Plasma blob formation by ion kinetic Kelvin–Helmholtz and interchange instabilities

Abstract

The near-edge region of a tokamak is observed to generate radially propagating, coherent filamentary structures (‘blobs’), which transport particles and heat from the confined region and across the scrape-off layer. The distribution of blob sizes may include a currently unresolvable population with radii comparable to the ion gyro-radius. Here, we conduct large-scale numerical simulations to study mechanisms for the creation of ion gyro-scale blobs via the ion kinetic Kelvin–Helmholtz and interchange instabilities, using a hybrid (kinetic ion, fluid electron) model. We present statistics of the sizes of blobs created by these instabilities, and radial particle displacement data. We find that ion gyro-scale blobs constitute a significant portion of the blob population, and that an increase in ion gyro-radius results in an increase in radial transport. Results are contrasted for pure proton plasmas and for 50 : 50 deuterium–tritium mix, relevant to burning plasmas. We conclude that ion kinetic physics plays a significant role in the transport of energy and particles by ion gyro-scale blobs in the near-edge region of low-field tokamaks.