Role of Microtearing Turbulence in DIII-D High Bootstrap Current Fraction Plasmas

We report on the first direct comparisons of microtearing turbulence simulations to experimental measurements in a representative high bootstrap current fraction ($f_{\mathrm{BS}}$) plasma. Previous studies of high $f_{\mathrm{BS}}$ plasmas carried out in DIII-D with large radius internal transport barriers (ITBs) have found that, while the ion energy transport is accurately reproduced by neoclassical theory, the electron transport remains anomalous and not well described by existing quasilinear transport models. A key feature of these plasmas is the large value of the normalized pressure gradient, which is shown to completely stabilize conventional drift-wave and kinetic ballooning mode instabilities in the ITB, but destabilizes the microtearing mode. Nonlinear gyrokinetic simulations of the ITB region performed with the cgyro code demonstrate that the microtearing modes are robustly unstable and capable of driving electron energy transport levels comparable to experimental levels for input parameters consistent with the experimental measurements. These simulations uniformly predict that the microtearing mode fluctuation and flux spectra extend to significantly shorter wavelengths than the range of linear instability, representing significantly different nonlinear dynamics and saturation mechanisms than conventional drift-wave turbulence, which is also consistent with the fundamental tearing nature of the instability. The predicted transport levels are found to be most sensitive to the magnetic shear, rather than the temperature gradients more typically identified as driving turbulent plasma transport.

Figure 1

Profiles of safety factor $q$ and electron temperature $T_e$. The uncertainties of the $q$ profile (indicated by the shaded region) are evaluated using a Monte Carlo analysis based upon diagnostic uncertainties.

Figure 2

(a) $\gamma /k_y{\rho }_s$ (where $\gamma$ is the linear growth) of the electrostatic drift waves for different scaling factors of experimental $\alpha$. The (b) frequency and (c) growth rate of dominant electromagnetic instability of $k_y{\rho }_s=0.2$ versus ${\alpha }_{\mathrm{scale}}$.

Figure 3

The (a) frequency and (b) growth rate spectrum of the MTM. (c) The eigenfunction of $A_{\parallel }$ ($k_y{\rho }_s=0.2$) in the ballooning space. The growth rate of $k_y{\rho }_s=0.2$ versus (d) $q$, (e) $s$, and (f) ${\nu }_e$. “${\alpha }_{\exp }$” and “${\alpha }_{\mathrm{cons}}$” in (d) mean, respectively, that $\alpha$ is fixed to the experimental value and that it varies with $q$ self-consistently.

Figure 4

(a) The trace traces of the gyro-Bohm normalized energy flux of different species; “D,” “C,” and “$e$” represent deuterium, carbon, and electrons, respectively. (b) The energy spectrum of $Q_e$ induced by different fields. The results of $k_{x,\max }{\rho }_s$ equal to 11.8 and 19.0 are shown.

Figure 5

(a) The $s$ profile with uncertainties evaluated from Monte Carlo analysis. (b) Linear MTM spectra of different magnetic shear. (c) Comparison between the cgyro predicted $Q_e$ and the experimental one. (d) Variation of $Q_e(k_y)$ with $s$ for $\mathrm{\Delta }{k}_y{\rho }_s=0.08$.

Figure 6

(a) Maximum growth rate of MTM in the ITB region. (b) Contour plot of the growth rate of $k_y{\rho }_s=0.2$ in the $q$ and $a/L_T$ space.