Ballooning Mode in a Stochastic Magnetic Field—A Quasi-mode Model*

Recently, Minjun Choi et al. employed entropy-complexity analysis to pedestal temperature fluctuations with RMP switched on, revealing a reduction in the predictability of pedestal turbulence. Given that peeling-ballooning mode is a likely mechanism of ELM, studying how a stochastic magnetic field affects ballooning mode is beneficial to our understanding of these results. Due to the fact that models for ballooning mode are set up in a torus while theories involving RMP focus on resonant surfaces in a cylinder, reconciling these two different geometries is necessary to develop a comprehensive theory. Since quasi-mode is the counterpart of ballooning mode in a cylinder, we choose to study quasi-mode first, and then extend the results to ballooning mode.

In this work, a multi-scale model of quasi-mode in a stochastic magnetic field is presented. The insights we have gained regarding the ballooning mode are:

1. To maintain ▽·J=0 at all scales, a microturbulence is driven.

2. The scaling of the resultant turbulent viscosity is larger than that in our study on interchange mode. This can be attributed to the variations in mode structure.

3. Stochastic magnetic fields can stabilize mode growth by enhancing the effective inertia, reducing the effective drive, and boosting mixing.

4. A non-trivial correlation develops between velocity fluctuations and magnetic perturbations. This could explain the reduction in the predictability of the pedestal turbulence.

Experiments for future investigations are also proposed.