[en] Superconductors for the helical coils of the Large Helical Device (LHD) must simultaneously satisfy the requirements of high current density and sufficient stability. An Nb - Ti/Cu superconductor with pool boiling is selected on the basis of mechanical toughness against the large electromagnetic force and the feasibility of the complicated helical winding. Several conductors with large current capacities from 20 to 30 kA have been designed and proposed. Scaled-down R and D conductors with operational currents from 7 to 10 kA were made on an experimental basis with the aim of studying how a difference in the position of pure aluminium in the conductors affects the stability and mechanical properties. Details of the design and the test results of the R and D conductors are described. (author)