Towards Understanding the Mechanism of Heat and Particle Transport Decoupling in I-mode Edge Plasmas

I-mode is an important alternative operational scenario for burning plasmas. A possible theoretical understanding is presented for a unique turbulent transport phenomenon in the I-mode regime, i.e., the so-called transport decoupling between heat and particle in tokamak edge plasmas. Based on our particle simulations by running gyrokinetic toroidal code (GTC), we found that a particular instability can account for such experimental phenomenon, which makes it the major candidate for experimentally observed weakly coherent modes (WCMs) turbulence. This instability is driven by steep electron temperature gradient up to a certain value, with characteristic time scale around transit time of passing electrons while the spatial scale falling in the range of ion's poloidal gyro-radius. A crucial feature about this instability is that neither ions nor passing electrons can be treated adiabatically, which distinguishes it from the conventional drift-like instabilities such as ion temperature gradient modes (ITGs), trapped electron modes (TEMs) and normal electron temperature gradient modes (ETGs). Those non-adiabatic responses for both ions and electrons indeed excited this unique instability and could be saturated by typical flow shearing suppression mechanism including low frequency zonal flow and geodesic acoustic modes (GAMs) instead of poloidal mean flow to give rise to the considerable particle and heat transport compared with experimentally observed values. A detailed discussion about this instability and its nonlinear saturation mechanism which may account for experimentally observed WCMs and associated turbulent transport will also be presented in this talk.
Keywords: I-mode, WCMs, Transport decoupling.