Persistence of Ion Temperature Gradient Turbulent Transport at Finite Normalized Pressure

Plasma $\beta$ dependence of electromagnetic turbulent transport is investigated by means of gyrokinetic simulations with self-consistent change of the equilibrium magnetic field. It is found that energy transport due to ion-temperature-gradient (ITG) driven turbulence does not decrease with increasing $\beta$; that is, the ion energy diffusivity does not decrease, and the electron energy diffusivity increases with $\beta$. This is because magnetic fluctuations are significantly influenced by the background magnetic field structure change with $\beta$ by the Pfirsch-Schluter current. The magnetic field change weakens the suppression effect of magnetic perturbations on the growth of the ITG mode, and it also suppresses nonlinear zonal flow production. The influence of the magnetic field change is significant as the global magnetic shear increases.

Figure 1

Linear growth rate as a function of normalized ion pressure ${\beta }_i$ in the magnetic field changed case (MFC, $\alpha \ne 0$) and in the magnetic field fixed case (MFF, $\alpha =0$) at $k_y{\rho }_{Ti}=0.2$. The magnetic field changed only in ${\omega }_d$ (${\omega }_{d}C$) and only in $k_{\perp }^2$ ($k_{\perp }C$) cases are added.

Figure 2

Ion and electron energy diffusivity in the MFC and MFF as a function of ${\beta }_i$.

Figure 3

Poloidal wave number spectra of (a) electrostatic potential $⟨|{\varphi }_k{|}^2⟩$ and (b) vector potential $⟨|A_{\parallel k}{|}^2⟩$ for MFC and MFF at ${\beta }_i=0.65\%$.

Figure 4

Time history of zonal flow production by the Reynolds and Maxwell stresses for the MFC at ${\beta }_i=0.65\%$.

Figure 5

Absolute values of the ${\omega }_{*e}$ term, the ${\omega }_{de}$ term, and the balance between the ${\nabla }_{\parallel }\varphi$ and ${\nabla }_{\parallel }{p}_e$ terms in the electron parallel velocity equation for the linear eigenfunction at $k_y{\rho }_{Ti}=0.2$ and ${\beta }_i=0.8\%$ for (a) the MFC and (b) the MFF.

Figure 6

(a) Ratio between linear growth rate for MFF and that for MFC ${\gamma }_{\mathrm{MFF}}/{\gamma }_{\mathrm{MFC}}$ as a function of ${\beta }_i$. (b) Ion energy diffusivity ${\chi }_i$ for MFC and MFF of AUG29224 with $s=1.06$.