By quantization of wave fields in a moving medium we have quanta of negative energy when the velocity of the moving medium exceeds the phase velocity of a wave in the moving medium. Wave amplification phenomena caused by negative quanta are classified in four types, to each of which several kinds of known amplification phenomena or instabilities are attached, e.g. a traveling‐wave tube, two‐stream instability, ultrasonic wave amplification in a CdS crystal, roton excitation in a superfluid helium, beam‐plasma instability, etc. A new type of amplifier, an inverse maser, is proposed.
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This type of ultrasonic wave amplification is often said to be a solid‐state analog to the traveling‐wave tube. Amplification with the traveling‐wave tube, however, is due to mode coupling between the plasma wave in an electron beam and the electromagnetic wave in a helix. On the contrary, in the ultrasonic wave amplification in a CdS crystal, there is no propagating wave to couple with the ultrasonic wave. Consequently, both amplification phenomena are different in principle.
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© 1964 The American Institute of Physics.
1964
The American Institute of Physics
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