Dynamo-free plasma in the reversed-field pinch: Advances in understanding the reversed-field pinch improved confinement mode

Abstract

Generation and sustainment of the reversed field pinch ͑RFP͒ magnetic configuration normally relies on dynamo activity. The externally applied electric field tends to drive the equilibrium away from the relaxed, minimum energy state which is roughly described by a flat normalized parallel current density profile and is at marginal stability to tearing modes. Correlated fluctuations of magnetic field and velocity create a dynamo electric field which broadens the parallel current density profile, supplying the necessary edge current drive. These pervasive magnetic fluctuations are also responsible for destruction of flux surfaces, relegating the standard RFP to a stochastic-magnetic transport-limited device. Application of a tailored electric field profile ͑which matches the relaxed current density profile͒ allows sustainment of the RFP configuration without dynamo-driven edge current. The method used to ascertain that a dynamo-free RFP plasma has been created is reported here in detail. Several confinement improvements during the accompanying periods of low magnetic fluctuations are observed. Namely, the magnetic fluctuation level is reduced to the point where stochastic-magnetic transport is no longer the dominant process in the core and nested flux surfaces are restored in the core of the dynamo-free RFP.

Statistical features of edge turbulence in RFX-mod from gas puffing imaging

“…configuration, where external circuitry acts in feedback to cancel locally the radial magnetic field measured by a set of coils on the shell [16,17]. (Similar to this technique, is the cancellation of dynamo-driven edge currents from the outside in MST [32]).…”

The Hall dynamo effect and nonlinear mode coupling during sawtooth magnetic reconnection

“…In MST, the imbalance between inductive electric field and current exists over nearly the entire plasma column. 31 It is predicted that various dynamo mechanisms may be playing a role in providing the required balance. Both Hall dynamo and MHD dynamo have been identified experimentally in MST with the relative weighting of these two effects throughout the plasma cross section being a topic of great interest.…”

Local measurements of tearing mode flows and the magnetohydrodynamic dynamo in the Madison Symmetric Torus reversed-field pinch

“…Specifically, the dynamo works to flatten the peaked parallel ͑magnetic field aligned͒ current profile by increasing current in the edge of the plasma while reducing current in the core. [9][10][11]45 Figure 11 illustrates the flat parallel current profile relative to the core peaked applied electric field. The positive toroidal direction is defined as the direction of the mean magnetic field on-axis, yielding an electric field and current in the negative toroidal direction on-axis in MST.…”