We show that previous calculations of the loss rate of ions from toroidally symmetric line cusps need substantial revision. The symmetry of the toroid produces a conserved generalized momentum which prevents a certain class of particles from penetrating the mirrorlike magnetic fields which exist above the cusp points. The importance of this reduction in loss rate is explained for several geometries including Tormac which can now have a loss rate substantially smaller than a mirror device.

• Received 12 January 1972

DOI:https://doi.org/10.1103/PhysRevLett.28.1323