Interpretation of heat and density pulse measurements in JET in terms of coupled transport

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Abstract
The perturbations of electron density and temperature profiles in a tokamak following a sawtooth collapse are considered. An analytic model for the interpretation of such perturbations is presented. It is shown that the perturbation can be decomposed into two contributions, which are eigenmodes of the linearized coupled diffusion equations for particles and energy. The approximations made in the analytic treatment are checked using computer simulations. Measurements of heat and density pulses in JET are used to illustrate the power of the new approach. It is shown that with the coupled equations an improved description of the heat and density pulses is obtained. The analysis yields the four diffusion coefficients in the linearized transport matrix. The non-zero off-diagonal elements explain certain salient features of the measurements, notably a marked decrease of the local density which occurs during the maximum of the temperature pulse.