Onset of internal transport barriers in tokamaks

TL;DR

This paper investigates the formation and breakdown of internal transport barriers in tokamaks, revealing new secondary shearless barriers and their dependence on plasma fluctuation amplitude and electric shear, with implications for plasma confinement.

Contribution

It introduces the discovery of secondary shearless barriers and their bifurcations, expanding understanding of plasma transport barriers in tokamaks.

Findings

Identification of primary shearless barriers and their dependence on fluctuation amplitude and electric shear.

Discovery of secondary shearless barriers created and destroyed by bifurcations.

Observation of double and triple secondary shearless barriers hindering chaotic transport.

Abstract

Barriers have been identified in magnetically confined plasmas reducing the particle transport and improving the confinement. One of them, the primary shearless barriers are associated to 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 time scale. 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 break-up are recurrent. Another noticeable feature, in these transitions, is the appearance of a layer of particle trajectory stickiness after the shearless barrier break-up or before its onset. Besides 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 which constitute a noticeable obstacle to the chaotic transport.