Observation of Double Impurity Critical Gradients for Electromagnetic Turbulence Excitation in Tokamak Plasmas

The impact of impurity ions on a pedestal has been investigated in the HL-2A Tokamak, at the Southwestern Institute of Physics, Chengdu, China. Experimental results have clearly shown that during the $H$-mode phase, an electromagnetic turbulence was excited in the edge plasma region, where the impurity ions exhibited a peaked profile. It has been found that double impurity critical gradients are responsible for triggering the turbulence. Strong stiffness of the impurity profile has been observed during cyclic transitions between the $I$-phase and $H$-mode regime. The results suggest that the underlying physics of the self-regulated edge impurity profile offers the possibility for an active control of the pedestal dynamics via pedestal turbulence.

Figure 1

Time evolution of the main parameters in a typical discharge with cyclic transitions: (a) plasma current $I_p$, toroidal magnetic field $B_t$ and NBI heating power $P_{\mathrm{NBI}}$, (b) divertor $D_{\alpha }$ signal, (c) inner stored energy $W_E$ and line-averaged electron density $n_{\text{el}}$, (d) the ratio of total radiation power to line-averaged electron density ($P_{\text{rad}}/n_{\text{el}}$) and intensity of carbon impurity ion C v 2271 Å. Profiles of (e) electron density and temperature, and (f) indication of the impurity density $I_{\text{bolm}}/n_e$ in the $I$ phase and $H$ mode.

Figure 2

(a) Divertor $D_{\alpha }$ signal and the indications of the $I$ phase and $H$ mode. (b) Spatiotemporal evolution of $n_z$, maximum $n_z$ gradient in the positive (c) and negative (d) gradient region.

Figure 3

(a) Divertor $D_{\alpha }$ signal, edge $n_z$ and the indications of $I$ ($I$ phase) and $H$ ($H$ mode), spectrogram of magnetic (b) and density (c) fluctuations, (d) coherency spectra between magnetic and density fluctuations in $I$ phase and $H$-mode phase, (e) time evolution of integrated coherency over the frequency domain of 50–100 kHz, (f) filtered magnetic fluctuations and (g) the intensity evaluated by the square of the root- mean-square (RMS) envelope in two $H$-mode phases.

Figure 4

(a) Contour plot of the electromagnetic turbulence spectrogram during the $H$-mode phase, (b) correlation between the frequency spectrum width of the turbulence and the turbulence intensity, (c) autobispectrum of magnetic fluctuations during the $H$-mode phase and (d) its summed bispectrum.

Figure 5

(a) Experimental result: the relation between the intensity of electromagnetic turbulence and the normalized impurity density gradient, (b) electromagnetic simulation by the HD7 code: the normalized linear growth rate of electromagnetic turbulence versus the normalized impurity density gradient.