Studies of fast ion outflow to the helical divertor of the U-3M torsatron

Abstract

In the l=3 Uragan–3M torsatron with RF-produced and heated plasmas (ω ≲ ω_ci), a two-temperature ion distribution with a suprathermal tail is formed. Faster ions (FIs) can be the characteristic of reactor-size stellarators in the long mean free path regime. The presence of the helical divertor offers new opportunities for studying FI loss by measuring ion fluxes and energies in the divertor plasma.

Ion energy distributions were measured in divertor flows in two symmetric poloidal cross-sections in several field periods. It is shown that FI flows out to the divertor mainly on the ion B×∇B drift side in accordance with the assumptions of a determinative FI contribution to the plasma divertor flow vertical asymmetry inherent to torsatrons/heliotrons.

Strong toroidal non-uniformities in flows and energies of ions outflowing into the divertor are observed. The island structure of the U-3M magnetic configuration and locality of RF power injection are considered as possible reasons for these non-uniformities.

Keywords:

• torsatron
• helical divertor
• fast ions
• divertor flow vertical asymmetry
• direct ion loss
• toroidal drift

Acknowledgements

The authors are deeply grateful to I.M. Pankratov, V.S. Voitsenya and participants of the Seminar on Stellarators of Institute of Plasma Physics NNC KIPT for detailed and fruitful discussion of the work and helpful comments. Also, we thank the technical personnel of the U-3M torsatron for ensuring reliable device operation.