Origin of Magnetic Stochasticity and Transport in Plasma Microturbulence

Nonlinear excitation of linearly stable microtearing modes—with zonal modes acting as a catalyst—is shown to be responsible for the near-ubiquitous magnetic stochasticity and associated electromagnetic electron heat transport in electromagnetic gyrokinetic simulations of plasma microturbulence.

Figure 1

Typical $A_{\parallel }$ POD mode structures. The $n=1$ mode (left) has ballooning parity, and the $n=2$ mode (right) has tearing parity.

Figure 2

The total electromagnetic electron heat flux spectrum (plus signs), summed over $k_x$ for $\beta =0.003$, decomposed into contributions from tearing modes (crosses), ballooning modes (asterisks), and all remaining fluctuations (circles).

Figure 3

The free energy in the POD tearing mode (solid black line) at $k_x{\rho }_s=0$, $k_y{\rho }_s=0.2$, and $\beta =0.003$, along with the total nonlinear drive (gray line—red online) and the nonlinear drive defined by coupling with zonal wave numbers (dashed line—blue online), plotted over a time segment of the nonlinear saturated state. The energetics of the tearing mode is dominated by the nonlinear drive which consists largely of the zonal coupling.

Figure 4

$Q_e^{\mathrm{EM}}$ plotted over the $\beta$ scan (plus signs), along with a model estimating the flux as a fixed fraction of ${\beta }^2{Q}_i^{\mathrm{ES}}$ (circles) and a model with an additional correction (crosses) defined by the quasilinear magnetic flux associated with the most unstable mode.