Control of Pfirsch-Schluter current by an external poloidal magnetic field in conventional stellarators

Abstract

The problem is analysed of whether it is possible to create net-current-free equilibrium configurations without Pfirsch-Schluter current (jzeta =0) in conventional stellarators with a planar circular axis. Recently, such an analysis has been presented for the case when only the vertical magnetic field was a control parameter. Here, the external poloidal field is considered as arbitrary, and it must be found from the condition jzeta =0. It is shown that such suppression of Pfirsch-Schluter current by means of an external poloidal field is possible, theoretically, in shear-free systems and in stellarators with l>or=3 only, i.e. in ordinary l=2 stellarators with a shear plasma equilibrium with jzeta =0 it is impossible with any choice of external poloidal field. In these systems, nevertheless, the dipole component of Pfirsch-Schluter current can be suppressed or strongly reduced by the joint effect of vertical and quadrupole fields. It is shown that in this case phase inversion of the Pfirsch-Schluter current could be obtained under realistic conditions. These results explain, in particular, experimental observations of configurations almost insensitive to plasma pressure in Heliotron E