1b may be termed an " end-fire " radiation, and in cases where the " broadside." radiation is the greater a modified apparatus may be utilized, in which the section of waveguide comprising the interaction volume is arranged at an angle to the lines of force of the magnetic field. The electrodes 3 and 4 are arranged at an acute angle to the longitudinal axis of the waveguide so that the electric field is still parallel to the magnetic field. Fig. 3 shows a vacuum tube arrangement in which a cylindrical cavity section 15 is surrounded by an electromagnet 16 having end plates 17 and 18 of high permeability material. A beam of electrons formed by a cathode 19 and focusing electrode 20 is projected through an aperture 22 in the plate 17 across the cavity 15 to an anode 23 insulated from end plate 18 by a dielectric ring 24. Auxiliary electrodes 27 and 28 provide a D.C. electric field transverse to the electron beam. Strong electromagnetic waves produced at resonance of the gyromagnetic frequency with the R.F. cavity frequency are coupled to a load by the loop 29. In an alternative arrangement a negative potential is applied to the electrode 23 which then forms a repeller electrode and repels the electron stream from cathode 19 so that an electron cloud is formed in the cavity 15.