144. blobs and drift wave dynamics
Name: Yanzeng Zhang
Grad Year: 2021
Blobs, high plasma density coherent filamentary structures propagating in tokamak edge toward outer wall with speed ~1 km/s, play an important role in the scrape-off layer plasma transport in both L-mode and H-mode in between ELMs. Although the blobs are studied for about 15 years, the mechanism(-s) of blob formation is still under debates. Meanwhile blobs are often observed inside the separatrix where they move mainly in poloidal direction and once in a while cross the separatrix and appear in the SOL. Recently, the Hasegawa-Mima equation was generalized by considering Boltzmann electrons and keeping all nonlinearities, which illustrated that the traveling wave solution F(x,y,t)=F(y-Ut) at large amplitude resembles blob-like structure. Based on this work, two conservative integrals, which are different from the generalized energy and enstrophy, are generated and discussed. From the first integral, it follows that the amplitude of wave-packet propagating in the direction of decreasing background plasma density will increase exponentially with the distance travelled until nonlinear effects become important, while the amplitude of the normalized electrostatic potential is limited by the second integral. The analytics and numerical simulations of the governing equation in both 1-D and 2-D cases are provided. In 1-D case (neglecting x-dependence), the amplitude of the potential increases as a result of modulational instability, while in 2-D case, it's mainly due to the conservation of the plasma density and will eventually saturates at a level ~1 by the nonlinearity. At this stage, normalized density bursts reaching factor ~3, which is close to experimental observations of blob plasma density in the vicinity of the separatrix.
Industry Application Area(s)
Energy/Clean technology | Plasma confinement, Controlled thermal nuclear fusion