Unraveling Quasiperiodic Relaxations of Transport Barriers with Gyrokinetic Simulations of Tokamak Plasmas

The generation and dynamics of transport barriers governed by sheared poloidal flows are analyzed in flux-driven 5D gyrokinetic simulations of ion temperature gradient driven turbulence in tokamak plasmas. The transport barrier is triggered by a vorticity source that polarizes the system. The chosen source captures characteristic features of some experimental scenarios, namely, the generation of a sheared electric field coupled to anisotropic heating. For sufficiently large shearing rates, turbulent transport is suppressed and a transport barrier builds up, in agreement with the common understanding of transport barriers. The vorticity source also governs a secondary instability— driven by the temperature anisotropy ($T_{\parallel }\ne T_{\perp }$). Turbulence and its associated zonal flows are generated in the vicinity of the barrier, destroying the latter due to the screening of the polarization source by the zonal flows. These barrier relaxations occur quasiperiodically, and generically result from the decoupling between the dynamics of the barrier generation, triggered by the source driven sheared flow, and that of the crash, triggered by the secondary instability. This result underlines that barriers triggered by sheared flows are prone to relaxations whenever secondary instabilities come into play.

Figure 1

Temperature (blue circles) and $E\times B$ shear (red triangles) during steady-state (open) and a TB phase (solid). The kinetic sources are labeled by the black and green areas.

Figure 2

Top. Turbulent heat flux (color map) and $E\times B$ shearing rate contours (white contours: the dotted contour is $8{\gamma }_{\mathrm{lin}}$, and the plain contour is $-20{\gamma }_{\mathrm{lin}}$). Bottom. $E\times B$ shearing rate ${\omega }_{E\times B}$ (plain blue line) and temperature gradient $R/L_T$ (dashed red line) radially averaged close to ${\rho }_{\omega }$. The red area corresponds to the zoom window used in Fig. 3.

Figure 3

Time evolution of the local resonant Fourier ($m$, $n$) modes left to the TB ($\rho =0.41$), during the second quenching. The temperature anisotropy is displayed in thin black line.