Plasma edge

The measured losses of particle and heat from the confinement devices for fusion are observed to be larger than predictions from theories based on inter-particle collisions.

The discrepancy is believed to be due to transport governed by highly non-linear turbulent processes, occurring on multiple spatiotemporal scales, driven by different energy sources, mainly the inhomogeneity of plasma pressure profiles and confining magnetic field, and with a number of regulating mechanisms.

Since good confinement is necessary to achieve self-sustained, energy producing plasmas, understanding, controlling, and predicting turbulent transport across a wide variety of plasma conditions is an issue of crucial importance. In particular, understanding turbulence in the edge of magnetic confinement device is an outstanding open issue in magnetic fusion. In this region, plasma interacts with the solid wall of the device, determining the boundary conditions for the core plasma, and controlling the plasma refueling, heat losses, and impurity dynamics.

The study of edge turbulence is quite challenging. First, a very wide range of spatiotemporal scales is involved and a number of approximations that are used in the study of the core plasma (e.g., small amplitude fluctuations) are not valid. The edge magnetic geometry is also particularly demanding, being constituted by a closed flux surface region, and by the Scrape-off-Layer (SOL) region, which extends outside the last closed flux surface and is characterized by open magnetic field lines ending on the tokamak limiter or divertor.

Moreover, different turbulence regimes are observed in the edge. In particular, when the plasma is heated above a characteristic power threshold that depends, principally, on the plasma density, magnetic field amplitude, plasma configuration, and machine dimensions, turbulence is suppressed, and a characteristic sharp temperature gradient forms, the so-called edge pedestal, associated with the presence of a transport barrier. The transport barrier causes an increase of the plasma confinement time and leads to a transition from a low to a high mode (H-mode) confinement regime, the so-called L-H transition.

A key characteristic of these transport barriers is the presence of a localized, non-uniform electric field with associated cross-field flow shear. The H-mode pedestal can be subject to Edge Localized Modes (ELMs) that impact the global confinement and fusion performances and induce transient heat load on the tokamak first wall, constraining its lifetime.

Although the success of ITER will critically depend on achieving the H-mode regime and controlling its stability, the theoretical understanding of the transition to it and its dynamics, in particular the ELM physics, are still an open issue.

Leveraging on the unique possibilities offered by SPC, in particular the availability of a set of state-of-the-art numerical codes, the tokamak TCV, and the basic plasma physics experiment TORPEX, we are approaching the study of plasma edge turbulence, with the goal of contributing to the achievement of a viable design for a thermonuclear fusion reactor.

The present research is funded by a FNS Professeur Boursier Fellowship.