Stability of a poloidal field coil in rapidly changing magnetic fields

A poloidal field model coil of 3 m in diameter was designed and built according to the following specifications: stability was required during an exponential field variation of /spl Delta/=0.4 T with a time constant of 5 ms, corresponding to a maximum field change of 80 T/s. The superconducting cable developed for this purpose was tested in a short sample experiment, yielding a stability limit beyond this requirement. The test results of the model coil are in good agreement with the stability limit expected from the cable experiment. They can be explained in terms of a simple stability model which compares the energy input due to coupling losses with the transient heat removal capability of supercritical helium. A more detailed stability calculation was performed with a computer code taking into account the temperature dependence of helium and conductor material properties. The coil reached the stability limit expected from the measured cable parameters and showed no unexpected ramp rate limitations.