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An electron injected into a longitudinal magnetic field with some initial transverse motion will rotate at the cyclotron frequency in the transverse plane. If an RF electric field, which is polarized in the transverse plane and which oscillates at the cyclotron frequency, is present, a cumulative energy interchange between the electron and the field will occur. This principle of cyclotron resonance interaction has been applied to the construction of a backward-wave oscillator and amplifier which requires no slow-wave circuit. The elimination of the slow-wave circuit provides several advantages, particularly if one is interested in the generation of energy at millimeter wavelengths. At these short wavelengths, it becomes extremely difficult to fabricate a slow-wave circuit, and, in addition, the interaction is weak because the fields decay exponentially away from the conducting surface. The present tube has approximately 100 times the transverse area of a helix operating at a comparable beam voltage and has strongest RF fields in the region of the electron beam. The tube has been constructed for operation at S band, and the circuit consists of sections of standard S-band rectangular waveguide with a field pattern of the TE10 mode. An unbunched beam with some initial transverse motion is injected into a constant, longitudinal magnetic field. Bunching occurs in the interaction region, and, for sufficiently high beam current, oscillation power is produced at the gun end of the tube.