The Positive Column in a Longitudinal Magnetic Field

A magnetic field parallel to the axis of the positive column of a discharge is known to impart a rotary component on the radial motion of electrons. This reduces the flow of both positive ions and electrons to the wall, causing a corresponding decrease in the radial and longitudinal components of the electric field and in the electron temperature. It is shown here that under certain conditions Langmuir probes can be applied to explore a plasma in a magnetic field. A new method of measuring the concentration of charges is used: first relative values of ion concentration are obtained from the ion saturation currents in various magnetic fields. This result together with the electron saturation currents in zero magnetic field gives absolute values of concentration of charges in a magnetic field. Such measurements have been carried out in helium at low pressures. The effect of the magnetic field on the radial distribution of charges, the rate of ionization, the electric field, the electron temperature, the average energy loss in a collision, and the radial potential distribution is discussed. It is concluded that in zero magnetic field the Langmuir theory of free ion fall describes best the properties of the discharge plasma whereas in a magnetic field of sufficient strength Schottky's theory of ambipolar diffusion applies. When the gas becomes highly ionized, the effect of the partial pressure of the electron gas may become important so that finally both the longitudinal component of electric field and the electron temperature should become independent of the magnetic field. Previous observations of a low pressure positive column in caesium at high current densities seem to support this view.