Thermal structural analyses during cool down of the ITER toroidal field coil in the magnet cold test bench

Abstract

A Magnet Cold Test Bench is being built in ITER to test some of the Toroidal Field Coils (TFC) and the Poloidal Field Coil PF1, before they will be assembled and installed in the tokamak. The purpose is to test the performance of the superconducting magnet in cryogenic conditions and with current.

The TF coil and its structures weigh about 300 tons and will be cooled down from 300 K to 4.5 K, with helium flow supplied by one of the three refrigeration cold boxes of the ITER cryogenic system, with a capacity of 25 kW at 4 K. The helium flows in the winding packs of the coil, through the bundle and central channel of the superconducting cables, and it also extracts heat through the numerous cooling pipes of the Casing.

From 300 K to 100 K, one major concern is the thermal stresses due to the different thermal contraction coefficients of the materials in the coil and its structure: superconducting cables composed of Nb₃Sn and Copper strands, Stainless Steel for the conductor jacket, the radial plates and the casing, electrical insulation materials such as Glass Kapton Glass and epoxy resin.

The usual cool down process requirement is to control the temperature difference between helium outlet and inlet and to ensure that it does not exceed 50 K. For the Magnet Cold Test Bench (MCTB), a selected cool down scenario was simulated to comply with the thermal hydraulic criteria and to check the resulting thermal stresses are acceptable for the coil, insulation material, structure and supports.