Transport barrier for spinning blobs in magnetically confined plasmas

In this work, we use the full-f gyrokinetic particle-in-cell code XGC [1] to investigate the blobs in the electrostatic limit. The blobs are seeded in the core region close to the separatrix with the realistic magnetic field from C-Mod H-mode discharge. We found the blob with a large amplitude has a stable shape with a large spin and a small radial motion. We also found the spinning blob with a certain amplitude in the midplane is split into two blobs in the separatrix region: the inner blob has a bounce motion (switch between the negative and positive radial velocity in a certain radial region), and the outer blob moves outward radially. It is characterized as a transport barrier for spinning blobs.

Here, we construct a new theoretical framework to interpret the transport barrier. Rather than the conventional fluid blob theory, our theory includes the ExB motion in the parallel Ohm’s law since the spin dominates the blob motion. The potential from the adiabatic response is in the zero-order while the dipole potential is in the next order. It shows that blob radial motion in response to the zonal electrostatic potential acts as to be attracted to a potential well as if the blobs were a positive charge, which is consistent with our simulations.

In this work, based on the simulations, we construct a new theoretical framework to explain the transport barrier is induced by the zonal field effect on the spinning blob motion, which is the potential explanation for the negative radial velocity of blobs in experiments [2,3]. The transport barrier is also calculated with realistic parameters in experiments, which agrees with our theoretical prediction.



[1] S. Ku et al, Phys. Plasmas 25, 056107 (2018)

[2] C. K. Tsui et al., Physics of Plasmas 25, 072506 (2018)

[3] S J Zweben et al., Plasma Phys. Control. Fusion 58, 044007 (2016)