Development of an experiment to study the effects of transverse stress on the critical current of a niobium-tin superconducting cable

Abstract

Superconducting magnets will play a central role for the success of the International Thermonuclear Experimental Reactor (ITER). ITER is a current driven plasma experiment that could set a milestone towards the demonstration of fusion as a source of energy in the future. Cable-in-Conduit is the typical geometry for the conductor employed in superconducting magnets for fusion application. The cable is composed of over 800 strands. Once energized, the magnets produce an enormous electromagnetic force defined by the product of the current and the magnetic field. The strands move under the effect of this force, and the force accumulates against one side of the conduit thereby pressing transversally against the strands. The experiment proposed here has the goal of assessing the functionality of the apparatus designed to study the effect of transverse load on a cable composed of 36 superconducting strands (with a 3x3x4 pattern) by mechanically simulating the ITER Lorentz stress condition. The apparatus was assembled at MIT and preliminary tests at 77 K and room temperature were made to improve the design prior to carrying out the actual experiments. These were done at the National High Magnetic Field Laboratory (NHMFL) located in Florida. Ideally, the transverse conditions simulating the ITER conditions should be created by Lorentz forces due to current and magnetic field. Unfortunately to create such a high level of stress, currents higher than the power supply capability at NHMFL (10 kA) would be required. This is the driving reason to have an apparatus simulating the same stress condition mechanically.

The first test was conducted in October 2005. It was possible to test the structure and its range of operation. Critical current measurements were made as a function of different fields. However during the first measurement, under the loading conditions, the sample was irreversibly damaged and no other measurements were possible. The successful test of the structural behavior of the apparatus motivated a second test carried out in January 2006. With the improvements made between the two experiments, it was possible to successfully measure the degradation of the cable as a function of the transverse pressure applied, measuring degradation as high as 50% with a transverse load of 100 MPa. The ultimate goal of these studies is to characterize the critical current behavior as a function of transverse load in order to predict the response of a full sized Cable-in-Conduit. The work in this thesis was used to explore a setup for measurements and measurement technique. A set of empirical equations describing the behavior of full size cables is needed and should be addressed with a new project that extends the work done so far.