June 12, 2019

Courant researchers A.J. Majda, D. Qi, and A.J. Cerfon propose a two-field flux-balanced Hasegawa-Wakatani (BHW) model [1,2] to improve the understanding of plasma edge turbulence and the creation of zonal structures. The model provides an improved treatment for the balanced electron dynamics on magnetic flux surfaces. With the new treatment, complex multi-scale dynamics are observed with multiple jets interacting with one another, and intermittent bursts. A detailed statistical analysis is used which highlights how the changes in the aspect ratio of the computational domain affect the third-order statistical moments, and thus modify the turbulent dynamics.

The researchers also develop theory to address the generation and persistence of large-scale coherent flow structures that are always observed in flux-balanced models. First, the maintaining of a dominant zonal flow structure is related with the selective decay principle that dissipates energy among all the other scales in a much faster rate than a particular single selected scale [3]. Second, the generation mechanism of zonal jets is displayed from the secondary instability analysis via nonlinear interactions with a background base state [4]. Together with the selective decay effect from the dissipation, the secondary instability offers a complete characterization of the convergence process to the purely zonal structure. Finally, a systematically derived unstable forcing is introduced to the modified model mimicking the role of the inherent instability near the low resistivity limit [5]. The insight gained from the simple model analysis can also offer guidelines for the development of model reduction methods for more complicated systems.


References :

[1] Majda, A.J., Qi, D., and Cerfon, A.J., "A flux-balanced fluid model for collisional plasma edge turbulence: model derivation and basic physical features", Physics of Plasmas (https://aip.scitation.org/doi/abs/10.1063/1.5049389)

[2] Qi, D., Majda, A.J., and Cerfon, A.J., "A flux-balanced model for collisional plasma edge turbulence: numerical simulations with different aspect ratios", submitted to Physics of Plasmas (https://arxiv.org/abs/1812.00131)

[3] Qi, D. and Majda, A.J., "Transient metastability and selective decay for the coherent zonal structures in plasma edge turbulence", J. Nonlinear Sci. (https://link.springer.com/article/10.1007/s00332-019-09544-5)

[4] Qi, D. and Majda, A.J., "Zonal jet creation from secondary instability of drift waves for plasma edge turbulence", Chin. Ann. Math., (https://arxiv.org/abs/1901.08590)

[5] Qi, D. and Majda, A.J., "Linking the two-field dynamics of plasma edge turbulence with the one-field balanced model through systematic unstable forcing at low resistivity", Physics of plasmas (https://aip.scitation.org/doi/10.1063/1.5095406)