The positive column of a D.C. dicharge becomes unstable when a longitudinal magnetic field exceeds a critical value Bc. The measurements have been extened, with emphasis on the properties and growth of the instability, to permit a quantitative comparison between experiment and the theory of Kadomtsev&Nedospasov. The oscillating floating potentials were observed through the Langmuir's probes, of which tips were spaced at the tube will. The properties of the helical state have been measured in He gas as functions of magnetic field and discharge current. The frequency and the wave length are in fairly good agreement with the predictions made by the theory of kadomtsev & Nedospasov.
Measurements of the electron density and electron collision frequency of cylindrical plasma at free space by Microwave transmission are reported. The electron density and collision frequence were obtained from the transmitted microwave phase shift and attenuation with the microwave interferometer bridge. The apparent phase shift and attonnation measured under the same plasma condition were varied with secondary factors such as e.m. horn-plasma. coupling condition. it was found. that the error seems to be due to the effect of scattering, and diffraction. The error in phase shift was found to depend on the relation between the plasma tube axis and the, polarization plane of the electromagnetic wave. The electron density in cylindrical plasma at free space obtained from microwave phase shift is in fairly good agreement with the value obtained from electron collecting Langmuir probe if suitable considerations are paid for microwave horn-plasma geometry.