Improvement of Magnetohydrodynamic Stability and Beta Limit by Toroidal Plasma Current in Heliotron DR

The effect of toroidal plasma currents on magnetohydrodynamlc stability has been studied for the currentless ECH plasmas (n̄ e =(2–3)×10 19 m -3 , T eo =300–400 eV, <β>=0.35–0.5%) in Heliotron DR ( R =0.9 m, a p ∼0.07 m, B =0.5–0.6 T). It is shown that small plasma currents (500 A) which reduce vacuum rotational transform improve the MHD stability. The experimental result is explained theoretically by the change of the magnetic configuration due to the plasma current (position of =1 surface, shear, well).