Effect of magnetic field topology on quasi-stationary equilibrium, fluctuations, and flows in a simple toroidal device

Abstract

In a simple toroidal device, the plasma profiles and properties depend on toroidal magnetic field topology. For example, the toroidal connection length crucially controls the adiabatic or non-adiabatic nature of electron dynamics, which in turn governs the nature of instabilities, fluctuations, and transport, the latter of which governs the plasma mean profiles. We present the results of extensive experiments in a simple toroidal device obtained by controlling the mean parallel connection length L¯c, by application of external vertical component of magnetic field Bv, where Bv≤2% of toroidal magnetic field BT. Interestingly, for nearly closed field lines, which are characterized by large values of L¯c, it is found that flute like coherent modes are observed to be dominant and is accompanied by large poloidal flows. For small values of L¯c, the mean density on the high field side is seen to increase and the net poloidal flow reduces while a turbulent broad band in fluctuation spectrum is observed. Upon a gradual variation of L¯c from large to small values, continuous changes in mean plasma potential and density profiles, fluctuation, and poloidal flows demonstrate that in a simple toroidal device there exists a strong relationship between Lc, flows, and fluctuations. The net flow measured is found independent of the direction of Bv, but an asymmetry in the magnitude of the flow is found. The observed imbalance between the mean flow, fluctuation driven flow, and net flow is also discussed.