A Backward-Wave Surface Mode in a Plasma Waveguide

The phase characteristics of a backward-wave pass-band mode have been measured in a plasma waveguide, consisting of a circular waveguide that is coaxial with the positive column of a mercury-vapor discharge. The mode has been identified as an n = 1 surface mode similar to that described by Trivelpiece. It is shown that zero-temperature plasma-waveguide theory predicts this mode accurately. This theory predicts that a simple relation exists between the lower cutoff frequency of the n = 1 mode and the electron plasma frequency. It is demonstrated that the calculated electron plasma frequency obtained from this relation, where the cutoff frequency is determined from a measurement of the phase characteristics, agrees well with an independent measurement of electron plasma frequency by the well-known cavity-perturbation method. Both the n = 1 and symmetric n = 0 modes were excited by a novel coupler called the double-ring coupler. Noise measurements on the n = 1 and symmetric n = 0 modes indicate that the output signal-to-noise ratio is independent of input signal power and is about 7 dB for the n = 1 mode and varies from about 7 to greater than 30 dB for the n = 0 mode. A simple model of the plasma is proposed which explains all the noise observations reported here. Crawford and Lawson independently proposed a similar model.