Turbulence in the Positive Column

An oscillation that is helical in shape is known to be present when an axial magnetic field applied to the positive column of a glow discharge is greater than the critical magnetic field Bc for the onset of the current convective instability. According to a theory proposed by Kadomtsev, this oscillation will be replaced by a turbulent spectrum of oscillations when the field is several times Bc. In this experiment with a helium plasma the magnetic field has been raised to as much as 20 times Bc to determine if and to what degree random oscillations characteristic of strong turbulence will replace the helical instability. It was found that strong turbulence dominated the positive column when the magnetic field was greater than 15 times the critical field. The radial mixing length as determined from spatial correlations with movable Langmuir probes and the radial density profile were found to agree with the results calculated from Kadomtsev's theory for strong turbulence. Particle transport rates calculated from the theories of Dupree and Kadomtsev for a turbulent plasma agree within a factor of 2 with the values deduced from this experiment.