Long-term high voltage tests of the conductor insulation of a superconducting magnet

The superconducting magnets of the ITER fusion reactor will be excited with high voltage during fast discharge. Especially fast voltage excitations of these large magnets can lead to high local electrical stresses caused by a non linear voltage distribution or oscillations within a coil. Investigation of the dielectric properties of the ITER model coils delivered valuable experience for the specification of quality assurance tests for ITER coils. Calculations of the transient behaviour of the ITER TF coils provide additional information for the choice of test voltages and waveforms. Conductor and radial plate insulation of the ITER TF model coil were exposed at room temperature for 10 hours to DC and AC voltages in agreement with proposed test values for ITER TF to investigate the long-term behaviour. Breakdown appeared during the highest AC voltage step after more than 9 hours. The partial discharge activity increased significantly about 12 minutes before breakdown.