A study of tokamak energy and particle transport, based on modulated electron cyclotron resonance heating
A dynamical technique for the study of tokamak energy and particle transport has been developed. The plasma in the medium-sized DITE tokamak was perturbed by the application of modulated electron cyclotron resonance heating, with wave-launching from the high-field side. These experiments were carried out with absorption at various distances from the plasma centre, over a range of densities. Energy transport through the electron channel was dominant, and the variations in electron temperature and density were measured using the soft X-ray, electron cyclotron emission and microwave interferometer diagnostics. Analysis in the frequency domain enabled the propagation of the thermal wave to be followed. The observed behaviour was generally indicative of diffusive propagation of the thermal perturbation. Further observations indicated a modulation of the horizontal plasma shifts, diffusive propagation to the edge and a low modulation level of line-averaged density. In some atypical cases, the observed behaviour was qualitatively different; this type of behaviour was accompanied by a pronounced sawtooth oscillation locked with the modulation. Two models have been employed for the interpretation of these results. The first model, based on the diffusive thermal transport of the perturbation, has led to results in good agreement with the experimental data. Values of the electron thermal diffusivity were deduced, in good agreement with those obtained from the alternative techniques of power balance analysis and sawtooth heat pulse propagation analysis; such agreement has not been universally obtained in similar experiments. The width of the absorption region has emerged as an important consideration in this analysis. A more complex model, including non-linear, coupled equations of particle and energy balance, has produced results in partial agreement with the experimental data, supporting, to some extent, the presence of coupled transport. It has been demonstrated how perturbation techniques can afford a useful means of testing transport models.