High-beta runaway transitions in a fluid model of electromagnetic ion-temperature-gradient turbulence

Researchers have developed a simplified fluid model to study electromagnetic ion-temperature-gradient (ITG) turbulence in tokamaks. This model aims to replicate key features observed in gyrokinetic simulations, particularly an abrupt increase in fluctuation levels.

The observed transition occurs when plasma beta exceeds a critical value, shifting the plasma state from zonal flow dominance to one characterized by radially elongated turbulent eddies known as 'streamers'. The new model allows for detailed numerical investigation of this phenomenon.

The study found that the transition is driven by an imbalance in stresses. When Reynolds stress, which reinforces zonal flows, is overcome by Maxwell and diamagnetic stresses that weaken them, the turbulence transition is triggered.