Theory of plasma blob formation and its experimental validation

Plasma blobs, that arise in the turbulent edge and scrape-off layer regions (SOL) of a tokamak, play an important role in the anomalous nature of plasma transport. A plasma blob can form from a radially elongated density structure when it breaks due to differential stretching in the radial and poloidal directions. Our model calculation shows that such a breakup can occur when the combined strength of the radial and poloidal electric field shear exceeds the growth rate of the interchange modes. This theoretical criterion has been validated against three-dimensional (3D) numerical simulation in the SOL region and is applicable for both  L-mode and H-mode plasmas. In the presence of an electron temperature gradient the poloidal gradient of the poloidal electric field is necessary to satisfy the criterion for H-mode plasmas. Experimental validation of the criterion is carried out using NSTX (shot # 141746) gas-puff imaging diagnostics data in the SOL region. The averaged values of the measured velocity shear at various locations as a function of time are scanned to look for coincidences between the occurrence of a blob due to the break-up of the radially elongated structure and the fulfillment of the criterion.