Backward-Wave Tubes

It has been surmised for some time that a travelingwave tube in which backward-traveling field components can be excited-such as for instance the "Millman" tube-may oscillate in a backward mode, the RF power emerging at the gun-end of the tube and its frequency depending only on the beam voltage. Experiments with the "Millman" tube show this to be so and oscillations have been observed in the first and second backward spatial-harmonic modes. The latter is excited between 600 and 900 volts, the tube oscillating between 5.9 and 6.4 mm. The former more powerful mode is excited between 1600 and 4000 volts, the tube tuning continuously between 6.0 and 7.5 mm, thus covering a frequency band of 10,000 mc. Power output of about 10 mw has been measured at 6.4 mm. The tube has also been studied as an amplifier and more than 20 db stable backward gain has been obtained. A simple theory of backward gain and of oscillation starting conditions is given.