On the resonant frequencies of a reactive structure coupled to a warm plasma

The resonant frequencies of a v.h.f. Lecher line coupled to a mercury arc plasma via a split-cylinder coupler have been measured as a function of electron density. Two strong resonances have been found for every value of electron density. The higher frequency resonance may be described as circuit resonance perturbed by the plasma, the lower one as plasma resonance influenced by the circuit. Cold plasma theory predicts qualitatively many of the observed properties of these resonances, including the appearance of upper and lower frequencies. It also predicts a forbidden frequency gap between the lower and upper frequency curves. It fails to predict the behaviour of the lower branch curves near their horizontal asymptote. Experimentally the curves approach this asymptote from above, whereas cold plasma theory predicts that the asymptote should be approached from below. Since the average transit time of thermal electrons is comparable with the period of the v.h.f. oscillations in this experiment, the results have been compared with warm plasma theory. It was found that warm plasma theory gives the correct behaviour near the lower branch asymptotes, and compares well with the experimentally measured resonant frequencies.