A universal mechanism for plasma blob formation

The formation of a density blob in the boundary region of a tokamak plasma is widely believed to be due to the presence of poloidal velocity shear that differentially breaks off a radially elongated streamer structure. However, blobs have also been experimentally detected in the absence of poloidal velocity shear. We ascribe the formation of such blobs to the differential stretching of the streamer arising from a radial velocity shear and thereby present a unified picture of the breaking mechanism that takes into account both the processes in a synergistic manner. Our universal condition for blob formation is validated from detailed numerical simulations of the edge turbulence in a typical L-mode scenario.