Electrothermal evaluation of conductors for the Compact Ignition Tokamak (CIT) PF coils

The coils in the PF (poloidal field) system for CIT will be cooled to 80 K by liquid nitrogen between each machine pulse. During a pulse, the conductor temperature rises essentially adiabatically. Several conductors have been under consideration. These include an Alloy-718/copper/Alloy-718 laminate, alumina dispersion strengthened (ADS) copper, a laminate of copper and CuNb, and C107 copper. Electrothermal data on these materials are limited; hence, a test program was performed to determine resistivity as a function of temperature from 80 K to 300 K, and to determine the adiabatic characteristic sometimes referred to as the G function or action integral. This is, equivalently, the integral of current density squared over time for the sample or the integral of (density)×(specific heat)/(resistivity) over temperature for the material. The measurements naturally include the nonlinear character of the properties with temperature. The integral is a direct measure of the adiabatic thermal capacity of the material for a specified RMS (root-mean-square) current density over a time interval. Data have been taken on three grades of ADS copper, on laminates of copper and CuNb of two ratios, and on C107 copper. Functions have been computed for copper/Alloy 718 of different ratios. Results are given over the temperature interval from 80 K to 300 K