Onset of internal transport barriers in tokamaks

Barriers have been identified in magnetically confined plasmas by reducing the particle transport and improving the confinement. One of them, the primary shearless barriers, is associated with extrema of non-monotonic plasma profiles. Previously, we identified these barriers in a model described by a map that allows the integration of charged particles motion in drift waves for a long timescale. In this work, we show how the existence of these robust barriers depends on the fluctuation amplitude and on the electric shear. Moreover, we also find control parameter intervals for which these primary barriers onset and breakup are recurrent. Another noticeable feature, in these transitions, is the appearance of a layer of particle trajectory stickiness after the shearless barrier breakup or before its onset. In addition to the mentioned primary barriers, we also observe sequences of secondary shearless barriers, not reported before, created and destroyed by a sequence of bifurcations as the main control parameters, the fluctuation amplitude and electric shear, are varied. Furthermore, in these bifurcations, we also find hitherto unknown double and triple secondary shearless barriers that constitute a noticeable obstacle to the chaotic transport.

Automated summary

This study identifies primary and secondary shearless barriers in magnetically confined plasmas, with the primary barriers dependent on fluctuation amplitude and electric shear. The operations and breakdowns of these barriers are recurrent within certain parameter intervals, with the appearance of sticky particle trajectories being a noticeable feature. Additionally, previously unknown double and triple secondary shearless barriers are observed in bifurcations, acting as obstacles to chaotic transport.