On the relative roles of neutral transport and plasma transport in the formation of the density pedestal in magnetically confined plasmas

Abstract

Currently, a much discussed question regarding the characteristic features of the plasma density radial profile—the ‘pedestal’—existing at or just inside the magnetic separatrix in tokamaks—is whether the neutral or plasma transport in that region is more controlling. In actual tokamak conditions, the neutral transport is generally quite complicated geometrically, making it difficult to understand the relative roles of plasma and neutral transport. Here, therefore, we consider a quite simplified configuration—one-dimensional slab geometry—in order to elucidate the essential aspects of the problem. It is found here, for the assumption of diffusive cross-field plasma transport, that simple quasi-one-dimensional modelling indicates that the spatial distribution of ionization of the recycling hydrogen neutral particles in the pedestal region, i.e. neutral transport, plays a strong role. It is also found, however, that the influence of plasma transport—specifically the effect of a ‘transport barrier’ just inside the separatrix where D⊥ (r) varies rapidly—can also play a significant role. It is found that a third—and roughly equally important—influence on the shape of the density profile inside the separatrix is the value of the density decay length outside the separatrix, λSOL, which enters as an important boundary condition of the problem; it is therefore important to know the relationship, e.g. from experiment, between λSOL and the pedestal density, nped, and to include that in the analysis. This paper ends with a discussion of what is required to analyse an actual tokamak plasma configuration. When undertaking the full three-dimensional interpretive analysis of a specific discharge condition in a specific tokamak, if the neutral transport—i.e. the spatial distribution of the ionization—can be accurately modelled in detail, and if the boundary condition information is available as input from the experimental data, then it should be possible to extract information on D⊥ and its spatial variation near the separatrix from detailed particle-balance analysis of the density profile.