Finding the Elusive $\mathbf{E}\times \mathbf{B}$ Staircase in Magnetized Plasmas

Turbulence in hot magnetized plasmas is shown to generate permeable localized transport barriers that globally organize into the so-called "ExB staircase" [G. Dif-Pradalier et al., Phys. Rev. E, 82, 025401(R) (2010)]. Its domain of existence and dependence with key plasma parameters is discussed theoretically. Based on these predictions, staircases are observed experimentally in the Tore Supra tokamak by means of high-resolution fast-sweeping $X$-mode reflectometry. This observation strongly emphasizes the critical role of mesoscale self-organization in plasma turbulence and may have far-reaching consequences for turbulent transport models and their validation.

Figure 1

Detail of the shear flow–mean profile–transport interplay next to a staircase step [a corrugation].

Figure 2

Local minima of the radial correlation length of the turbulent fluctuations at three different times efficiently track the staircase steps in Gysela.

Figure 3

The reflectometer coherence length plotted against radius shows clear experimental evidence of a staircase at locations $S_1$, $S_2$ and $S_3$, possibly also at $S_0$.

Figure 4

The low-order $q=1,3/2,2$, and $5/2$ rationals’ surfaces are superimposed on top of the $\mathbf{E}\times \mathbf{B}$ shear. No clear correlation with the corrugations can be inferred.