Effects of the radial electric field on the confinement of trapped fast ions in the Wendelstein 7-X and Helias reactor

Abstract

Confinement of trapped fast ions in the Wendelstein-line stellarators in the presence of the radial electric field, Er, is studied. It is shown that negative electric field improves the confinement; in particular, a radially localized field can play the role of a transport barrier for the ions escaping from the plasma when Er=0. In contrast to this, the positive electric field tends to deteriorate the ion confinement, unless its magnitude is very large. Such a field accompanied by the plasma rotation with the frequency around a certain magnitude, which we refer to as the resonance rotation frequency, leads to a quick particle loss. A possibility of using the plasma rotation with the resonance frequency for the ash removal in a Helias reactor is considered. The mentioned results are obtained analytically and numerically. The analytical consideration was done on the basis of the derived bounce-averaged equations of the particle motion. The numerical calculations were carried out for Wendelstein 7-X [G. Grieger et al., J. Plasma Fusion Res. Series 1, 53 (1998)] and a Helias reactor [J. Kisslinger et al., Proceedings of the 17th International Conference, Yokohama, 1998 (1999), Vol. 4, p. 1239] by the guiding center code ORBIS (ORBits In Stellarators) developed in this work.