Double poloidal field system with superconducting and conventional copper coils for induced high loop voltage: a new concept and a feasibility study for a RFP FFHR

A critical point for the use of the superconducting magnets in Reversed Field Pinch (RFP) fusion reactors is represented by their limitations in the allowed magnetic field derivative, which poses constraints in the maximum induced loop voltage for the gas breakdown and the initial rise of the plasma current. Also in Tokamaks the losses in the superconducting coils related to magnetic field variation need to be carefully considered. On the other hand the use of superconductors for the poloidal field magnets is mandatory in a fusion reactor due to the high volt-seconds required for plasma current rise, the flat-top in pulsed machine and to guarantee the plasma equilibrium. A new concept is proposed for a RFP reactor with a mixed use of different coils: some made with superconductors and others with conventional copper coils operating at low temperature (about 80- 100 K). The proposed solution allows to obtain high loop voltage in the start-up phase, followed by a reduced voltage in the following rise and flat-top plasma current maintaining the level of magnetic field derivative in the superconductors within the allowed limits. The basic principle is firstly discussed on the basis of a simplified model that allows to understand the coils - plasma operation and the evolution of the magnetic field into the superconducting coils. This new concept is then utilized for a feasibility study of the poloidal magnetic system of a pilot neutron source based on a Reversed Field Pinch (RFP) as fusion core. The preliminary design shows the feasibility of this mixed solution for the poloidal field coils of a RFP machine (R=4, a=0.8) with a plasma current of 12 MA, with a high initial loop voltage (100-200 V for about half second) for a fast rise of the plasma current with a reduced magnetic field derivative in the superconductors, less than 2 T/s.