Tokamak heat transport - A study of heat pulse propagation in JET

The propagation of heat pulses originating from sawtooth activity in JET has been investigated in a series of limiter discharges with the following parameters: plasma current, Ip ≅ 3 MA, toroidal magnetic field, BT ≅ 3 T and elongation, κ = 1.45. The auxiliary power was varied such that the total power ranged from 2 to 13.5 MW. Electron temperature perturbations in a 20 cm region around a minor radius of r = (2/3)a were recorded with high time resolution, using a 12 channel electron cyclotron emission polychromator. From these measurements the electron heat diffusivity χeHP was derived. Over the whole range of powers considered, χeHP was found to be independent of power and to lie in the range of 2.5 +- 0.5 m2·s-1. The quantity χeHP is compared to χeGLO as derived from global power balance analysis. For Ohmic heating, the latter is lower than χeHP by a factor of 2.5. For increasing auxiliary power, χeGLO approaches χeHP. A model for the dependence of the local χe on the temperature gradient is presented; it permits a unified description of the heat pulse behaviour, the deterioration of confinement and a certain degree of profile consistency. The model does not invoke non-local parameters such as the total power input. It is shown that the present heat pulse data, subject to this interpretation, contradict the τE scaling laws of the typical form τE varies as to P-0.5. (author). 26 refs, 10 figs, 1 tab