Edge and divertor plasma: detachment, stability, and plasma-wall interactions

Abstract

The processes involving edge plasma and plasma-material interactions in magnetic fusion devices are very multifaceted and include a wide spectrum of phenomena ranging from plasma turbulence and meso-scale stability, recycling and transport processes of hydrogen species in the wall material, to the modification of wall material properties. In many cases these processes are strongly coupled and exhibit synergistic effects. Here we present the results of our studies of a wide range of edge plasma related issues: Our numerical simulations solve a long standing dispute on the roles of impurity radiation loss, plasma volumetric recombination, and ion-neutral friction in the rollover of the plasma flux to the target, which is the manifestation of detachment. We show that the rollover is caused by the increase of the impurity radiation loss and volumetric plasma recombination while the ion-neutral friction, although important for establishing necessary edge plasma conditions, does not contribute per se. With numerical modelling and theoretical analysis we consider stability of detachment and show that the absorption and desorption of hydrogen and impurity species from the wall can be crucial for a global stability of detached plasma. We also identify different mechanisms of meso-scale thermal instabilities driven by impurity radiation and resulting in a self-sustained oscillations of edge plasma parameters. We consider a trapping of He, which is an intrinsic impurity of fusion plasmas, in the first wall tungsten material. Our newly developed model, accounting for the generation of additional He traps caused by He bubble growth, fits all available experimental data on the layer of nano-bubbles observed in W under irradiation of low energy He plasma. Finally, we report on an impact of sheared magnetic field on the dynamics of blobs and ELM filaments playing an important role in edge and SOL plasma transport.

Physics of ultimate detachment of a tokamak divertor plasma

“…In most of the 2-D numerical simulations performed so far (in particular, in support of ITER), the cross-field plasma transport coefficients are assumed to be constant in time. A bifurcation-like transition to detachment may also be related to a breakdown of the quasi-equilibrium between the absorption and desorption processes on the plasma-facing components (Krasheninnikov et al 2017b). A large amount of hydrogen, at least in carbon wall tokamaks, is usually stored in the first wall material.…”

Self‐consistent coupling of the three‐dimensional fluid turbulence code TOKAM3X and the kinetic neutrals code EIRENE

“…However, in order to bring the simulations closer to experimental situations in divertor operation, it is necessary to include recycling physics in the models. This is mandatory to address regimes relevant for next‐step machines, high recycling regimes, and even detached regimes, all the more because experimental evidence points towards changes of transport with the density regimes . Several groups have implemented neutral models in their code, either in 2D or 3D codes: a simplified kinetic model in GBS and a fluid model in BOUT++ .…”

Analysis of the plasma blob formation/transport and its effects on impurity transport in the scrape‐off layer regions

“…As a first step, we focus on the plasma blob formation caused by edge turbulence by applying a two‐dimensional (2D) interchange turbulence model to the edge and the SOL regions. The 2D model consists of the density and vorticity equations with E × B drift and diamagnetic drift, which are considered to have a direct relation to the plasma blob formation and transport . The computational domain is set as a simple 2D slab geometry.…”

“…Recent studies have shown that non-diffusive radial transport is also important. [7][8][9][10][11][12] The plasma blob has been observed in the various fusion devices. [7,13,14] However, the formation mechanism of the plasma blob itself and also the effect of the plasma blob on the impurity transport have not yet been understood well.…”

“…The 2D model consists of the density and vorticity equations with E × B drift and diamagnetic drift, which are considered to have a direct relation to the plasma blob formation and transport. [9][10][11] The computational domain is set as a simple 2D slab geometry. As a second step, the effect of plasma blobs on the impurity transport is studied using a simple test particle model of impurities with their equations of motion.…”