The ∇B drift velocity of trapped particles in stellarators

Abstract

In recent times, procedures for optimizing stellarator configurations have been strongly improved. New ideas and the development of pertinent codes allow an inclusion of various target functions into this optimization procedure. Here, new target functions which are related to collisionless α-particle confinement are introduced. They are based on specific averages of the bounce-averaged ∇B drift velocity of trapped particles across magnetic surfaces. This approach accompanies the frequently used computation of the effective ripple and it can be realized in a similar way using a field line tracing code. In addition, the poloidal motion of trapped particles is described in detail and results are discussed in the frame of an optimization study which aimed for a closure of contours of the second adiabatic invariant.