H-mode transition and pedestal studies in fusion plasmas

E x B shearing rate associated with vortex flow inside a macroscopic magnetic island is investigated. Due to the elongation of the MI and incompressibility of the E x B flow, the shearing rate near X-points is much lower than that near the mid-plane (x-axis of the local Cartesian coordinate) of the MI on the same flux surface.  This calculation of E x B shearing profile and, in particular, minimal E x B shear near the X-points [TS Hahm et al, Phys Plasmas 28, 022302 (2021)] is consistent with the recent experimental finding that turbulence tends to spread into an MI through regions around the X-points [K Ida et al, Phys Rev Lett 120, 245001 (2018)]. In addition, the researchers show that the vortex tends to be better sustained in a large MI, while toroidicity induced precession can break up the quasi-helical symmetry of the vortex, leading to more complicated flow pattern in a long term around a relatively thin MI [GJ Choi and TS Hahm, submitted to Phys Rev Lett (2022)].

Transport barriers (TBs), either those produced in edges (H-mode) or inside (ITBs), are crucial for achieving high performance plasmas. Here, the researchers study the origin for the formation of ITBs which have been found in magnetic-shear free toroidal plasmas in both nearly flat and/or reversed safety factor profiles where the eigen-value is not determined from the magnetic structure due to the luck of resonance. Based on the global electromagnetic gyro-kinetic simulation in finite beta-value plasmas, the researchers present a new law to reveal the higher order counterpart as a discrete set of independent dispersion groups by regrouping toroidal mode numbers so as the non-resonant free energy to be sequenced according to the mismatch from the resonance. The regrouped dispersion leads to a new class of turbulence expressed by quasi-independent hierarchized spectra due to the selective energy transfer among them. They also present a flux driven full-f gyro-kinetic simulation as an example of the self-consistent formation of ITBs with heat source and sink by exciting different ion and electron modes, ie ITGs and TEMs, incorporated with ion and electron heating utilising the spatial dimension in magnetic-shear free toroidal plasmas.

The two main concepts for magnetically confined fusion plasmas are the tokamak and the stellarator. The flexibility and external control of the stellarator configuration in combination with unique diagnostic capabilities convert stellarators into ideal systems for the study of the relation between magnetic topology, electric fields and confinement transitions. This talk addresses advances in the characterisation of plasma transport, with emphasis on the physics of radial electric fields and transport control in the TJ-II stellarator. The presenter pays particular attention to the experimental evolution of fluctuation levels, the cross phase between fluctuating variables and zonal flows, revealing the simultaneous spatiotemporal evolution of these quantities, during spontaneous and biasing induced transitions. Causality detection techniques provide a deeper understanding of the interaction between the various fluctuating quantities. The experimental results described here, using unique diagnostic capacities in combination with advanced analysis tools, provide further physics understanding of confinement bifurcations driven by radial electric fields in stellarators, complementing well-known empirical approaches in tokamaks. It is concluded that there are different paths to achieve confinement bifurcations, and each path might correspond to different conditions (ie power / density threshold) for accessing the H-mode in fusion plasmas.

It is well admitted that the shear of the radial electric field E_r plays a key role in triggering and sustaining the edge transport barrier of H-mode tokamak plasmas. Here the researchers first report, just inside the separatrix of WEST plasmas, on the formation of a deeper well of E_r when operating in lower single null as compared to upper single null. These results obtained close to the L-H power threshold are consistent with the idea that the magnetic drift pointing towards the X-point favors the L-H transition. Second they unravel – by means of flux-driven simulations with the gyrokinetic code GYSELA encompassing core, edge and a simplified modelling of the scrape-off-layer in limiter configuration – how both the Reynolds stress and more critically its diamagnetic counterpart play a key role in the buildup of an E_r well in the L-mode edge of tokamak plasmas. Leading to a mild steepening of the temperature profile at the edge, these results shed light on the possible dynamics at play in the L- to H-mode transition. Finally, a 1-dimensional nonlinear model allows one to explore the competition of the Reynolds stress, diamagnetic stress and neoclassical poloidal flow damping in the formation of a sheared E_r profile.

The talk presents results from various dedicated L-H transition studies at JET-ILW. In particular Dr Solano reports changes in the value of the density at which the L-H transition power threshold is minimised as a function of plasma species and plasma shape. The researchers have obtained results in H, D, T, He and various mixtures. In He and D plasmas, Doppler reflectometer measurements of perpendicular velocity (related to the radial electric field profile) in the plasma edge indicate that there is no critical value of the radial electric field value or the shear of the v_ExB rotation before the transition. More importantly, during the L-mode phase, while the input power is being increased up to the L-H power threshold, there is no evidence of evolution of the E_r profile.