Energetic particle transport in compact quasi-axisymmetric stellarators

Hamiltonian coordinate, guiding center code calculations of the confinement of suprathermal ions in quasi-axisymmetric stellarator (QAS) designs have been carried out to evaluate the attractiveness of compact configurations w hich are optimized for ballooning stability. A new stellarator particle fol lowing code is used to predict the confinement of thermal and neutral beam ions in a small experiment with R=145 cm, B=1-2 T and for alpha particles i n a reactor size device. As for tokamaks, collisional pitch angle scatterin g drives ions into ripple wells and stochastic field regions, where they ar e quickly lost. In contrast, however, such losses are enhanced in QAS so th at high edge poloidal flux has limited value in improving ion confinement. The necessity for reduced stellarator ripple fields is emphasized. (C) 1999 American Institute of Physics. [S1070-664X(99)00409-7].