Experiments on a Toroidal Screw Pinch with Various Field Programming

In the toroidal screw pinch ISAR-IV (large diameter 60 cm, aspect ratio 5, maximum storage, energy 140 kj) attempts were made to get an improved stability of the plasma by different kinds of field programming. The best results were obtained with positive trapped Bz-fields and simultaneous switching of main Bz-field and Iz-current. In this case the dense plasma column (ne ≈ 2-3 x 1016 , kT ≈ 50-100 eV, β ≈ 15-20%) is surrounded by a force-free plasma (β = 1%) with weak shear and it behaves stably for, at least, 25 μs. The resulting containment time nr of near 1012 s cm-3 remains a factor of 2-3 below the upper limit given by the classical diffusion. The following loss of the equilibrium position near the coil axis (Δ ≈ 1-2 cm) is connected to a strong damping of the axial plasma current which starts near the end of the containment. It may be assumed that the increase of the effective plasma resistance mainly results from a contact of the force-free regions with the tube wall. Attempts were made to improve the containment by suitable programming of a plasma z-current. The results are presented. Experiments with one quartz limiter inside the torus improved the equilibrium but introduced instabilities at the new surface of the dilute plasma. To obtain more information about the outer region, the dilute plasma was produced without a dense core and separated from the tube walls by weak adiabatic compression. Under these Tokamak-like conditions the q-value was varied. In the region of q ≈ 1 there appeared instabilities which seem to haver higher m-modes and rather short wavelengths. In a different kind of field programming the field distribution of the ''diffuse pinch'' was realized within an accuracy of 5-10% (kT ≈ 100 eV, β ≈ 30%). In contrast to the predictions of MHD-theory, stability was observed only for about 3 μs. Then the plasma is totally destroyed, presumably by resistive instabilities. (author)