Experimental study of scattered waves and cylindrical antenna sheaths in a microwave-driven plasma

A short-wire antenna is immersed in a quiescent, weak-gradient underdense plasma. The system is irradiated at low power with 1.2 GHz microwaves. Spectra received from the antenna show three strong down-scattered frequencies: the half-harmonic of the driver and a pair of frequencies equidistant above and below the driver. These frequencies are described as the symmetric pair. Growth and decay of these signals, their variation with the plasma density, and the effects of the antenna sheath are investigated with a view to distinguishing among various processes suggested for their generation. Two hypotheses in particular were tested: (1) can two-plasmon decay be distinguished from parametric sheath instability by controlling conditions in the probe sheath? (2) Is the high-frequency peak of the symmetric pair an electromagnetic wave, confirming it as Raman scattering? Based on the experiments reported here, it is postulated that gradient-driven two-plasmon Decay in the sheath and/or presheath of the probe is the source of the half-harmonic signal. The extreme plasma-density gradients found in a sheath offer high-growth-rate conditions for this parametric instability. The observed threshold lies between the thresholds calculated for the cases of collisional and convective losses. The very-low minimum density suggests that Landau damping is suppressed in the sheath