A Way to Increase the Critical Current in the Stainless Steel Jacketed Nb₃Sn Conductors for Fusion

Abstract

The critical current density of Nb₃Sn under magnetic field is highly dependent on the intrinsic strain ε, having a maximum value for ε=0 The performance of stainless-steel jacketed conductors is usually lowered by the differential thermal expansion between steel and the superconducting filaments (J is divided by 2 for B=12T, T=5.5K, and ε=-0.7%). This is due to the longitudinal compression appearing on the superconducting filaments during the cool down from the reaction temperature to 4K. To overcome this difficulty, in the reference design of ITER (International Thermonuclear Experimental Reactor), the conductors are jacketed with Incoloy. A progressive test program is foreseen to demonstrate the possibility to reduce the filament compression in conductors jacketed with A3 16 LN. This process is based on the use of a low thermal expansion material for the supporting tools during the reaction phase. The performance level of such a reacted conductor could, in principle, be similar to a conductor jacketed with Incoloy.