On the origin of rotation in magnetically confined plasmas

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Abstract
The main part of this paper surveys the mechanisms that can lead to rotation of plasma in an electrodeless discharge (commonly called a theta pinch) and in a mirror machine. Eight proposed mechanisms are examined, of which two are found to be incorrect in some way. Another mechanism, in which the reaction is purely electromagnetic, applies only to the tenuous plasma of a mirror machine. The remaining five theories apply to the highly compressed plasma of a theta pinch. These all rely fundamentally on the large difference in mass between an ion and an electron, i.e. either on Hall currents or the correction for finite ion Larmor radius in the stress tensor. The mechanisms which incorporate the Hall effect cause an equal and opposite mechanical reaction either on the wall (Roberts and Taylor), or on external conductors (Haines), or on the plasma at each end of the discharge axis (Bostick). The mechanisms which use the collisionless stress tensor predict either radial division into oppositely rotating plasma (Velikhov) or a transfer of angular momentum to the walls by collisions during the initial stage of physical contact (Haines). This last theory is new and arose as a result of a critical examination of an earlier theory (Jensen and Voorhies). Experimental evidence for rotation has been confined mainly to high speed photographic techniques, and methods of obtaining a more definite measurement of the rotation and its origin are suggested. The paper distinguishes carefully between the angular motions associated with the centre of mass, the guiding centres and the diamagnetic current, and also considers the stability of a rotating plasma including the Hall effect and the collisionless stress tensor.