Recent experiments on the behaviour of plasma in the theta pinch have shown that, although the plasma in reversed-field discharges is normally unstable, there are some cases in which the plasma is stable and contained. The present paper considers the following hypothesis: (a) that the mode of instability commonly encountered is that observed by Clark and Wuerker (in which the axis of the magnetic moment of the current supporting the trapped reversed field appears to rotate from its initial state of alignment with the field axis) and (b) that there exists a class of configurations of the plasma enclosed by the field lines that are stable against this rotation of the plasma axis.This class of configurations is characterized by a high length-to-radius ratio and by having both radial and axial pressure balance. Under these conditions the trapped field acts only to determine the plasma area and does not contribute significantly to the total energy of the system. Rotation of the plasma axis then leads to an increase of the total energy of the system, so that the plasma is stable.It is shown that the few observations of stability are consistent with plasma configurations of this type.
Plasma stabilization by a high-frequency rotational field