On the motion of trapped particles in a vintotron

Abstract

The motion is investigated of particles trapped in the vacuum magnetic field of a vintotron, i.e. a plasma trap of the stellarator type with a spatial magnetic axis whose magnetic configuration consists of a strong longitudinal field, together with the field of helical windings plus that of a ring current, or of a strong longitudinal field, together with the field of helical windings with non-symmetrical currents. Attention is chiefly devoted to the motion of particles trapped by inhomogeneities of the helical field. It is assumed that the inhomogeneity of the magnetic field due to the toroidicity is considerably smaller than that caused by the helical windings. Using an integral of the drift surfaces, the longitudinal adiabatic invariant and the equation of motion of the guding centre of the particle expressions are obtained for the departure of the trapped particle orbits from the magnetic surfaces, the period of oscillation of the trapped particles between the magnetic mirrors, and the period of oscillation of the orbits of the trapped particles ('bananas') with respect to the magnetic surfaces. Over a wide range of the vintotron parameters, these quantities, together with the coefficient of diffusion due to trapped particles, are of the same order as those for the stellarator with small toroidicity.