Nonlinear Interaction of an Electromagnetic Wave with a Plasma Layer in the Presence of a Static Magnetic Field. IV. Experimental Results

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Abstract
The nonlinear interaction of an electromagnetic wave with a uniform, weakly ionized anisotropic plasma layer has been investigated experimentally. The experiment was designed to comply as closely as possible with the assumptions used in the theory of the nonlinear interaction, which is given in parts I, II, and III of this series of papers. Experimental results on sum and difference frequency mixing and harmonic generation are described as a function of the ambient electron density, the electron-neutral particle collision frequency, the external dc magnetic-field strength, thickness of the plasma layer, and the field strengths and frequencies of the incident waves. The case of linearly polarized, small-amplitude microwave signals incident on a layer of helium plasma is examined. Within the plasma, the signals propagate as plane waves with their directions of propagation and polarization normal to the dc magnetic field (i.e., the extraordinary mode of propagation). A detailed comparison is made between the experimental results and the theoretical predictions. Quantitative agreement was obtained over the electron density range permitted by the experiment. The results show that the plasma model assumed in the theory is an accurate representation of the actual plasma, and that the small-signal analysis accurately predicts the effects of the nonlinear terms in the Boltzmann equation on propagation phenomena. However, special care is required to insure that the plane-wave, infinite-medium assumption employed in the theory is satisfied experimentally. The experimental results also establish the validity of using certain characteristics of the nonlinear phenomena as a tool for measuring the electron density in a plasma as proposed in II and III. A resonance was detected in the sum frequency wave which is not predicted by the theory. This resonance occurs for values of the dc magnetic-field strength corresponding to cyclotron resonance at the arithmetic mean of the two incident frequencies. The origin of this resonance is not understood at this time.