Experiments confirm energy transfer enhancement through shear flows

In magnetic confinement nuclear fusion, the transition from low to highly confined plasmas is potentially accompanied by a strong zonal shear flow that redistributes energy away from turbulence. The processes involving these zonal flows, driven by gradients in the Reynolds stress, are not well understood.

Ullmann et al. studied such processes in detail for the first time using a multitude of local measurements within a plasma. This builds on previous studies that were only able to take limited measurements augmented by simple assumptions on their spatial extent.

Using an array of 128 probes, the authors measured four different flux surfaces to calculate the Reynolds stress and the power of the zonal flows. These measurements allowed a comparison of the steady-state background shear with growth rates from an elliptical model. The results confirmed the dependence of the Reynolds stress and energy transfer on the shearing rate. The authors additionally found a redistribution of power away from the turbulence to the zonal flow, which is stronger, the higher the background shear.

“The results from two different diagnostics and different analysis routines finally present a consistent picture of the dependence between stationary shear, energy transfer and the relative zonal power,” said author Til Ullmann.

The study highlights the potential of studying fundamentals of plasma turbulence using small scale experiments with low temperature plasmas. The authors intend to continue analyzing such turbulence in similar experimental set ups, particularly with more complex cases like particle transport and magnetic field dependencies.

Source: “Turbulent energy transfer into zonal flows from the weak to the strong flow shear regime in the stellarator TJ-K,” by T. Ullmann, B. Schmid, P. Manz, G. E. M. Tovar, and M. Ramisch, Physics of Plasmas (2021). The article can be accessed at https://doi.org/10.1063/5.0039959 .