Isotope Effects on Trapped-Electron-Mode Driven Turbulence and Zonal Flows in Helical and Tokamak Plasmas

Impacts of isotope ion mass on trapped-electron-mode (TEM)-driven turbulence and zonal flows in magnetically confined fusion plasmas are investigated. Gyrokinetic simulations of TEM-driven turbulence in three-dimensional magnetic configuration of helical plasmas with hydrogen isotope ions and real-mass kinetic electrons are realized for the first time, and the linear and the nonlinear nature of the isotope and collisional effects on the turbulent transport and zonal-flow generation are clarified. It is newly found that combined effects of the collisional TEM stabilization by the isotope ions and the associated increase in the impacts of the steady zonal flows at the near-marginal linear stability lead to the significant transport reduction with the opposite ion mass dependence in comparison to the conventional gyro-Bohm scaling. The universal nature of the isotope effects on the TEM-driven turbulence and zonal flows is verified for a wide variety of toroidal plasmas, e.g., axisymmetric tokamak and non-axisymmetric helical or stellarator systems.

Figure 1

Collisionality dependence of the radial heat flux $\sum _{s=i,e}{q}_s$ with the mixing-length diffusivity $\gamma /k_{\perp }^2$ for the linear TEM (L-TEM) and ITG (L-ITG) modes in LHD inward-shifted H, D, and T plasmas, where $k_x{\rho }_{ts}=0$, $k_y{\rho }_{ts}=0.4$ ($s=\{\mathrm{H},\mathrm{D},\mathrm{T}\}$). For qualitative comparison, the nonlinear results from Fig. 2 are also displayed for the TEM case (NL-TEM).

Figure 2

Impacts of the hydrogen isotope mass on (a) turbulent heat fluxes normalized by the hydrogen gyro-Bohm unit ($\sum _s{q}_s/q_{\mathrm{GB}(\mathrm{H})}$), (b) turbulence ($W_{\text{turb}\mathrm{.}}$) and zonal-flow ($W_{\mathrm{ZF}}$) energy, and (c) normalized entropy transfer to zonal flows ($\sum _s{T}_s{\mathcal{T}}_s^{(\mathrm{ZF})}/\sum _s{q}_s{L}_{T_s}^{-1}$), where $s=\{i,e\}$, and ${\nu }_{ei}^{*}=0.07$.

Figure 3

Spatial structures of the potential fluctuations in the TEM-driven turbulence for (a) LHD H, (b) LHD D, (c) CBC-like tokamak H, and (d) CBC-like tokamak D plasmas, where the simulation conditions of LHD and CBC tokamak cases correspond to those in Figs. 2 and 4 (${\nu }_{ei}^{*}=0.035$), respectively.

Figure 4

Collisionality dependence of the radial heat flux $\sum _s{q}_s$ for (a) ITG and (b) TEM cases in CBC-like tokamak plasmas, where the linear mixing-length (L) and the corresponding nonlinear (NL) results are plotted by dashed lines and line symbols, respectively. The linear results are uniformly scaled for qualitative comparison of profiles. NL-TEM cases without the zonal flow are also displayed by the open symbols.

Figure 5

Radial profiles of TEM-driven zonal flows in CBC-like tokamak H and D plasmas for (a) ${\nu }_{ei}^{*}=0.018$ and (b) ${\nu }_{ei}^{*}=0.035$, where a larger relative amplitude of the steady zonal flow appears in D plasma with ${\nu }_{ei}^{*}=0.035$ in comparison to the others.