Turbulence Spreading into an Edge Stochastic Magnetic Layer Induced by Magnetic Fluctuation and Its Impact on Divertor Heat Load

Turbulence spreading into the edge stochastic magnetic layer induced by magnetic fluctuation is observed at the sharp boundary region in the large helical device. The density fluctuation excited at the sharp boundary region with a large pressure gradient does not propagate into the boundary region due to the blocking of turbulence spreading by the large second derivative of the pressure gradient. Once the magnetic fluctuation appears at the boundary, the density fluctuation begins to penetrate the edge stochastic layer and the second derivative of the pressure gradient also decreases. The increase of density fluctuation in this layer results in the broadening and reduction of the peak divertor heat load. It is demonstrated that magnetic fluctuation plays a key role in controlling the turbulence spreading at the boundary of plasma which contributes to the reduction of divertor heat load.

Figure 1

Radial profiles of (a) $L_c$ in the edge region. (b) 2D distribution of $L_c$. Corresponding locations of $r_{\mathrm{eff}}=0.48$ and 0.60 m are indicated in (b).

Figure 2

(a) Spectrogram of magnetic fluctuation, temporal evolutions of radial profiles of (b) $P_e$ and (c) density fluctuation (${\tilde{n}}_e$), (d) ${\tilde{n}}_e$ in the stochastic layer ($r_{\mathrm{eff}}=0.54\mathrm{m}$), (e) divertor peak heat load. (f) ${\tilde{n}}_e$ in the stochastic layer as a function of the magnetic fluctuation intensity of $m/n=3/3$ mode (#148584). Time evolutions of (g) radial profiles of ${\tilde{n}}_e$ and (h) divertor peak heat load in the case without the $m/n=3/3$ magnetic fluctuation (#148600). The divertor heat load during the MHD burst is masked.

Figure 3

Radial profiles of (a) $T_e$, (b) $n_e$, (c) electron pressure, (d) 1st radial derivative of the pressure, (e) 2nd radial derivative of the pressure, (f) $V_{ExB}$ shearing rate, (g) density fluctuation, and (h) radial derivative of the density fluctuation. Blue (open) symbols: $t=4.10\sec$, red (closed) symbols: $t=4.33\sec$ (#148584).

Figure 4

(a) Divertor heat load profile normalized by the peak values, fitted with a Gaussian function (dotted lines). (b) $L_c$ profile at the divertor plate. Blue (open) symbols: $t=4.09\sec$, before the turbulence spreading. Red (closed) symbols: $t=4.40\sec$, after the turbulence spreading. The peak heat load are 0.81 and $0.23\mathrm{MW}/{\mathrm{m}}^2$ at 4.10 and 4.40 sec, respectively. (c) Divertor peak heat load, (d) the FWHM of the heat load profile, (e) Wp, and (f) ${\overline{n}}_e$, as a function of density fluctuation in the stochastic layer ($r_{\mathrm{eff}}=0.56\mathrm{m}$). (g) Divertor peak heat load as a function of the edge pressure gradient at $r_{\mathrm{eff}}=0.47-0.49\mathrm{m}$. Blue circles: $t<4.15\sec$, before turbulence spreading, red circles: $t>4.17\sec$, after turbulence spreading, green square, $t=4.16\sec$. The dashed line shows a linear fit to the data before the spreading ($t<4.15\sec$).