A continuously pulsed Reversed Field Pinch core for an ohmically heated hybrid reactor

Recent results on RFX-mod, the largest Reversed Field Pinch (RFP) experiment, have confirmed a positive increase of the electron and ion temperature vs. plasma current with a trend that allows hypothesizing significant D-T fusion processes in larger devices with higher plasma current. Such an operation, in which the plasma is purely ohmically heated, could give an efficient, robust and cheap fusion neutron source to be used as the basis for a fusion-fission hybrid system. It is worthwhile to remember the advantages of the RFP with respect to Tokamaks and Stellarators: plasma ohmically heated, toroidal winding designed for a magnetic field lower than one-two orders of magnitude with respect to Tokamaks, no divertor, very simple, robust and cheap construction, simple access for remote handling and maintenance. To guarantee an almost continuous neutron production, like required in a hybrid reactor, a solution for a continuously pulsed operation is presented, compatible with a purely magnetic induced plasma current rise and sustainment, without the complication of additional current drive systems. The aim of the paper is to investigate the relationship between the machine size (major and minor plasma radius), the attainable volt-seconds and, consequently, the maximum plasma current and pulse duration. The analysis starts from the experimental RFX-mod data (volt-seconds necessary for plasma current rise, loop voltage during flat top, plasma current rise time) extrapolated towards higher current and plasma size up to values compatibles with a condition of significant neutron production from D-T reactions. The paper will present the results which show a realistic possibility to reach fusion condition in an inductively operated RFP with a proper machine size.