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In the past a considerable amount of experimental evidence presented by various workers has indicated that gain apparently occurs in a traveling-wave type device even though the voltage may be so high that growing waves cease to exist. This means, in terms of Pierce's traveling-wave tube theory, that the growth constant of the growing wave x1 is zero in this regime of operation. A theory explaining this phenomenon has been worked out and both small-signal and large-signal calculations have been carried out to investigate the characteristics of this type of operation. The gain occurs in this region due to a beating effect produced between the three small-signal forward waves described in traveling-wave tube theory as they travel along the RF structure. The maximum achievable gain is determined by the injection velocity and not by the length of the tube as in the normal case. Based on the above theory a device named the Crestatron, which utilizes this new mode of operation, has been built and tested to verify the theory, and it has been found that moderate gain (10-20 db) and high operating efficiency coupled with a very short length (4-6 wave-lengths) characterize this mode of operation.