Role of MHD dynamo in the formation of 3D equilibria in fusion plasmas

This work shows via experiments and modelling that dynamo, or magnetic flux pumping mechanisms play a key role in the formation of helical core equilibria in fusion plasmas. Dynamo effects impact the current density profile and thus the safety factor profile of the final 3D equilibrium, with important consequences on MHD stability and transport. We compare results from multiple machines (RFX-mod, MST, DIII-D) and nonlinear MHD modelling. Two paradigmatic cases of helical state formation are considered and common physics is identified: spontaneous formation in high-current reversed-field pinch (RFP) plasmas and the hybrid scenario in high-? tokamak plasmas. Helical cores form in both cases, either spontaneously via saturation of MHD modes, or due to the marginally-stable ideal MHD response to external 3D fields. Direct measurements of the dynamo e.m.f. associated to 3D plasma distortions are presented for a database of helical RFP plasmas from RFX-mod and MST, covering a wide range of plasma parameters. Similar measurements were also made in helical states forming in response to external 3D fields in Ohmic RFX-mod tokamak plasmas and in DIII-D high-? hybrid plasmas. These experimental results qualitatively agree with nonlinear MHD modelling performed with the SpeCyl and PIXIE3D codes. They indicate that central current is redistributed by a dominantly electrostatic MHD dynamo. The underlying physics common to RFP and tokamak is thus revealed: a helical core displacement modulates parallel current density along flux tubes, which requires a helical electrostatic potential to build up, giving rise to a helical dynamo flow.