Zero-turbulence manifold in a toroidal plasma.

Area of Science:

• Plasma Physics
• Fusion Energy
• Turbulence Dynamics

Background:

• Sheared toroidal flows in tokamak plasmas can lead to bifurcations into states with zero turbulent transport.
• Subcritical turbulence, driven by the parallel velocity gradient, limits the maximum achievable temperature gradients.

Purpose of the Study:

• To investigate the role of the magnetic field pitch to inverse aspect ratio (q/ϵ) as a critical control parameter for sheared tokamak turbulence.
• To determine if reducing q/ϵ allows for higher temperature gradients without inducing turbulence.

Main Methods:

• Mapping the zero-turbulence manifold in the zero-magnetic-shear limit.
• Analyzing the parameter space including perpendicular flow shear (γ(E)), q/ϵ, and normalized inverse temperature gradient scale (R/L(T)).

Main Results:

• Demonstrated that q/ϵ is a critical control parameter for sheared tokamak turbulence.
• Showed that reducing q/ϵ enables significantly higher temperature gradients without triggering turbulence.
• Observed temperature gradients comparable to those in experimental transport barriers.

Conclusions:

• The parameter q/ϵ is crucial for controlling turbulence and achieving high-gradient states in tokamaks.
• Reducing q/ϵ offers a pathway to access improved confinement regimes, potentially relevant for fusion energy.
• The study maps the accessible parameter space for zero-turbulence states and discusses the role of linear theory.