• Energy Research

Abstract Reconnection events in high current reversed field pinch plasmas are often associated to the partial or total loss of the helical magnetic topology. The electron temperature collapse during these phenomena is investigated in RFX-mod thanks to high time resolution soft-x-ray diagnostics; these data are used, together with magnetic energy reconstructions, for energy balance analysis. The paper shows that the energy released during reconnection events, similarly to astrophysical plasmas, might be involved in ion heating, the latter being estimated by the energy distribution function of neutral atoms, a rather interesting feature in a reactorial perspective. These issues will be further investigated in RFX-mod2 , an upgrade of the present device starting its operations from 2022, where the modified boundary conditions are expected to increase the helical states duration and reduce the frequency of reconnection events.

Experimental results and theoretical studies call for Reversed Field Experiment (RFX) machine and power supply improvements to allow studies that go beyond those of a conventional Reversed Field Pinch (RFP) with passively stabilized turbulent MHD dynamo. The RFX experiment, in operation at Padua (Italy) since 1992, is the most important RFP machine in the world. The RFP magnetic configuration is actively studied in Europe, USA and Japan for its perspectives in reach the fusion conditions at low magnetic field and for the particular scientific interest of plasmas in this configuration. The new paths on the MHD mode studies and control, opened by recent results in RFX and other RFP machines, are introduced. Then the goals and the design lines of the technical modifications of RFX, mainly addressed to actively interact with the MHD modes through the control of the plasma magnetic boundary are reported. The main modifications introduced are related with the improvement the first wall, the addition of 192 saddle coils around the shell and to increase the operational flexibility of the toroidal field circuit power supply.

The studies on the development of fusion–fission hybrid reactors (FFHR) have gained consensus in recent years as an intermediate step before fusion energy. This work proposes a possible approach to FFHRs based on the coupling of a Reversed Field Pinch fusion machine and a Molten Salt Subcritical fission test bed. The proposed test bed is characterized by the coexistence of a fast-neutron fission core and a dedicated thermal-neutron zone, allowing the performing of tritium breeding and actinides transmutation studies. The neutronic design solutions and the results obtained by the irradiation of FLiBe salt (inside the thermal-neutron zone) and of an actinide target (inside the core) are shown. The outcomes of the analysis reveal the potential of FFHR systems as breeding/burner systems. In particular, the results regarding tritium breeding are very encouraging as the system is demonstrated to be able to reach a very high Tritium Breeding Ratio.

The paper describes the control system of the International Thermonuclear Experimental Reactor (ITER) Vertical Stabilization converter, which has been optimized to achieve the best performance in terms of response time. The VS converter is rated for 8 kV no load output voltage and 22.5 kA maximum output current; it is based on two identical basic units connected in series, each made of two identical subunits connected in parallel. The converter four quadrant operation is first described, and the undesired current unbalance between the converter subunits, due to fast voltage transients, is discussed; then the design criteria and the regulator structure to avoid current unbalances are described. Finally, the simulation results of the complete circuit operation in the worst conditions are reported to evaluate the converter performances. The results show that also in these conditions the specified performances are satisfied, while in a wider operating region the dynamic behavior is much better than requested. Current unbalances are avoided during the inversion phase; it is also pointed out that transients in the system can be controlled without chokes, which therefore can be avoided reducing the system cost.