Low-Density Plasma Flow at a Magnetic Barrier

Abstract

The encounter of a low‐density stream of plasma with a barrier in an otherwise uniform axial magnetic field is treated theoretically on the basis of particle dynamics. Conditions are considered for which the magnetic moments which result from the transverse velocities of the particles are adiabatically invariant within the barrier region. At the barrier there is an increase of magnetic field caused by external currents and increase in electric potential φ caused by charge separation in the plasma. Within the barrier the electric potential brings about an exchange of energy eφ between ions which carry the kinetic energy of the stream and electrons so that both may penetrate in equal number. The penetration of the barrier, the force on it, and the change of electric potential in it are given in terms of the ratio of barrier to guiding‐field, stream velocity, and the temperatures of the ions and electrons. A comparison is made with results which have been obtained for an encounter in which the barrier rises so rapidly that the moment of the ions is not adiabatically invariant and for which the Rosenbluth‐Garwin sheath model has been employed. This model is appropriate to cases in which a rapidly varying current flows on the surface of the plasma (skin effect) as in the pinch effect. Much greater penetration takes place with the slowly varying magnetic field which may be encountered when the field is produced by current‐carrying conductors external to the plasma.