Observation of Reduced Electron-Temperature Fluctuations in the Coreof H-Mode Plasmas

Core electron-temperature fluctuations [$0.5\%\le {\tilde{T}}_e/T_e\le 2\%$, $k_{\theta }{\rho }_s\le 0.3$ in neutral-beam-heated low confinement-mode (L-mode) plasmas] are observed to decrease by at least a factor of 4 in standard and quiescent high-confinement-mode (H-mode and QH-mode) regimes in the DIII-D tokamak ($r/a=0.7$). These fluctuations are attributed to ion temperature gradient (ITG) modes stabilized by rotational shear at the H-mode transition. The simultaneous reduction in electron heat diffusivity (${\chi }_e^{\mathrm{QH}}/{\chi }_e^L<0.25$) suggests that ${\tilde{T}}_e$ fluctuations can contribute significantly to L-mode electron heat transport.

Figure 1

Schematic of the experimental arrangement.

Figure 2

(a) Injected neutral-beam power and plasma current;(b) line-averaged density; (c) electron temperatureat $r/a=0.7$; (d) $D_{\alpha }$ line emission intensity; (e) normalizedelectron-temperature fluctuation level for shot no. 125113. L-Hand H-L transitions are indicated by vertical dashed lines, the QH-mode phaseis shaded.

Figure 3

Correlation coefficient and cross-power spectra before and afterthe H-mode transition ($t_{\mathrm{L}\mathrm{\text{−}}\mathrm{H}}=900\mathrm{ms}$). The dataare integrated over 200 ms (a)–(b), (e)–(f), and100 ms (c)–(d).

Figure 4

(a) Cross-power spectrum $P_{12}(f,t)=[{\tilde{T}}_R(f,t)/T_R(t){]}^2$ and mean frequency $f_M$ (white line); the vertical dashed line indicates the L-Htransition; (b) mean frequency $f_M$ and frequency FWHM of spectrum in (a); (c) $\mathit{E}\times \mathit{B}$ frequency $f_{\mathit{E}\times \mathit{B}}=v_{\mathit{E}\times \mathit{B}}(r/a=0.7)k_{\theta }/2\pi$ for $k_{\theta }=1.25{\mathrm{cm}}^{-1}$, and $f_M$ (dashed line); (d) $D_{\alpha }$ line intensity indicating the timing of the L-Htransition.

Figure 5

Electron heat diffusivity in L mode (solid line) andQH mode (dashed line). Also shown is the ratio of the lineargrowth rate ${\gamma }_L$ (calculated with the TGLF code for $k_{\theta }=1.25{\mathrm{cm}}^{-1}$) to the $\mathit{E}\times \mathit{B}$ shearingrate ${\omega }_{\mathit{E}\times \mathit{B}}$.