Helical field stabilization of plasma devices

Abstract

A toroidal plasma device has a toroidal confinement vessel defining a toroidal space and confining ionized gas therein. A solenoid which links the toroidal space induces a toroidal electric field therein to produce plasma current. A plurality of first windings are wound substantially helically around the vessel substantially equally spaced around its minor circumference. A plurality of second windings are wound substantially helically around the vessel substantially midway between successive first windings. Direct current is passed through the respective first and second windings in opposite directions with the current in the respective first and second windings equal or slightly unbalanced. The currents in the first and second windings produce a helical magnetic field. The combination of the poloidal magnetic field from the plasma current with this helical magnetic field produces a separatrix in the toroidal space, this separatrix defining a closed surface which limits and encloses a region within which closed and nested magnetic flux surfaces exist. The sense of rotation of the first and second windings and the direction of the plasma current produces a variation in the safety fractor q with minor radius at any poloidal angle, whereby the sign of q reverses near the outer edge of the plasma, q being an average over a flux surface of the number of transit made around the torus in the toroidal direction by a magnetic flux line in making a single transit in the poloidal direction. The sign of q is determined by the sense of the direction in which the toroidal transit is made.