The authors have studied the stability of helical modes in a current carrying stellarator, ignoring the curvature of the axis and plasma pressure. They show that it is possible to fully stabilize a current column with a given arbitrary current value against both ideal and tearing modes by superimposing an external stellarator rotational transform with a sufficiently large value of its difference along the radius, |Δth| = |th(a) − th(0)|, in configurations of two types, namely with the total rotational transform increasing or decreasing with the radius. The value of the difference |Δth| needed for stabilization increases with the value of the total current.

A description is given of theoretical and experimental research into the possibility of using, an automatically regulated transverse field to increase the gas-kinetic pressure limit of the plasma in an fi = 3 stellarator. In terms of a simplified theoretical model the various laws governing the control of a transverse field by means of feedback are analysed. It is shown that a control system based on magnetic measurements can provide the accuracy required for maintaining an equilibrium close to optimum. Experimental data relating to equilibrium effects obtained on the Uragan stellarator are in close agreement with theory.

An attempt is made to find an optimal quasi-helically symmetric (QHS) four period magnetic configuration with a Heliac-like structure of the magnetic surface cross-sections. This means that the cross-sections rotate in phase with the magnetic axis principal normal in contrast to the Helias-like systems where the magnetic surface cross-sections rotate half as fast as the principal normal. The equilibrium and local stability are studied with the VMEC and TERPSICHORE codes. It is shown that, in the configuration found, the equilibrium beta limit is about 9%, the Mercier beta limit is about 4% and the ballooning modes restrict the beta value to 1%. The accuracy of the fulfilment of the QHS condition is given by X~3 (i.e. the largest amplitude of the magnetic field strength B harmonic that violates the quasi-symmetry is a third that of the main helical harmonic of B in Boozer co-ordinates)