Alpha-particle losses in compact torsatron reactors

Abstract

Loss of alpha particles in compact torsairon reactors is studied. For 6,9,and 12 field period reactors, the direct loss is a relatively weak function of radius and energy and varies from 2= S3 % for M = 6 to ?z 18 % for M = 12. Loss of alpha particles through scattering into the loss region ts calculated using the Fokker-Plank equation for fast ions and found to contribute an additional alphaparticle energy loss of cz 15 %. The consequences of these relatively large losses for torsatron reactor design are discussed.The relationship between the direct particle losses and the magnetic field structure is also studied. Orbit losses from a variety of stellarator configurations are calculated and a figure-of-merit that characterizes the orbit confinement of a magnetic configuration is deduced from these calculations. This figure-of-merit is used to show how the direct losses might be reduced at low aspect-ratio. Effects of finite beta on the direct particle losses are also addressed, and are shown to significantly increase the direct losses in some configurations.The compact torsatron sequence 1 is a family of low aspect-ratio 1 = 2 torsatron configurations optimized for high-beta operation. These configurations make interesting reactor candidates because they have the capablity for high-beta operation in the second stability regime, possess natural divertors, and have relatively open coil geometry for access to the plasma. Compact torsatrons are, however, optimized for MHD properties, not orbit confinement. A large fraction (== 5) of particles in these devices are trapped in the relatively large helical ripple in the magnetic field strength B. Toroidal effects at low aspect ratio cause orbits of these trapped particles to deviate significantly from flux surfaces leading to increased transport and in some cases to loss of the particles from the confinement region. Radial electric fields which develop to maintain quasineutrality of the plasma can reduce the deviation of the trapped particles from the flux surface through E x B orbit rotation, but only for those particles which have kinetic energies less than their potential energies