Suppression of drift wave turbulence and zonal flow formation by changing axial boundary conditions in a cylindrical magnetized plasma device

Abstract

For drift wave turbulence, due to charge conservation, the divergence of the parallel current is coupled to the divergence of the perpendicular polarization current, which determines the effective radial momentum flux, i.e., the Reynolds stress. Changes in the current flow patterns also affect the nonlinear energy transfer from smaller to larger scales. Here, we show that by changing the end plate boundary conditions in a cylindrical plasma device, the radial currents through the plasma and hence the net momentum transport and the nonlinear coupling for the inverse energy transfer are strongly modified. The transition to drift wave turbulence and the formation of low frequency zonal flows can be either suppressed with conducting boundaries or enhanced with insulating boundaries.

Understanding the impact of insulating and conducting endplate boundary conditions on turbulence in CSDX through nonlocal simulations

“…In the IBC case, density blobs are mostly being formed in the region of the maximum density gradient, propagate in the electron diamagnetic direction, and they exhibit a radial extension that becomes tilted azimuthally as the structure approaches the shear layer. Similar qualitative behavior has been reported experimentally, 23 as well. Moreover, in Fig.…”

“…These observations of the inverse energy transfer dynamics are also qualitatively consistent with the previous experimental measurement of frequency space kinetic energy transfer in various CSDX experiments. 20,22,23 The saturated amplitude profiles of the fluctuations for both the cases are shown in Fig. 14.…”

“…Equilibrium profiles were determined by fitting a general functional form n 0 ¼ ð n À n base Þ expðÀðr=l n Þ n a =n a Þ þ n base using the least square method to the measurements taken from experimental conditions. 23 The general function fits to the experimental equilibrium profiles are shown in Figs. 1(a) and 1(b).…”

“…Data measured at B ¼ 1 kG; P rf ¼ 1500 W, with argon gas pressure of 3:2 mTorr. Data taken from Thakur et al 23 Black line is the fit to the profiles. (d) Radial profiles of inverse gradient lengthscale for the electron density and temperature fits.…”

“…Thakur et al 23 experimentally investigated the effects of changing the endplates from insulating boundary condition (IBC) to the conducting boundary condition (CBC) on the turbulence induced zonal flow. The experiment was conducted by changing the type of endplates while keeping other input parameters such as input gas pressure, magnetic field, and source power constant.…”