This article is devoted to problems of stability in a plasma column which has a hybrid current (constant and variable in time) flowing through it. The variable component of the current is, generally speaking, a helical multipole with zero skin layer. The constant current component exhibits either a skin effect or a uniform distribution over the column cross-section. The perturbations considered here are of the surface-wave type, since it is known that this type of perturbation plays a major role in a current plasma with a free boundary.Analysis of the dispersion equation obtained indicates that the application of high-frequency multipole fields reduces the critical q-value in current systems. From the physical point of view, stabilization is brought about by the presence of a time-averaged magnetic well and by the effect of dynamic shear. It was found that the effectiveness of stabilization increases as the multipolarity of the high-frequency field, a fact which is explained by increased depth (on average) of the magnetic well. This requires an increase in the radio-frequency circuit currents, however, which means that circuits with a high Q-factor must be used.The authors also show to what extent the high-frequency circuit (helical conductors) can assume the role of a stabilizing sheath. In the model adopted here the circuit is taken to be an anisotropically conducting sheath and the helical pattern is reflected; perturbations are found on which the circuit exercises a stabilizing effect. These are perturbations whose helix coincides with that of the high-frequency circuit currents projected on a cylindrical plasma surface of radius “a”. All other perturbations remain unaffected by the circuit: the perturbed currents do not form a circuit because the helices of the perturbation and the circuit are crossed.

The influence of ion-acoustic instability on the penetration of a high-frequency field into a plasma is investigated. The slowing-down of the plasma electrons as the instability develops is taken into account by means of a non-linear Ohm's law jz = j0 [1-exp(-αE)] which enables one to consider continuous variations of the depth to which the field penetrates the plasma as a function of the parameter α = E0/Ek, where E0 is the peak electric field value and Ek the field value at which ion-acoustic instability occurs in the plasma. The depth of penetration by the field into the plasma is determined as a function of α. For large α the penetration depth increases by a factor α½ relative to the skin layer thickness of the linear theory (where α ≪ 1).