Gyrokinetic Microtearing Turbulence

The nonlinear dynamics of microtearing modes in standard tokamak plasmas are investigated by means of ab initio gyrokinetic simulations. The saturation levels of the magnetic field fluctuations can be understood in the framework of a balance between (small poloidal wave number) linear drive and small-scale dissipation. The resulting heat transport is dominated by the electron magnetic component, and the transport levels are found to be experimentally relevant. Microtearing modes thus constitute another candidate for explaining turbulent transport in such toroidal systems.

Figure 1

Microtearing modes saturate at a fluctuation level of ${\tilde{B}}_x/B_0\sim {\rho }_e/L_{T_e}$. Using the nominal resolution and others, each of which represents a reduction by $1/3$ or $1/2$ in one of the phase space directions, leads to only moderate scatter.

Figure 2

Dependence of the magnetic field fluctuation amplitude on the plasma parameter ${\beta }_e$ for $R/L_{T_e}=4.5$.

Figure 3

Poincaré plots of magnetic field lines intersecting the perpendicular plane on the outboard side for weak drive ($R/L_{T_e}=2.5$, left-hand plot) and stronger drive ($R/L_{T_e}=3.5$, right-hand plot). The observed features are discussed in the text.

Figure 4

Electron heat diffusivity as a function of the relative fluctuation level of the magnetic field. The Gene results are described well by the Rechester-Rosenbluth model ${\chi }_e^{\mathrm{em}}=\eta v_{\mathrm{te}}qR({\tilde{B}}_x/B_0{)}^2$ with $\eta =1.37$. At small fluctuation amplitude, the model breaks down, as explained in the text.

Figure 5

Time-averaged (magnetic) electron heat flux spectrum in $k_y$ space for $R/L_{T_e}=3.5$. The linear growth rate spectrum is shown for comparison. Net free energy dissipation is found at $k_y{\rho }_i>0.2$.