Effect of a double-shell structure on the reduction of field errors in the STP-3(M) reversed field pinch

In the STP-3(M) reversed field pinch, a thick conducting shell is surrounded coaxially by a one-turn poloidal coil, which also acts as a shell. Field errors at the shell gap in this double-shell structure are shown to be an order of magnitude smaller than those in a conventional shell structure. Reduction of the field errors resulted in a remarkable improvement of the plasma parameters through control of the average plasma position with a quasi-stationary vertical field, without any compensation of local field errors. (author). Letter-to-the-editor. 16 refs, 7 figs