Evidence and modeling of turbulence bifurcation in L-mode confinement transitions on Alcator C-Mod

N M Cao,J Irby,M A Chilenski,P H Diamond,A E White,P Rodriguez-Fernandez,J W Hughes,Alcator C-Mod Team ,P C Ennever,J E Rice,M L Reinke

doi:10.1063/1.5144444

Abstract

Analysis and modeling of rotation reversal hysteresis experiments show that a single turbulent bifurcation is responsible for the Linear to Saturated Ohmic Confinement (LOC/SOC) transition and concomitant intrinsic rotation reversal on Alcator C-Mod. Plasmas on either side of the reversal exhibit different toroidal rotation profiles and therefore different turbulence characteristics despite the profiles of density and temperature, which are indistinguishable within measurement uncertainty. Elements of this bifurcation are also shown to persist for auxiliary heated L-modes. The deactivation of subdominant (in the linear growth rate and contribution to heat transport) ion temperature gradient and trapped electron mode instabilities is identified as the only possible change in turbulence within a reduced quasilinear transport model across the reversal, which is consistent with the measured profiles and inferred heat and particle fluxes. Experimental constraints on a possible change from strong to weak turbulence, outside the description of the quasilinear model, are also discussed. These results indicate an explanation for the LOC/SOC transition that provides a mechanism for the hysteresis through the dynamics of subdominant modes and changes in their relative populations and does not involve a change in the most linearly unstable ion-scale drift-wave instability.

Highlights

Confined plasmas are active, turbulent nonlinear systems, which demonstrate a complex dependency between external actuation and the plasma response
The deactivation of subdominant ion temperature gradient and trapped electron mode instabilities is identified as the only possible change in turbulence within a reduced quasilinear transport model across the reversal, which is consistent with the measured profiles and inferred heat and particle fluxes
Experimental constraints on a possible change from strong to weak turbulence, outside the description of the quasilinear model, are discussed. These results indicate an explanation for the LOC/SOC transition that provides a mechanism for the hysteresis through the dynamics of subdominant modes and changes in their relative populations and does not involve a change in the most linearly unstable ion-scale drift-wave instability

Summary

INTRODUCTION

Turbulent nonlinear systems, which demonstrate a complex dependency between external actuation and the plasma response. Since increasing collisionality does not have as large of an effect on the ITG drive, which tends to pin ion temperature gradients near marginal stability, the confinement scaling saturates This argument appears to agree with reduced modeling of LOC/SOC using both analytic estimates of transport[7] and TGLF.[8,9] The roles of TEM and ITG instabilities are implicated in producing the observed change in the core temperature response to edge cold pulses in perturbative laser blow off injection experiments in Ohmic plasmas,[17,18] where a gradual transition from TEM to ITG dominance correlates with the experimentally observed trend of transition from the core non-diffusive temperature inversion response to the core cooling response. We will demonstrate how changes in turbulent fluctuation are seen at experimentally indistinguishable density and temperature profiles and that circumstantial evidence supports an interpretation of these changes reflecting changes in the underlying turbulence

CHARACTERIZATION OF ROTATION REVERSAL HYSTERESIS ON ALCATOR C-MOD

LINEAR AND QUASILINEAR GYROKINETIC MODELING

SUMMARY AND FUTURE WORK