Transitions to improved confinement regimes induced by changes in heating in zero-dimensional models for tokamak plasmas

TL;DR

This paper demonstrates that rapid changes in heating can trigger transitions to better confinement regimes in zero-dimensional tokamak plasma models, revealing new insights into turbulence suppression and chaos driven by oscillating heating rates.

Contribution

It is the first study to analyze the effects of step and oscillating heating rate changes on confinement regimes in coupled zero-dimensional plasma models.

Findings

Rapid heating changes induce improved confinement regimes.

Oscillating heating can lead to chaos via period-doubling bifurcations.

Micro-turbulence is suppressed during improved confinement states.

Abstract

It is shown that rapid substantial changes in heating rate can induce transitions to improved energy confinement regimes in zero-dimensional models for tokamak plasma phenomenology. We examine for the first time the effect of step changes in heating rate in the models of E-J.Kim and P.H.Diamond, Phys.Rev.Lett. 90, 185006 (2003) and M.A.Malkov and P.H.Diamond, Phys.Plasmas 16, 012504 (2009) which nonlinearly couple the evolving temperature gradient, micro-turbulence and a mesoscale flow; and in the extension of H.Zhu, S.C.Chapman and R.O.Dendy, Phys.Plasmas 20, 042302 (2013), which couples to a second mesoscale flow component. The temperature gradient rises, as does the confinement time defined by analogy with the fusion context, while micro-turbulence is suppressed. This outcome is robust against variation of heating rise time and against introduction of an additional variable into the model. It is also demonstrated that oscillating changes in heating rate can drive the level of micro-turbulence through a period-doubling path to chaos, where the amplitude of the oscillatory component of the heating rate is the control parameter.