Giant Oscillations of Electron Temperature during Steady-State Operation on Tore Supra

During fully noninductively driven discharges in the Tore Supra tokamak, large spontaneous oscillations of the core electron temperature ($\Delta T_e/T_e>50\%$) have been observed for the first time. They occurred during the standard O regime, which is itself characterized by periodic oscillations of much smaller amplitude. The “giant” oscillations appear to involve distinct mechanisms with respect to the O regime and provide a spectacular example of the complex nonlinear interactions between energy confinement, noninductive current sources, and MHD that may occur in a tokamak plasma during steady-state operation.

Figure 1

(a) Time traces of $T_e$ (core ECE channels, blue: $\rho =0$, green: $\rho =0.1$, and red: $\rho =0.2$), plasma current $I_p$ (purple, multiplied by 12), LH power $P_{\mathrm{LH}}$ (black), and central electron density $n_{e0}$ (light blue), shot #33983. In this discharge, the toroidal field decreases slowly from $B_t=3.81$ to 3.76 T between $t=34$ and $t=55\mathrm{s}$, which may influence the repetition rate of the GOs. (b) Zoom on the start of the O regime for the same time traces. (c) Zoom on the first GO for the central ECE channels.

Figure 2

(a) Time traces of central electron temperature $T_{e0}$ (blue), LH power $P_{\mathrm{LH}}$ (black, top), parallel refractive index of the two LH launchers $n_{\parallel 1}$ and $n_{\parallel 2}$ (black, bottom), and central electron density $n_{e0}$ (light blue), shot #33986. (b)–(d) Zoom on various phases of the shot showing the different behavior of $T_{e0}$ as $n_{\parallel }$ increases.

Figure 3

Dynamics of $T_e$ and core ${\chi }_e^{\mathrm{eff}}$ profiles during a GO at $n_{\parallel }=1.8$ of shot #33986. (a) and (b): slow decrease phase starting from the O regime level, $t=26.4$ to 26.5 s; (c) and (d): increase phase up to the top of the GO, $t=26.5$ to 26.7 s. On (a) and (c), solid lines correspond to the ECE signal, displayed every $\Delta t=0.01\mathrm{s}$. Blue circles are Thomson scattering measurements, time averaged over 100 ms. On (a), the small $T_e$ increase near $\rho =0.25$ (small bump in the red profile at $t=26.48\mathrm{s}$) shows the outward energy flow. On (b) and (d), the effective diffusivities are displayed every $\Delta t=0.02\mathrm{s}$ using the same color code as in the left column.

Figure 4

Color contour plot of $T_e$ (keV) showing the growth of a large ($m=2$, $n=1$) tearing mode (between $t=31.1$ and 31.15 s, magnetic islands around $R=2.63$ and 2.55 m, indicated by white arrows) at the transition from the O regime to a GO event (shot #33982).

Figure 5

Dynamics of various profiles during a GO at $n_{\parallel }=1.8$ of shot #33986 (same GO as in Fig. 3), at $t=26.4\mathrm{s}$ (blue, O regime just before the GO), 26.5 s (green, minimum $T_e$), and 26.7 s (red, top of the GO). (a) LH current density (deduced from Abel inversion of the FEB signal), (b) bootstrap current density, and (c) $q$ profile deduced from current diffusion simulation, (d) electron temperature from ECE. Note that the $q$ profiles should be taken with caution, owing to the presence of MHD activity during the various phases of the GO.