IRFM > Highlights > Power modulation in Tore Supra clarifies the role of plasma turbulence in particle transport
Power modulation in Tore Supra clarifies the role of plasma turbulence in particle transport
Particle transport is a key issue in magnetized fusion plasmas. Understanding its physics mechanism is essential for predicting the fusion performance in ITER and for the design of future fusion reactors. The relation between particle transport and plasma turbulence has been characterized in the Tore Supra tokamak by a set of original experiments carried out by a CEA team where the plasma turbulence is modified by heat source modulation using Ion Cyclotron Resonance Heating (ICRH) power. The interpretation of the experimental results recently published in Physics Review Letter shows that the particle diffusive flux increases sharply and the particle convective velocity changes its direction inside the plasma torus when a critical plasma pressure gradient is exceeded. The new observations are in agreement with first principle calculations of plasma turbulence.
Particle transport has been studied in the Tore Supra tokamak using modulated Ion Cyclotron Resonance Heating (ICRH) power to generate perturbations of density and temperature. The diffusivity D, and, the convective velocity, V, have been separately determined by fitting the experimental data of the amplitude and phase of the Fourier transform of the modulated density with an analytical linear transport model. As shown in Fig.1, results from ICRH modulation experiments show that when a critical pressure gradient (combination of density and temperature gradient) is exceeded, the diffusive flux increases sharply and the particle convective velocity direction is reversed from inward to outward. Agreement between the experimental results and the Quasi-linear gyro-kinetic simulation is qualitatively satisfactory. It demonstrates that the strong diffusive flux increases and the convective velocity reversal corresponds to the transition of known instabilities namely the trapped electron mode (TEM) and the ion temperature gradient (ITG) instabilities. This suggests that the behavior of the density profile is governed by a feedback loop, as a self-regulating system. When the density gradient is low, only the ITG turbulence exists, which induces an additional thermo-diffusion inward particle pinch leading to a density gradient increase. The density gradient increases until it exceeds the threshold by triggering TEM turbulence, which leads to an outward thermo-diffusive particle convection. In return this action causes a density gradient decrease. When the density gradient goes below the threshold again, the TEM disappears, and only the ITG remains.
