Speakers
Description
Leveraging the results of a series of $3$D flux-driven $2$-fluid simulations in a diverted equilibrium with GBS, it is shown how a regime of high confinement can develop as the power crossing the separatrix exceeds a critical value. As the edge temperature increases, the resistive-ballooning turbulence characteristic of L-mode conditions becomes subdominant, and turbulence is mostly driven by the electron drift-wave instability. Electromagnetic effects then act to suppress drift-wave turbulence by enhancing the electron adiabatic response. For the resistive branch of the drift-wave instability in particular, the strength of suppression is proportional to the background gradients driving the instability. Under a set of specified conditions, the plasma can therefore become unstable to the spontaneous formation of an edge transport barrier. In this regime, a steepening of the edge profiles in $n, T_e$ leads to a further decrease of turbulent flux and a feedback loop develops, driving the transport barrier formation. Furthermore, the transition to a high-confinement regime is impeded when the toroidal magnetic field points in the unfavourable direction for H-mode access. The power required to access this regime $P_{\text{th}}$ is analytically derived in a simplified geometry, via a local quasi-linear estimate for the $E \times B$ turbulent transport rate in the electromagnetic drift-wave regime. The resulting scaling law for $P_{\text{th}}$ is compared with the ITPA experimental database for the threshold power $P_{\text{LH}}$ to access H-mode in tokamaks, yielding good overall agreement $R^2 \simeq 0.7$.
