Momentum transport from nonlinear mode coupling of magnetic fluctuations

The authors have determined experimentally that magnetic fluctuations can drive anomalous transport of momentum via internet electromagnetic forces. In the reversed field pinch, the effect arises from forces generated by three-wave nonlinear interactions predominantly involving two m = 1 modes and an m = 0 mode. They observed that the correlated triple wave products vary in time and peak during the period of strongest core mode (and plasma) deceleration. By selectively removing the (0, 1) mode the sudden change deceleration of the core modes at sawteeth can be essentially eliminated. Finally, by selectively driving the (1, 6) mode the rotation of the other m = 1 modes is affected. These active experiments further support the presence of nonlinear interactions. Other mechanisms, such as parallel particle diffusion along stochastic magnetic field lines, may produce anomalous momentum transport as well.