How does the I-mode function?

The I-mode is an attractive confinement regime for future tokamak based fusion reactors.

The I-mode is an attractive confinement regime for future tokamak based fusion reactors. It is characterized by steep edge electron temperature profiles, but rather flat density profiles. This is attributed to a decoupling of heat and particle transport, which is not understood so far. Drift-Alfvén turbulence allows for a decoupling of density and electron temperature fluctuations by electron thermal conductivity dissipating electron temperature fluctuations. To reach such a regime strong interchange turbulence such as ITG turbulence has to be avoided. Heat conduction from the separatrix to the divertor can decouple ion and electron temperatures, the ion temperature gradient across the separatrix flattens reducing the drive of ITG turbulence and allowing for the development of drift-Alfvén turbulence.

Such a regime has been simulated using gyrofluid simulations showing several experimental I-mode observations. Even detailed dynamical fingerprints as the occurrence of bursts with precursors  (see figure) are present in the simulation. The model can explain the widening of the operational window with magnetic field strength and challenges met when detaching the plasma. This work has been recently published in Nuclear Fusion.