Ohmic heating in the W VII A-stellarator

First results of experiments with Ohmic heating in the W VII A-stellarator are reported. At B0=2.5T, a comparison of discharges with large (rotational transform0=0.23) and small (rotational transform0=0.055) rotational transform of the external helical field is made. In the parameter regime, nsub(e)=5X1018-6X1019m-3, Tsub(e)=200-900eV, Tsub(i)=100-220eV, energy confinement and MHD-instabilities are investigated. The energy density nT follows the scaling nT approximately rotational transform Jsub(p)B0 (Jsub(p)=plasma current, B0=toroidal field), the energy confinement time increases with nsub(e) and Tsub(e). Disruptive instabilities do not occur in discharges with large helical fields (rotational transform0=0.23). Gas puffing during the discharge leads to higher densities (nsub(e)0 approximately 8X1019m-3), low Zsub(eff) and heating of the ions. Internal relaxations are not only observed on X-ray signals but also on various other diagnostic signals outside the plasma. (author)