Disruption Avoidance in the Frascati Tokamak Upgrade by Means of Magnetohydrodynamic Mode Stabilization Using Electron-Cyclotron-Resonance Heating

Disruption avoidance by stabilization of MHD modes through injection of ECRH at different radial locations is reported. Disruptions have been induced in the FTU (Frascati Tokamak Upgrade) deuterium plasmas by Mo injection or by exceeding the density limit (D gas puffing). ECRH is triggered when the $V_{\mathrm{loop}}$ exceeds a preset threshold value. Coupling between MHD modes ($m/n=3/2$, $2/1$, $3/1$) occurs before disruption. Direct heating of one coupled mode is sufficient to avoid disruptions, while heating close to the mode leads to disruption delay. These results could be relevant for the International Thermonuclear Experimental Reactor tokamak operation.

Figure 1

Comparison of two discharges with Mo-injection: (a) $I_p$, (b) central $T_e$ from ECE, (c) MHD (envelope of oscillations of ${\dot{B}}_{\theta }$, midplane, low field size), (d) width and (e) radius of $m=2$ and $m=3$ islands from soft x-ray tomography. The disruption is avoided in 29 479 by application of ECRH; the deposition location is shown in (e).

Figure 2

Abel-inverted soft x-ray contours (discharge 29 473).

Figure 3

Mirnov coils analysis for discharge 29 473 using phases of the various coils to extract the $m$ (poloidal mode number) and $n$ (toroidal mode number) of the modes. The circles indicate the time when the $3/2$ mode is just visible.

Figure 4

Correlation between mode locking time ($t_{\mathrm{lock}}$) and MHD activity starting time ($t_{\mathrm{MHD}}$).

Figure 5

Dependence of current quench time delay on ECRH deposition position for (a) boron- and (b) lithium-conditioned walls. The $q$ values are obtained from island visualization through soft x-ray tomography (except for $q=1$ determined from sawtheeth inversion radius).

Figure 6

Soft x-ray tomography at $t=0.818\mathrm{s}$ for discharge 29 473 (no ECRH); sight lines shown as dashed lines.

Figure 7

Soft x-ray tomography at $t=0.825\mathrm{s}$ (a) and $t=0.836\mathrm{s}$ (b) for discharge 29 984 ($r_{\mathrm{dep}}=5.2\mathrm{cm}$).

Figure 8

Comparison between Rutherford model and experimental evolution of islands (a); calculation of $m=2$ island width versus absorbed $P_{\mathrm{ECRH}}$ (based on experimental input data for discharge 29 479); also shown the experimental point measured during the stabilization phase [see Fig. 8] (b).

Figure 9

Density limit disruptions: (a),(c) Mirnov coils traces; (b),(d) corresponding FFT analysis; (e) $I_p$ with $P_{\mathrm{ECRH}}=0.8\mathrm{MW}$ (dashed, 27 802) and without (solid, 27 799).