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Description
Transport barriers in magnetically confined plasmas pose an intriguing thermodynamic problem: large gradients, and hence substantial free energy, can coexist with strongly reduced turbulent transport. We examine this phenomenon from the perspective of constrained nonequilibrium dynamics. A macroscopic thermodynamic model treats the plasma boundary layer as a heat engine in which incoming power drives ordered flows and currents, producing a bifurcation to a stable high-gradient state above critical thresholds [1]. This picture is complemented by gyrokinetic considerations. Low-frequency fluctuations are subject to a constraint requiring the fluctuation-driven charge-weighted radial particle flux to vanish. In sufficiently strong-gradient regimes, this constraint can become incompatible with growing fluctuations, preventing access to available free energy even when conventional energetic considerations suggest instability. Gyrokinetic simulations demonstrate this constrained stabilization and associated changes in mode structure [2]. Recent DIII-D pedestal studies provide an experimentally relevant example of related threshold behavior, including microtearing stability boundaries and second-stable kinetic ballooning modes [3]. Together, these results suggest that transport barriers may arise not simply through reduced thermodynamic drive, but through restrictions on the dynamical pathways by which free energy can be relaxed.
[1] S. M. Mahajan, D. R. Hatch, Z. Yoshida, and M. Kotschenreuther, “A unified theory of transport barriers (TBs) in magnetically confined systems,” arXiv preprint arXiv:2603.26919, 2026.
[2] M. Kotschenreuther, X. Liu, S. M. Mahajan, D. R. Hatch, and G. Merlo, “Transport barriers in magnetized plasmas—general theory with dynamical constraints,” Nucl. Fusion, vol. 64, no. 7, Art. no. 076033, 2024, doi: 10.1088/1741-4326/ad4c75.
[3] D. R. Hatch, L. A. Leppin, M. T. Kotschenreuther, S. Houshmandyar, S. M. Mahajan, J. Schmidt, and P.-Y. Li, “Microtearing thresholds and second-stable ballooning in the DIII-D pedestal: Reduced modeling and core-edge implications,” Phys. Plasmas, vol. 33, Art. no. 072512, 2026, doi: 10.1063/5.0337081.