Description
We present two examples of unconventional dynamics of turbulent transport in spherical tokamaks.
First, we elucidate a novel interaction between mean E×B flow shear and Ion-Temperature-Gradient (ITG) turbulence. In contrast to the well-established stabilizing role of strong E×B shear in conventional aspect-ratio tokamaks, our work reveals a mechanism through which it can actually enable ITG turbulence. High-fidelity nonlinear gyrokinetic simulations, supported by a fluid model, demonstrate that the underlying physics originates from the Dimits regime persisting across an extended range of temperature gradients. In this situation, the mean E×B shear disrupts the zonal flow structures inherent to the Dimits regime before it can completely quench the turbulence, thereby facilitating the destabilization of ITG turbulence. This mechanism is further confirmed in realistic simulations considering MAST-U equilibria.
Second, we report that microturbulence with finite β can exhibit a negative turbulent Prandtl number. After implementing and benchmarking the full electromagnetic toroidal angular momentum flux, we show that a negative Prandtl number is attainable in spherical tokamaks. This represents spontaneous symmetry-breaking, which can drive intrinsic flow shear without external momentum input or up-down asymmetry in the magnetic equilibrium. The underlying physics is tied to the decoupling of the parallel velocity moment from the electrostatic potential as plasma β increases. This finding demonstrates a new way to generate intrinsic rotation in future large spherical tokamaks.
Collectively, our works uncover unique turbulent transport phenomena intrinsic to the spherical tokamak geometry, with direct implications for confinement and stability.