Inductive Current Density Perturbations to Probe Electron Internal Transport Barriers in Tokamaks

Improved electron energy confinement in tokamak plasmas, related to internal transport barriers, has been linked to nonmonotonic current density profiles. This is difficult to prove experimentally since usually the current profiles evolve continuously and current injection generally requires significant input power. New experiments are presented, in which the inductive current is used to generate positive and negative current density perturbations in the plasma center, with negligible input power. These results demonstrate unambiguously for the first time that the electron confinement can be modified significantly solely by perturbing the current density profile.

Figure 1

Time traces of the (a) plasma current and EC power, (b) Ohmic transformer current, (c) electron thermal confinement time, and (d) loop voltage for typical scenarios. We show cases with no perturbation at 1.4 s (#25 956), $+30\mathrm{mV}$ (#25 957), and $-30\mathrm{mV}$ (#25 953). (e) TCV poloidal cross section with the plasma boundary and the rays representing the gyrotrons beams. The beams $A$ drive co-CD off axis, while $B$ is in the poloidal plane, ECH on axis.

Figure 2

Pressure profile before any perturbation, averaged on $[1\mathrm{s},1.4\mathrm{s}]$ (star symbols), with $+30\mathrm{mV}$, $[1.8\mathrm{s},2.4\mathrm{s}]$ #25 957 (triangles), $-30\mathrm{mV}$, $[1.8\mathrm{s},2.4\mathrm{s}]$ #25 953 (squares). The profile before the central heating beam is added is also shown $[0.6\mathrm{s},0.8\mathrm{s}]$ (circles).

Figure 3

(a) $H_{\mathrm{IT}98L}={\tau }_{Ee}/{\tau }_{\mathrm{IT}98L}$ for a series of discharges with different constant slopes in the Ohmic transformer current imposed at 1.4 s. (b) Electron pressure profiles for the discharges shown in (a) averaged over the time interval $[1.8\mathrm{s},2.4\mathrm{s}]$.

Figure 4

(a) $q$ profile corresponding to the case #25 956, solid line, as calculated with ASTRA using only experimental inputs for profiles, $P_{\mathrm{EC}}$, $j_{cd}$, and ${\chi }_e$. The $q$ profiles obtained by imposing $V_{\mathrm{loop}}=+30\mathrm{mV}$ (dashed line, #25 957) and $-30\mathrm{mV}$ are also shown (dash-dotted line, #25 953). (b) Time evolution of $I_p$ and magnetic shear near $q_{\min }$ when a $V_{\mathrm{loop}}=+30\mathrm{mV}$ perturbation is imposed at $t=1.4\mathrm{s}$, as calculated with ASTRA and corresponding to the evolution from the solid line shown in (a), #25 956, to the dashed line, #25 957. $I_p(t)$ can be fitted with $\exp (-t/0.15)$ yielding ${\tau }_{\mathrm{crt}}=0.15\mathrm{s}$.

Figure 5

Contour of the soft x-ray signals for the cases with $+60\mathrm{mV}$ (#25 958) and $-60\mathrm{mV}$ (#25 952) perturbations. In the first case, the barrier improves when the positive inductive current enhances the nonmonotonic $j$ profile and then it degrades the barrier when the central $j_0$ increases. The opposite happens with a negative current induced from the plasma edge, #25 952.