Probing Internal Transport Barriers with Heat Pulses in JET

The first electron temperature modulation experiments in plasmas characterized by strong and long-lasting electron and ion internal transport barriers (ITB) have been performed in JET using ion cyclotron resonance heating in mode conversion scheme. The ITB is shown to be a well localized narrow layer with low heat diffusivity, characterized by subcritical transport and loss of stiffness. In addition, results from cold pulse propagation experiments suggest a second order transition process for ITB formation.

Figure 1

(a) Experimental profiles at $t=8\mathrm{s}$ (maximum performance) of $T_e$, $T_i$, $n_e$, and $q$ for shot 59 397 ($3.45\mathrm{T}/2.8\mathrm{MA}$, ${}^3\mathrm{He}\sim 12\%$, ICRH $f=33\mathrm{MHz}$). The ITB region is highlighted. Uncertainties for $T_e$, $T_i$ are in the order of 10%. (b) profiles of Fourier component of $A$ [squares (red online)] and $\phi$ [circles (blue online)] at the modulation frequency (15 Hz) during the time interval 6.2–6.48 s. Estimated rf power deposition profiles are also plotted (dashed black line).

Figure 2

(a) Experimental profiles at $t=5.5\mathrm{s}$ of $T_e$, $T_i$, $n_e$, and $q$ for shot 62 077 ($3.25\mathrm{T}/2.6\mathrm{MA}$, ${}^3\mathrm{He}\sim 20\%$, ICRH $f=37\mathrm{MHz}$). The ITB region is highlighted. (b) profiles of Fourier component of $A$ [squares (red online)] and $\phi$ [circles (blue online)] at the modulation frequency (20 Hz) during the time interval 5.5–5.7 s. Estimated rf power deposition profiles are also plotted (dashed black line). (c) same as (b) for the time interval $t=8–9\mathrm{s}$, when the ITB has been lost.

Figure 3

Schematic of 2nd order transition for ITB formation. The turbulence threshold is higher than in conventional plasmas. Two situations can be hypothesized, as discussed in the text.

Figure 4

Experimental (dots) and simulated using the CGM model [4] (lines) profiles of amplitudes (black full symbols) and phases [open symbols (red online)] at fundamental modulation frequency for shot 62 077. In the inset also the ${\chi }_e$ profile (black full line) used in the simulation is plotted at one time during the modulation ON phase, together with the time constant profile of the threshold ${\kappa }_c$ [dashed line (red online)].

Figure 5

Schematic of 1st order transition for ITB formation.

Figure 6

Contour plots of $\Delta T_e$ during cold pulse in ITB plasma: (a) experimental discharge 51 581; (b) simulated with an empirical 2nd order transition model (as in Fig. 3), (c) simulated with an empirical 1st order transition model (as in Fig. 5). Units of $\Delta T_e$ color codes are eV. $t=0\mathrm{ms}$ corresponds to the application of the cold pulse in the edge region.