Investigating the effects of radial density gradient length scale on the nonlinear saturation of drift wave turbulence and zonal flow formation

Controlled Shear Decorrelation eXperimental (CSDX) is a linear magnetized plasma device for drift wave (DW) turbulence and zonal flow (ZF) formation research. In addition to experiments, we shall present numerical studies of the influence of the radial density gradient length (ρ_s/L_n) on saturated DW turbulence. The simulation was carried out with BOUT++ (BOUndary Turbulence 3D 2-fluid edge simulation) model of CSDX. The evolution of plasma is very sensitive to ρ_s/L_n. As ρ_s/L_n increases, there is a rapid turning on of the turbulence features along with the formation of a mean E×B shear flow (ZF). The DW turbulence enhances, gets saturated and then decreases rapidly when the ZF forms and grows. Meanwhile, the ZF behaves similarly: increases, gets saturated and then collapses, even changes sign. Experimentally, increasing the magnetic field (B) has a similar effect. Previous experiments have shown the growth, saturation and relaxation of mean E×B profiles with increasing B [1]. However, due to limited RF power, the relaxation of the profiles was also accompanied by axial detachment at high B, which is not included in the simulations. In the upgraded CDSX (with up to 20 kWatts) we can decouple the two phenomena and compare with the numerical results.

[1] S. C. Thakur, PSST (2014)