Amplification, current-voltage variations, and refraction in the interaction between millimeter-wave radiation and the glow-discharge plasma

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Abstract
The amplification of millimeter electromagnetic radiation within the cathode region of a cold-cathode glow discharge is correlated with the variations in the current-voltage characteristic of the glow discharge. The transfer of energy from the fast electrons to the electromagnetic wave amplifies the incident radiation and also causes a decrease in discharge current. The latter is called the "negative response." This negative response is superimposed on a "positive response"; i.e., an increase in current and decrease in voltage, which follows from the reduction of electron-ion recombination rate by the incident electromagnetic radiation. The negative response was extracted from the measured current-voltage variations, and the measured amplification is shown to be proportional to the negative-response voltage multiplied by the electron density. Divergence of the incident electromagnetic wave by the plasma (whose refraction index is smaller than one) was measured and compared with the theory. The effect of this divergence on the measurement of amplification is discussed. The theory of stimulated emission of bremsstrahlung has been developed to explain the above experiments. Two methods of calculations are compared: one based on differences in populations of levels, as is conventional in laser physics, and the second method which exploits the dependence of the scattering cross section on the incident electron energy. Possible improvements of the theory are discussed.